How to determine the current required to balance battery pack?

[redgear]

How do I determine the right amount of current to use to balance the battery packs? Should I go with Active Balancing for small battery packs or should I just stick to passive balancing?

Thanks

[Daixiwen]

There are two causes for cell unbalance to occur inside a pack if is was previously balanced:

• difference in individual cell self-discharge
• difference in currents due to the monitoring circuit

You need to have an idea of those two factors and have a balancing current that is higher than this.
For the self discharge, usually a lithium ion/polymer cell self discharges at a rate of about 2-3 % per month. It will get higher with high temperatures, so let's say 10% to have a good margin. This gives about 1.4*10^-4 C. For a 2000 mAh cell this means 0.28 mA.
The monitoring circuits typically draw microamps, but it depends on which one you are using. Let's say 0.1 mA. This means a total discharge current (both self and external due to monitoring circuit) of 0.38mA for a 2000 mAh cell. The discharge difference between two cells will actually be a lot lower, but it's very difficult to evaluate. As a worst case scenario lets say that the difference will actually be up to 0.38mA.

Then you need to know how often you will charge the cell. If for example a cell charge lasts for one hour and you recharge the battery pack every 4 days (100 hours), you have a 1/100 ratio so the balancing current needs to be at least 38 mA. The balancing will only actually work towards the end of the charge process, so to get a good margin you can assume 200mA, or C/10.

This is of course just an example and you need to calculate with your own numbers, but it gives you an idea on how you can calculate this. Note that I've used very pessimistic hypothesises here, so in most cases you will not need such a high balancing current. The only cases where you may need a higher one are if you assembled a battery pack with cells at different state of charges, or if you kept it in storage for a long time. In both those cases, if the balancing current is not high enough you just need to keep the battery pack connected to the charger for a longer time to let the balancing circuit do its job.

As for the type of balancing circuit I'd say the simpler the better, especially if you do it yourself. It's more than enough in most cases.

[voltsandjolts]

For 2S the TI BQ29200 is simple and works nicely. I use it with external shunts.

[Siwastaja]

In theory,

Self discharge current of the most self-discharging cell, minus the self discharge current of the least self-discharging cell, divided by the ratio of time you can spend balancing.

An example:
Highest leakage: 30µA.
Lowest leakage: 20µA.
-> Imbalancing difference: 10µA
Stupid balancing algorithm that only works in CV state - battery fully charged once a month - CV phase average length 1 hour: balancing time duty cycle = 1/(30*24) = 0.0014
-> Balancing current needed: 10µA/0.0014 = 7 mA

In reality:
* Cell self-discharges are unspecified.*
* Balancing algorithm is unspecified, if you use an IC
* Even if you knew the algorithm, usage pattern is extremely difficult to predict

*) I made my own extensive study over 1.5 years and ~40 samples, and can say with a confidence that any proper 18650 cell leaks less than 10%/year or approx. 30µA, when stored 100% charged, at elevated (40 degC) temperature, but most much less, and the leakage is practically zero below about 50% SoC, and around 3-4%/year for room temperature even at 100%. But leave room for error.


Obviously, balancing algorithms that only work at the charge end (CV) phase - still very usual - completely fail to do any balancing whatsoever if the user never fully charges the battery, which is usually recommended for good battery life 

I have dissected imbalanced packs with balancers, and balanced packs without balancers. It's very hard to guarantee balancing (which is also why, for worst edge case safety analysis, a balancer is modeled as imbalancer!)

A good algorithm can enable 100x lower balancing current to be used, by doing the work over 100 hours instead of 1 hour! Like I said in an earlier thread, I had no problems whatsoever keeping an abused (overdischarged and revived) 250Ah-ish 20kWh pack in balance with mere 40mA balancing currents, while the competitors do sell products with 1A balancing currents (and stupid balancing algorithm) for the exact same market - they require proper thermal design!

As a result, balancing current is almost always hand-waved (yes, even by very professionals), based on experience and feelings. This is an error-prone process, but because cell balancing is not a safety feature (if someone claims so, don't let them close to designing battery protection systems!), this is not a big issue.

The most important thing, by far, is to guarantee that whatever balancing circuit you use, it won't cause problems. It has to have very low leakage characteristics (if non-negligible, then it has to be balanced with little unit-to-unit variation) so that it doesn't cause imbalance, or, worse, overdischarge cells in long-time storage.

If you can guarantee no ill effects, then any balancing would be a plus, and the more you have, the more capable it is in restoring energy storage of strange, corner case packs.

If you mean redistributive by "active", no, it's practically useless, but remains an academic subject. Looking only cost, the ROI is in thousands of years or infinite; complexity and system cost goes up with either no benefits at all, negative "benefits", or very minor energy or energy density gains.

[redgear]

Then you need to know how often you will charge the cell. If for example a cell charge lasts for one hour and you recharge the battery pack every 4 days (100 hours), you have a 1/100 ratio so the balancing current needs to be at least 38 mA. The balancing will only actually work towards the end of the charge process, so to get a good margin you can assume 200mA, or C/10.

