The battery game

Discussion in 'Electric Bicycles' started by Fuzzo, May 31, 2009.

  1. Fuzzo

    Fuzzo Member

    Given that batteries are by far the most expensive, heavy, and tricky part of an electric bike setup, it seemed to me that a good approach would be to build a battery pack incrementally, rather than just have one pack for all occasions.

    Then you could:
    A. Add more batteries as you could afford them, or find them for a decent price.
    B. Take only the batteries that you need for any particular trip
    C. Charge up some of your batteries whilst still using the bike, leaving them at some power point for a few hours.

    This thread continues my introduction thread - http://www.motoredbikes.com/showthread.php?t=20392 without continually bumping and pushing out the new introductions - it seemed like there was a lot to discuss in what I was aiming to do, and was thus worthy of it's own thread.

    I was very rapidly made aware that chucking more batteries in is by no means as simple as that. In fact, it borders on the impossible, by some accounts. There are many complications, firstly in mixing battery pack voltages (don't do it at all), mixing battery pack capacities, and mixing battery chemistries.

    Now I don't need to be told that the safest way is to just buy the very best batteries with the capacity you need, all at once, all to exactly the same spec, and Bob's your Uncle. But unfortunately, that means:

    1. Knowing exactly what capacity you need. Since my trips are not all the same length, under the same load, I don't know this in advance.
    2. Taking a risk on the entire pack - you could buy thousands of dollars worth of batteries and then find they are not up to scratch.
    3. Discarding or using for something else any previous batteries, which seems wasteful.

    Now, this could turn out to be the most practical advice in the end anyway, but I'm not prepared to give up on the idea of incremental building just yet, for the reasons A, B and C, and 1,2, and 3. Every battery is still a source of power, and, in theory, circuits can be contrived to allow different capacities and chemistries to live together in harmony. Is it practical? That's what this thread is for.

    The introduction thread has a few pages of discussion already, so make sure you check that out first, but please don't leave any more comments there.
     

  2. Fuzzo

    Fuzzo Member

    To contextualize, I have a pedal-electric hybrid, which is 200W and has 2x12V 12Ah SLA batteries. This gets me about 30km in Auckland, NZ, if I'm just riding around the city. Which is cool, that gets me to the city and back easily. But I'd also like to be able to ride to the surf beaches on the West Coast, particularly Piha. There is a mountain range in between. A couple of weeks ago I had a go, just to see how far I'd get, and was pleasantly surprised to find that I could get to the summit of the range on the current setup. But the battery was totally exhausted. From the summit to the beach there are some more ascents before a long steep downhill. So I figure I need about double or triple the current capacity. Double if I can recharge on the other side, triple if I want to do it all on one charge. My SLA pack is fully detachable, so it is feasible that I could exhaust that on the trip to the summit, and then just hide it somewhere until I get back (it's a forest, so that wouldn't be hard), so as not to carry it most of the way as dead weight. I could even charge it up at the summit, there is a cafe there, but I don't know yet how they'd feel about me leaving it there for hours.

    So ideally I want battery packs that can be used either together, or individually. If they are used individually, it strikes me that the complications of merging the packs drops dramatically, because I basically won't - I'll be switching between, and the only circuitry required will be switches. But there are some advantages to using them together - particularly for SLAs, which drop capacity drastically if put under heavy load.
     
  3. Fuzzo

    Fuzzo Member

    Complete aside, I tried a less ambitious ride over the last couple of days, to get in training for this assault on the West Coast. I decided to ride over to my parent's beach house on Waiheke Island with my 3 year old son, Marcus. This involved the well-known 10km trek into the city along the Northwestern Cycleway, a 1 km downhill through the heart of Auckland City, a 20km ferry trip to Waiheke, and then a totally unknown 6km ride to the beach house, which has no road access. A nice way to celebrate the birth of our glorious Monarch HM Queen Lizzie 2 who in Her Majesty's royal wisdom extended the weekend by one day.

