# Battery charging in parallel

#### RodneyB

Joined Apr 28, 2012
697
I don't understand why a battery I have in parallel with another battery does not charge.

The bigger of the batteries is connected to the charger. The smaller battery a 7Ah battery it is used as back up when the big battery is not connected.

The small battery goes flat I then connect it to the big battery, the voltage goes up slightly but not to the same voltage as the big battery, I would really like to understand why

#### dl324

Joined Mar 30, 2015
17,011
Charging batteries in parallel, or series, isn't a good idea.

#### Lestraveled

Joined May 19, 2014
1,946
A healthy battery generally has a lower voltage than a weaker battery. A charger sees only one battery, the one with the lower voltage. Connect a weak battery and a healthy battery together to charge and the healthy battery gets all the charge and the weak battery gets very little. Charge them separately and they will both be happy.

#### RodneyB

Joined Apr 28, 2012
697
A healthy battery generally has a lower voltage than a weaker battery. A charger sees only one battery, the one with the lower voltage. Connect a weak battery and a healthy battery together to charge and the healthy battery gets all the charge and the weak battery gets very little. Charge them separately and they will both be happy.
Is there no circuit i can build to be able to assist the charging. If the charger is charging at 13.8 volts and the small battery has dropped to 10.6 volts what i don't understand is why the smaller battery won't charge to the 13.8 volts

#### Lestraveled

Joined May 19, 2014
1,946
A voltage that low suggests a bad cell. I suggest you over charge the battery for a while. Do you have a power supply that you can charge at 15.5 volts?? This might fix the battery.

#### RodneyB

Joined Apr 28, 2012
697
A voltage that low suggests a bad cell. I suggest you over charge the battery for a while. Do you have a power supply that you can charge at 15.5 volts?? This might fix the battery.
Thank you i will try that and post the result a bit later

#### WBahn

Joined Mar 31, 2012
30,251
What kind of batteries are we talking about? Lead acid?

#### RodneyB

Joined Apr 28, 2012
697
What kind of batteries are we talking about? Lead acid?
The batteries are lead acid.

#### MrAl

Joined Jun 17, 2014
11,583
Hi,

If the two batteries have a different series resistance then the one with the lower resistance gets most of the current.

As an extreme example, consider one having 1 ohm and one having 100 ohms, and both connected to a voltage that is 2 volts higher than their initial voltage. One battery gets a current of 2 amps while the other gets only 0.020 amps. Quite a difference. The lower current may charge the battery a little, or it may never charge at all due to the minimum charge current requirement of some batteries like lead acid.

#### RodneyB

Joined Apr 28, 2012
697
Hi,

If the two batteries have a different series resistance then the one with the lower resistance gets most of the current.

As an extreme example, consider one having 1 ohm and one having 100 ohms, and both connected to a voltage that is 2 volts higher than their initial voltage. One battery gets a current of 2 amps while the other gets only 0.020 amps. Quite a difference. The lower current may charge the battery a little, or it may never charge at all due to the minimum charge current requirement of some batteries like lead acid.

Thank you, If the battery was fully charged, the one with the highest resistance would the lower resistance one then not charge..

If I was to put a Resistor in series to make sure the battery had a similar resistance would that help at all.

#### hp1729

Joined Nov 23, 2015
2,304
series, isn't a good idea.
But we charge battery packs with cells in series all the time. Home phones, R/C cars and planes, car batteries, lead acid batteries ... all cells in series.
I understand your argument about batteries in parallel. Would putting a low resistance in series with each battery help?

#### dl324

Joined Mar 30, 2015
17,011
But we charge battery packs with cells in series all the time. Home phones, R/C cars and planes, car batteries, lead acid batteries ... all cells in series.
I understand your argument about batteries in parallel. Would putting a low resistance in series with each battery help?
It doesn't matter whether you use or charge batteries in series and/or parallel. It's always a compromise.

For series batteries, you always replace them all at once; hoping that the individual batteries are well matched. Invariably, one will be weaker than the rest and that becomes the weakest link. It will discharge faster and will no longer contribute to the "pack" voltage. When that voltage drops low enough, the pack is at EOL. If it's a rechargeable battery pack, the weakest battery will charge the slowest; so runtime will decrease continually until that cell fails completely. More batteries means lower service life.

Similar arguments for parallel batteries.

