Can I run 2 x 48v bms in series to make 96v?

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Nov 27, 2020
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Good afternoon everyone.

I've run into a bit of a problem in designing my DIY electric motorcycle. Turns out the CBR125 isn't the best frame to hold a single large format battery so it seems I have to go with two smaller batteries.

I've decided to go with pre made 18650 batteries and I want to have a 96v (26s 109.2v) system with 40-50Ah and it seems the best way to hit those numbers is to buy two packs and run a 2s configuration.
(I know 4-5kw is not much capacity for an electric motorcycle but I'll have two and make the packs removable)

The only thing I don't know about is how the BMS's will handle the configuration.
I'm not overly worried about discharging as both BMS would be rated for 120A and the max i would be pulling continuous would be 100A, so it seems like it would work fine, but I don't know if charging will be an issue. Ideally I'd like to just use one single charge port but I'm not sure if connecting 2 different BMS in a 2s configuration will work or if it would just confuse the BMS and end up not charging at all.
Is this possible or would I be better off biting the bullet and getting a custom (read as "expensive") battery made up for my specific application.

Thanks in advance
 
Get a BMS that handles your full pack voltage

Just need to get the wiring right for the two sub packs at 13S 48V each.

Best to "permanently" wire them together, make the BMS removable for when you want to go into storage mode

or disassemble for testing maintenance.
 
Thanks for the input.

Just to make sure I've understood this all correctly. If I want to go with two small packs, its best to open them up, remove the two included 48v BMS boards, and replace them with a single 96v BMS treating the packs as one?
Sounds straight forward enough. I wanted to avoid messing with batteries, but I suppose in building a custom motorcycle I wasn't going to get away without learning at least one new skill.

As for the removable BMS for storage, I'm a little confused.
Why is it advantageous to have a removable BMS. As for winter storage, wouldn't I just keep it at a medium charge state? I don't see why removing the BMS would help that.
 
Thousands of packs have been wrecked by a rogue Battery Murdering System, maybe more than have been protected.

Stuff happens, cells in storage really are best 100% isolated.

Most BMS cannot be turned off completely, thus accelerate self-discharge rate by an order of magnitude

some even draw unevenly thus causing gross imbalances after just a few days.

BMSs are often the first component to fail, you want to be able to easily test/ monitor/ replace it.

You have to do a full rewiring of both the power and the balance leads anyway...
 
john61ct said:
Thousands of packs have been wrecked by a rogue Battery Murdering System, maybe more than have been protected.
Stuff happens, cells in storage really are best 100% isolated.
Most BMS cannot be turned off completely, thus accelerate self-discharge rate by an order of magnitude
some even draw unevenly thus causing gross imbalances after just a few days.
BMSs are often the first component to fail, you want to be able to easily test/ monitor/ replace it.
You have to do a full rewiring of both the power and the balance leads anyway...

Huh, the more you know I guess.

Perhaps I could make the BMS completely separate from the battery and treat it more like hobby LiPo batteries with both a power connector and another balance connector. I'd only be able to charge on the bike, but then I could easily remove the bms from the equation with a simple plug.

Anyway, new plan.
As much as I didn't want to, its looking like the best option. I'll make 2 full custom DIY battery pack made from 32650 battery hookup LiFePo4 cells.
Both 32s8p packs. One integrated into the frame and the other completely removable. Both with separate BMS and wired in parallel so I can build them independent of each other according to space requirements.
From what I've read so far on solar forums it seems that running two BMS in parallel is perfectly fine for discharging and also charging with a single charger.

It's a lot of wiring, but that's my previous professional background so I'm not worried about doing that, but I just want to be sure that running a 2 BMS system is going to work before I go spending a lot of money on something that won't be guaranteed to work.
 
It also depends on the quality of the battery cells that are in the battery and how large the battery is the bigger the battery in ah the easier it is to monitor over time in use.
What 48v pack a link ?
I have a 24s A123 20ah battey for 8years as two 12s packs with no bms just balance wires and a C.A. to monitor. Your's at 40ah should be EZ to monitor. Plus I just bulk charge and very rarely balance charge. 1,480 cycles
 
Millhouse_5 said:
Perhaps I could make the BMS completely separate from the battery and treat it more like hobby LiPo batteries with both a power connector and another balance connector. ...
then I could easily remove the bms from the equation with a simple plug.

Whether you can parallel the BMSs or not, very much depends on the BMS.

You will in that scenario, only be "balancing" the two sub-packs at the pack level voltage

whereas by using just one higher voltage BMS the subpacks will act as one pack all the cell/groups balanced and protected as one.


> I'd only be able to charge on the bike

There is no reason to build it so that it must be mounted on the bike to charge

and many reasons not to.

 
The issue with putting two batteries in series is that if one BMS shuts off, its transistors have to be able to withstand the entire series voltage. That requires your 13S BMS to have 130V transistors, as 26S is 109V, and you want some margin.

I probably hang around the wrong BMS sellers, but the units I buy don't have a max voltage listed for the NOSFET's. Perhaps your seller will list them.
 
OK,

Here's the current plan, with a wiring diagram too.
Now that i've learned some more here and also done some outside reading I think going full DIY with the battery hookup 32650 cells is my best bet, even though it will be the most work by far.

The plan for the battery is to make two identical packs both with their own 150A BMS, that way I can safely run the full load with one battery if needed.

Does my block diagram look right or have I missed something critical and need to go back to the drawing board.

lMWeAa2.png
 
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