    The trip over was lovely, with my boy singing as we rode the cycleway under a cool but clear sun, and looking around goggle eyed at the buildings as we descended to the harbour. The ferry was free for both him and the bike, a pleasant surprise. We sat on the back deck beside the bike, and watched the city recede to the horizon, as the volcanos of Auckland marched past on either side. I sipped a passable cup of coffee, and Marcus munched his way through a small bag of crisps. Then we cruised onto Waiheke, the most notorious nest of hippies in all of Auckland, another hilly volcanic island. The trip through the main town was a breeze, and then the last ascent was faced. I got about a third of the way up, and I certainly could have made the entire climb, but I've got a bit of a cough so I figured I better not stress myself out too much, and walked the rest. A nice side-effect of electric bikes, is that even when they are not really up to carrying you up a hill, they're still strong enough to carry themselves, so pushing up a hill is no more effort than just walking it, if you can keep your hand on the throttle. Anyways, 5 minutes of walking and I was looking out over my parent's little slice of heaven from the top of a cliff with a panoramic view of the Hauraki Gulf. I opted to walk the bike down the path to the house - too steep for rim-brakes, especially carrying a little boy at the time.

    A lovely day was spent with my parents and some of their friends in the beach house, and Marcus was so tired that when he fell out of bed in the middle of the night, I came in to find he had not even woken up from the fall, and was sleeping soundly rolled up in the duvet on the floor.

    The trip home was not quite so pleasant. I took too little heed of the weather warnings, and opted to catch the 4pm ferry home. When I got to the ferry, a strong, cold southerly had whipped up, and the ride home was very rocky. The bike took a tumble at one point, despite being tied up with a bungy cord, so I lay it down for most of the trip. Other cyclists were typically inquisitive, and when I told them how far I was riding it, they looked astonished, being all dressed in gimp suits themselves, they looked almost disbelievingly at me standing there in jeans and a sweater, with a backpack and carrying a child to boot. Anyways, because of the weather the boat went very slowly, and by the time I was back to Auckland, it was raining steadily, and the sky was darkening. I decided to catch the train home, but was foiled badly in the plan by Auckland's truly pathetic train service, which was yet again not working, and diverting passengers to buses (which I can't put my bike on). My wife is a very nervous driver in the wet and dark, and in the city, so I opted to just grit my teeth and ride home. By the time I'd got to the top of the city, my hands were so cold and wet that I couldn't change gears! I simply couldn't squeeze the gripshift hard enough. Which upped the workload for human and motor significantly. But I pushed on, and got to the cycleway, by which time the exercise had warmed me up enough make it less unpleasant. But not so for poor little Marcus, who was starting to get cold. The poor little mite was very brave, and we sang songs to keep his spirits up, but the last 5 km were pure misery for him, shivering as a strong cold wind and rain sledged him mercilessly. He forgave me after a hot meal and a warm bath, but I haven't forgiven myself, and will be getting him much more appropriate clothes for the cold and wet (which can strike unexpectedly), particularly some windproof pants and some warm gloves. And more importantly, I'll make sure we don't end up getting caught by advancing weather and night nor relying on trains ever again.
     
  4. duivendyk

    duivendyk Guest

    I was once on a cycling trip up the West Coast of Scotland,even in a rainsuit it was miserably wet&cold, and that in the summer. On top of that my SA hub got stuck in middle gear.Not fun but J&B helped.
    I took a look at your Piha? route on Google.Looks about 25-30km from S Auckland, you get to Titirangi and from there a steepish climb judging from the zigzags about 10km to Waiatarua then something like 15 km to the W coast.What is the relative elevation/distance to the level? portion on the way to W,and what gives on the climb back southfrom Piha.Is the top part East back to W fairly level?
    I guess that you might do better relegating the SLA's, to riding around A. I also believe that the ride to Piha prob needs a more powerful motor and more&better batteries.What sustained output are you yourself realistically capable of (200-250W ?).I would like to do some power/energy calcs, to get better handle on the coast trip situation esp these climbs.That's where the batteries take a beating.(low speed, high torque and poor eff.).What about are the weights ,bike+rider+eqt ,and motor+battery?
     
