Batteries: Parallel vs Stand alone w/ transfer switch

[Krist003]

Hello all,

I have (1) 100AH LiTime battery in my FWC Hawk. The battery is a year old, but has only been discharged a half dozen times. Would buying another new identical battery to install in parallel give me any problems? Would there be balancing issues due to the age difference?

On another note, I thought about putting in a transfer switch to swap between each 100AH battery independently. Once one battery gets too low, I would just transfer to the other. My thinking is that it would eliminate the balancing issues between the two and maybe even give me some more security should something go wrong with just one of the batteries.

I only use the camper about 6 times a year and 100AH is usually sufficient, but sometimes I need more for a longer trip. I don’t have solar or alternator charging and am not interested in adding either.

Thanks!

 

[crashmaster]

According to Li Time you should only wire 2 identical batteries in parallel if they are purchased within 1 month.

My experience with wiring 2 Li Time 100ah Bluetooth batteries in parallel is that one battery will discharge, and the other will stay in standby as shown on the Li Time app. I have no experience with their non-bluetooth batteries though.

2 LiTime 100ah Bluetooth LiFePO4 in parallel

I have 2 LiTime 100ah bluetooth LiFePO4 batteries wired in parallel and it appears by looking at the BMS of both batteries that only one battery is under load. I attached photos of both BMS. If anyone has these batteries wired in parallel do you have the same issue when looking at both BMS...

www.wanderthewest.com

 

[Jon R]

Limiting to one month apart in age is very conservative. Maybe in addition to cell age concerns they have BMS software version compatibility concerns given the odd BMS behavior we’ve read about in other threads?

 

[crashmaster]

Agreed, that seem very conservative. I still cant make much sense of the BMS behavior.

 

[Jon R]

Probably another concern with a manufacturer like this is that using batteries from different production batches may mean the batteries have significantly different battery cells installed. This could cause some of the same problems that are talked about for connecting batteries of significantly different age or different types.

 

[Krist003]

Thank you. That does seems very conservative to me as well.
I am really starting to like the idea of having two batteries isolated by a transfer switch anyway. I can’t really think of any downsides other than having to manually use the switch when one battery gets too low. Anyone have any thoughts on this?

 

[Jon R]

You should be careful about connecting two batteries in parallel when one has a significantly higher level of charge. You should understand and build for the possible current between the batteries to avoid damage to your wiring or batteries, Make sure you select your transfer switch carefully and select one that doesn’t connect the batteries as you switch from battery A to B unless you intend it to do that.

 

[Krist003]

Sound advice. I think a transfer switch that is off in the middle position would be best for this scenario.

 

[Lunch]

Good qstn. b/c battery SOH is a function of both cycles and age. Curious how close your battery performance is after 1 year and 6ish cycles. Of course, the best parallel scenario is to buy the same battery at the same time from the same manufacturer (and, of course, the same capacity). Also Keep your pos and neg leads the same length and connect each from different batteries.

How did the transfer switch work. I'm guessing that some people upping their capacity are buying a heated battery to connect with an older non-heated battery. I don't see any manufacturer discouraging this 1) b/c it's mixing old and new and 2) b/c mixing heated and not heated can cause problems. It's even worse if the battery cannibalizes it's own energy for heat (Canbat has heated batteries that draw from the charging source, rather than the battery). I'm wondering if a transfer switch could be a solution in these cases.