# Unequal Batteries in Parallel and Series - what's wrong with this picture?

#### Lumenosity

Joined Mar 1, 2017
614

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#### Ian0

Joined Aug 7, 2020
9,432
The 17AH and 10AH batteries are actually labelled 35Ah?

#### Lumenosity

Joined Mar 1, 2017
614
The 17AH and 10AH batteries are actually labelled 35Ah?
hah!
Nice try.
The actual batteries ARE 35Ah batteries.
The indicated capacities are their "present" capacities.

It's not a trick question. I figured most people would gather it's an electrical question.

#### MrChips

Joined Oct 2, 2009
30,421
If the two 12V 35Ah batteries are equivalent then you don't need the smaller battery.
In any case putting two different batteries in the same circuit (series or parallel) is not a good thing to do.
One battery will drain faster than another.

#### Lumenosity

Joined Mar 1, 2017
614
If the two 12V 35Ah batteries are equivalent then you don't need the smaller battery.
In any case putting two different batteries in the same circuit (series or parallel) is not a good thing to do.
One battery will drain faster than another.
Even though the total capacities on both sides of the equation are equal ?
Under equal testing parameters, the capacities of the three batteries are as shown.

would it not work due to different IR's or what exactly?
Is the smaller battery incapable of keeping up? would it go empty first? If so, why? It has more capacity than the larger battery shown above it?

The smaller battery is capable of handling a 4 amp load.
What if the load were only 2 amps ? 1 amp?

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#### MrChips

Joined Oct 2, 2009
30,421
Even though the total capacities on both sides of the equation are equal ?
Under equal testing parameters, the capacities of the three batteries are as shown.

Is that due to different IR's or what exactly?
You may do as you wish. With 4A load you should bank on only 2h operating service.

#### dl324

Joined Mar 30, 2015
16,649
I wouldn't count on that combination working well. The 17Ah battery will likely end up abusing the smaller ones.

#### Lumenosity

Joined Mar 1, 2017
614
You may do as you wish. With 4A load you should bank on only 2h operating service.
It's not really about doing as I wish. It's a learning thing.
Also, keeping these batteries in service as long as possible saves the planet (on a microscopic scale, in a manner of speaking)
I'm trying to get expert opinions on what would happen in this scenario and why.

I realize two new batteries is the optimal solution if that's all I was after.

#### Ian0

Joined Aug 7, 2020
9,432
To continue this thought experiment - if you were to take a 17AH battery, and remove the plates, then cut them into two (using a hypothetical non-conductive saw) at 10/17 of the way along, then put each piece into a separate plastic tank, would you not have achieved the situation you describe?
So, provided that all three batteries are constructed identically with the same plates, strength of acid etc. I don't see hypothetically, why it shouldn't work.

#### Lumenosity

Joined Mar 1, 2017
614
I wouldn't count on that combination working well. The 17Ah battery will likely end up abusing the smaller ones.
I tend to agree.
But, why?

If you had a 100Ah LiFePO4 battery in series with a 100Ah LiIo (18650 cells) battery, would one abuse the other?

Since the smaller battery actually has nearly the same capacity as the one it is paralleled with, shouldn't it be ok?

The problem I could see running into here is related to the load demand.
If the load was high, then the internals of the smaller battery might be overloaded.

But, if the load were light, say less than 4 Amps, then where would a problem most likely develop in this scenario?

#### Lumenosity

Joined Mar 1, 2017
614
To continue this thought experiment - if you were to take a 17AH battery, and remove the plates, then cut them into two (using a hypothetical non-conductive saw) at 10/17 of the way along, then put each piece into a separate plastic tank, would you not have achieved the situation you describe?
So, provided that all three batteries are constructed identically with the same plates, strength of acid etc. I don't see hypothetically, why it shouldn't work.
Some of us are trying to learn about electronics here.

#### dl324

Joined Mar 30, 2015
16,649
I tend to agree.
But, why?
This is my thinking. If the 7Ah battery reached a discharged state earlier than the rest, the 10Ah battery would try to charge the 7Ah battery. That would mean that the 10+7 combination would discharge faster than the 17Ah. Continuing to have a load would cause the 17Ah battery to force the 10+7 combination in to a reverse voltage condition. That would damage most batteries.

You'd want the parallel batteries to share the load current evenly, but that's not likely to happen.

#### MrChips

Joined Oct 2, 2009
30,421
The main factor that has adverse effect on performance is battery internal resistance.

Let us begin with two identical batteries in series. When one battery degrades faster than the other, both charging and discharging characteristics will differ. In particular, the weaker battery will charge and discharge faster than the other which will accelerate its degradation.

#### Lumenosity

Joined Mar 1, 2017
614
This is my thinking. If the 7Ah battery reached a discharged state earlier than the rest, the 10Ah battery would try to charge the 7Ah battery. That would mean that the 10+7 combination would discharge faster than the 17Ah. Continuing to have a load would cause the 17Ah battery to force the 10+7 combination in to a reverse voltage condition. That would damage most batteries.
Ok. That makes sense.

Do you think it would work if the bigger battery on the right was at 7AH capacity and the smaller battery was also at 7Ah capacity and the battery on the Left at 14Ah capacity?

#### Lumenosity

Joined Mar 1, 2017
614
The main factor that has adverse effect on performance is battery internal resistance.

Let us begin with two identical batteries in series. When one battery degrades faster than the other, both charging and discharging characteristics will differ. In particular, the weaker battery will charge and discharge faster than the other which will accelerate its degradation.
This is EXACTLY what happened to the two 35Ah batteries I have in series.
For whatever reason one has degraded to 17Ah and the other to 10Ah.
They have lived their entire lives in series kept at a float of 27.2v

I think the original 35Ah claim was based on some very stringent test conditions and unrealistically low loads for testing.
MightyMax ML35-12. I have personally had consistently poor performance from MightyMax batteries and no longer buy them.
I did not capacity test them when new but the highest test capacity I ever got from them was 20Ah.

#### dl324

Joined Mar 30, 2015
16,649
Do you think it would work if the bigger battery on the right was at 7AH capacity and the smaller battery was also at 7Ah capacity and the battery on the Left at 14Ah capacity?
If the batteries were matched. Even if you started with matched batteries, over time, one will become weaker than the other. That's when abuse of the weaker will accelerate.

#### MrChips

Joined Oct 2, 2009
30,421
When batteries are configured in series it is better to charge them individually.

I have two 12V batteries in my lawn mover. I opened the battery pack and brought out a connection to the junction between the two batteries. Now I can charge each battery separately.

#### Lumenosity

Joined Mar 1, 2017
614
When batteries are configured in series it is better to charge them individually.

I have two 12V batteries in my lawn mover. I opened the battery pack and brought out a connection to the junction between the two batteries. Now I can charge each battery separately.
I think that would be ideal.
These two batteries are used in a battery backup and are very difficult to get to.

Don't you have to separate your batteries when you charge them?

#### MrChips

Joined Oct 2, 2009
30,421
I make an extra connection to where the two batteries are connected.
I then charge each battery with a 12V battery charger.
I have to be extra careful when and how I connect the charger.
For battery #1, +ve from the charger goes to the +ve of the pack, -ve from the charger goes to the extra connection.
For battery #2, +ve from the charger goes to the extra connection, -ve from the charger goes to -ve of the pack.