  5. Fuzzo

    Fuzzo Member

    It's about 30km from my door to Piha. I don't know how to work out elevation other than with a GPS which I don't own. My guess is that the climb to Waiatarua, and the climb from Piha to the first descent at the top of the range are about the same. What is unknown is how much up and down there is in the range itself. I have done the trip a thousand times in cars, but you don't really get a feel for the actual climbs that way.

    More and better batteries I totally agree with. A stronger motor? Not so sure. Won't that just chew the battery up faster?

    At the moment I'm not really sure of my sustained output. 200W sounds on the high side, I'd be guessing sustained 100-150. But no idea how to calculate it.
     
  6. duivendyk

    duivendyk Guest

    150 W may well be the upper limit,unless you're a trained cyclist and in good shape.Higher power motors are more efficient at the same load than lower powered ones.You can throw a lot of power away due to the inherent low motor efficiency at low speed.So if you have to slow down because of power output limitation,you end up in the worst possible situation,low speed high current,a long assent too and lots of energy loss due to low efficiency,certainly not where you want to be if you can help it.
    If you go flat out on level ground (no asist) what is your max speed about?.May be there is a topographic map around to get an idea of the elevations involved.
     
  7. Fuzzo

    Fuzzo Member

    So, anyway, my current thinking of how to increase the total battery power goes right back to my original plan of adding battery packs in parallel until I have enough. As an approach this seems sound in every way, except for the danger of destroying the weaker packs. As I understand it, there are 3 dangers:

    1. Backflow. If one pack is delivering less voltage than the other, then current is being forced into it. This is firstly a waste since there will be losses, particularly if the weaker pack is fully charged - it will have to just burn off the excess as heat. Secondly it can damage the weaker battery if the current is too high or it's already full and burning off the excess.

    2. Complete depletion. One of the packs could become completely depleted and if its chemistry doesn't like this, it could be harmed.

    3. Excessive draw. If the weaker pack is not actually up to the job of delivering enough current to power the bike, and for some reason it has the higher voltage at some time, it will overheat trying to deliver the power and could die.


    Can these dangers be addressed? It seems that some of them can.


    1. Seems to be soluble via diodes protecting each parallel pack from backflow. These diodes would ideally not be hard-wired to the batteries, because they will prevent charging. They would be plugged into on the bike.

    2. Depleted batteries can be disconnected. The big danger here is that some battery types can go to lower voltages than others whilst still delivering useful power. The way I see it, if you know how low each type can go, then you can disconnect them when they get that low. Requires a voltmeter across the entire array, but I think that's a worthwhile meter to have anyway, it's as close to a fuel gauge as there is for batteries.

    3. Is the trickiest one, as I see it. For my 24V 200W bike, I could stick 20 1.2V NiMH batteries in series and put that in parallel with the existing SLA pack to give an extra 2.5-3 Ah. But if the voltage of this pack was higher than the SLA pack by even a little bit, then the bike on full throttle will draw 200W straight from the NiMH pack, with a current around 20A. This is about 8C for the batteries, and if they're not rated for that kind of draw, they'll burn up. So the solution for that is either:
    A. Make sure the batteries can handle the draw. This means expensive batteries.
    B. Get a lot of batteries. If my batteries were rated for 1C, then I'd need 8 packs of 20 to handle the draw. Which would, of course, give me 8 times the extra Ah too, so I'd get around 20-24 Ah more, which would approximately triple my current 12Ah build.

    So this gives me 3 potential plans for increasing battery Wh.

    i. Just add SLAs exactly like what I have.
    ii. Add small expensive high rated packs in parallel with protective circuitry and put a voltmeter across everything (to be honest I'd like to put a voltmeter in anyway).
    iii. Add many cheap low rated packs in parallel with protective circuitry and the voltmeter.
    iv. Completely replace battery pack with alternative.

