# I Batteries connected in parallel

1. Dec 25, 2016

### Astronaut500

When 5 batteries are connected in parallel, how can I find the equivalent current that it gives? What is the formula for calculating that?

2. Dec 25, 2016

### Staff: Mentor

Do not connect batteries in parallel!!! Ever.

3. Dec 25, 2016

### hilbert2

When you connect batteries in parallel, you will get a combined power source that has a smaller internal resistance than a single battery. The main significant effect is that that the combination is a greater fire hazard if you short circuit it.

4. Dec 25, 2016

### Astronaut500

Oh I get it, you are right. The current output depends on the resistance of the circuit, it cannot be found without sufficient information

5. Dec 25, 2016

### sophiecentaur

It's not as bad as that. If the batteries are all new, the same capacity and design (and manufacturer) and they stay connected all their lives then they will survive to a ripe old age. It's standard practice in large standby systems. I remember visiting an old telephone exchange and the battery room had shelves full of large accumulators, connected in series / parallel. They could supply the exchange for the longest power cut ever envisaged and the batteries lasted for many years.
otoh, an old, discharged and knackered battery will soon knacker a new charged battery if placed in parallel with it.

6. Dec 25, 2016

### anorlunda

Thanks for the Christmas present. You expanded my vocabulary.

7. Dec 25, 2016

### sophiecentaur

A well known Engineering term, applicable to anything that's on the way out - including humans.
The "knacker's" or "knacker's yard"was a term used for someone who disposed of old horses and cattle, i believe.

8. Dec 25, 2016

### mrspeedybob

Most light and medium duty diesel trucks use 2 batteries in parallel in order to provide enough current to start the engine. Heavy trucks may use more, IDK. Batteries in parallel are fine as long as they're the same chemistry. 2 batteries in parallel is essentially the same as 1 battery with plates that are twice as big. Circuit protection is required against short circuits, but that's required even for single batteries.

I have 3 extra full sized lead-acid car batteries installed in my truck in parallel with the 1 that's designed to be there to support an inverter. Fuses on both ends of the positive cable that connects them, and on the cable that supplies the inverter.

9. Dec 26, 2016

### derek10

Why? There are battery packs with cells connected in parallel and without a controller circuit like those used in RC

10. Dec 26, 2016

### Nidum

Problems can definitely arise when some types of rechargeable batteries are connected in parallel .

NiCad batteries are particularly prone to damage when connected this way .

11. Dec 26, 2016

### CWatters

It would be better to say...

Never connect ideal voltage sources in parallel (unless they are exactly the same voltage).
Never connect ideal current sources in series (unless they are exactly the same current).
Never short circuit an ideal voltage source.
Never open circuit an ideal current source.

Real world voltage and current sources may sometimes behave close enough to ideal ones to cause safety hazards.

12. Dec 26, 2016

### sophiecentaur

The operative word there is "ideal". There are not many ideal sources - unfortunately.

13. Dec 26, 2016

### oz93666

No one has yet answered this very simple question ..just a lot of unfounded nervousness about parallel connection.

First you need to find the internal resistance of the battery . 12v car batteries have an internal resistance of about 0.006 Ohms ... if you put a 0.006 Ohm load across it the total resistance is 0.012 and the current will be a thousand amps ...

If you connect 5 of these batteries in parallel you have the same 12 volts , but now with an internal resistance of 0.006/5 = 0.0012 Ohms so whatever load you put on this , you'll get a higher current than from just one battery .

14. Dec 26, 2016

### sophiecentaur

Trouble can start when the batteries do not have equal levels of charge and one battery can be discharging through another. The nervousness is not totally unfounded - just a bit over-dramatised, perhaps. Trying the trick with a mixture a AA cells out of a drawer can be expensive and new batteries could often be a better solution.

15. Dec 26, 2016

### oz93666

Well , all that's happening in that case is that one that's more full charges up the more empty , not a problem , unless one is very flat and the type of battery has a low internal resistance , then the charge exchange might be a little excessive ...

Ah I've just seen my error I assumed we were talking about rechargeables !!! You are correct!

The real disappointment is connecting Li batteries in series , they can be discharged in series , but established wisdom is that they should have a 'balanced charger' which has to have a connection to each cell to track the voltage of each cell ....that said , some don't bother with this without too many problems.

16. Dec 26, 2016

### oz93666

If the cells are connected when new this is OK ... but if a new cell is connected with a half full cell the full one will try to charge up the half empty , not good if the cell is not rechargeable.

17. Dec 27, 2016

### lychette

You should give reason/explanation why not !!!!!! People do this all the time,,,,,,