Balance Charging? 100% Charging?

[Gionnirocket]

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

 

[AHicks]

I can think of one reason. Let's say the charger shuts off at 54.6 and the battery starts leveling itself out internally. You decide to go for a ride several hours later, but now, your fully charged 54.6v battery has 53 volts - and there's no guarantee the cells are completely balanced yet!

Results will be reduced performance, as well as reduced mileage availability, both due to lower pack voltage.

Possible too that cells with higher voltages within the pack may be more highly stressed if called upon for max output. This vs. ALL cells supporting the big load.

 

[Gionnirocket]

Hi Al.
Yes I get that, though I haven't seen my battery drop more than a tenth or two even after 48 hours of resting after a charge.
But that really doesn't address the question. My question is... Does a battery self balance at rest, and if not, why?

 

[AHicks]

I think it does try to self balance. As far as I know there's nothing to prevent that from happening. A properly balanced battery shouldn't drop any voltage to my way of thinking - unless there's some sort of internal housekeeping going on in the controller or something.

 

[Gionnirocket]

I think it does try to self balance. As far as I know there's nothing to prevent that from happening. A properly balanced battery shouldn't drop any voltage to my way of thinking - unless there's some sort of internal housekeeping going on in the controller or something.

That's what I'm thinking as well but was hoping someone with a technical background could shed some more light.

 

[harryS]

Most BMS work by shutting off charge as soon as any cell group hits a set point, usually 4.2 volts. A balance BMS will turn on a bleed circuit that burns off current with a resistor from this group. Then the other cells can catch up. You're wasting power by burning off the high groups, but you get it back from the charger.

Could someone design a system where the excess voltage is diverted to the lower cells? Sure, but without a charger, you're not gaining any extra energy. So you don't want self balancing at rest. It also would complicate the design. As long as you have "free" power with the charger, do it then,

By the way, many batteries don't have balance BMS. That isn't that bad, if you have good quality cells. I've been running a pack like that since 2017. Charged 83 times alone in 2020. No balance BMS, but it still gets to 41.8V (42.0 is ideal) fresh off the charger, which I believe indicate it's pretty much in balance.

How do you know if your battery will balance? If you use a current meter and see it pulling a trickle of current after charging, like .05A, that is about the balance current is. Of course, a leaky cell can do that too, In any case, I have put in balance BMS into batteries and that's what I see when charging.

Here's the original BMS in a UPP bottle battery, It's so tiny, there's no room for balance circuits.

 

[George S.]

Most BMS work by shutting off charge as soon as any cell group hits a set point, usually 4.2 volts. A balance BMS will turn on a bleed circuit that burns off current with a resistor from this group. Then the other cells can catch up. You're wasting power by burning off the high groups, but you get it back from the charger.

Could someone design a system where the excess voltage is diverted to the lower cells? Sure, but without a charger, you're not gaining any extra energy. So you don't want self balancing at rest. It also would complicate the design. As long as you have "free" power with the charger, do it then,

By the way, many batteries don't have balance BMS. That isn't that bad, if you have good quality cells. I've been running a pack like that since 2017. Charged 83 times alone in 2020. No balance BMS, but it still gets to 41.8V (42.0 is ideal) fresh off the charger, which I believe indicate it's pretty much in balance.

How do you know if your battery will balance? If you use a current meter and see it pulling a trickle of current after charging, like .05A, that is about the balance current is. Of course, a leaky cell can do that too, In any case, I have put in balance BMS into batteries and that's what I see when charging.

Here's the original BMS in a UPP bottle battery, It's so tiny, there's no room for balance circuits.

View attachment 66101

I think they call them active cell balancers, but like you say, there is not net charging. Will Prowse, who has an active YouTube channel for RV solar and lithium, tested some active chargers and found they were passive. People get a little crazy on this subject. My packs without a BMS stay in balance, but it's nice to have balance leads to test. If you have a battery that is dropping in voltage on a bike, or the display is acting weird, it never hurts to just put a load on the battery (off the bike) and watch how the voltage drops. It should be steadily downward.

https://www.power-sonic.com/blog/batteries/the-smart-battery-lithium/

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[Gionnirocket]

Could someone design a system where the excess voltage is diverted to the lower cells?

