Effect on LiFePO4 battery stability when wired in series

[Ocracoke]

Hi Everyone!

I've received a pair of 2SP1 30C LiFePO4 batteries for The Honey Badger. A single battery pack is 6.6v so the drive motor spins slower than that in the 12v 1.2ah SLA battery that was previously connected to the motor (the robot did have 1 battery per motor).

Would tying up the battery packs in series (to make 13.2v) be an issue for the stability of the cells in the packs at all (assuming they are charged at the same time)? My plan was to tie the two packs together to get to 13.2v so the motor spins at a similar rotational speed to the SLA batteries and then split the circuit after the removable link before a pair of fuses.

[overkill]

That is absolutely fine, they don't even really have to be charged exactly the same. Its when you connect Lipos in parallel that you have to be careful.

[maddox10]

I guess we have a little confusion about cells in series and cells in parallel.

Good LiPo and LiFePo4 packs have balancing leads, especialy to charge each cell.
That is also the reason for the rule to use dedicated chargers/balancers for those packs. There are tricks

Drawing power from the packs is another matter.

[Ocracoke]

That is also the reason for the rule to use dedicated chargers/balancers for those packs.

Thanks, I do understand that - I wasn't planning to have both batteries on charge when wired up in this manner (though having re-read my original post, it does look as if I was asking about charging them at the same time as well), just when inside the robot (i.e. whilst in use). From the sounds of it, would be OK to do.

[overkill]

This part of your post is confusing me; 'split the circuit after the removable link before a pair of fuses'

Does that just mean you want a separate fuse before each drive motor ESC? Not many people bother with that.

[Ocracoke]

Does that just mean you want a separate fuse before each drive motor ESC? Not many people bother with that.

Yes, that is what I mean though I don't see why not. Yes, it is another fuse to keep on top of but if it means one side continues to move whilst the other side is blown, I'd be happier with that than one fuse after the battery.

[overkill]

Nothing wrong with having as many fuses as you want. The fuse is only in the rules to protect the Lipo from the rest of the bot and prevent fires not to protect the electronics.

[Ocracoke]

That is fair enough... though why does the fuse rule only appear to apply to LiPo batteries? LiFePO4 batteries appear to not be subject to this requirement under rule 7.8.

I would have thought it was at least good practice to have at least one fitted to the wiring for a LiFePO4 powered circuit (unless it is specifically mentioned because the chemistry is safer than LiPo?)

[maddox10]

The LiFePo4's are not under the Fuse Rule because the battery Chemistry is inherently safer than LiPo.

Worst that those can do is puff up and emit a lot of smoke.