Answer a question about parallel batteries?

[benmwv]

I need to know if it is safe to put two li-ion or li-po batteries in parallel.

I have heard that if you want to do this you need to use a special circuit to keep them from discharging into eachother. If this is true where can i find one? I have searched around and have no results.

What are the side effects of not using this type of "balancing" circuit?

Even if i am supposed to use one of these circuits can a get away without it? Will i be alright as long as i always use the same pair of batteries, charging them at the same time and such so they are always pretty much the same amount charged?

-Ben

 

[anselm]

I'm not sure, not having tried this yet, but:
You might get away with it, but I would check voltage often, at least in the beginning,
until you feel comfortable enough that they both discharge at the same rate.

In order for them to do that, you would also want your pair of batteries to have
closely matched internal resistance.

 

[Eudaimonium]

What would be your application for them?

Parraleling typical lithium ion cells we all use in our lasers is not a problem.

Not sure about high power polymer packs as I have not used them, but lithium ions can be parraleled as long as one isn't full and another one empty.

 

[Benm]

As long as they are identical cells, there should not be a problem. The best method would probably be to fully charge cells before connecting them in paralel. Even if their voltage is slightly different, the equalizing current would be limited at the moment of connection.

Laptop batteries often have sets of 2 cells in paralel, and this does not pose much of a problem.

Just don't try with batteries of different chemistry - though i would even advise using ones from the same batch just to be sure.

 

[benmwv]

The purpose is to make a USB battery pack.

I was wanting to use some spare batteries I have with a boost circuit based around a chip like this one:
http://pdfserv.maxim-ic.com/en/ds/MAX1771.pdf

Right at the bottom of the datasheet it already has the circuit I want on there. 5v, 1 amp output. I was think of using that circuit with two 18650's or 2 ~2000mah li-po's in parallel in a nice little case I can fit in my pocket. It would have a USB port on the side ( or maybe 2) and it would have to have a fold out hinged lid so i could hide a few shortened cell phone and other gadgets USB cord inside. Maybe I would even add solar panels folded up under the lid so I could open it up and unfold like 4 solar panels for more surface area.

I'm sure if I was going to buy all the components from digi key I would buy enough for at least a few sets just so it's worth my while and the extras would make good laser drivers, and I could even tweak the output a little (it says up to 2A at 90% efficiency and it seems like they are implying it is capable of more)