Internal Battery Balance BMS Board?

[simey_binker]

Hello all,

Does anyone make a small PCB to balance the cells in a common hobby battery (perhaps check temperature as well?) that would be installed permanently next to the battery? Right now all that logic is within the charger, but I would like it with the battery.

This makes the interface to charge the battery much more simple: just apply your high current DC +/- voltage to a plug, and the internal BMS would manage the balance within the battery. I just want to eliminate the nuisance of that balance cable also needing to be hooked up whenever I charge.

--

Ultimately... this is for a OneWheel Plus battery swap. My OneWheel BMS was fried when some water got into the case, and OneWheel won't sell me just the BMS replacement. The OW Plus uses LiFePO4, which isn't particularly energy dense, so I was thinking of moving over to Li-ion (like the OW XR uses) for doublish the range. I'd be looking at 14S or 15S (2 x 7S or 3x 5S), so you can understand that hooking up a 14 or 15 leads to balance the cells every time you needed to charge would become obnoxious.

Thankfully the OW Plus doesn't have any smarts from the controller requiring the stock BMS to communicate. You can just apply voltage to the controller, and off you go.

 

[simey_binker]

Ah, I see from another thread that "Daly" or "Ant" are some options... checking them out now...

 

[Voltron]

I've got some leftover BMS boards from Solo wheels that were for split 14s packs. I'll send you one for experimenting if you want.

P.s. none of the stuff for balancing and temp monitoring is in a charger usually.

 

[Voltron]

Another p.s.... it's not as safe, but you can run the battery without a BMS, and manually check it for balance periodically. The biggest thing is making sure the low voltage cutoff in the controller is high enough to not take the battery voltage too low during running. I get lots of batteries that have dead BMSs, and a good quality battery that doesn't get run super low on voltage will stay pretty balanced, and only take occasional manual balancing. But, again, def not as foolproof as a proper BMS.
That being said, maybe the easiest thing is finding a premade battery with bms that fits in there, or going hobby king type lipo batteries and using an rc charger, that would then have the balancing done by the charger, unlike regular lithium battery chargers.

 

[john61ct]

There are dedicated balancers, aka "non-protective BMS" some are very fast and accurate.

No reason to have them hooked up all the time while in use.

In fact a healthy pack may only need rebalancing every 20-30 cycles or more, even annually may be enough.

 

[Voltron]

Ideally you should have a BMS hooked up all the time, that monitors each group during use, to prevent one from sagging too low before the overall battery low voltage cutoff happens, and they usually have amp limits to keep hooking it to a controller that draws too much current from damaging the entire battery.
But if it doesn't have those features, then maybe you don't need it mounted full time.
That's from my experience of killing several BMS-less batteries by both methods
A well designed unit that does both of those, and puts itself to sleep during disuse to avoid killing the battery from parasitic draw is the ideal.

 

[simey_binker]

Thanks guys for the responses.

The type of "balancing charger" I'm thinking about is a common one from Hobby King, where they would have two large gauge wires for power, and a small balance plug or board that is also plugged into the battery. So the cells are balanced on each charge cycle, but not discharge. And yes, there would be no temperature monitoring.

The trick is that I'd like to eliminate the necessity of plugging in this balance plug - I just want two DC lines into the device. Which means I move the BMS over to the OneWheel.

I perhaps assumed that the motor controller would have a low-voltage cut off, but perhaps that's a bad assumption. I'm going to continue researching my options for an on-board BMS (with temperature monitor, low voltage cut off, balance on charge etc).

Looks like I'll be building a 14-cell 21700 (4900mah?) series battery. It would be.... similar to this item (although it must use smaller cells with a 14S2P configuration:
https://chibatterysystems.com/services/cbplus

And your question is: "why don't you just buy that battery?" Two reasons: #1, I think I can build it for less, and #2, it's an educational experience. I'm a mechanical engineer who has worked on electric vehicles most of my career, and I've never built a battery yet.

 

[john61ct]

Protective BMS functions while essential in many cases, have nothing to do with balancing,

and even if fantastic at the former usually do a very poor job at the latter.

There are many top notch balancing chargers that can bulk "fast charge" with balancing disabled.

But you still want protection against higher voltage groups going dangerously high, while pack level is still far from the setpoint.

Only if you've verified recently that the pack is well balanced at the top of SoC wiuld that be safe.

A BMS per-cell HVC can offer that protection though

but that is separate from balancing.

 

[simey_binker]

It seems like there is a recurring theme here; am I wrong in thinking that the common off-the-shelf BMS, or the BMS within a "balance charger" does not balance the cells each time it charges?