    Cost-risk-benefit analysis of the various plans:
    i. Cheapest Wh, indeed cheapest overall. Probably the safest plan. Very simple. And I can use the existing charger or get another cheap one. But very heavy and bulky. The current pack is 7kg. So I'd have to add 7kg or 14kg to double or triple. SLAs are also problematic when drawing at 1-2C - so I probably don't really get my Wh.
    ii. The second lightest solution. I could go for LiPo or something like it. In the middle in terms of safety - probably the packs will be able to handle the load, even if they have quite low Wh. Cost is the big drawback - the batteries are expensive in NZ, and not easy to get either. I'll need a charger for each pack.
    iii. Wh/$ in the middle of i. and ii. Weight also in the middle. Fiddly work involved in building the packs, unless purchased premade. Will need at least one but probably more dedicated chargers.
    iv. The lightest solution, probably the safest too. And also the most costly by a large margin. Current quotes I have just to equal my existing setup Wh are around $700 NZD (battery and charger). To actually improve upon it, to get me to the beach, I might need to outlay something like $2000. Contrast this with about $200 to just buy 4 more 12V 12Ah SLAs.

    Currently I simply can't afford plan iv. In NZ, that can buy me a decent motorbike, which, to be honest, I'd rather have than a bunch of battery packs.

    Plan ii. is only a third cheaper - by basically one $700 pack. Again, I'd rather buy a motorbike.

    Plan i. is too heavy and bulky. I might have space on my bike for another 7kg pack, but not 2 of them. So realistically, this plan can only double my range. I don't actually know if this would be enough. Maybe it would. That would cost around $100, and the only challenge is "where to put the batteries". The bike will be noticeably heavier.

    Plan iii. is in the middle in most ways. Cost wise, AA NiMHs at 2.5AH are about $3 if in bulk, so 160x$3 = $480. The weight of that is about 5kg. The cost of the chargers is currently unknown. At 5kg extra, I could triple the Wh of the current setup. I'm pretty sure this would meet my required energy to get to the coast and back. It also has the bonus that, if it works, it might inspire me to drop the SLAs altogether, in favor of another 80 NiMHs, thus shedding 5kg of SLA weight (and allowing all the batteries to fit nicely into the existing carrier).


    Which is a very long way of showing why I'm in favour of plan iii at the moment. Can anyone see a major flaw in my reasoning? Or perhaps an improvement? The part which is most unknown is the cost of the chargers for both NiMH, and LiPo. A potential improvement would be some way of getting around issue 3 above - excessive draw. If there were a way to limit the current drawn from a pack, then I could experiment without having to outlay around $500 before finding out if the idea isn't just a total lemon right from the start. I've already got plenty of NiMH batteries I could experiment with.
     
    Last edited: Jun 3, 2009
  8. Fuzzo

    Fuzzo Member

    No pedaling speed on the flat is around 23kmh. With light pedaling, around 26kmh, but I'm pretty sure the motor stops contributing around 24kmh. This is based on the observation that at 26kmh, if I turn the motor off, I can't feel any difference, and continue at that speed. I can keep this output up all day long. With moderate pedaling I can do 29kmh, which I could probably keep up for about 20-30 mins, by which time I'd be totally exhausted. With hard pedaling, I can take it to about 35kmh, but only for bursts of a minute or so. Curiously, this can be repeated, if I get sufficient rest in between, for over an hour. So when going up a steep hill, I can alternate between putting in at this rate, and hopping off and walking the bike, about 1-1. I put this down to the nature of my sports training - I was also a waterpolo player, mostly as goalkeeper, and also I've always done a lot of martial arts, both of these are high intensity, with long rests. So the hill ascent ends up being like HIIT for me - High intensity interval training. They are not 'constant high burn', which I just can't do. Which, ironically, might be just what the bike needs on a hill ascent - when I'm on it, it's on full sauce, as am I, and we can ascend quite steeply at between 10 and 20 kmh. Then I tire, and the speed drops hugely, and I figure that I'm just wasting power sitting on the bike, so I walk it for a bit. The bike can carry itself up the hill on 1/3 throttle so the walking part is easy on both of us. When I feel the muscle sugars have built up enough, I jump back on and knock out another burst.

    If there was a super-low gear, I might not have to do all this jumping off and on, but I already have a "mega-range" cassette, designed for hill climbing, and the lowest gear is not sufficient for the steepest inclines on this particular range. I could add a front derailer to extend the ratio a bit.

