Connecting batteries in parallel.

[Jabs]

I am just about to connect two 12v leisure batteries in parallel. They are identical make and model.

In order to get the max life and max capacity do I need to do anything beforehand, such as charge them both to the same voltage or same SG?

Advice would be most welcome.

Thanks

Tony.

 

[VicS]

I am just about to connect two 12v leisure batteries in parallel. They are identical make and model.

In order to get the max life and max capacity do I need to do anything beforehand, such as charge them both to the same voltage or same SG?

Advice would be most welcome.

Thanks

Tony.

It would make sense to have them both charged but there's no need to get them exactly the same SOC. Same acid concentration in the two would be sensible I suppose.

I'd guess you are asking questions that very few people would think of.

When connected in parallel you should make the negative connection to one and the positive connection to the other to eliminate unbalance due to volts drop in the interconnecting cables.
http://www.smartgauge.co.uk/batt_con.html

 

[Jabs]

When connected in parallel you should make the negative connection to one and the positive connection to the other to eliminate unbalance due to volts drop in the interconnecting cables.
http://www.smartgauge.co.uk/batt_con.html

Yes, had worked that one out, but thanks.

I expect it won't make a huge amount of difference so long as they are both fully charged anyway!

Tony.

 

[RichardS]

I'm sure that when I was at Uni we were taught not to connect rechargeable batteries in parallel because the slight voltage differences due to charge state will lead to the higher voltage batttery charging the lower voltage one which sets up a sort of pendulum/overshoot scenario where all the batteries end up discharged fairly quickly.

Was this advice incorrect?

Richard

 

[Jabs]

I am sure that 'in theory' this is probably correct.

I need to get 200 amp hours in a space that will only take two 100 amp hour batteries. There is no alternative.

Wish there was.

I am sure I have seen commercial packs that contain a series/parallel configuration.

Tony.

 

[VicS]

I'm sure that when I was at Uni we were taught not to connect rechargeable batteries in parallel because the slight voltage differences due to charge state will lead to the higher voltage batttery charging the lower voltage one which sets up a sort of pendulum/overshoot scenario where all the batteries end up discharged fairly quickly.

Was this advice incorrect?

Richard

Pretty common for people to have battery banks made up from several smaller batteries connected in parallel.

 

[TQA]

I'm sure that when I was at Uni we were taught not to connect rechargeable batteries in parallel because the slight voltage differences due to charge state will lead to the higher voltage batttery charging the lower voltage one which sets up a sort of pendulum/overshoot scenario where all the batteries end up discharged fairly quickly.

Was this advice incorrect?

Richard

Correct BUT it only matters if you are leaving them connected and not charging them everyday. I have 6 x 6 volt connected to make a large 12 volt bank with a solar package that charges them every day. So no probs.

However if I were to leave them connected and not charge them the 'see saw' effect kicks in and in a week or so I would have flat batteries.

 

[Vara]

Correct BUT it only matters if you are leaving them connected and not charging them everyday. I have 6 x 6 volt connected to make a large 12 volt bank with a solar package that charges them every day. So no probs.

However if I were to leave them connected and not charge them the 'see saw' effect kicks in and in a week or so I would have flat batteries.

Is that true? I have left my 3 battery bank connected in parallel, charged for a month and as far as I could see only lost .2v, which I would have expected from a single battery.

Further query isn't a 12v battery essentially a collection of cells connected in series, wouldn't one expect a similar effect?

 

[RichardS]

Further query isn't a 12v battery essentially a collection of cells connected in series, wouldn't one expect a similar effect?

That's correct - but it's only cells connected in parallel that can cross-charge each other.

I am very interested in the comments from you guys as this has been puzzling me ever since I started reading these forums! (perhaps I need to get out more! )

Richard

 

[VicMallows]

I'm sure that when I was at Uni we were taught not to connect rechargeable batteries in parallel because the slight voltage differences due to charge state will lead to the higher voltage batttery charging the lower voltage one which sets up a sort of pendulum/overshoot scenario where all the batteries end up discharged fairly quickly.

Was this advice incorrect?

Richard

Was this perhaps particularly in relation to NiCd (and NiMH) batteries which have a very sharp 'knee' in the discharge voltage characteristic? In that case I can certainly see that once one cell in a parallel configuration reached that point it would discharge all the others.

Lead-acid batteries have a much smoother discharge curve and are commonly connected in parallel. However, if one cell in one of the batteries were to fail s/c, then the result would be similar.

 

[Plevier]

However if I were to leave them connected and not charge them the 'see saw' effect kicks in and in a week or so I would have flat batteries.

I'm glad nobody has told my batteries that and consequently it doesn't happen even after a couple of months.

Seriously it isn't a problem provided both batteries are healthy. If one gets an internal short it will drain the other.

Many military and commercial aircraft use sealed lead acid batteries in parallel and that isn't a field where you take chances.

 

[claudio]

I'm glad nobody has told my batteries that and consequently it doesn't happen even after a couple of months.

Seriously it isn't a problem provided both batteries are healthy. If one gets an internal short it will drain the other.

Many military and commercial aircraft use sealed lead acid batteries in parallel and that isn't a field where you take chances.

I've left mine fully charged, 6 off 220AH in parallel for over 3 months without trickle charge and without problems.

 

[Chris_Robb]

I'm glad nobody has told my batteries that and consequently it doesn't happen even after a couple of months.

Seriously it isn't a problem provided both batteries are healthy. If one gets an internal short it will drain the other.

Many military and commercial aircraft use sealed lead acid batteries in parallel and that isn't a field where you take chances.

Having 2 100AH in parallel rather than 1 200AH (which in theory is better), gives you a safety margin if one fails; just isolate it. At least you have something left.

 

[sarabande]

However if I were to leave them connected and not charge them the 'see saw' effect kicks in and in a week or so I would have flat batteries.

That's in theory, but somehow it's not how it works in practice....

 

[ghostlymoron]

Having 2 100AH in parallel rather than 1 200AH (which in theory is better), gives you a safety margin if one fails; just isolate it. At least you have something left.

And also won't give you a slipped disc when you lift it out. I'd go for 3 x 75aH just for manual handling reasons.