Proper way to connect two 4 cell Overkill Solar BMS LifePO4 12 Volt batteries together in Parralel ?

[ak99]

Proper way to connect two 4 cell Overkill Solar BMS LifePO4 12 Volt batteries together in Parralel ?

Do i just connect the positive to the positive on each 12 Volt battery and then one of the positives on one battery to the positive bus bar in my RV? And then the two negatives to the negatives coming from the BMS’s to the bus bar in my RV?


Will that confuse the BMS’s as normally you should just attach one of the negatives on the second 12 Volt battery to the negative bus terminal in the RV.


Or should i connect a jumper wire from both the positive to positive on each battery and the negative to the negative on each battery, then connect the two negative leads coming off each BMS to the negative terminal?

 

[DThames]

Connect one BMS to battery #1 and one BMS to battery #2. Connect the Common of the BMS (the power wire not connected to the battery) of both batteries to your bus. Connect both positives to your bus.

I suggest you get the phone app so you can turn the battery output off on both BMS. After you get them wired up, you can turn on one and then when the voltage match (at a high or at a low state of charge), turn on the second one.

 

[ak99]

I have built two 12Volt batteries with two separate 4S Overkill BMS units that i've built with Fortune cells. This gives me two negative cable terminals (one coming from each BMS) that i need to attach somewhere.

Are you saying that i should not parallel these two batteries together, but hook up the positive terminal of each to the RV positive bus bar and each of the two negative terminal cables coming off each batteries BMS to the RV negative buss bar?

Yes i have an app for each overkill solar battery active in the software. Got it on turning them both off via the software then turning on one at a time via the software.

 

[squowse]

I use a rotary isolator rated at 350A on the positive side. Allows you to swap out a battery if necessary. You would need to turn off the discharge and charge on the app to isolate the battery.
The negatives are just connected to the bus bar. Each positive and negative cable is 35mm2. The combined output from the rotary isolator goes straight onto the 75mm2 bus bar.

 

[Ampster]

Are you saying that i should not parallel these two batteries together, but hook up the positive terminal of each to the RV positive bus bar and each of the two negative terminal cables coming off each batteries BMS to the RV negative buss bar?

That electrically does put the batteries in parallel if I understand what you are saying. I agree with @squowse that an isolator is an enhancement that can aid in diagnosis. You should have a fuse or fuses in there somewhere to protect the wire. My preference is close to the battery on the positive side.

 

[ak99]

That electrically does put the batteries in parallel if I understand what you are saying. I agree with @squowse that an isolator is an enhancement that can aid in diagnosis. You should have a fuze or fuzes in there somwhere to protect the wiire. My preference is close to the battery on the positive side.

I'm going on a trip in a couple days with my RV so i need to have this set to go now, so i don't have time to go with the Isolator option although it's very interesting. Thank you so much!

Can i just hook up the two batteries in Parallel as you would any Battle Born or other battery by connecting both positives and negatives. Keep in mind that i'm using two Overkill solar BMS for each of two 12 volt batteries,

So... positive to positive battery terminal and negative to negative........Then with a Battle Born or other regular LIFEPO4 battery you would then attach one negative of one battery to the RV negative bus bar and the positive of the other battery to the postive bus bar. This would be a proper Parallel connection.

However with overkill solar BMS i have a cable coming off each batteries BMS that says i'm supposed to hook up each cable from each BMS to "Negative output". This is the part that is confusing me.

So if i hooked up like a regular two 12 volt situation, would i parallel the two batteries together as described above, then hook up positive and negative from one of each of the batteries and also hook up the two cables coming off each of the two BMS to the negative RV bus bar? So then i'd essentially have three cables going to the negative bus bar.

I need to run two batteries in Parallel as i need the power as i've hooked up a split AC to my RV. I've run it before successful off two batteries, so i know it will work. Just not sure about the proper set up with those two extra cables coming off Overkill solar BMS that say "Hook up to Output Negative".

 

[DThames]

Look at the manual image, page 10. Built each battery according to the picture below. The red and the black terminal bar is the "bus". Connect both batteries ("battery" meaning battery-BMS build) to the same terminal bar and they will be in parallel. Only connect when the both are at the same state of charge and the same voltage.

 

[mikefitz]

Regard the 'battery' as the cells together with the BMS.
To connect in parallel charge each battery separately to full first.
Connect in parallel, battery positive to battery positive, BMS negative, black cables, to BMS negative, black cables.
Idealy the parallel connection point would be on a buss bar with equal cable lengths from the battery.

Mike

 

[icyroads]

Imagine your BMS setup for each bank as a separate off the shelf complete battery with an internal BMS. As others have said, you do want a fuse on each bank to protect things in case of a short (eg a dropped wrench on just the wrong spot), and a switch on each (or one off, a, b, a+b switch) for easier maintenance.