My question or thought is that isn't it inherent in the build? All the cells, or more accurately cell groups as far as balancing goes are interconnected already. What is needed beyond that?

And the subsequent question would then be, then wouldn't a battery charged to 80% self balance overnight by itself?

 

[harryS]

The BMS has wires going to the positive/negative of every cell, but that's just the connectivity. Your passive system monitors those cells to make sure they stay in the safe range. You have ten voltage checkers on a 36V pack and 13 on a 48V pack. If it balances, you have 10 or 13 bleeder circuits.

They also want to make the BMS a $5-10 part. Not a lot of money there to put in the circuitry that NASA might want.

Still, you can get a bluetooth BMS for $40 and see all the internal voltages on your phone App. For extra cost, I'm sure someone could make the balance voltage programmable for your 80% charge. The battery maker would say, "why do I want to support 80% charge? I'm here to sell performance and sell more batteries."

 

[Taylor57]

How often should I fully charge my XP and my Sport. Lately, I have been only charging the XP to about 90% and the Sport to ??. Is it ok to once in awhile go max and charge till my charger turns green? Also, do I do any damage if I leave the charger on overnight?

 

[BarryS]

How important is any of this in reality : In real world usage, power and life of your battery : My owners manual

How often should I fully charge my XP and my Sport. Lately, I have been only charging the XP to about 90% and the Sport to ??. Is it ok to once in awhile go max and charge till my charger turns green? Also, do I do any damage if I leave the charger on overnight?

MO after doing extensive reading : All this concern about Charging is way over blown . My Specialized Owners manual says Try no to let my battery go below 10% to often and to Charge until the light on the charger is Green. I'm going to enjoy the Bike and use it according to the Owners manual it comes with : I have a 300 charge cycle warranty or 2 years whichever comes first : I live in Michigan : The 2 years will expire before teh 300 charge cycles does : It also warns not to leave the battery exposed to Temps over 85 Degrees or Under -4 for extended periods If I were you I'd just enjoy the bike for what it is : If you followed all this silly rules it may get you a result of what ? A Hand full of extra charges :
Where you need to pay attention is not allowing the battery to stay on the Charger long after it's charged up. Other then that Enjoy : PS: There was a post I can't find at the Moment Where specialized forum members reported over 500 charges and still going strong :
Don't Sweat the small stuff It's anal as hell . IMO of course Just keep in mind : There are some who getting every ounce of Juice out of their batteries is a challenge they find interesting : For me and others like me : That's to much effort for to little and possibly no extra results : My last cell phone I used to be so careful to follow all teh tech charging rates : It was gone after just 2 years : My current phone is just 2 years old and still lasts along while : And I've just charged it up full depending on what My needs are : Good luck

 

[AHicks]

How important is any of this in reality : In real world usage, power and life of your battery : My owners manual

MO after doing extensive reading : All this concern about Charging is way over blown . My Specialized Owners manual says Try no to let my battery go below 10% to often and to Charge until the light on the charger is Green. I'm going to enjoy the Bike and use it according to the Owners manual it comes with : I have a 300 charge cycle warranty or 2 years whichever comes first : I live in Michigan : The 2 years will expire before teh 300 charge cycles does : It also warns not to leave the battery exposed to Temps over 85 Degrees or Under -4 for extended periods If I were you I'd just enjoy the bike for what it is : If you followed all this silly rules it may get you a result of what ? A Hand full of extra charges :
Where you need to pay attention is not allowing the battery to stay on the Charger long after it's charged up. Other then that Enjoy : PS: There was a post I can't find at the Moment Where specialized forum members reported over 500 charges and still going strong :
Don't Sweat the small stuff It's anal as hell . IMO of course Just keep in mind : There are some who getting every ounce of Juice out of their batteries is a challenge they find interesting : For me and others like me : That's to much effort for to little and possibly no extra results : My last cell phone I used to be so careful to follow all teh tech charging rates : It was gone after just 2 years : My current phone is just 2 years old and still lasts along while : And I've just charged it up full depending on what My needs are : Good luck

Couldn't agree more on every point.