I would think that the large gauge wires in the charger throw high amperage in to the full pack, and then the small gauge wires (which have each of the cells individual voltages) would either add or remove volts at low amps as needed to balance the pack. Or, as I've seen it, when a cell becomes full, it routes power into a resistor or switches to the other cells until the rest catch up.

Is this not the case? If they do not actively balance, are they simply just monitoring in case of a failure, and then stop charging if a cell goes too high?

 

[john61ct]

It seems like there is a recurring theme here; am I wrong in thinking that the common off-the-shelf BMS, or the BMS within a "balance charger" does not balance the cells each time it charges?

Keep terms straight, there is no BMS within any charger.

That is usually just a balancing charger. Different chargers work differently.

Same with a BMS, you can of course charge without balancing. Most cheap ones balance very poorly, and their parameters are not adjustable.

There are at least five fundamentally different methodologies for balancing, plus you have top vs bottom, and even midpoint balancing as well.

The usual is at the top, and called "resistive", burning off the excess from the topcell(s), usually at a very low current rate and often not starting until a very high voltage that is not good for cell longevity.

Charging usually comes to a halt while that burning off takes place, so while a "bulk" only charge might take 90min, a full balance charge can take five hours, or sometimes days.

> are they simply just monitoring in case of a failure, and then stop charging if a cell goes too high?

That is a separate **protective** function, aka HVC.

 

[simey_binker]

ok. So balance chargers exist. and they balance the cells. Sometimes they balance in the way I was thinking (which, as I understand, can be in the least efficient/effective more-traditional "burning-off-excess-power-in-a-resistor-type")

Now, the function of a BMS. The BMS stays with the battery (that is, to get terms straight; never with the charger), and... I was under the impression that it too would "balance" the cells through some sort of means, no? Does not a standard BMS balance cells as well?

So a balancing charger will balance the cells on charing. And from my understanding, a BMS can balance the cells on charge and discharge, no? Yes, there are voltage cutoffs, and reporting cell voltages on a screen etc - but both a balance charger, and a BMS, have a function of cell balancing. yes?

 

[john61ct]

the least efficient/effective more-traditional "burning-off-excess-power-in-a-resistor-type")

Correct, can just call it bleeding-resistive balancing

BMS functionality is primarily protective.

HVC, LVC, pack level vs cell level

sometimes OCP, sometimes temperature.

Balancing is also optional, aka protective-only BMS.

Then you have dedicated balancers, aka non-protective BMS.

Balancing should take place only at one selected voltage / SoC, and does not at all need to be done at the top while charging, just that's how the cheaper IMO poorly designed bleeding-resistive gear does it.

It can be implemented well that way, the keys are a higher current rate and ideally independent of the cell V delta ("active balancing")

and user adjustability of the voltage-related setpoints

 

[simey_binker]

Any BMS that I've been looking into (some of which include bluetooth connectivity) have high voltage and low voltage, short circuit protection AND cell balancing capabilities (and not the resistor type).

For instance:
https://www.aliexpress.com/item/32878976763.html?spm=a2g0o.productlist.0.0.2e2423efROmol1&algo_pvid=8decdec2-ff03-44e1-8a89-2b79cee3fe72&algo_expid=8decdec2-ff03-44e1-8a89-2b79cee3fe72-4&btsid=0b0a556c16054524699371071ede27&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

or:
https://www.aliexpress.com/item/4001158535780.html?spm=a2g0o.productlist.0.0.2e2423efROmol1&algo_pvid=8decdec2-ff03-44e1-8a89-2b79cee3fe72&algo_expid=8decdec2-ff03-44e1-8a89-2b79cee3fe72-1&btsid=0b0a556c16054524699371071ede27&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

And also this review where he tests all the functions:

https://youtu.be/bj7abaCrwvI

I don't know about this "a BMS is either protective or non-protective", or "BMSs commonly won't balance". Because according to my experience, that's one of their primary functions.

 

[john61ct]

I don't know about this "a BMS is either protective or non-protective", or "BMSs commonly won't balance". Because according to my experience, that's one of their primary functions.

Non-protective BMS is just a term sometimes used for dedicated balancers. I use it to distinguish between the protective functions as you list, and the balancing function, which is not protective, more of a routine maintenance function.

I never stated anything like "BMSs commonly won't balance".

My point is that 99% of them do a very poor job at it, based on keeping costs low.

So if your pack actually needs better / faster balancing, then you should get a dedicated balancer and

either use a non-balancing BMS

or disable that function on your "regular" one

or just avoid reaching its start-balancing voltage setpoint.

Hope that is more clear.

 

[999zip999]

You can hookup a low voltage alarm to your sense wires.