Balance Charging? 100% Charging?

[AHicks]

Thanks for that too Harry!

 

[Ravi Kempaiah]

I'm hoping someone ( maybe @Ravi Kempaiah : ) can help me understand some aspects of Balance Charging.
I understand with most simple chargers and BMS's this is done at the end of a full charge and I understand the benefits of why it needs to be done.

But where I would like a little clarification is why doesn't a battery self balance once unpluged if all the cells are interconnected and higher voltage will always flow to lower voltage?
Is it because individual cell resistance can be greater than the potential at smaller differentials

Somebody please school me

why doesn't a battery self balance once unpluged if all the cells are interconnected and higher voltage will always flow to lower voltage?

Thanks for tagging me in.
I spoke with a friend of mine who has many years of experience designing BMS for battery packs before replying here.

You are right about the parallel groups but you need to balance the cells in series as well. There are many ways to do that and it is not as easy as it may seem.

The summary is:

1. Passive balancing is more common in BMS boards where cost is a major design constraint and it makes use of bypass resistors. This kind of balancing works when you are charging the battery pack and not when you are discharging. Packs that can be be purchased for ~$250, it may most likely use one of these bypass resistor based balancing circuits.
   
   
2. Active balancing on the other hand is more complex and needs more components in the BMS board. What you are referring in your question comes under the gamut of active cell balancing where one cell transfers the excess charge to the next cell. To transfer this charge, the circuit needs to know when to initiate this process, when to stop this process, how much to transfer etc. This necessitates a few more components, larger footprint and I frankly don't think run-of-the mill E-bike batteries can afford that.

To explain the complexity of your question, I am attaching this document by Analog Devices ( which is a well respected company just like Texas Instruments). They explain it much better than I could do now.

 

[AHicks]

This needs to be a "sticky" somewhere.....

 

[arcom]

我希望有人（也许@Ravi Kempaiah：）可以帮助我理解余额计费的某些方面。
我了解使用最简单的充电器和BMS，这是在充满电结束时完成的，并且我了解为什么需要这样做的好处。

但是，我想澄清一下的是，如果所有电池都互连并且更高的电压始终流向更低的电压，为什么一旦拔下电池就不会自平衡呢？
是因为单个电池的电阻可以大于较小差异时的电势

有人请学校我

I could not agree more. You hit the nail on the head!

 

[Gionnirocket]

I've been running a few tests and here are some preliminary observations.
Starting with the battery at 40% I charged to 80%... and then the next morning I charged to 100% in anticipation of my daily ride.
On the second leg of charging I made note that at 54.30v the current began to steadily decline from the initial 1.97a. This is expected with CC-CV charging though a bit earlier than I expected.
At 54.45v there was a voltage bounce that registered 54.50 (though I believe it was actually higher) and the charger shut off. Just prior to the bounce the output current was down to 0.69a.
I waited 30 minutes and restarted the charger and it continued to charge at which point the start readings were 54.42v and 0.93a output. It continued to charge until reaching 54.58v and 0.17a output. Between the voltage bounce and the end of the second attempt the battery absorbed an additional 12 watt/hour of energy.

So preliminary hypothesis is that there may be some reason to try a second attempt at a full charge, especially if you are not reading close to 54.6v after the initial attempt. I've had my charger shut down initially at 54.0v - 54.2v several times

My goal is not to pack the battery with the absolute highest amount of power but to better facilitate the balance process as it is accomplished at the end of the charge cycle. This is initiated around the t1 mark with most basic Passive Balance BMS's and premature charger shutdown may interfere with this.
One of my next attempts will be to use a bench top power supply to charge as it has no automatic Voltage Off, though I will limit it by time for safety and allow the charging process to be strictly handled by the BMS.

 

[harryS]

Most of my chargers will flip the green led to red when the battery gets near full voltage but they don’t shut off, but keep applying voltage. That allows the balance to happen if the BMS is capable.

I believe the cells naturally throttle the current as they approach full charge, but we do want the charger to go into constant voltage mode no higher than the full voltage. If you connect a 60V supply to your 13S battery, it will not be able to force much more current than what the battery will accept, but it can try to overcharge the cells and your only protection is the overcharge circuit in the BMS.

 

[Gionnirocket]

Most of my chargers will flip the green led to red when the battery gets near full voltage but they don’t shut off, but keep applying voltage. That allows the balance to happen if the BMS is capable.

