2 lithium ion batteries in series...

[bike4life]

https://www.ebay.com/itm/36V-20AH-Li-ion-Battery-Volt-Rechargeable-Bicycle-1000W-E-Bike-Electric-Li-ion/274134211710?hash=item3fd3ac1c7e:g:NQwAAOSwe-td7cdh

Can i add 2 of these lithium batteries in series, to get 72 volts... Each of these batteries has a BMS, but would i need a bms thing for both of them, could each of them have different potential differences and mess eachother up....

Thanks...!

 

[amberwolf]

There's a number of considerations when doing this.

There is a lot of information about this in the various previous threads, many of which are in this list:
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
not everything is relevant, but you can tell mostly by title which are what you are after.

 

[MadRhino]

To make it short: batteries that you assemble in a series must have the same capacity, and C rate. Otherwise, well, you’ve heard about the weakest link of the chain?

 

[amberwolf]

That's only one consideration. The other threads have more info.

 

[Jonno]

You can put them in series but you would need a 20s bms wired into the now 20s2p new configuration in order to keep your 2 10s2p packs the same.

I have 3 packs of 26650 5000 ma cells in mine 2 x 10s7p and 1 x 2s7p all connected to 1x 22s BMS. This is charged with a 92V charger. Difference in cell voltage after a 40 mile ride = 0.013v. After charging 0.000v across the 154 cells

Pretty good for a bunch of cheap chinese cells

Jonno

 

[markz]

You risk depleting one battery too much if the batteries are not charged both to around the same voltage, both batteries being equal.

For example, one battery is say 10S and the other battery is 9S, both fully charged the 9S will be slightly more depleted making it not so healthy. But charge 10S fully and lets make the other one 10S but half charged, or 3/4 charged, then you risk damaging the battery because it will be depleted more reaching the LVC of the controller. This is of course using batteries without a BMS. LiPo batteries dont have BMS installed in them. There are BMS' available to buy.
Back in the old ancient days, lead acid batteries didnt have BMS' so not sure what they did in prehistoric dinosaur age.

If they do have a BMS then the BMS will cutoff power and you wont go as far when you are riding.

That is one risk factor to consider.

 

[docw009]

Haven't we talked about those crap batteries? They're still claiming 20AH out of a 10S-5P arrangement, and the cells don't exist that can do that. They're lying to you.

Crap batteries mean a crap BMS. If they only put 60V MOSFET's in the BMS, if you put two in series and one battery happens to shut off, its MOSFETs will blow up.