My last ever battery question (for this week!)

[bluedragon]

If I mix Ah ratings between two service batteries (say 110Ah in parallel with a 135Ah battery), same type wet lead-acid, both new, is there a problem? It's been suggested that the 135Ah might not fully charge as the other will charge quicker and shut-down the charge controller to float voltage. I'm sceptical about this as I think they'll equalise. But...???

 

[pvb]

If I mix Ah ratings between two service batteries (say 110Ah in parallel with a 135Ah battery), same type wet lead-acid, both new, is there a problem? It's been suggested that the 135Ah might not fully charge as the other will charge quicker and shut-down the charge controller to float voltage. I'm sceptical about this as I think they'll equalise. But...???

It'll work fine, so there's no problem.

 

[VicS]

+1

the larger one will draw more current than the smaller one so they will reach their max state of charge together.

 

[LadyInBed]

It's been suggested that the 135Ah might not fully charge as the other will charge quicker and shut-down the charge controller to float voltage.

That is correct

I'm sceptical about this as I think they'll equalise.

They will equalise, but at the capacity of the lower one.

Take an extreme case - say 20Ah and a 130Ah
The 20 will charge up quite quickly and push the terminal voltage up to 14v
The 'smart' charger senses this and goes to float more
The 130 is nowhere near charged, and the 20 hasn't got enough guts to bring the 130 up to capacity.

In the case you site, the capacity difference is not too great, so you wont have a real problem.

 

[bluedragon]

Hmmm, some slight difference of opinion here. But won't the fully charged one discharge into the lesser charged one to balance everything up in the case I've mentioned here?

 

[VicS]

Take an extreme case - say 20Ah and a 130Ah
The 20 will charge up quite quickly and push the terminal voltage up to 14v
The 'smart' charger senses this and goes to float more
The 130 is nowhere near charged, and the 20 hasn't got enough guts to bring the 130 up to capacity.

No it wont.

I could argue that the larger one will hold the voltage down. The smart charger senses this and continues at full charging rate and results in overcharging of the small one.

In reality since they are in parallel they will both have the same voltage at their terminals. They will happily charge together.

 

[pvb]

That is correct



They will equalise, but at the capacity of the lower one.

Take an extreme case - say 20Ah and a 130Ah
The 20 will charge up quite quickly and push the terminal voltage up to 14v
The 'smart' charger senses this and goes to float more
The 130 is nowhere near charged, and the 20 hasn't got enough guts to bring the 130 up to capacity.

In the case you site, the capacity difference is not too great, so you wont have a real problem.

No, that's incorrect. The batteries will take charge current according to their needs. If batteries are connected in parallel, you can't have a situation where one will reach a higher voltage than the other!

 

[bluedragon]

I said they were the same type, but that's not strictly true in that one one has filler caps and the other (110Ah) is sealed low-maintenance. Shouldn't make any difference should it, except I can't check the water in the latter?

 

[pvb]

I said they were the same type, but that's not strictly true in that one one has filler caps and the other (110Ah) is sealed low-maintenance. Shouldn't make any difference should it, except I can't check the water in the latter?

Basically, no. The low-maintenance ones are essentially the same flooded lead-acid construction. I use these in my domestic bank.

 

[LadyInBed]

 

[bluedragon]

I LIKE IT!!! Pity we don't get this in more posts on this forum

 

[Plevier]

Not a problem at all with the capacities you quote.
Not such a good idea with hugely different capacities.

 

[VicMallows]

Too late!... the WHITE FLAG has been hoisted!