My first project. 12.6V 70Ah

Hello everyone!
My name is Remus and im writing from Sweden.
I want to build a battery for my electric boat motor and i need your help. This my first battery that i want to build so im extremely new in this field so...be gentle with me.
My target is a 12.6v 70-75A battery that i want to build connecting two batteries with less amps.

Battery A - i have 40 pieces LG18650 3.2Ah that will be 3s13p 12.6V 40Ah BMS1
Battery B - i have 37 pieces LG18650 2.9Ah that will be 3s12p 12.6V 34Ah BMS2
Battery A + Battery B in parallel will be 12.6V 74Ah, my target.

My motor is taking max 40Ah.
I this the right design for what i want to build?
I have some questions related with the BMS
- what is the bms i need? 3s 40Ah or 3s 70Ah?
- how many 3s in parallel can manage a BMS?
What kind of charge do i need? 12.6V or 16.8V?

Thanks for all the support i get
Remus

Please read the FAQ (https://secondlifestorage.com/forumdisplay.php?fid=42) about 12V and lithium battery configuration. This may answer quite a few of your questions, and may answer some you didn't even know you had.

The bms amp rating depends on what the max output your batteries can put out. The bms will protect the batteries from overheating because they are putting out more amps then they are rated for. Find out what the max discharge is for each cell then add it up to get the max discharge of the pack. If your batteries can handle 40 amps, get a 40 amp bms, but also check what the peak amps are on the bms, some motors when they startup they require more amps, some bms can handle 40 amp constant and maybe 100 amps peak for a few seconds, some even more.

You only need one bms for both packs, just connect the balance connectors together so they equalize. Then you have one large 3s pack. And you can parallel more 3s as you need to. Just make sure all batteries are the same voltage when connected together.

You need a 12.6 volt charger for your pack. (4.2 volt max per cell x 3s = 12.6 volts), get a cv/cc charger its the best for a lithium. It gives you maximum amps when it starts charging and when the battery is almost full it reduces the amps, gives you battery a very good safe charge.

For extra safety get one of these lcd monitors and leave it connected all the time to your battery. Its lcd so it waste very little power but it sounds a very loud alarm if any cells goes above 4.22 volts or below 3.0 volts. Some packs just sitting around will self discharge to 1 volts if they have a weak cell in them, this will warn you in plenty of time to a problem whether overcharging or discharging. I have these on all my 3s packs.


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