Two batteries in series with different voltages

[jwunder]

Hi, I currently have a 52v battery pack on my ebike (14s6p). Can I make another battery (7s6p) with the same type of 18650 cells with its own bms, and connect it in series with the first battery to get a 21s battery? The batteries would be charged separately, but discharged simultaneously in series. I know this is not an optimal 21s configuration, but gives me the option of running 14s or 21s without a making a whole new 21s battery (I'm cheap and lazy). Are there any gotcha's in this setup that I'm missing? Thanks.

 

[amberwolf]

the bms must be designed to handle the entire full voltage of *both packs* in total, because when one bms shuts off, the fets in it will then have the full voltage of both packs across them.

if the fets aren't capable of that full voltage, then they'll fail or be damaged, and then the bms either has no output, or more commonly the fets fail shorted, so the bms cannot turn the output off anymore, and the cells are now unprotected against overdischarge. in that event, you probably won't know until the cells are damaged enough to show symptoms on their own (and externally visible symptoms are usually very bad. ).


so your best bet is to rebuild the battery so it is a single 21s battery with a 21s bms.


your next best bet is to stick diodes in parallel with the packs so they'll bypass the voltage across a pack when a bms tries to open up the circuit, so the bms doens't get slapped with voltage it's not meant for. there are a number of threads about seriesing battery packs that discuss this and talk about the kind of diodes and how to wire them.

not all the threads in these lists are relevant, but the titles should indicate which are what you're after

https://endless-sphere.com/forums/search.php?keywords=batter*+series*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

https://endless-sphere.com/forums/search.php?keywords=pack*+series*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

 

[jwunder]

Thanks, good point. The thing is... I don't have any room for more cells in the triangle of my bike, but I do have room in front of the downtube. I was thinking I could use that room for more voltage, or for another separate 14s battery for more capacity (not wired in parallel). I suppose I could wire both packs together into one 21s pack with one 21s bms (klugy), or just build a smaller 21s battery for the triangle, but I was hoping for a more flexible solution that would allow me to use the pack I currently have as 14s or 21s. 21s would have more power, while 14s would have more capacity.

 

[amberwolf]

you coudl do it either of those ways...just that if you use separate bmses, you either have to be sure the fets on each bms can handle the full 21s+, or you have to use the bypass diodes.

people have used split packs in various ways on various bikes over the years, with sense leads/etc from one going to the bms that's in the other part of the pack somewhere else, so it can be done.

these may have some info on that
https://endless-sphere.com/forums/search.php?keywords=split+pack*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
but i am pretty sure there are others i couldn't find (different title words than i can think of).

 

[amberwolf]

fwiw, if you mean just the 14s would have more capacity than a 21s, given same number of parallel cells (same ah total), then that's not correct. the 21s pack would have more wh total.

if you mean two 14s packs vs a single 21s pack would have more capacity, given same number of parallel cells in each pack (same ah total), then yes, that's true.

the 21s pack would have more voltage, which gives you a higher top speed (assuming your controller can take it without exploding), and depending on the motor and controller that could also give you higher power than a 14s.