Battery suggestions for my gas/electric bmx..

Discussion in 'Electric Bicycles' started by HybridHaro, Feb 8, 2010.

  1. HybridHaro

    HybridHaro New Member

    Im weighing the options (literally) for a swap out of the batteries on my tribrid. Right now, the three 12v SLAs are fine. Even the range is acceptable with the gas motor helping.

    The batteries are wayy to heavy. The added weight of the batteries make short work of the rear tire. I get about 250miles on a rear. I need a lighter battery for sure. The battery bag runs in about 40 lbs. Id like to shave that in half by either buying a newer LifePO4, Nimh or laying off beer and snacks for about three months. Im leaning towards the battery option.

    What are the benefits of either? The LifePO4 and the Nimh packs are pretty even in price. I'm trying to find the one that fits my particular application. The Nimh packs Ive seen look like long strips of AA batteries and seem to weigh a little less. Ive seen somewhat cheaper LifePO4 batteries on the 'bay out of Hong Kong. Any opinions of these?

    I only need the battery in starting off and climbing hills (the occasional breeze thru town on electric) so I wouldn't need something that would be heavily drawn on all the time.

    If it matters, the bike has a WE 36v brushed hub..


  2. grouchyolfart

    grouchyolfart Member

    Personally, the new tech batteries are priced out of my reach. I use SLA batteries. Sounds like you have a batt pack with 3 12v 12ah batteries. Try switching them out for the 10ah or even the 7ah batteries instead. They're smaller and lighter. They won't have the duration of the 12ah batteries, but if you're running with the gas motor and also pedaling, they should still fit your needs.
  3. HybridHaro

    HybridHaro New Member

    Good point, I like the price of the SLA batteries too. I was wondering if the shorter life span of SLAs would offset the cost of the more expensive tech batteries.

    Also, how does one know when SLAs start to go south? When I first got the pack months ago, after charge the multi was reading 41.1v now it reads 39.6v off the charger. Is this method a good way of measuring battery life?

  4. AussieSteve

    AussieSteve Active Member

    It's an indication that they're no longer at their prime, but that's all. Varying temperatures, state of discharge at the beginning of the charge cycle, length of charge, charging temperature etc are all factors.

    Regarding battery types, you can't go past Lithium, LiFePo4.
    Sticking with SLAs and downsizing to 7Ah or 10Ah isn't really the solution. Range will suffer and as you say - bloody heavy.

    GrouchyOldFart guessed that you have a 12Ah capacity. Is that right?
    I'd recommend 14Ah LiFePo4. For weight, performance and usable life, they're very hard to beat.
    Start saving your bikkies.

    I should have added that open-circuit, (no load), tests on a battery voltage can be misleading. If a 12V battery has a high internal resistance but all 6 cells are still functioning, the battery will still show a decent voltage on a DMM, due to it's high input resistance, typically 100K to 10M, depending on quality.
    Under even a small load, though, the battery voltage can drop off dramatically.

    .. Steve
    Last edited: Feb 21, 2010
  5. professor

    professor Active Member

    Ryan, you crack about laying off the beer and snacks was a good one. I am delighted to have gained a few pounds this winter-maybe it was the chips. I'm up to 140.
    I am wondering why your tire life is so short. Are you a drag racer? Is the tire all squished when youre on it? Tire contact area is less on the 20 incher but still, 250 miles seems incredibly short tire life. How about stiffening up the tire with a liner or an inner tire as a liner?
  6. HybridHaro

    HybridHaro New Member

    Thanks for the help, I took the bike out today for a quick run. Temp was around 35 deg f (Thats about 2 deg c I think). I the battery pack wasnt quite performing up to par. Im thinking temp was the culprit here. Would anyone suggest using those disposeable hand warmers as battery warmers. I got that idea while on the ride chewing up the last twenty miles of a tire today.

    The reason for the short lives of my rear tire is simply this:

    220lb man + 80lb bike + friction roller setup = 12 bucks every 250 miles

    None of the Goped spindles are the right ratio, so I had to custom make one. I used an old spindle and placed it in the center of a chunk of pvc. Then I whipped up a batch of JB weld mixed with sand and fiberglass. Then poured it in and let set up. It works great and took the contour of the tire. It seems to hook up better than other custom spindles Ive tried. I firgure lightening the load will cut down on the friction on the rear tire.

    Looks like Ill be looking into LifePO4 set up to replace the three 12v 12a SLAs.

  7. AussieSteve

    AussieSteve Active Member

    I was lucky with my LiFePo4 battery. (36V, 10Ah)
    8Ah lead-acid was standard on my bike, but it was only an extra $80AU for the 10Ah Lithium upgrade. They didn't have 14Ah available.
    The base model was steel-framed, too, with an alloy-frame upgrade for $100AU. Of course, I bought the alloy version.

    The weight difference between lead-acid and Lithium has to be felt to be believed. (Not sure of the actual weight of my battery - just checked the ZBike site and it's not listed).
    I can easily lift the battery pack with my little finger.

    ... Steve
    Last edited: Feb 21, 2010