Nice, simple battery question.

[ThereAndBack]

Hi everyone.

I currently have 2 x 125 amp domestic batteries but one is on its last legs. Must I replace it with another 125 amp battery?
The ides is lodged in my mind that domestic batteries have to be of a matched size (i.e. the same ampage). Can someone tell me if I am right about this.
If this is the case, can anyone (briefly) explain why, just so I know .

Thanks
Alan

 

[sarabande]

it depends if they are being charged separately as

a) 2 separate 125 AHr batteries at 12v, or
b) combined to make a bigger 24v bank, or
c) being used as one big 250 AHr 12v battery.

If being charged separately (i.e. one output from the charger going to each battery) the charger will think nothing of it and charge each one to its best ability.

If they are set up as a parallel or serial bank, there is a problem with having two batteries of different size. Assuming they are both partially depleted to the same % level, the charge will fill up one battery (the small one) before the other, and keep trying to fill the larger one. So the small one will overcharge; this may or may not be serious depending on the extent of the mismatch.

You should aim to have the same type of battery and the same size of battery if you are charging them together in one unit.

 

[pvb]

I currently have 2 x 125 amp domestic batteries but one is on its last legs. Must I replace it with another 125 amp battery?

If they are wired in parallel (to give a 12v 250Ah bank), it's not necessary for the batteries to be identical capacity. The reason for this is that the batteries will each take whatever proportion they need of the total charging current.

However, with batteries wired in parallel, it is necessary to use identical types of battery (ie both wet lead/acid, both gel, both AGM, etc). This is because different types of batteries have different maximum charge voltages.

 

[bobgoode]

I would go further than Sarabande....change both batteries at the same time. Differences in the internal resistance could give rise to problems if you don't.

 

[Csail]

Bob is right change both. I had loads of probs when i changed just 1.

 

[Paul_R]

The question might be simple but............

Firstly I'm assuming that we are talking about a single voltage system, e.g. 12V batteries and a 12 V system. Are the batteries permanently wired in parallel to provide a single bank with a single common main switch? If so, I've always understood that they should be of same age, type and capacity and preferably have been part of the same bank since new. Otherwise you will have problems with cross discharging from one battery to the other when not in use and won't be able to take full advantage of your total capacity. So in this case I'd recommend changing both. If the batteries are only connected in parallel for charging or have independent charging arrangements, for example by a VSR or charge splitter and have separate loads (e.g. fridge on battery 1 and other domestic loads on battery 2), then this is less critical and you can mix batteries as indicated in previous posts.

 

[ThereAndBack]

Apologies for not providing more information.

Two domestic batteries (start battery separate) wired in parallel to create one 12v 250 amp bank.

Hope that clarifies things.

It would seem from the replies that I need to have batteries of the same ampage and type.

Wish there was space for a third one

 

[sarabande]

check out Merlin

http://www.merlinequipment.com/markets/group.asp?groupid=20

for sophisticated batteries with a small footprint.

 

[pvb]

It would seem from the replies that I need to have batteries of the same ampage and type.(

Same type, yes; same capacity, not necessarily.

I ran a previous boat for years with a domestic bank of 2 x 110Ah and 2 x 75Ah batteries (all wet lead/acid). Worked fine, no problems.

 

[vyv_cox]

Without wishing to dispute that some people have had problems when adding a new battery to a used one, my experience is different. I did exactly that, adding one new 110 Ah to two four-year old ones. No problems at all and each of the three continues to behave very well two years later. Prior to doing this I asked on the forum whether there is any documented advice or experience against doing it and the response from knowledgeable electrical people was that there was none.

 

[Beadle]

Yes Vyv

My experience is much the same.

I'm not sure how two batteries connected in parallel can have different voltages

 

[bobgoode]

Without wishing to dispute that some people have had problems when adding a new battery to a used one, my experience is different. I did exactly that, adding one new 110 Ah to two four-year old ones. No problems at all and each of the three continues to behave very well two years later. Prior to doing this I asked on the forum whether there is any documented advice or experience against doing it and the response from knowledgeable electrical people was that there was none.

I am not saying that you will get problems if only one battery is changed....but you might.
The older battery may have sulphated cells resulting in a markedly different internal resistance to the new battery. Now, if the batteries are in parallel and placed under load the one will experience a differnt lost volts to the other and thus the higher load voltagebattery will dischage into the lower load voltage battery...effectively charging the battery andthus discharging itself more quickly. Now when the load is removed and the cells recover, one battery will recover to a lower voltage and draw charge from the other battery.

 

[pvb]

I am not saying that you will get problems if only one battery is changed....but you might.
The older battery may have sulphated cells resulting in a markedly different internal resistance to the new battery. Now, if the batteries are in parallel and placed under load the one will experience a differnt lost volts to the other and thus the higher load voltagebattery will dischage into the lower load voltage battery...effectively charging the battery andthus discharging itself more quickly. Now when the load is removed and the cells recover, one battery will recover to a lower voltage and draw charge from the other battery.

I'd say stick to photography and rowing!

 

[PeterGibbs]

I would go further than Sarabande....change both batteries at the same time. Differences in the internal resistance could give rise to problems if you don't.

You know when you have a battery powered appliance, the instructions always say "replace the whole battery set at the same time" With reason.

Batteries of different readiness wired in common are going to affect their neighbours in service. Not so important perhaps in the case of a bedside radio, but at sea you want reliability - install new, and they will run down together like Darby and Joan, with smiles on their faces (well, mutually -supporting rather than draining then!)

PWG

 

[Plevier]

Having some years of experience in the lead-acid battery industry, I promise you there are no simple, straightforward queries! Maybe you were being ironic.

Perfection says you shouldn't mix different or new and old batteries but it's a matter of degree. If the size difference isn't too great or if the older battery is not on its last legs you'll be OK for a while. The main problem is the older/smaller one will have higher internal resistance and consequently the big/new one will do most of the work. It will discharge and recharge preferentially. So it will wear out quicker.

As the old one ages more of course the situation becomes worse until in effect only the new one is working. So you must replace the old one before it gets too bad. You'll always be out of phase in replacing them. A bit like car tyres. When you write off a half worn one, do you replace one or two?

Definitely don't mix flooded and recombination (AGM) batteries.
"Sealed maintenance free" car batteries however, not real recombination sealed ones, can be considered the same as ordinary flooded ones. The difference is more marketing than technical!

The comparison with dry batteries is invalid, different situation. First they will (almost) always be in series not parallel so one duff one in a chain of maybe 6 stops the item working at all, second you aren't dealing with cycling.

 

[sarabande]

FWIW, I emailed Cardex in the USA (the Battery University people)

http://www.batteryuniversity.com/index.htm

about the advisability of mixing types/sizes/ages in parallel banks.

They whizzed a reply back:

"Look at it this way: You can use a chain with weak links to pull light weight. However, the weak links will break on a heavy load. The same with a battery with cell of different capacity. On charge, the smaller cells will be charged before the good ones and heat up.

Best wishes,

Isidor Buchmann
Cadex Electronics Inc."


I guess that they are looking at one battery in a bank equating to one cell in a battery. It would be nice to nail the science behind this. I'll keep ferreting.

It seems that "best practice" for high-dependency systems would indicate that batteries should be same type and capacity. If you are not treating your boat battery bank as a mission critical component, then pragmatists can get away with mixing types.

 

[pvb]

If you are not treating your boat battery bank as a mission critical component, then pragmatists can get away with mixing types.

Well said, and surely pragmatism is the essence of PBO?