Mixing battery Amp hours question. (1 Viewer)

[Cliffdale]

My motorhome battery is 95Ahr, can I add a 120 Ahr in parallel with it? The dealer said it would not work as one battery will rob the other. He said stick with a second 95Ahr unit.

I thought you could add what ever amp battery you want, they are just storage tanks to me. A bit like adding in parallel a 100L tank of water connected a 50L tank of water. Any thoughts on this please?

 

[Reallyretired]

Perfectly Ok although recommended to try to have matched batteries it is more important that they should be of similar age as if one is knackered (technical term) it will pull down the good one.

 

[Teuchter]

IMO if kept charged no problem however if well discharged the terminal voltage of the 95 will eventually fall below that of the 120 one causing it to "try to charge" the lower PD one.

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[pappajohn]

Capacity makes no difference whereas voltage, age and condition do.
If the old battery is failing, even slightly, it will effect the overall capacity and performance of both.
Connected together the pair simply become one larger battery albeit in two seperate casings.
You cannot overcharge the lesser one and undercharge the larger one.
Its just the same as putting four batteries in a torch except in that case you increase the voltage, not the amperage.

 

[pappajohn]

IMO if kept charged no problem however if well discharged the terminal voltage of the 95 will eventually fall below that of the 120 one causing it to "try to charge" the lower PD one.

You dont draw current independantly from each individual battery.
If you have a draw of 10a it is taken from the combined capacity, not 5a from each battery.
Both batteries will be fully charged or fully discharged at the same time.

 

[mjltigger]

Think of it like % .. When you use 1ah you are using 2.15ah of your total. 1.2ah from one and 0.95 from the other.

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[funflair]

Think of it like % .. When you use 1ah you are using 2.15ah of your total. 1.2ah from one and 0.95 from the other.

I think you mean when you use 2.15ah it splits 1.2 and 0.95.

Martin

 

[mjltigger]

I think you mean when you use 2.15ah it splits 1.2 and 0.95.

Martin

Yeah darn it. I meant 1%....

 

[PeteH]

I have been running with the Original battery (85Ah) and a new battery (110Ah) for the last 18 Months, with no issues. Just spent the holiday weekend "off grid" 4 full days. With 2 Solar panels in use, we left on Monday with the same voltage showing as when we arrived Thursday.

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[Steve]

Not a problem been doing it for years.

 

[Teuchter]

You dont draw current independantly from each individual battery.
If you have a draw of 10a it is taken from the combined capacity, not 5a from each battery.
Both batteries will be fully charged or fully discharged at the same time.

Not too sure about that - IMO the battery with less capacity (whether it is because of a different ah rating or it is in poor condition) will tend to discharge quicker but will in effect be "charged" by the stronger one thus "appearing" to discharge at the same rate as the other one - just my opinion for what it's worth

Similarly when charging (and this I actually know from having worked in a battery charging room for 2 years in my youth ) - if both batteries are "flat" then the one with the lesser ah rating will reach its fully charged state quicker.

Think of it this way if a 10 ah battery is connected in parallel to a 250 ah battery and they are charged from a common supply of say 14v then the 10ah battery would reach its fully charged state first

 

[Doctor Dave]

Not too sure about that - IMO the battery with less capacity (whether it is because of a different ah rating or it is in poor condition) will tend to discharge quicker but will in effect be "charged" by the stronger one thus "appearing" to discharge at the same rate as the other one - just my opinion for what it's worth

Similarly when charging (and this I actually know from having worked in a battery charging room for 2 years in my youth ) - if both batteries are "flat" then the one with the lesser ah rating will reach its fully charged state quicker.

Think of it this way if a 10 ah battery is connected in parallel to a 250 ah battery and they are charged from a common supply of say 14v then the 10ah battery would reach its fully charged state first

In this case the alternator - or any other charging device - will be looking at a single battery of 260 Ah.


Dave

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[Teuchter]

In this case the alternator - or any other charging device - will be looking at a single battery of 260 Ah.


Dave

Agreed but the voltage across the terminals of each battery will remain at a constant alternator supplied voltage (say 14 v)

With 14v applied to a 10ah battery it will reach its fully charged state after a certain time

With 14v applied to a 250 ah battery it will also reach its fully charged state after a certain time but a much longer time than it takes to charge a 10 ah one

 

[scotjimland]

Many do it, many say it works for them ..but few can point to the theory or explain how batteries of different ah capacities connected in parallel actually perform.. even so called experts disagree.

I don't advise , because batteries with different capacities have different internal resistances, Ohm's Law tells us that the stronger battery will discharge through the weaker battery so even with no load you will always have a small current flowing, wasted current that does nothing but warm batteries, as this happens, there will be battery discharge. Even identical batteries, same age same type same maker will have slightly different internal resistance..

One self builder I knew insisted that his new batteries came off the same production line , with consecutive serial numbers..


all the advice I have read says that for optimal performance they should be the same ah, you certainly wouldn't buy two different ah batteries for a new installation... adding retrospectively it probably doesn't matter much as they will probably be different ages anyway .. your choice