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Thread: Building wheelchair based bot, need battery advice

  1. #1
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    Building wheelchair based bot, need battery advice

    Hi,

    I'm starting design of a bot based on a power wheelchair I acquired a few years ago. The wheelchair was powered by a 24 volt, 75 Ah SLA (well, a pair of 12V in series). The batteries are dead so they need replaced.

    I'm wondering if I should just replace with SLAs or if I should go with LiFePo's?

    And does anybody know a good brand and website to buy either sort of batteries from?

    Thanks in advance.

    This is for my newest bot, Steam Automaton Groucho.
    Last edited by DangerousThing; 04-08-2014 at 03:28 PM. Reason: Spelling

  2. #2
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    Re: Building wheelchair based bot, need battery advice

    I'm not a fan of lead-acid because of the weight and short life. Might be nice for a gigantic UPS because of the lower initial cost, but generally crap for mobile uses or frequent deep cycling. The 7.2Ah pack in my old APC UPS demanded its replacement within a year or so of purchase while experiencing only a few <10 second outages. The 7.2Ah packs in the lab for the IR light source barely lasted a summer of use before losing significant capacity and were never very capable as the stable voltage source they were intended to be. We will probably just hook them, or darsha's LiFePO4, up in parallel with a 12V supply the next time the light source is needed.

    The 12V 20Ah LiFePO4 in darsha is quite nice even though it has not been used much because of my continued brain issues. Should probably get into the lab sometime soon and check the voltage since I have not touched it in months. It should be fine since the cells were at 3.2V nominal for long-term storage, but better safe than sorry. The A123 26650 cell I bought many years ago but never really used, and maybe topped off one time ~1 year ago, is still sitting at 3.29V.

    batteryspace and all-battery are two of the few places that I know to stock prismatic LiFePO4 batteries in >10Ah ratings, but I've only purchased from batteryspace. Did receive a single dud 18500 LiFePO4 cell (1 of 2 purchased) for a freebie LED flashlight and was not worth the shipping to replace (was able to charge, but did not hold that charge more than a couple months), but that is the only issue I've had with their batteries.
    Last edited by tician; 04-08-2014 at 07:58 PM. Reason: 4 cells at 3.2V each
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  3. #3

    Re: Building wheelchair based bot, need battery advice

    As I said in the other thread: I, too, recommend LiFePO4. I got mine from all-battery.com, and it worked fine for two years, but the controller circuitry (not the batteries) died just recently, so my reliability estimate of that part has gone down.
    I also don't know of a really good source for LiFePO4 batteries. The hobby RC people all use mostly older LiPo. The main availability of LiFePO4 for the public seems to be in SLA replacement cells -- you can buy 12V LiFePO4 packs that have the same form factor as previously used SLA cells used in UPS-es and similar. Maybe some of those would help?
    http://www.electric-bike-kit.com/lit...placement.aspx

  4. #4
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    Re: Building wheelchair based bot, need battery advice

    Yes, I think I've searched that site when looking. I love the bamboo casing.

    There are so many options, and I don't know which are worth it or not. Most of these options have to do with keeping the cells balanced, which sounds like a good thing.

    I wish it were possible to charge while the batteries were still connected, but there was an open source project mentioned that has a good description of the process that I can program easily once I find a good and safe place in my house for a charging station.

    I'll look at all-battery, I've seen batteryspace and their prices look OK.

    Thanks,
    Jay

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