Charging multiple 14500's in parallel

[lightseeker2009]

I want to use 3X 14500 batteries in series. I don't like to use these type of batteries in series but the RC crowd charges and uses lipos in series, up to 10 cells in series without any problems if treated correctly.
But even though they will be used in series at a low ampdraw, something like 0.2A, I want to charge them in parallel on my hobby charger.
Just a question about this.
Since cells must be balanched and using them in series might cause them to be slightly ''out'', I will only start charging them after connecting them in parallel and waiting a few minutes for them to equalise. But How much voltage difference between them is allowed before I can safely connect them in parallel?

 

[mvyrmnd]

I've found .1V difference is fine when attaching cells in parallel.

The trick is to make sure that the current that will flow between the cells is no more than the maximum safe charge rate of the cells ie. if the cell holds 1000mAh (I know your cells have less, but round numbers are easier!) and has a max charge rate of 0.5C then you want no more than 500mA flowing when you hook up the cells. You could use a multimeter to test the current at 0.1V difference and measure to see how far out of balance you can safely go.

 

[lightseeker2009]

Í will have to do a test like this. But how many seconds does it normally take to equalise.

 

[mvyrmnd]

Í will have to do a test like this. But how many seconds does it normally take to equalise.

That depends entirely on how far out of balance they are :)

 

[tatasal]

I've found .1V difference is fine when attaching cells in parallel.

The trick is to make sure that the current that will flow between the cells is no more than the maximum safe charge rate of the cells ie. if the cell holds 1000mAh (I know your cells have less, but round numbers are easier!) and has a max charge rate of 0.5C then you want no more than 500mA flowing when you hook up the cells. You could use a multimeter to test the current at 0.1V difference and measure to see how far out of balance you can safely go.

If I want to charge 4 liIon 18650s in parallel @ .6a, do you multiply the amperage rates by the no. of cells, or it stays @ .6a, regardless of the no. of cells in parallel?

 

[lightseeker2009]

If I want to charge 4 liIon 18650s in parallel @ .6a, do you multiply the amperage rates by the no. of cells, or it stays @ .6a, regardless of the no. of cells in parallel?

Charge will be devided. Each cell will only get .15A. that is the beauty of charging cells in parallel. You can increase the charge rate and save a lot of time on charging. If you want to charge them at .6A each, set your charge rate at 2.4A

 

[mvyrmnd]

Charge will be devided. Each cell will only get .15A. that is the beauty of charging cells in parallel. You can increase the charge rate and save a lot of time on charging. If you want to charge them at .6A each, set your charge rate at 2.4A

Exactly.

 

[tatasal]

Exactly.

Thanks, gentlemen

 

[lightseeker2009]

Hi guys, just some feedback on this matter for those of you that might be interested what happens when you connect 14500's at different voltages together in parallel.
I made a 3AA parallel charging cradle. I inserted a 14500 with a voltage reading of 4.06V and the weakest one I had of 3.94V in parallel. I only connected 2X 14500's for the test. Then I connected them via a multimeter. The moment I connected the multimeter the current began to flow from the stronger battery to the weaker one, to be expected of course.
But what is unexpected, to me at least, is the small flow of current. I really thought there will be a rather big spike, then it will come down as the time goes on, but not so. The moment I started the test there was a current of only 0.15A. Very much an acceptable charge current so its very very safe to connect 14500's in parallel if voltages is not to far different. Then my second observation. I innitially thought cells will ballance each other within seconds. Not so. After about 30 seconds the current between the cells droped from 0.15A to only 0.12A. So it will take a couple of minutes at least for cells to be completely balanched. And this is with a small voltage difference of only 0.12V between the cells.

 

[HKJ]

I did this test with two 18650 some time ago, using this setup:

With one full and one empty battery I got a short spike of 6A:

 

[45/70]

There have been several threads here on the Forum concerning charging Li-Ion cells in parallel. I think this one, started by SilverFox, probably covers the topic best.

After setting up converted (from original NiCd chargers) 4 bay cradles to plug directly into a hobby charger, in both series and parallel configurations, anymore I just use the parallel setup. It's simpler and you always end up with balanced cells, without having to use the balancing feature along with a balancing harness, necessary when charging in series.

The only drawbacks to parallel charging are that you are limited, as far as charge current available, as this is divided up among how ever many cells you are charging at once. I've found even with only a 5A capable charger, that it is rare that more charge current is needed, as four cells can be charged at 1250mA/cell. Normally I charge most 18650 cells at 1000mA/cell anyway so...... If I need more than that, such as when plugging my two cradles together (8 cells in parallel), I can just use my 10A charger. The other limitation, is that each individual cell cannot be monitored during charging (or discharging). This is handy to check for a bad/under performing cell, but can only be monitored when series/balance charging.

Anyway, personally, I prefer to have all cells to be charged within 0.1 Volt of each other. While 0.5 Volts seems to be OK, I'm just more comfortable with the cells a bit closer. If your charging cells that are used together in series, as I often do, you're in trouble anyway, if there is a difference of more than 0.1 Volt between cells, unless they are nearly completely drained (eg. less than 3.5 Volts OC).

Also, I don't think it's necessary to let the cells "balance" before hooking them up to the charger. This happens so fast they're close enough to go ahead and start charging. The "self balancing" feature when parallel charging will continue anyway, as the cells are charged.

Dave