My last ever battery question (for this week!)

bluedragon

Active member
Joined
7 Apr 2004
Messages
1,773
Location
Cardiff Bay
Visit site
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

Well-known member
Joined
16 May 2001
Messages
45,604
Location
UK East Coast
Visit site
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

Well-known member
Joined
13 Jul 2002
Messages
48,311
Visit site
+1

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

LadyInBed

Well-known member
Joined
2 Sep 2001
Messages
15,227
Location
Me - Zumerzet Boat - Wareham
montymariner.co.uk
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

Active member
Joined
7 Apr 2004
Messages
1,773
Location
Cardiff Bay
Visit site
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?
 
Last edited:

VicS

Well-known member
Joined
13 Jul 2002
Messages
48,311
Visit site
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

Well-known member
Joined
16 May 2001
Messages
45,604
Location
UK East Coast
Visit site
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

Active member
Joined
7 Apr 2004
Messages
1,773
Location
Cardiff Bay
Visit site
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

Well-known member
Joined
16 May 2001
Messages
45,604
Location
UK East Coast
Visit site
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

Well-known member
Joined
2 Sep 2001
Messages
15,227
Location
Me - Zumerzet Boat - Wareham
montymariner.co.uk
0511-0702-0211-2545_Businessman_Waving_a_White_Flag_Behind_His_Desk_clipart_image.jpg
 
Top