Two different lithium batteries in series?

Scootdan

100 W
Joined
Mar 19, 2021
Messages
113
Ok Amberwolf, here is one I think you, or one of the gurus, would know. Can I safely connect the 72v and 48v batteries pictured here in series to make a 120v battery? They each have their own BMS and are also different amp output, 100a for the 72v, 70a for the 48v. My controllers are normally set at 300a, for a large battery, but they had me change them to 100a max. for this 72v battery. I really want to try this, but I am too young to die, so expert help would be most appreciated! :D
 

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You can but it’s not adviseable. The weakest cells in the series sets the limit so controller can max be programmed to 70A and you will only be able to use the smaller pack capacity at 31.5 Ah.

The less capable pack will be higher stressed, will quit faster and i am not really sure what happens if any of the bms’s cut the power. It might take out the other bms and start a fire..
 
Thank you brother Lars. I had similar thoughts I should set the controllers to 70a and first try it with both batteries at half charge. It would just be a most temporary way to get a valuable reference point of the power I need. I have ridden about 10 full charges on the 72v battery, so I know about what it does. It would be awesome to see what it could do, even for just a minute, being very careful, with a half charged 120v 31ah 70a battery! I am pretty sure I will be more than happy with a 96v battery because I am getting almost 50mph with the 72v battery. Of course it takes a long time to get there, and I only have about 25 miles range, but that is the funnest 25 miles ever! I just can’t wait to see how it will be with a little more kick. So has anyone else tried this with two batteries? I am still tempted, because I could place the batteries where I could fully access, disconnect, and God forbid - extinguish them! :roll: Will work for lipo! 8) 11463AC2-2B34-418B-A2C2-A736790D73F4.jpeg
 
The lowest amps is the limit for the system, so you would need to decrease current draw to the lowest of the two; 70a in this case.

If a BMS of a battery that is in series with another battery shuts down for any reason, it now has the full voltage of the entire system across it the now-open-circuit FETs. If it does not have FETs capable of handling this voltage, they will probably fail. Usually FETs fail shorted, stuck on, so now the BMS cannot ever shut off to protect the battery. It is a silent failure in most cases, so you won't even know this has happened, and are now at risk of damaging the battery by overdischarge (and possibly overcharge, if there is a single port instead of separate ones).

If it doesn't fail stuck on, it'll probably fail open, and your system will just stop working (no current flow from the batteries).

It's unlikely that either BMS has FETs capable of handling the total series'd voltage, so the likelihood of failure is high.

If you look around the forum you'll find a number of similar threads about putting multiple batteries with BMS in series, with possibly more detailed info.

Scootdan said:
Ok Amberwolf, here is one I think you, or one of the gurus, would know. Can I safely connect the 72v and 48v batteries pictured here in series to make a 120v battery? They each have their own BMS and are also different amp output, 100a for the 72v, 70a for the 48v. My controllers are normally set at 300a, for a large battery, but they had me change them to 100a max. for this 72v battery. I really want to try this, but I am too young to die, so expert help would be most appreciated! :D
 
As usual I owe you Amberwolf. Thank you. I don’t even need to read more about it. I will not be doing that. Besides, I hit 50 freaking mph on the beast just now! I don’t know what it is, but these motors seem to be going a little faster each time I am going out. Electric motors don’t run faster when their ‘broken in’? Maybe batteries take a little more charge with steady use? I am loving it whatever it is! I really need to gear up before I think about more batteries anyway. Will work for full face helmet :wink: 50MPH! :bigthumb:
 
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