    Which bring me to one of my biggest gripes with pedal-electrics - I don't use most of my gears. It's quite annoying that I have 7 gears, and yet I don't have a really low and a really high gear. I had to alter the bike to get the super low gear, but there simply was not any cassette that gave me wider range at both ends. I'm mighty glad to have the super low gear, any time I hit a really steep hill I can pretty much power up it now (around town that is - mountain range ascents are different). But going downhill the bike easily gets up around 50kmh on quite typical hills in Auckland, and I just can't pedal that fast. And yet there's about 3 gears that I just don't use on the bike, which could have been available for higher speeds.

    I think the reason for that is because the cassettes are designed around non-electric use, where minor adjustments in a narrow cadence band are common. But with electric use, the minor adjustments aren't needed. You can (and do) take up the slack with the motor.
     
  9. Fuzzo

    Fuzzo Member

    I'm sure there's truth to that, but seriously, if I'm going to be replacing the motor, then I've also got to replace the controller. Considering that I'd also be replacing the batteries, the only thing I haven't replaced is the bike itself. Since the bike is actually the cheapest part, I might as well do that too, to get myself some weight back. Which is exactly what everyone flogging off LiPo batteries has been saying "Why don't you just buy a new bike?", after which they quote me $2500 NZD for their lamest one, or twice that for the fully geared up one. Given that my bike cost me $200, with so far about another $150 spent on various little improvements that I can keep (odometer, mega-range cassette, better tyres, headlamp, child seat, carrier, repair kits, clothes), I'm loathe to multiply my outlay to the price of decent car, just to get a bicycle that I sit on without exercising (which seems to be the ultimate ambition of most people looking to sauce their bikes up). If I wanted that, I could easily get it in a $500 scooter, and $2000 worth of gas would get me around 30,000 km, which is probably about 10 years worth of scooting about for me. Or I could get a $2500 motorbike and rip around at 200 kmh until I got killed, which would probably be somewhere on my first tank of gas.

    Now I appreciate the point that my bike might not be up to this mega marathon I've proposed, a 60km round trip over a very steep mountain range. But so far, it's got me to the top once. That suggests that if I triple the battery Wh, I should be able to make it. Maybe just doubling it will actually triple it, since it's currently likely that I'm losing a goodly proportion to the high rate of discharge. In which case, I can save myself a couple of grand and get the motorbike *as well* as have my primo exercise equipment, only mildly modified.

    Hence my focus on the batteries. So far the power has been enough. It's the fat-bottomed SLAs holding me back, I'm sure of it.
     
  10. safe

    safe Active Member

    I just switched from SLA's to my own home made NiCad battery pack using a design that is "solderless". It's a lot of work to put together, but being solderless I can remove the batteries as needed to replace the bad cells. All battery chemistries tend to have 5% of the cells a little weaker than the rest and those tend to die off first which brings the whole pack down. Replace the "runt" cells and the pack comes back to life.

    SLA's are either all or nothing... they are 12 volts and all 12 of the volts goes bad at once. (or at least you have no ability to correct an SLA subcell since they are actually made up of 6 subcells)

    NiCad's are about half the weight of SLA's and last the longest of anything out there. LiFePO4 cells are much lighter, but you need some fancy electronics to keep them from self destructing themselves.

    If you've followed ebikes for a while you will hear stories of people spending $2000 for LiFePO4 cells only to have them self destruct and be useless.

    This is a "hole in the wall" Chinese company that sells LiFePO4 cheaply (relatively) and many have had acceptable results:

    http://stores.shop.ebay.com/PingBattery__W0QQ_armrsZ1

    My NiCads look like:

    [​IMG]

    ...it's made up from PVC tubes and uses spring loaded SubC NiCads.
     
