To balance or not to balance packs?

[Momato]

Hi all,

I normally build my 14.X V packs with a PCB and balancer leads. However, I notice that most (all?) commercially available packs (e.g. from all-battery or batteryspace) are without balancer leads. Are they not needed (I do not need an explanation of their function)? What do you guys do?

Best regards, Morten

 

[dom]

Hi Morten
With low drain application at 1A or less which these type batteries are mostly used for, the batteries don't usually get that unbalanced.

Usually the hi drain lipos for RC stuff need the balancing leads as that can put the batteries out of whack very quickly.

Cheers
Dom

 

[Momato]

Thanks - so how many charge cycles would you expect before there's problems when not balancing?

And one extra question: I have been using 4 2600 mAh Tenergy cells, and 4-cell PCB's. I would like some packs with higher capacity - will the 4-cell PCB still see "one" cell even if two are parallel connected (i.e. 4 x 2 cells total)?

 

[dom]

There's no worries with the parallel connection as that balances itself out.

I've had my 4 cell 14.8V 2400Ah (LG cells from Batteryspace) with PCB running at 700mA - 1A for maybe 40 cycles with no apparent unbalance.
No worries to run 2 of these parallel if that was what you were asking.

Not that i can check them individually -but they charge back up and stay at their original voltage -so the PCB is doing a good job.

Cheers
Dom

 

[SemiMan]

It is a matter of long term battery life. While you may do well in the first 100+ charges, after that, you may run into rapid decline of the pack. This issue is starting to be noticed on packs and at least in high volume applications where battery life is important (or at least the manufacturer cares). Laptop warranties are usually 1 year.... and likely less than 100 charge cycles occur. However, those of us that are heavy battery users are not at all unused to being 6-12 months into our battery with a very significant drop in capacity. Right now the suppliers are likely salivating over expensive replacement battery (and laptop) replacement. I think some laptops, and perhaps even a good percent now may be putting in balancing as poor life and customer dissatisfaction is not something they can live with long term.

I have read some technical articles that show that cell balancing in a pack can double the number of recharge cycles and specifically will extend the number of cycles where you are getting 70%+ of the capacity of the complete pack.

Semiman

 

[SilverFox]

Hello Morten,

In a battery, the cells will get unbalanced at some point. If the battery is not abused and the cells well matched, it will take longer to reach this point, but you will get there.

As you know, Li-Ion cells in series can have a cell end up overcharged. In cases of slight overcharge, the cell simply degrades and the overall performance of the pack drops. The purpose of balancing is to prevent this overcharging.

I think the reason you don't see packs available with balance leads is because the lower end chargers don't have a provision for balancing a pack. A business that offers a $20 charger would have to put forth a major effort in educating their customers why they should instead purchase their $100 charger that allows for balancing...

Tom

 

[dom]

You can notice when batteries get out of balance just by checking the voltage after charging and making a note of the original voltage.

I had 24V e-bike Li-Ion battery packs and could quickly tell when a cell was on the way out with a 0.1V difference on the end charge.

As to your original question -you have to work out if it's worth it to make your cells last longer with the cost of the balancing module/charger.

Cheers
Dom

 

[Momato]

Thanks guys

Ok, then I will extend my question:

1) I have a balanced charger, so that is not the problem. It balances up to 6 cells.

2) I want an 8-cell 14.8 V battery (because I want to double my capacity)

3) Can I parallel 2 strings of four cells and just use a 4-cell PCB - will the PCB just "see" 4 cells, even if there is actually 4*2 cells?

4) Can I parallel 2 strings of four cells and just use a 4-cell balancing wire - will the balancer just "see" 4 cells, even if there is actually 4*2 cells?

I appreciate that you are willing to educate me

Best regards, Morten - Denmark

 

[SilverFox]

Hello Morten,

When you parallel Li-Ion cells, the paralleled cells will self balance so you can treat them as a single cell with a larger capacity. You can both balance and protect pairs of Li-Ion cells in a battery made up this way. The limitation comes from the protection circuit. The battery pack may be capable of higher currents than the protection circuit allows.

Tom

 

[Momato]

The limitation comes from the protection circuit. The battery pack may be capable of higher currents than the protection circuit allows.

Thanks Tom - that was the answer I was looking for :thumbsup:

Since I will be using this for a triple led design I will only drain around 1A, so I should be good. Will order some more cells from All-battery.

Best regards, Morten