I believe the cells naturally throttle the current as they approach full charge, but we do want the charger to go into constant voltage mode no higher than the full voltage. If you connect a 60V supply to your 13S battery, it will not be able to force much more current than what the battery will accept, but it can try to overcharge the cells and your only protection is the overcharge circuit in the BMS.

Yes Harry I did notice that the charger light did turn green from red at near full voltage but that it continued to have a current output and that it continually decreased.
When I said that the charger shut off in my experiment, the output current was 0, so it was done changing. One of my chargers will cycle back ON if the voltage drops but when it does the bounce usually shuts it right back down. The other charger once OFF needs to be unpluged to restart.
When charging with a 60v bench top power supply if capable as mine is I can set it up for CC-CV output. So it will function the same as my charger never going above 54.6v but without the 54.6v shutdown. I do realize that I'll be mostly relying on the BMS for overcharge protection but that is one of the reasons for the experiment. I feel that the cheap chargers are shutting down to early. I will have it protected by the time function and also the over volt protection (set to 54.7v) of my Hack for run away, but most importantly it will be closely monitored. I'm running these experiments with a temp probe and webcam

 

[MikeDee]

So battery balancing occurs at the end of a full charge. Now, wouldn't charging your battery to the recommended 80% to extend the life f-up the battery because the balancing operation wouldn't occur?

 

[AHicks]

That's what I was thinking, pretty much exactly, until getting involved in the discussion here (last couple of pages if you're in a hurry).
https://electricbikereview.com/forums/threads/electric-bike-battery-care.36481/

It turns out that once balanced, the cells don't need to be balanced frequently, mostly due to the fact they are not being discharged at a very high rate. The 1C rate Ravi suggests is pretty common for an e-bike is a pretty low discharge rate considering what these cells are capable of. We got into a pretty detailed discussion, as I wasn't buying into it at first. It was a tough sell, but he managed to change my mind. Link above might be worth reading if you'd like to know more about it. -Al

 

[Gionnirocket]

Most of my chargers will flip the green led to red when the battery gets near full voltage but they don’t shut off, but keep applying voltage. That allows the balance to happen if the BMS is capable.

I believe the cells naturally throttle the current as they approach full charge, but we do want the charger to go into constant voltage mode no higher than the full voltage. If you connect a 60V supply to your 13S battery, it will not be able to force much more current than what the battery will accept, but it can try to overcharge the cells and your only protection is the overcharge circuit in the BMS.

I happen to do another full charge on my battery yesterday as I had a long ride planned and it has been a while since I had gone above 85%
Again my chargers struggle to get the battery above 54.3v as they shut down for safety at 54.6... but then the battery instantly settles lower once the current input is cut.
So I then connected my bench top power supply and it took it the rest of the way with the current dropping all the way down to 0.01a/0.5w
I'm curious what a more expensive chargers like the Grin Satiator or something say from Yamaha that have in factory matched BMS's and chargers achieve.

I've double checked my display's voltage with a Fluke and another less expensive multimeter several times and they are all within 0.02v

On a side note... I had forgotten how a few extra volts adds to the aggressive feel of this motor.

 

[AHicks]

If you're wanting to fully peak a battery with a stock charger, you can let it kick off normally, then wait an hour or so for the cells to settle a bit, and then plug the charger back in. It'll only run for a few more minutes (15 maybe?) and shut off again as the pack reaches full voltage. Do that until you see the 54.6 when checked. There's no way the factory chargers will go beyond that point....

 

[Gionnirocket]

If you're wanting to fully peak a battery with a stock charger, you can let it kick off normally, then wait an hour or so for the cells to settle a bit, and then plug the charger back in. It'll only run for a few more minutes (15 maybe?) and shut off again as the pack reaches full voltage. Do that until you see the 54.6 when checked. There's no way the factory chargers will go beyond that point....

Yeah I understand the safety concerns built into the chargers.. and it's really not a problem. This is more of an experiment and mental exercise for me.
The charger that came with the battery seems very sensitive and the initial bounce when close to full voltage shuts it right back down. A one second delay on shut down would solve the problem.
A backup charger requires an unplug to restart.
The bench top supply gets it done more efficiently and with no waiting. Costing more than both chargers combined, it's no surprise. I only do this supervised and with other high voltage cutoff implemented for safety so I'm not worried about over charging. It seems the BMS does an excellent job of protecting from overcharging as well.
The bench top supply is very stable and accurate.. You just need a clear head when setting it up.

 

[AHicks]

Yup, have some lab quality equipment here too.