I will be using this bms in LEV, so it is very difficult to determine how often the cell gets charged. What would be the best approach in that case?

This is of course just an example and you need to calculate with your own numbers, but it gives you an idea on how you can calculate this. Note that I've used very pessimistic hypothesises here, so in most cases you will not need such a high balancing current. The only cases where you may need a higher one are if you assembled a battery pack with cells at different state of charges, or if you kept it in storage for a long time. In both those cases, if the balancing current is not high enough you just need to keep the battery pack connected to the charger for a longer time to let the balancing circuit do its job.

As for the type of balancing circuit I'd say the simpler the better, especially if you do it yourself. It's more than enough in most cases.

Thank you for the detailed reply. Can you link me to some simple balancing circuits to look at?

For 2S the TI BQ29200 is simple and works nicely. I use it with external shunts.

It is a 3P6S battery pack!

Obviously, balancing algorithms that only work at the charge end (CV) phase - still very usual - completely fail to do any balancing whatsoever if the user never fully charges the battery, which is usually recommended for good battery life 

I wanted to have balancing at both charge and rest. But, I am not sure if I would end up wasting more energy this way(if I choose passive balancing)

The most important thing, by far, is to guarantee that whatever balancing circuit you use, it won't cause problems. It has to have very low leakage characteristics (if non-negligible, then it has to be balanced with little unit-to-unit variation) so that it doesn't cause imbalance, or, worse, overdischarge cells in long-time storage.

If you can guarantee no ill effects, then any balancing would be a plus, and the more you have, the more capable it is in restoring energy storage of strange, corner case packs.

How can I make sure I don't mess up anything? Is there a checklist to follow?

If you mean redistributive by "active", no, it's practically useless, but remains an academic subject. Looking only cost, the ROI is in thousands of years or infinite; complexity and system cost goes up with either no benefits at all, negative "benefits", or very minor energy or energy density gains.

By "active" I meant transferring current from one to cell to another instead of dissipating it as heat through resistors.
Thank you for the detailed reply.

[Daixiwen]

I will be using this bms in LEV, so it is very difficult to determine how often the cell gets charged. What would be the best approach in that case?

I would say make a hypothesis with an average use case and see what kind of current it gives you. If the BOM cost stays reasonable you can increase it, but at some point you will just have to say to the user that if the battery is kept in storage for a long time, it may have to be put on the charger for a longer time (or charged multiple times) until it recovers its full capacity.

Thank you for the detailed reply. Can you link me to some simple balancing circuits to look at?

Well I thought that would be easy, but I haven't designed balancing circuits in at least 15 years and all the chips I tried to find just now with a quick search were active balancing circuits with redistribution from cell to cell. They are more efficient but get quickly more complicated than a "passive" balancing circuit, that just connects a bypass resistor when the cell voltage gets over a certain level. I would advise you anyway to use a ready made chip solution for your balancing circuit, because if you try and develop one on your own, if you have the slightest difference in balancing voltage between two cells, as said by Siwastaja you will cause more imbalance than correct it.

I wanted to have balancing at both charge and rest. But, I am not sure if I would end up wasting more energy this way(if I choose passive balancing)

It depends on the circuit, but be careful during rest that you don't start emptying the battery. I prefer to only have balancing during charge to ensure a good storage time without loosing capacity. Some cheap hobby chargers use external balancing circuits that are only connected to the cells at the same time than the charger and it can be a solution.

How can I make sure I don't mess up anything? Is there a checklist to follow?

I don't have a check list but at least make sure the current drawn from each cell when not balancing is as low as possible (and as close as possible from cell to cell) and that the voltage measurement used for balancing is as close as possible from cell to cell.

[redgear]

Well I thought that would be easy, but I haven't designed balancing circuits in at least 15 years and all the chips I tried to find just now with a quick search were active balancing circuits with redistribution from cell to cell. They are more efficient but get quickly more complicated than a "passive" balancing circuit, that just connects a bypass resistor when the cell voltage gets over a certain level. I would advise you anyway to use a ready made chip solution for your balancing circuit, because if you try and develop one on your own, if you have the slightest difference in balancing voltage between two cells, as said by Siwastaja you will cause more imbalance than correct it.

Thanks, I am going with a TI solution.

It depends on the circuit, but be careful during rest that you don't start emptying the battery. I prefer to only have balancing during charge to ensure a good storage time without loosing capacity. Some cheap hobby chargers use external balancing circuits that are only connected to the cells at the same time than the charger and it can be a solution.

Scrapped the idea of balancing at rest. Will be balancing only while charging.

I don't have a check list but at least make sure the current drawn from each cell when not balancing is as low as possible (and as close as possible from cell to cell) and that the voltage measurement used for balancing is as close as possible from cell to cell.

Ok