    Last edited: Jun 4, 2009
  11. duivendyk

    duivendyk Guest

    Paralleling dissimilar batteries of significantly different capacities is a bad idea,and does not accomplish all that much either,they ought to have at least approximately equal capacities
    Monitoring the output voltage of such a parallel combination does not tell you all that much about the current division between them.Looking at the voltage changes (under load !)that is A, B, A+B gives some info.The individual current monitoring using small resistors in series with the batt. tells you a lot more.
    The homogenizing scheme of a 12V SLA + 12V NMH (or Nicads) in series and two of these in parallel with the crossover switch to turn them into two separate 24 V SLA/24V NMH batteries has a lot going for it,by sidestepping the crossfeeding& loadsharing problem.
    Nicad batteries have a memory effect,if not completely discharged before recharging their capacity tends to decrease.
    Low speed hill climbing is the pits for these kind of motors,high torque at low speed means poor eff..But total power available (rider+ motor) will limit motor speed.A lot depends on how steep the incline is.
    I have derived a useful expression relating the power P (in Watts) to climb a hill to the weight (W in kg), the incline I (in %) and the speed S (in km/hr).Here goes:
    P= 0.027xWxIxS Watt.For instance W= 130kg ,I= 4% and S= 10 km/hr ,P=140 Watt
    This does not include the power needed to overcome the rolling&aerodynamic drag,prob. another 25-30W?
     
    Last edited by a moderator: Jun 4, 2009
  12. Fuzzo

    Fuzzo Member

    What's your recharger solution? Dui also suggested that you had some experience mixing battery chemistries. How did that go?

    I take it to find bad cells you can firstly measure each output to find the weak one, then just slide the batteries out of that tube to test each one?

    Where does any vented gas go?
     
  13. Fuzzo

    Fuzzo Member

    Dui, your series scheme is worth trying, particularly from the safety point of view, although the complications of charging will need to be solved. You already suggested a double-pole-double throw could switch from parallel to series - seems to me like it's worth trying, and goes to one of my key aims - incremental pack building. Am I correct in assuming that I could add as few as 10 1.2 volt batteries in series to each 12V SLA and not risk overdrawing from the little batteries? If so, that's ideal - then I can add more 10-battery packs as I see fit, in parallel with each other. Crossfeed and balance will be less of a problem because they are the same spec and chemistry. They'll still be in series with the SLAs.

    Your power equation surprises me. It is linear on speed, so going half the speed should draw half the watts. Which suggests it doesn't matter what speed you are going, the efficiency will be the same - half the watts over twice the time is the same total energy used. Extrapolating a little further, it would actually be most efficient to go slowly, as the drag factors are at a minimum, and the Peukert effect is less.

    Have you left something out?
     
  14. safe

    safe Active Member

    Did you know that's a myth? (obviously not)

    Apparently NASA sent up a satellite with some NiCads and had them set up in such a way that they discharged and recharged in a pattern that was identical from day to day. It was the regular routine that caused the problem. Ebikes tend to use their energy in unpredictable ways... you can never be sure of exactly how much energy is going to be used and how fast.

    I've found with my NiCads that at first they tend to behave rather sluggishly until I start to use them more often. The harder you push them the more they like it... NiCads like hard abuse.

    NiCad's only get a memory effect if you do the opposite and use them gently and predictably.

    Treat them like a woman... they like a rough ride now and again or they bored... :grin5:

    -----------------------------------

    http://www.repairfaq.org/ELE/F_NiCd_Battery.html#NICDBATTERY_014

    2.9) Does the memory effect exist?

    <Flame shields on> YES

    Just as everyone is running around and saying that the memory effect is a myth, here I am, saying that it is true. OK, so, why is this? First of all, the term memory effect is quite unscientific. People tend to attribute any failure of a NiCd to memory.

    Let us define memory as the phenomenon where the discharge voltage for a given load is lower than it should be. This can give the appearance of a lowered capacity, while in reality, it is more accurate to term it voltage depression.

    Memory is also hard to reproduce, which makes it hard to study. Originally, memory effect was seen in spacecraft batteries subjected to a repeated discharge/charge cycle that was a fixed percentage of total capacity (due to the earth's shadow). After many cycles, when called upon to provide the full capacity, the battery failed to do so. Since we aren't in space, the above is not really relevant...