#### hp1729

Joined Nov 23, 2015
2,304
It doesn't matter whether you use or charge batteries in series and/or parallel. It's always a compromise.

For series batteries, you always replace them all at once; hoping that the individual batteries are well matched. Invariably, one will be weaker than the rest and that becomes the weakest link. It will discharge faster and will no longer contribute to the "pack" voltage. When that voltage drops low enough, the pack is at EOL. If it's a rechargeable battery pack, the weakest battery will charge the slowest; so runtime will decrease continually until that cell fails completely. More batteries means lower service life.

Similar arguments for parallel batteries.
Of course what you say is true. So what do you do about battery packs? Do you break them apart and charge each one individually?

#### dl324

Joined Mar 30, 2015
17,011
So what do you do about battery packs? Do you break them apart and charge each one individually?
I charge them with the provided charger and replace them when the first cell dies. Even if I wanted to charge cells individually, it's generally not possible as disassembly can't be reversed...

#### RodneyB

Joined Apr 28, 2012
697
Getting a bit off topic I think

#### hp1729

Joined Nov 23, 2015
2,304
I charge them with the provided charger and replace them when the first cell dies. Even if I wanted to charge cells individually, it's generally not possible as disassembly can't be reversed...
Then, as a general rule, we do charge batteries in series knowing full well the problem that will certainly come up ... years down the road.

#### bwilliams60

Joined Nov 18, 2012
1,450
In this case, I would take a note from the trucking induatry. When you have multiple pack batteries, separate them and charge them individually. Does not matter series or parallel, weak batteries won't charge properly. Little story.
Four guys digging a hole and one gets tired. He goes home but hole still needs to be dug by deadline. Three guys work harder to get it done. Boss sends in new guy to help. New guy does most of work because he's fresh and othe three slack off. New guy gets tired faster than normal.
Moral of storyne bad battery in pack, replace all. I work in this industry and have seen all the battery ideas there are. The guys here have not lead you astray. Separate, charge and test. Replace as necessary.

#### Lestraveled

Joined May 19, 2014
1,946
Those of you who have worked with batteries know what an "equalization" charge is. This is where you purposefully over charge a lead acid battery. Equalization does two major things, desulfation and charging weaker cells. Equalization should be done periodically (monthly) and is healthy for the battery. In the course of normal or inadequate charging, weak cell get weaker and strong cells get stronger. Equalization brings cells closer together as far as state of charge, and a lot of sulfation gets turned back into electrolyte.

In post #5 I asked the TS to charge his problem battery at 15.5 volts. This is an equalization charge. I hope it works.

#### MrAl

Joined Jun 17, 2014
11,583
Thank you, If the battery was fully charged, the one with the highest resistance would the lower resistance one then not charge..

If I was to put a Resistor in series to make sure the battery had a similar resistance would that help at all.

Hi,

That's a very good idea, but only if the resistances are similar and if the added resistances dont have to be too high in value.

The best way however is to use a higher voltage for the higher resistance battery, and see if that brings the resistance down (it may). If so then the problem may get better and you wont have to do this again for a while.

If not, then the best way is to use a separate voltage for each battery. If you need a higher voltage for the 2nd battery you can use a small boost circuit. Yes that makes things a little more complicated, but batteries are devices that are not as simple as they appear to be.

#### hp1729

Joined Nov 23, 2015
2,304
Hi,

That's a very good idea, but only if the resistances are similar and if the added resistances dont have to be too high in value.

The best way however is to use a higher voltage for the higher resistance battery, and see if that brings the resistance down (it may). If so then the problem may get better and you wont have to do this again for a while.

If not, then the best way is to use a separate voltage for each battery. If you need a higher voltage for the 2nd battery you can use a small boost circuit. Yes that makes things a little more complicated, but batteries are devices that are not as simple as they appear to be.
Re: charging the battery to 5.5 V
If he is using a conventional charger that comes with a device that isn't an option. Roll-your-own chargers are needed to set the voltage to some desired level. I have my own designs for such things but they are far from being equal to a commercial product and need constant human monitoring.
Are there any specific designs you might suggest? Mine are simple voltage regulators with some degree of current limiting. Do any designers have good rules to go by for designing battery chargers? Mine are usually "low current over a long time" designs. At max I probably charge at 1/10th the Amp-Hour rating. SLA I might charge at a low current (100 mA) for days at a constant voltage.
Charging batteries in parallel? I've never done it.

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