# Parallel Batteries and their effect on Current.

1. Dec 15, 2012

### clm222

Hello, I would like to know how you can calculate the current in a circuit with cells in parallel. When searching for this on the web all I get is current in parallel circuits, and not my specific question about cells in parallel.
If there is maybe an equation, method, or link somebody could post, I would appreciate it.
Thanks.

2. Dec 15, 2012

### Studiot

That is because you should not operate batteries in parallel, without special circuitry which has the disadvantage of reducing their output voltage.

If you need more current, get a larger capacity battery.

3. Dec 15, 2012

### justsomeguy

Batteries are often connected in parallel in automotive and marine applications without any special circuitry. This should not be used to try and draw more current, which I think was Studiot's point -- it's simply used to extend operating time (amp-hours). You should not exceed the draw on any single battery in the group without a current limiter on each one before they are joined.

4. Dec 15, 2012

### Studiot

And what is a current limiter if not special (=additional) circuitry?

It is just the most rudimentary form.

A forum search will reveal several threads where the formal calculation is presented for power sources(more generally than just batteries) in parallel.

To effect such calcualtions you need additional information about the sources, the simplest being the internal resistance as well as the EMF.

5. Dec 15, 2012

### nsaspook

You don't need a group limiter just equal connection and internal battery resistances to the load to parallel same voltage sources at the max current of both sources. The problem with parallel connections of high power sources is that the interconnect resistance values at common battery voltages are so low a tiny fraction of a ohm anywhere could shift the current balance by a huge amount causing one battery carry most of the load.

When doing a battery power design I use the 1/N rule for max current from parallel connections to account for balance problems.

One battery: 1/1 for current
Two batteries: 1/1+1/2
Three batteries: 1/1+1/2+1/3

I agree it's usually better to have one (big) battery.

6. Dec 15, 2012

### CWatters

Perhaps because they are lead acid? Not all types of battery can be safely connected in parallel.

First you need to understand what you can and can't do with ideal voltage sources... Lets suppose you had two ideal voltage sources one 11.5V and the other 12.5V. It would be very unwise to connect them in parallel using an ideal wire. That's because the current flow from the higher voltage source to the lower would be infinite.

Fortunately some types of battery (eg lead acid) are not ideal voltage sources. The voltage can vary with state of charge and they have some (but not a lot) of internal resistance. If you have two lead acid batteries that are nominally 12V but one has been discharged and the other is full then you will probably get away with connecting them in parallel. Current will flow in a direction that tends to equalise the charge state and hence the voltage. The current therefore tends to self limit. If the cells are allways connected in parallel then they will tend to stay matched and self equalise.

You can also parallel some types of lithium cell but it's unwise to try the same trick with NiCad or NiMH cells which tend not to self equalise.

7. Dec 15, 2012

### justsomeguy

My brain immediately goes to lead acid batteries when anyone says "battery" unqualified, can't say why, but of course you are correct. I don't tend to think in terms of ideal systems or components unless there's a specific mathematical problem at hand though; this sounded like a rather practical question by the OP, so I suppose the question is "what kind of batteries?"