    Let us look at various causes of "memory" or voltage depression.

    Memory can be attributed to changes in the negative or cadmium plate. Recall that charging involves converting

    Cd(0H) to Cd metal.
    2

    Ordinarily, and under moderate charging currents, the cadmium that is deposited is microcrystalline (i.e. very small crystals). Now, metallurgical thermodynamics states that grain boundaries (boundaries between the crystals) are high energy regions, and given time, the tendency of metals is for the grains to coalesce and form larger crystals. This is bad for the battery since it makes the cadmium harder to dissolve during high current discharge, and leads to high internal resistance and voltage depression.
    The trick to avoiding memory is avoiding forming large crystal cadmium. Very slow charging is bad, as slow growth aids large crystal growth (recall growing rock candy). High temperatures are bad, since the nucleation and growth of crystals is exponentially driven by temperature. The problem is that given time, one will get growth of cadmium crystals, and thus, one needs to reform the material. Partial cycling of the cells means that the material deep with the plate never gets reformed. This leads to a growth of the crystals. By a proper execution of a discharge/charge cycle, one destroys the large crystal cadmium and replace it with a microcrystalline form best for discharge.

    This does NOT mean that one needs to cycle one's battery each time it is used. This does more harm than good, and unless it is done on a per cell basis, one risks reversing the cells and that really kills them. Perhaps once in a while, use the pack until it is 90% discharged, or to a cell voltage of 1.0V under light load. Here, about 95% of the cells capacity is used, and for all intensive purposes, is discharged. At this point, recharge it properly, and that's it.

    The more common "memory effect" isn't memory at all, but voltage depression caused by overcharging. Positive plate electrochemistry is very complicated, but overcharging changes the crystal structure of the nickelic hydroxide from beta-Nickelic Hydroxide to gamma-Nickelic hydroxide. The electrochemical potential of the gamma form is about 40 to 50 mV less than the beta form. This results in a lower discharge voltage. In a six cell (7.2v) pack, this means a loss of 300 mV. Trick? Don't overcharge. Leaving cells on a trickle charger encourages formation of gamma nickelic hydroxide. Expect the cells to discharge at a lower voltage.
     
    Last edited: Jun 4, 2009
  15. safe

    safe Active Member

    My NiCad SubC's can deliver 10C (that's ten times the rated amp hour capacity) current. They do this with a minimal Peukert's Effect so that means you are much more free in your ebike design as you can drain them rapidly if you so choose. (if you actually try to use 10C all the time the NiCad's get hot)

    SLA's have a really bad Peukert's Effect and in general you definitely want to get as close to 1C as possible. So let's do the math:

    Four 12 Volt 18 Ah SLA's batteries.

    Connect them in series to make a 48 Volt 18 Ah pack.

    Now use a controller that has a 20 amp current limit (1C) and you can now have total power going to your motor as:

    48 volts * 20 amps = 960 watts

    Minus motor losses:

    960 watts * (average) 75% = 720 watts

    -------------------------------------

    So you can get a one horsepower bike this way.

    Weight of the batteries:

    14 lbs * 4 = 56 lbs

    ...so you need about 56 lbs of lead to build a basic ebike using SLA's.

    -------------------------------------

    My NiCad pack stores about the same energy, but weighs about 35 lbs and can deliver a lot of power if you wanted to:

    260 amps * 24 volts = 6240 watts (4680 watts output... 6 horsepower)
     
    Last edited: Jun 4, 2009
  16. Fuzzo

    Fuzzo Member

    NiCd looked like a good option, but you get a lot less Wh/kg, and Wh/$ than NiMH. I can appreciate if you're loading them up big time, drawing 6kW then yup, you have to have batteries that can handle it. But I'm not doing that, I'm hitting them with 200W. My aim is currently range, not speed.

    I'm just extrapolating from the equation Dui presented. I'm sure we both know it's not as simple as that - keeping the motors at efficient RPM doesn't figure in there anywhere.
     