 

[harryS]

People get a little crazy on this subject. My packs without a BMS stay in balance, but it's nice to have balance leads to test. If you have a battery that is dropping in voltage on a bike, or the display is acting weird, it never hurts to just put a load on the battery (off the bike) and watch how the voltage drops. It should be steadily downward.

Hi George. I haven't seen one of your posts for quite a while. Nice to hear you're still RV'ing..

 

[Gionnirocket]

I'm not stressing about this at all
And I'm sorry but no one is reading the original question correctly.
It's that I like to know things and I find it interesting to learn about my hobby rather than assume... or even worse, just repeat what everyone says

@harryS ... Please show me a BMS with with 65 pairs of wires for a 13s5p 48v battery

 

[tomjasz]

Partial Charge and Cell Balancing
One of the only downsides to partial charging is that many inexpensive battery management system (BMS) circuits will only do active bleed balancing of the cells when they are at or near the full charge voltage of 4.2 V/cell. This means that with partial charge profiles that don’t reach that voltage, the BMS circuit will never be able to rebalance cells if they are drifting apart. Over time you may have less available capacity from the pack as certain cells will hit the low voltage cutoff on discharge well before others.

If this is an issue it can be easily remedied by occasionally (like once every month or two) leaving the pack connected to a 100% charge cycle overnight.

Good quality programmable BMS circuits will usually attempt to balance the cells whenever they see more than a certain voltage spread between the highest and lowest cell in the group, and in that case there is no problem with partial charges. Similarly, good quality cells rarely drift out of balance in a series string, and can easily handle 100 or more cycles and maintain a perfect voltage matching even if the BMS circuit doesn’t do any active balancing. But if you aren’t sure of the makeup of your battery pack, then the protocol of occasionally giving a 100% top-up is a good bet to ensure both a long cycle life and evenly matched cell voltages.

 

[Gionnirocket]

Seems everyone likes to drift to the same debate and that's really not what I'm asking.
So let me try something different

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

 

[Gionnirocket]

Ok... I found a partial explanation for which I may need to put down on paper for a visual to have it sink in.
And on Wikipedia of all places.... who knew

"Balancing is only necessary for packs that contain more than one cell in series. Parallel cells will naturally balance since they are directly connected to each other, but groups of parallel wired cells, wired in series ( parallel-series wiring ) must be balanced between cell groups."

So.. The parallel groups of the battery do self balance but the series groups can't because current can only flow in one direction is my assumption.
@harryS ...and that's why the BMS doesn't have a pair going to each cell and just one wire going to each series group.

If anyone has further detail or my assumption is incorrect, please school me.
But for now, I'm So happy I can stop stressing about it!!

 

[AHicks]

Thanks for posting! I wasn't thinking we have parallel AND series in play. Head stuck on parallel..... -Al

 

[Gionnirocket]

Thanks for posting! I wasn't thinking we have parallel AND series in play. Head stuck on parallel..... -Al

Me too
Who says you can't teach ol' dogs!

 

[harryS]

Here's a schematic of a BMS for a 36V ebike battery, which has 10 series groups. Your bike is directly connected to the positive terminal of the batteries in series, but the bike's negative connection has to go to P- thru the transistor switches controlled by X2. For charging, the positive of the plug also goes to the positive of the batteries, and the negative goes to C-, controlled by X4.

The X2 contains the circuitry that shuts off the power from the battery when a cell group goes too low in voltage. or if the current is too high.

The X4 has circuits that shut off charging if any cell group hits the maximum voltage. It also would have the balance capability if that is supported.

Better batteries also have temperature sensors that shut the battery if too hot, and I have seen one that shuts off charging if it's too cold.