General information on BMS / Cell balancing as well as battery packs

Ilan from Metroboard did a great video discussing a tear-down of his 20 mile battery pack. I love this guy, he really enjoys educating others and finds that a customer who has more knowledge on their board is one who will enjoy their board even more.

The interesting part of his discussion is around battery balancing and how the BMS achieves this. I asked Ilan if I needed to have my Metroboard unplugged or plugged in so that it could properly balance. He indicated it needed to be plugged in for the balancing to occur. In this video I see why this is so. The BMS after its finished charging will monitor the cells, it then will see what cells are under the nominal 4.2 volts will keep charging up those cells until 4.2volts is reached.

I need to email Evolve to see if we need to keep our GTs on the charger in order for the cell balancing to correctly function.

 

Am I imagining this or is there a certain amount of "natural passive balancing" that takes place as well?
(not from the charger actively - just be being interconnected - maybe it matters in parallel vs in series etc)

 

Evolve wrote back saying that the BMS on the GTs don't do any cell balancing.

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The BMS is more of a management to let you know if a cell is out of balance. It doesnt bring a cell up if its way off.
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This reply really doesn't make any sense to me honestly. What good is a BMS is it tells you that its out of balance but not to balance the cells?

From my observation I believe the GT's BMS is like the one in the video, where you need to keep it plugged in. My board never reached its rated top speed until I left the battery / board sit on the charger overnight for a couple of nights. Then only did it reach the top speed. And when I view my cell's voltages in the remote they're very close in tolerance and not far off at all.

 

yea if you look at an RC Style LIPO balancer / charger it has a power connection to the whole pack and then a balance lead with access to the individual cells - has anyone noticed this between the motor controller board and the battery pack ? (it must be there to get the individual cell voltage in that hidden menu) - I guess they can simply choose to use this for balancing or not

I know in RC its kind of a split camp - but the general consensus is you don't need to balance every single charge

I have a Shorai battery in my motor bike with a special Shorai balancer and charger and it connects to the balance leads and always balances - for what its worth

To me balancing never seems like a bad idea - it just takes longer and needs more "smarts" in the charger

 

No, that's called active or dynamic balancing and very expensive to implement. That type of balancing will balance cells as they discharge as well which is very cool. We only can balance when we're fully charged and on the charger using the BMS, the input voltage and bleed resistors.

 

so basically it stops charging the cells that are already full and keeps going with the less-full ones? (hence the bleed resistors to dump the extra power - likely via a bit of heat)

 

Exactly.

 

FYI, Evolve responded back regards to my BMS battery balancing question. They misunderstood my question and agreed that the BMS does balance the battery cells and it does need to be plugged in in order for the BMS to balance the battery pack.

 

Soooooo...how long does it need to be plugged in to balance the pack? Does the type of charger matter? Slow or the new black fast one? Or did you say the charger doesnt balance, the BMS does?
I was always told to unplug the charger as soon as it goes green.

 

The BMS does the balancing and *needs* the charger to be plugged in so that the BMS can supply voltage to the cells of lower voltage and bring them up to 4.2v. How long should you leave it on? Well it all depends on how off your cells are out of balanced and their mV spread from each other. But we do have that ability to see our cell's voltage in the remote, so look at that for metrics. I always leave my board plugged in over night. If you keep your board balanced like that as a regular habit the cells can't stray too far and it won't take long to bring them back. But as I mentioned my board was out of balance when new and took 2 nights of balancing to bring it back to where it should.

As for types of chargers, any charger quick or slow works as long as it supplies voltage to the board. I always use my slow charger whenever possible. I use my fast charger only when I need to charge fast but I don't make a habit of using that as a standard charger. Small packs like ours like lower amps, higher amps over time can heat and stress the cells. I bought my quick charger for when I riding up in the city and need a charge fast. But at home I use the 2 amp slow.

 

when the light turns green, and board is fully charged, doesn't the charger simply stop charging the board, I don't understand how leaving it on for many hours or overnight after you reach the green light can balance the cells, It may do that while charging, but when you reach 100%, the charger shuts off to prevent over charging. Unless someone can do tests, i don't think leaving it on for many hours after 100% is actually doing anything.

 

The charger is still supplying voltage to the board, even when its green. Fully charged and fully balanced are two different things. This is simple BMS management.

The charger does NO battery management, the BMS does. The charger does not prevent overcharging, the BMS does.

 

yeah what i meant was let it charge till green light goes on, check info, then leave it on over night, and see if anything what changes

 

I do agree that it would be good to validate if you wait for the charger to just turn green, if the pack is balanced there or after sitting on the charger for a duration of time that it gets balanced. You'd have to get your pack out of balance, record the cell voltage screen, then do method a where the charger immediately turns green, check the cell voltages. If not balanced, then do method b of letting it sit on the charger. Not sure if you need to allow the board to drain and then charge again to initiate that or not.

 

So having a "tech talk" with Ilan on the MetroBoard Facebook page about battery balancing, he indicated that balancing only begins when the pack voltage has reached 42Volts and typically that's when the charger light turns green. He also said that it can happen before the light turns green as well since the standard charging algorithm for a lithium battery is a constant current until the pack reaches 42Volts. He said that depending on the charger, some chargers can "disconnect" that 42Volts as soon as it reaches the "Green" state. They do this as a safety feature since they don't want the charger to totally rely on the BMS in case of a BMS malfunction. And with that, cell balancing can be difficult. I then discussed if the charger on the MetroBoard shuts down the 42V after it turns green or if it holds it at 42V. He said he believes the slow 2 amp charger disconnects but feels the fast charger, the 4 amp does not. He said that we can test this by monitoring the output voltage of the charger once it has completed the charge and disconnected from the battery. I think we need to do this same testing on our GTs using both the 2amp slow and 4amp fast charger to fully understand what's happening.

So with that, we're back to what do we have with our board and how do we properly balance our pack again question..... So we either need to hook up a multi-meter during and after the charge, see if the charger disconnects abruptly after it turns green or keeps the 42volts going to the BMS/pack and slowly tapers it down. We also can look at that cell voltage as we discussed earlier to monitor the balancing of the cell/pack.

It seems like we still have some homework to do in order to fully understand our BMS and charging system.

 

I think we just need to put a multimeter inline between the charger and the board and see if there are still a few milliamps passing to the BMS on the board (after its gone green) - I don't think balancing would take much (not like full faster 2 amp charging) - if you notice the green light appears on the charger if the board is not even plugged in.

But balancing can take some time - from doing this with RC batteries just a regular charge that turns off on peak VS a balance charge can be quite a bit quicker - again depending on how out of balance the cells are - so where a plain charge session may be done and turn off in 15mins - a balance session may take 45mins (or more) - this is up to the bms of course

 

Will balancing my Bamboo GT battery give it more range with AT wheels?

 

others will chime in here but I don't think so - its more for the health and longevity of the whole battery pack

 

Battery balancing brings up stronger cells that take longer to charge up to a full charge, hence you get more voltage and capacity. That translates to more power and range. If battery balancing didn't occur, your battery would be as good as your weakest cell when charging. However battery is as good as your weakest cell when discharged since again, the weakest discharge the quickest and the strongest hold on to their charge longer.