  17. safe

    safe Active Member

    NiCads and NiMh end up being the same price.

    http://cgi.ebay.com/100-NiCd-Sub-C-...photoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

    NiMh cost twice as much for the same size cell, but then deliver twice the energy in total. However, the NiCads win in the end because they last twice as long. But the NiMh win it back because you are getting effectively double the battery for the same weight.

    My solderless tubes hold SubC cells of WHATEVER I want to load into them... so if I want to spend more money I can always upgrade. With NiCads and my advancing age (I'm nearly 50 years old) the odds are that the batteries outlive my own useful lifetime. (by 60 years old I won't be able to do much with these road racers anymore)

    -----------------------------------

    I've written a full scale ebike road course simulation using the GNU language. This is the formula I used in that simulation:

    #
    # PowerHillClimbing(w)
    # --------------------
    #
    # Global Parameters
    # -----------------
    #
    # slp : slope of hill (percent)
    # wt : weight of bike plus rider in kg
    #
    # Function Parameters
    # -------------------
    #
    # w : rotational speed (rpm)
    #
    # Returns
    # -------
    #
    # watts
    #

    PowerHillClimbing(w) = slp * wt * 9.8 * Velocity(w)


    ...one of these days I'll post that again. There are some bugs and upgrades I want to do on it. (maybe next winter)

    "w" is the rotational speed of the motor in the simulation.

    The Velocity has to go through a bunch of other calculations in order to connect motor rpm to the rear wheel speed. That's where you're going to run into troubles because you need to consider things like gears and tire size. That 0.027 constant probably only applies to his bike. (it's not a universal constant or anything like that)

    Anyway... the only way to really get much information of value is to do the complete simulation of an ebike and run it over an imaginary course and see how it performs. I use it to find ideal gearing for the courses I ride on.
     
    Last edited: Jun 5, 2009
  18. duivendyk

    duivendyk Guest

    This equation has nothing to do with the means of propulsion ,it could be a steam engine.It's just basic physics.It takes so many Watts to move a certain weight up a certain incline at a certain speed,disregarding rolling &air resistance and the drive train eff. including the motor. You can rework it if you like HP for power instead of Watts (1.0 HP= 745 W, so the constant becomes 0.027/745=0.000036,or if speed S in mph instead of km/h, multiply constant by 1.61. It becomes 0.0435.
    I can give an expression of the probable motor eff. at different speeds based on the eff at max.voltage. (a bit more complicated )
     
  19. Fuzzo

    Fuzzo Member

    Yup, I got that it was a very rough figure of the energy required. You're talking about watts at the axle rather than watts coming out of the battery. The relationship between the two won't be straightforward.

    The hard part is working out the incline. It's a winding hilly road with lots of ups and downs. I can give a total ascent over distance to give an average gradient, but that's not going to account for extremes in any way. I could do the same for a ten kilometer ride on the flat with a vertical cliff at the end, but it's pretty clear that the vertical ascent will require more maximum power than a ten kilometer long incline to the same height. Furthermore, Piha is at sea-level, as is the starting point at home, so the average incline there is zero. The ride from Waiatarua to the top of the Piha hill actually drops quite a lot, but that doesn't mean I can coast the whole way - it's a mountain range, with many steep hills along it.

    Also, at this point, I'm not really considering changing the power of the motor. Which means that if more power is required at any point than I have available, I simply have to drop my speed, right back down to walking the bike in the extreme cases. I'm fine with this, since actual experience has shown me that the length of those stretches is not that great. It's not hard work walking alongside an electric bike on 1/3 throttle carrying itself and my bag up a hill. If it all gets too much I can always stop and have a rest. Or give up and go home.

    What I can't do is put any more power into the battery. So the battery needs to have enough in it for the entire trip, or I'll be walking it most of the way.
     
  20. Fuzzo

    Fuzzo Member

    I guess overvolting is another possibility. I haven't really considered it much, having this gut feeling that it could just totally wreck the motor and/or controller. But it would make the battery game a little easier. I could make 12 volt packs which are parallel with each other, but added in series to the SLA, giving me 36 volts, which I would guess could lift the power from 200 to 300 watts (still legal in NZ). Any thoughts on this possibility are welcome.
     
Loading...