What limits LiFEPO4 "12v drop-in" wired in series to 48v?

iconoclast

1 mW
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Jul 4, 2008
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San Diego
I need to replace the six 12v batteries (6S, 72v) in a GEM NEV.

Almost all of the drop-in 12v LFP suppliers recommend not connecting more than 4 in series. I understand about balancing to .2v before connecting, and charging each of these 12v batteries independently because the BMW will cut off power to the whole string when the first battery reaches full charge.

What components in the BMS limit use in a configuration to 48v? Just the MOSFETs?

I saw a comment in one thread that mentioned that the MOS components of the BMS have a voltage withstand value of 80v. What kind of headroom is needed? Does anyone have experience upgrading these 12v BMS to support 6S / 72v or higher voltages?

If a diode was used on the interconnects to eliminate battery backflow, would that be sufficient protection? (I can easily charge the batteries individually).

I can solder and like to upgrade controllers, have limited experience with BMS.

(Ogrphy is selling a 12v 100ah LiFEPO4 battery on Amazon for $340. Chin and others have similar batteries for $380. That's very competitive with FLA batteries, but of course 2x usable capacity, 10x cycle life, 1/3 weight. If these batteries can be made to work it would be a big win for the NEV community)
 
That is a good tip, and I did see that site after logging in here after many years away.

My thought was that at the price, the Ogrphy is a pretty good deal for a drop in package. If I can uses these in 6S configuration with diodes of some kind, and/or an 80v voltage cutoff circuit protection, it would be a very quick install and easily maintainable. It would also be a more off the shelf solution that other GEM owners could use

(at the price of the Ogrphy, with warranty, no NEV owner who knew about this would ever want to buy lead again, but the install has to be clean and easy).
 
If the batteries have a BMS, the FETs in the BMS must be able to handle the full voltage of *all* the batteries in series with it (the full system voltage). If not, then as soon as any of them turns off for any reason, the FETs will be damaged or destroyed; they usually fail shorted, so the BMS is then stuck on, and the cells in the battery will be damaged too, because there was a reason the BMS was trying to turn it off, but it can't anymore. It's also a "silent" failure, so you won't know it has happened until something (possibly dramatic) happens to that battery because of this.

If the batteries do not have a BMS (or other internal protections), only cells inside the casing, then it shouldn't matter how many are in series unless the wiring of the batteries internally doesnt' have insulation rated high enough to handle the full series voltage of the pack (unlikely, unless they went ultra-cheap with them).

If you don't know for sure, follow the manufacturer / etc recommendation of not seriesing more of them than whatever limit they provide. (or else you risk whichever form of damage could happen because of the system voltage vs the failure mode).
 
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