NZer on hybrid

[Fuzzo]

Must get me one? I don't relish the idea of charging up 30 batteries in a standard charger every time I go for a ride.

 

[heathyoung]

No, it isn't practical. To charge large NiMh packs too (especially in a series/parallel configuration) the parallel packs should be charged separatly.

I know its a PITA but its doable with shottky diodes to isolate the packs - this allows you to plug in chargers for each pack, and charge each pack in isolation, but also allows the packs to combine and - more importantly - not to try and charge weaker packs (cell reversion - death to NiMh). I designed a setup like this that also did LVC on each pack using a Mosfet and a Zener diode.

Don't get discouraged with this, I know its sounds complex, but its worth it.

 

[duivendyk]

How the current between two parallel batteries divides depends on their internal resistance.For instane if one has 0.1 Ohm and the other 0,075 Ohm resistance,and the first battery delivered 7.5 A,the other would supply 10A (7.5x0.1=10x0,075),not only that ,the discharge characteristics of leadacid and these other batteries are different also.So it all becomes a chancy proposition IMO.It would make more sense to 'homogulate' the batteries from an operational point of view,that is put a 6V Nicad in series with a 6V SLA and put the 2 12V batteries thus created in parallel.This would work better when using them,but might create problems charging them,unless you come up with some fancy switching arrangement (is doable and a good mental exercise).Hint : you can actually do it with a one double pole double throw switch

 

[Fuzzo]

I don't really have a problem charging the SLAs separately to the NiMH. I just don't want to have to charge the NiMH packs separately from each other, if possible. My ultimate aim is to drop the SLAs, and use them for something else. But until I get my desired range, I'm keeping them!

Heath do you have a diagram of what you did? I'm pretty sure I know what you mean, but then again...

 

[duivendyk]

To seperate the SLA's&NMH's from the composite arrangement SLA+ NMH in series.Imagine 2 parallel strings,each consisting of 2 SLA and 2 NMH batteries in series.At the interconnection point in the middle of each string put a double throw switch that allows you to connect to either the original battery(of the same kind) or to the one of the other kind in the other string,crossing over so to speak.So with a double pole 2 position switch in the middle ,you can either have 2 batteries of the same kind in series,or two of different types.So that is not all that hard to do.I know little about charging NMH or Nicads in parallel strings except that it's not a good idea.

 

[Fuzzo]

If I'm understanding you duivendyk, you're suggesting I make the NiMH into 6V packs and string those in parallel to each other and in series to a 6V SLA each. What's the advantage to that over just having 2 12 volt packs, SLA and NiMH in parallel? I can see that it would balance the internal resistance better.

I have never tried this, but my assumption was that 2 batteries of the same voltage in parallel should end up pretty much balanced in their depletion under load, as the load would draw more from the battery with the higher voltage. Is this not correct?

 

[duivendyk]

Your assumption is not necessarily true,not if the two battery types have different internal resistances.Do you remember a previous post in which I pointed out that if you put two dissimilar batteries,A and B with the same nominal voltage and the same AH capacity in parallel but which have different internal resistances the current drawn by each divides as follows :
IaxRa= IbxRb or Ia/Ib= Rb/Ra, so if for instance Rb= 0.8 Ra then Ia= 0.8 Ib,so the B battery with the lower internal resistance gets discharged more quickly.Of course if that battery had a higher AH capacity lets say 25% higher this would havebeen fine&dandy
The internal resistance of a battery goes up as it discharges in fact that how the battery condition manfests itself,the noload voltage does not change all that much but the voltageunder load plummets.It so happens that Nicads don't show much voltage change under load until almost fully discharged and then collapse (Nmh might be similar)
So you can appreciate that if you come up with a series combination af A and B batteries you have a composite battery AB if you make another one you have two identical batteries they will share the load equally.Imagine two 6V SLA's in series and two 6V Nicads in series side by side if you cut the connections in the middle and crossconnected such shat the left top SLA is connected to the right bottom Nicad and conversely the right top Nicad is connected to the left bottom SLA.you have created to identical composite batteries with (pretty much) identical internal resistances,and if you stick a double pole(2circuits) double throw (2 positions) switch in the middle you can switch back&forth between the both configurations.I must admit I'm not an authority on how to play this sort battery game,this seemed to me a way to play it safe.The other possibility would be to use them sequentially.You could ask SAFE, he has been mixing SLa's& Nicads and has a lot of practical experience messing around with batteries.Send him a PM and ask his advice,he seems pretty helpful.

 

[Fuzzo]

It's worth trying all of that, thanks JJ.

Would another possibility be to tweak the resistance on the less resistant pack by adding resistors? I guess the aim is to have all of the batteries mostly depleted at the same time, so the bigger Wh pack needs to draw more current. How much more exactly can be both calculated in theory, and measured in practice.

 

[duivendyk]

That appears rather doubtful I'm afraid.In theory if the capacities were inversely proprtional to the internal resistances they would lose their charge at an equal rate.The discharge characteristics of different types of batteries are not the same and furthermore are also dependent on the discharge rate,nor is that dependency the same to the best of my knowledge.Looks like a can of worms to me.Throwing power away just to equalize relative discharge rates,somehow does not seem appealing.As I pointed out just by flipping that crossover switch you can switch configurations,alternatively you could do the alternating battery routine.You could put a diode in series with each battery (of one kind) to isolate them from one another,if you parallel them,so that they can't crossfeed .The best type would be a power Schottky diode (if such a thing exists),a silicon diode has close to 0.7V drop which is not insignificant ,the Schottky about half that.Again remember I am not the foremost battery expert on this forum probably,and I hate to pontificate about things I don't know a lot about.But you should also be aware there are people who have firmly stated opinions which are not backed up by either knowledge and/or experience, I'm sorry to say.You could start a thread on this subject,it could elicit useful info from members who have real-life experience with inter-mixing battery types.

 

[Fuzzo]

Thx JJ. I'll search the site a bit before I charge ahead and start a thread on this. I can't work on it for a while anyway, while I'm still fitting out the garage, so I'm just collecting information and ordering parts at the moment. While I understand the crossover switch idea, I have to say that any arrangement where I can keep the wiring of the SLAs and the NiMHs separate appeals more, as it is likely that I will detach one or other pack at various times, perhaps to leave them somewhere to charge, while I continue to ride on the other pack.