LiFePO4 parallel charging and cold

[Lollandster]

I'm currently debating if it's worth going to LiFePO₄ for my boat project. I initially thought AMG was a good start as weight isn't a big issue on a boat. But investing in chargers is going to cost quite a bit so upgrading to LiFePO₄ later costs more than just the batteries.

If I use LiFePO₄ then I will need to connect a lot of them in parallel which I know is no problem. I'm wondering if I can put multiple chargers in parallel. I'm thinking that the LiFePO₄ chargers are very dumb and just let the BMS control the charging, so I'm not expecting any problems with doing that. All the batteries will be in parallel and should balance themselves. The reason I'm thinking about getting two chargers and charge in parallel is to be able to decrease the charging time when in a hurry by connecting both at the same time, or just use one charger for normal charging and when using the 2kw generator I'm going to have as a backup. I will never go over 3500w charge power as that is the limit at my marina. If one BMS has a voltage limit on the battery slightly higher than the rest of the packs it should still not get more than about 3C charge current and that would be over a very short period.

The other question I have has to do with cold weather. Can a LiFePO₄ pack be subjected to freezing temperatures without any lasting effects? If I use LiFePO₄ in my boat then they are going to see cold nights near the end of the boating season, but they are not going to be used in freezing temperatures. I also would like to store the batteries in the garage rather than indoors during the winter (usually not colder than -20°C), but that is negotiable.

Thanks

[Bikemad]

Using two chargers in parallel should be fine as they will bot work on a constant current/constant voltage principle.
If their end voltages are slightly different, it just means that one will stop charging slightly sooner than the other, but they will both be contributing throughout the majority of the charge.

So you could in theory use a 54.6V LiMn charger and a 58.4V LiFePo₄ charger in parallel (Although it might require a diode on the LiMn output) and the LiMn charger would simply stop providing assistance when the battery voltage reached 54.6V (3.41V per cell) leaving the LiFePo₄ charger to finish the charge on its own.

From what I've read, a LiFe PO4 battery should be ok down to -40°C before the electrolyte actually freezes, and although it is commonly recommended not to charge if they are below 0°C, I understand that they can still be charged but at a greatly reduced current (C/100) to prevent damage. Charging at the normal charge current in sub freezing temperatures can seriously damage the cells.

I suggest to have a good read of this post on cruisersforum.com.

Alan
 

[Lollandster]

Good to know they should survive a cold night or two. I'll probably manage to find some indoor space for the batteries during the coldest months.

I was thinking that the BMS has a cut-off voltage and that was the one I was worried that might be different one each pack. But I guess there is no need for the BMS to have a cutoff voltage (except for the low voltage cut-off of course). My worry was unfounded.