32.4v batteries on 36v system.

[UnclePuncher]

Hi all,

I've managed to get hold of a few 32.4v batteries for free. They appear to work fine. However they're meant for an external BMS (it's part of the bike they came off). I want to use them on my 36v ebikes.

My question is will my 36v system run ok from a 32.4v battery? Albeit with a bit less power. Obviously I'll have to add a BMS but that's less work than the other option... But will this just feel too underpowered? Or simply not work?

The other option is I break the batteries down and build up a 36v battery with the cells. Batteries are currently in a 9s4p configuration. So for example I could get 2 batteries and build a massive 10s7p battery and still have 2 spare cells left. But obviously that's a fair bit more work and breaking batteries down is not fun work (imo too much risk of puncturing cells when removing tabs etc...)

I suppose the most obvious 36v battery would be 10s4p (so I'd just need to separate the cell groups of 4). But then I've still got 32 cells left.

Thanks.

 

[E-HP]

The low voltage cutoff on most 36V systems is around 30V, sometimes a bit higher, depending on your controller. Most controllers will state the LVC on the label. Depending on the LVC, you'll have reduced range (no ability to utilize the full charge range of the battery).

Example with a 31.5V LVC.

 

[UnclePuncher]

Thanks. I feared as much. I'll probably just break them down and build a 36v pack.

I appreciate this is just an off brand controller. But how does this one work? If it can operate at 24v then surely it doesn't have a lvc? Or at least not one much above 20v.

 

[UnclePuncher]

Also as I'll have to add the BMS won't the BMS be rated 36v? That was more my concern that that would cut out. Or does it only cut out if the individual cell packs aren't at nominal voltage?

 

[E-HP]

Thanks. I feared as much. I'll probably just break them down and build a 36v pack.

I appreciate this is just an off brand controller. But how does this one work? If it can operate at 24v then surely it doesn't have a lvc? Or at least not one much above 20v.

Depends. The nominal voltage of the battery connected to many (most) budget multi voltage controller is usually auto sensed/guessed by the controller. Then the lvc for that nominal voltage is set accordingly. There are also controllers that can be set through the display, usb, or Bluetooth, but those tend to be more expensive.
If it’s auto setting, there are some forum threads with methods to get the controller to re-guess the battery voltage.

 

[E-HP]

Also as I'll have to add the BMS won't the BMS be rated 36v? That was more my concern that that would cut out. Or does it only cut out if the individual cell packs aren't at nominal voltage?

If it’s 9s, no. If it’s 10s, yes. You’ll buy a BMS that matches whatever configuration you decide on.

 

[docw009]

You can buy a BMS that runs at lower voltage. You'll have to figure out what your 32.4 battery really is. It could be a 9S lithium ion pack with a nominal voltage of 32.4, or it could be a LiFEPO4 pack with a max voltage of 32.4V, With the latter, I believe their sweet spot is 28.8V, which is totally unsuited for your ebike.

SInce you have to add a BMS anyway, if it turns out to be a 9S li-ion battery, you could consider adding one more group of cells to make it 10S and it would act like a normal ebike 36V,

Look at the voltage label on the charger that came with the battery, or measure it with a voltmeter, See if it says 36.8V or 32.4V.

 

[amberwolf]

The other option is I break the batteries down and build up a 36v battery with the cells. Batteries are currently in a 9s4p configuration. So for example I could get 2 batteries and build a massive 10s7p battery and still have 2 spare cells left. But obviously that's a fair bit more work and breaking batteries down is not fun work (imo too much risk of puncturing cells when removing tabs etc...)

I suppose the most obvious 36v battery would be 10s4p (so I'd just need to separate the cell groups of 4). But then I've still got 32 cells left.

Don't remove the tabs. Just cut the tabs between cells, so you can keep at least small groups of cells together, and save existing interconnects.

For instance, if you have 2x 9s4p and want 10s7p, leave one battery completely intact,
The second just remove a long 9s1p segment from the other by cutting just the paralleling tabs between that set and the rest.
Then parallel those two, the 9s4p and 9s3p.
Then cut the 9s1p tabs between cells, and parallel 7 of those, then series those to the 9s7p you made above.

 

[amberwolf]

I appreciate this is just an off brand controller. But how does this one work? If it can operate at 24v then surely it doesn't have a lvc? Or at least not one much above 20v.

If it has no LVC, or one well below the total LVC your pack should have, then there is no protection against overdischarge of a BMSless pack (other than you manually monitoring the voltage while you ride, every time, all the time, to be sure it doesn't go too low.

That's distracting and can be unsafe if you really do it as you have too much attention taken from riding, and if you get blase about it and forget, you can damage your battery.

If the controller has an LVC that's correct for the actual battery in use, that's not so much an issue as long as the cells are all matched and drain identically.

If they're not matched, a BMS is a good idea to prevent overdischarge of and damage to the least-capable cells.


The BMS just has to be the right kind for the chemsitry of the cells you are using, so that it's LVC and HVC are appropriate for those cells. Or programmable so that you can set it correctly.

So you don't use a BMS for LiFePO4 (LFP) for cells that are NMC or Li-Po, Li-Ti, etc etc, and vice-versa.

 

[Chalo]

I'm told, but haven't tried it, that a controller that has calibrated itself to a given battery voltage but is capable of using a lower voltage, will reset to the lower voltage by means of the self-learn wires when a lower voltage battery is attached. That sounds like a nuisance if it has to happen in the middle of every long ride, but at least it can happen.