Paralleling 'drop-in' LFP batteries

magicbus

One of the LFP solutions I'm looking at for our RV are 'Battle Born' drop-in batteries, which come with an internal / built-in BMS.

From their literature (backed by a 10-year warranty), they state you can parallel these batteries. And, since their 24V versions only come in 50AH, that's exactly what I will need to do: parallel 4 of them (for 200AH total).

If this was LA, there's no way I'd be considering 4 parallel strings. Is there a reason I shouldn't be equally concerned about paralleling 4 of these? I pretty much know nothing about BMS other than what the letters stand for. Can the individual BMS in each battery mitigate the problems with different resistance paths in a parallel configuration?

mcgivor

The integral BMS will prevent imbalances but it would be wise to follow the diagonal post connections as per LA, or use a bus to common the negative / positive terminals. Attached is the Battle Born manual.

chewface

Hi,

Being new (late)comer to solar, I'm taking baby steps to learn more about the system components. Recently bought 2 used Valence U1 12RT lithium iron manganese batteries to build a small solar generator. Now waiting for the PV panels to arrive. In the interim, I want to charge them using a 12V car battery charger. It puts out 3A charge current.

My question is, can I charge these 2 batteries in parallel using the same diagonal connection technique mentioned in the earlier thread?  The Valence datasheet doesn't mention anything about it. The BMS on these RT batteries should be working since they were formerly used in a hospital. 

If this is ok, then want to add 2 more batteries in parallel to increase storage capacity. Can I charge these using the same diagonal connection technique without having to buy an expensive BMS? 

Any expert advice appreciated. Thanks

mcgivor

chewface said:

Hi,

Being new (late)comer to solar, I'm taking baby steps to learn more about the system components. Recently bought 2 used Valence U1 12RT lithium iron manganese batteries to build a small solar generator. Now waiting for the PV panels to arrive. In the interim, I want to charge them using a 12V car battery charger. It puts out 3A charge current.

My question is, can I charge these 2 batteries in parallel using the same diagonal connection technique mentioned in the earlier thread?  The Valence datasheet doesn't mention anything about it. The BMS on these RT batteries should be working since they were formerly used in a hospital. 

If this is ok, then want to add 2 more batteries in parallel to increase storage capacity. Can I charge these using the same diagonal connection technique without having to buy an expensive BMS? 

Any expert advice appreciated. Thanks

Since each has its own BMS there should be no reason why they can't be connected in parallel, the specs (attached),  recomend 20A charging current, a small 3A charger may overheat and fail, lithium batteries are extremely low impedance. Checking with the manufacturer is always the way to be sure.

Marc Kurth

mcgivor said:

.........   a small 3A charger may overheat and fail, lithium batteries are extremely low impedance.

Help me understand what drives this opinion.

The charger will simply modulate the output voltage to hold its maximum output current. Every small charger that I have seen is designed to supply the maximum rating for long periods of time. Eventually, the current draw will drop low enough to reach the design voltage - that is the definition of the transition from Bulk to Absorb.

Marc

mcgivor

The word "may" as opposed to "will" overheat, is used because some chargers are rated to a maximum battery capacity, a 3 amp charger would take extremely long to recharge a 200Ah battery if at a low state of charge, perhaps beyond its capabilities. My  30A automotive specs are below, stating both maximum and minimum capacity, I have used it on one occasion to charge a 400Ah bank, it will hold 30A for about an hour before shutting down via thermal fuse. Because it's impossible to know what ever charger is capable of, think of it as a safety first thought.

LFP batteries do not have an absorbtion stage, they take maximum current until fully charged, those who have used an alternator to charge them have discovered the alternator overloads causing winding failure, so a current limiter is needed to prevent such issues.