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Using ebike lifepo4 battery packs for solar

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  • Using ebike lifepo4 battery packs for solar

    Hi all, I am looking at buying a 48v 40ah lifepo4 battery pack off eBay. Has anyone got any experience of using these in a solar rig? Having read the sticky on charging lipos, I am going to disconnect the BMS and I'm prepared to be hands on with the charging regime. My worry is the charge rate. All the ones I've seen say 8A max charge but I want to charge it at more like 20A. Is this 8A figure to do with the BMS or is it the pack? Are these ebike packs cheap rubbish? Does anyone have a source for lifepo4 prismatic cells in the UK?

    8 x pvl-136 unisolar amorphous roll out panels in 4 strings of 2
    FM60 charge controller
    Studer AJ-400 48V inverter
    Midnight Solar fuse box
    Currently 4s 70ah agms.

    This is my first post in this forum. Many thanks in advance for any info.

  • #2
    Originally posted by Green Dragon View Post
    All the ones I've seen say 8A max charge but I want to charge it at more like 20A. Is this 8A figure to do with the BMS or is it the pack?
    No it is maximum charge current the pack can safely be charged up without exploding into flames. It is a thermal limit the manufacture is comfortable with. So yeah you can charge at 20 amps.
    MSEE, PE

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    • #3
      I believe he is saying that it is undoubtedly prudent to pay attention to what the manufacturer says and keep it at 8 amps or less.

      I have a tendency to pay attention to directions. I know someone from 173rd Airborne LRRP who did not pay attention to "Front towards Enemy" signage on his Claymore and wounded 3 out of 6 on the recon patrol in December 1966. His reputation and popularity plummeted drastically.

      Reed

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      • #4
        Originally posted by Green Dragon View Post
        Hi all, I am looking at buying a 48v 40ah lifepo4 battery pack off eBay. Has anyone got any experience of using these in a solar rig? Having read the sticky on charging lipos, I am going to disconnect the BMS and I'm prepared to be hands on with the charging regime. My worry is the charge rate. All the ones I've seen say 8A max charge but I want to charge it at more like 20A. Is this 8A figure to do with the BMS or is it the pack? Are these ebike packs cheap rubbish? Does anyone have a source for lifepo4 prismatic cells in the UK?
        You are asking the right questions, but as always, how good is that ebay source? Can you trust that you aren't getting old stock, or rejected cells?

        You are on track about the limited charging rate, although we don't know exactly what cells you are looking at. That being said, for a bms system that includes passive resistive balancing, this can put a major restriction on the charge rate in order not to burn up up the mosfets or resistors doing the bleeding. In a solar application where you don't NEED to take these cells to 100% SOC all the time, you can be safe and still have enough capacity by only charging up to about 3.5v per cell. Now you have a little room for imbalance, but of course one needs to check on the very first charge to see if they are extremely out! Seems like you'll be checking this.

        That being said, I'd steer away from high-rate cylindricals, as they are designed for extremely high rates, and in a solar storage application you'd be spending more for a feature you won't be using. Far better and more cost-effective for our application are large-prismatic cells. Leave the high-rate cylindricals to wheelchairs, high-end EV's, starter/racing/rc-modeling and the like.

        I can't vouch for these guys since I have never dealt with them:
        http://www.ev-power.eu/Winston-40Ah-200Ah/

        You'll also want to see the owner's manual which goes into charging in much more detail:
        http://en.winston-battery.com/index....ownload-center

        That owners manual is great, although I caution against taking cells to 4.0! Knock that down to 3.5v max since we don't NEED to go to 4 or even 3.6v for that matter. Starting out, set a goal of 3.5v max per cell to keep things under control.

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