Homemade LifePo4 battery

Discussion in 'Mech Tech' started by pkrboo, Mar 26, 2020.

  1. I decided before Christmas to build a lithium battery. Partly due to the cos of the purchase of ones form a company. And partly cos I like doing stuff like that, and I'm fed up of FLA batteries lasting 1-2 seasons before being next to useless.

    So far this has cost me less than £400, most lithium batteries are £800-£1200

    Parts used are a BMS (Battery Management System), 4 x 105aH aluminium cased LifePo4 cells, a battery box with built in terminals. I also used a some 10mm2 cable. And some Crimp connectors.

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  2. Nice. I assume this is to use instead of a leisure battery?

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  3. Yes
     
  4. Excellent. I have solar on my roof which split chargers both my batteries. I think my leisure battery is 110 ah... Is yours like... 4* that capacity?

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  5. No, Sorry, I should have been clearer, the 4 cells are 3.2 volts (nominal) each so 4 in series creats a 12.8v battery. still with the same 105aH capacity. Big difference here is that it can be discharged to 0aH with no ill effects. an FLA like yours can actually only use about 50% of the stated capactiy.

    It is also much lighter, (half the weight at least) and will last at least 5000 cycles before dropping to 80% capacity. FLA are 2-300 cycles before being next to useless
     
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  6. Ah, thanks for the explanation. I will definitely bear in mind if I have to replace my leisure battery. Cheers.

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  7. How are you going to charge it? I know they win there without the need for the long tail charging, but I gather it can be quite dangerous, you can't just attach them to your split charger.
     
  8. Yeah, you've got to be very careful about not over-charging, I assume that's the purpose of the charge controller/BMS?
     
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  9. thats why you have a BMS, it controls it all. you can split charge with iof you wnat to as again the BMS is in control. i wont be split charguing as i see no need. Just chatging via normal solar. LiFePo4 is incredibly stable.
     
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  10. charging raw cells is bad, The BMS is the key here. i could charge this with a 40 amp power supply straight into it if i wanted to(or even had one!!)
     
  11. Be interesting to see how your experience works out, Matt. Quite a lot more money than a normal leisure battery.... justifying the extra cost is important, time will tell.


    Also, I expect if you go down this route again in future the cells could be cheaper.
     
  12. the cells alone were £308 delivered, these particular ones have gone up in price but they are relaeasing new ones all the time. just have to keep an eye on aliexpress. and buy the right ones!!
     
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  13. And what would happen if you forgot to switch off your charger? Bang? Not scare mongering but the people need to be warned.

    Someone on a boaty forum is also designing and building his own battery managing thing but I think more for the fun of it than cost saving.
     
  14. No the BMS disconnects at the pre determined set voltage

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  15. Ha! you dodged the question. The thread I quoted was about a 40A charger and no BMS or at least that's what I meant. Just for the fullness of knowledge.
     
  16. My wording was poor, I didn't mean 40amps straight into the battery without a BMS. I meant with a BMS still, just that it could be a bench top power supply without any charge circuitry, like a ctek for example.

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    Last edited: Mar 26, 2020
  17. Oh and to be clear, I haven't built the BMS just bought a proven one and fitted all the other parts together.

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  18. As you just won't bite I'll say it then. If you connect to these things without proper control they will explode.
     
  19. Does the BMS also handle the discharge condition, so that no cell is discharged below 2.5v?
     
  20. Yes correct, I have at no point said you should, it's why I have fitted a BMS, I was planning on going into more detail once I have done the screen shots in what the BMS does.

    Also, I am a man, as you know, so unless you spell it out there's no point hinting at something, just ask it outright!!

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