batteries in series with different Ah ratings

[brownt]

Hi,

What happens if a 6v 4 Ah lead-acid battery is used in series with a 12v 8 Ah battery?

thanks

[Doctorandus_P]

Normal recommendation for lead-acid is to never discharge them to below 80% or so, that would mean that your battery is "empty" after you took about 3.5Ah out of it. Going further would severely stress the smaller battery, and when go over 4Ah, the voltage over the smaller battery will get reversed and this would very quickly lead to irreversible damage.

[BeBuLamar]

In another word don't do it.

[Doctorandus_P]

In another word don't do it.

That is a simpler answer, but it's not an answer to the question

Also:
I only reacted to the difference in capacity, not the difference in voltage. Voltage differences do not matter much, you can just add them together. A 6V Lead acid battery will already have 3 cells in series and a 12V battery will have 6 in series. It is the difference in capacity that makes this a bad setup for long term use. If you need a bit more voltage for some reason it can be acceptable as long as you are careful not to discharge the small battery too far.

[RoGeorge]

It's all OK, as long as you don't over discharge them (particularly the smaller Ah one).  Another thing you need to take care is to make sure you are not drawing more current (A, not Ah) than the max amps specified for any of them.

When two or more batteries are connected in series, the one that discharges first will become reverse polarized by the other one(s).

[bidrohini]

Not a good idea actually: https://battlebornbatteries.com/100ah-batteries-in-line-with-my-270ah-batteries/

[tszaboo]

In another word don't do it.

That is a simpler answer, but it's not an answer to the question

So an answer would be: "Things get bad, don't do it."

[Doctorandus_P]

Nope that is also not an answer.
I'm also one of the "stubborn" people and I need to understand things. If someone just tells me "Things get bad, don't do it." without further explanation, there is no understanding, and the suggestion is meaningless to me.

[Psi]

What happens if a 6v 4 Ah lead-acid battery is used in series with a 12v 8 Ah battery?

For discharging

You now have a combined voltage of 18V,  it will work totally fine initially, until you have used up 4Ah and the 6V battery is flat. At that point the bad stuff happens.

You now have a 6V battery that is flat and a 12V battery that is got 4Ah left.
Current from the 12V battery will continue to flow to your circuit but must pass through the dead 6V battery.

Series cells are connected negative to positive so this current flow into the 6V battery has reverse polarity! So your 12V battery starts trying to recharge your 6V battery backwards!!!
Nothing good comes of this.


For charging

If you try to recharge this 18V battery the 6V will become fully charged too soon, before the 12V is finished. At this point you will start 'overcharging' the 6V battery and causing damage.
This overcharging may occur automatically as your battery charger won't correctly detect when the entire pack is charged

[BeBuLamar]

In another word don't do it.

That is a simpler answer, but it's not an answer to the question

Also:
I only reacted to the difference in capacity, not the difference in voltage. Voltage differences do not matter much, you can just add them together. A 6V Lead acid battery will already have 3 cells in series and a 12V battery will have 6 in series. It is the difference in capacity that makes this a bad setup for long term use. If you need a bit more voltage for some reason it can be acceptable as long as you are careful not to discharge the small battery too far.

You already gave the answer and I just said "In another word, don't do it"

[golden_labels]

Using capacitors as a crude approximation for batteries: simulation. Using two “batteries” of 5V, as differences in voltages are a separate thing.

Basically the larger battery pushes current through already deplated smaller battery.

Real batteries are not capacitors. They rely on chemistry to store charge and with that chemistry not happening in reverse, they will not be literally charged like a capacitor would. But they will be subjected to that negative voltage, deplated beyond acceptable limits, and undergo destructive changes in their chemistry. Depending on the type, some will actually acquire small charge in opposite direction, showing negative open-circuit voltage.

[DavidKo]

If you are not able to monitor voltage of each battery, than do not do it. If yes, than you can do it - you need to solve the charging of 2 batteries to ensure not overcharging the one of them and disconnect the whole battery when one of them is depleted.

[wasedadoc]

It is really just an extreme case of the issues encountered when using Lithium based rechargeables in series.  Consider the measures implemented in those packs even when the cells do have the same nominal voltage and capacity.