Two Batteries Of Different Size

[doganboncu]

Hi all,

 

I want to increase my battery capacity in the van from 90 Ah to some higher value. The cabine is big enought for another of 60 Ah.

Is it then OK, or is there a restriction to have identical batteries to use them in a parallel configuration?

 

Regards

 

Dogan

[RADIOTWO]

Hi all,

 

I want to increase my battery capacity in the van from 90 Ah to some higher value. The cabine is big enought for another of 60 Ah.

Is it then OK, or is there a restriction to have identical batteries to use them in a parallel configuration?

 

Regards

 

Dogan

 

Hi Dogan

 

It is not good to do that, one will try and discharge before the other, and also when charging one will charge up before the other.

 

If you want to use two batteries you need them both the same and same age (useage) new would be the best

 

the best is to have just one battery, and if you have space I think they do a 220Ah battery (opps over hear)

 

Radiotwo

[neil]

Hi all,

 

I want to increase my battery capacity in the van from 90 Ah to some higher value. The cabine is big enought for another of 60 Ah.

Is it then OK, or is there a restriction to have identical batteries to use them in a parallel configuration?

 

Regards

 

Dogan

 

Hi Dogan

 

I agree with what Radiotwo says about mixing battery sizes, a changeover switch would allow a workaround

but this would be a lot of trouble. Also it really isn't a good idea to have the battery in the living area, the

best place is in a dedicated locker sealed from the interior but vented to the exterior to allow any hydrogen

gas to dissipate safely.

 

neil

[aaman]

Hi all,

 

I want to increase my battery capacity in the van from 90 Ah to some higher value. The cabine is big enought for another of 60 Ah.

Is it then OK, or is there a restriction to have identical batteries to use them in a parallel configuration?

 

Regards

 

Dogan

 

Yes you can, but you won't get 150 amps from them.

What you will get is 120 amps. I can't explain the science but the same question was asked by a reader of Practical Caravan a few months ago.

Probably better to bite the bullet and fit one large capacity battery.

[doganboncu]

Thanks for the answers.

 

If I should have two smillar batteries then I think I will wait until this one dies.

 

Having two batteries looks more reasonable to me than having one big . It may not be as efficent but perhaps more practical. Besides I occasionally expect to use one of them for the electric motor of the boat.

 

For the cabinet of the battery, I insulated it from the living area and cut the side for a ventilation hole (picture). A small fan operates when the charging starts.

 

Ragards.

 

Edited April 21, 2009 by doganboncu

[neil]

Thanks for the answers.

 

If I should have two smillar batteries then I think I will wait until this one dies.

 

Having two batteries looks more reasonable to me than having one big . It may not be as efficent but perhaps more practical. Besides I occasionally expect to use one of them for the electric motor of the boat.

 

For the cabinet of the battery, I insulated it from the living area and cut the side for a ventilation hole (picture). A small fan operates when the charging starts.

 

Ragards.

 

 

Hi Dogan

 

Not sure the fan is really a good idea if it only runs during charging as it will stop when chraging ends but the battery may

still be gassing, when you turn the chargier back on it could be a source of ignition if the explosive gases are still present.

 

neil

[doganboncu]

Hi Dogan

 

Not sure the fan is really a good idea if it only runs during charging as it will stop when chraging ends but the battery may

still be gassing, when you turn the chargier back on it could be a source of ignition if the explosive gases are still present.

 

neil

 

Is there an estimation of how long further it gasses? I may add a kind of timer.

[neil]

Is there an estimation of how long further it gasses? I may add a kind of timer.

 

Hi Dogan

 

No idea how much longer it gasses for but just make sure the area is well ventilated

and no need to bother with a fan at all.

 

neil

[harry.m1byt]

Yes you can, but you won't get 150 amps from them.

What you will get is 120 amps. I can't explain the science but the same question was asked by a reader of Practical Caravan a few months ago.

Probably better to bite the bullet and fit one large capacity battery.

 

I cannot agree with that. Two 60amp/hour batteries will provide 120amp/hour, always providing the batteries are in their spec condition and working conditions. Discharge batteries at high rates and their capacities will not meet the specified amp/hour. Discharge them at much lower rates and they might well exceed the amp/hour rate.

 

Obviously one 120amp/hour battery will take up less space than two 60amp/hour batteries and be lighter.

[aaman]

I cannot agree with that. Two 60amp/hour batteries will provide 120amp/hour, always providing the batteries are in their spec condition and working conditions. Discharge batteries at high rates and their capacities will not meet the specified amp/hour. Discharge them at much lower rates and they might well exceed the amp/hour rate.

 

Obviously one 120amp/hour battery will take up less space than two 60amp/hour batteries and be lighter.

 

Two 60 amp batteries will give 120 amps but a 60 amp and a 100 amp for example won't give 160 amps, they'll only give 120 because the 100 amp battery won't charge beyond the capacity of the smaller one.

[harry.m1byt]

Two 60 amp batteries will give 120 amps but a 60 amp and a 100 amp for example won't give 160 amps, they'll only give 120 because the 100 amp battery won't charge beyond the capacity of the smaller one.

 

Why might that be? I must admit I wasn't expecting that answer, could you provide a reference?

 

A battery will continue to charge while ever a sufficient voltage is applied across it. The first battery to reach a full charge will stop charging and the larger one will continue until it attains its fully charged state. Mixing a poor battery in parallel with a good battery is a bad idea, because the fully charged voltage of the poor one will be lower and thus prevent the good one attaining its fully charged.

[aaman]

Why might that be? I must admit I wasn't expecting that answer, could you provide a reference?

 

A battery will continue to charge while ever a sufficient voltage is applied across it. The first battery to reach a full charge will stop charging and the larger one will continue until it attains its fully charged state. Mixing a poor battery in parallel with a good battery is a bad idea, because the fully charged voltage of the poor one will be lower and thus prevent the good one attaining its fully charged.

 

 

Read it in a Practical Caravan article.

And its logical if you think about it.

[doganboncu]

Read it in a Practical Caravan article.

And its logical if you think about it.

 

 

Please somebody explain the point that I cannot see.

 

If two batteries are connected in parallel whether they are the same capacity or not, they should come to the same valtage after a while.

Then, when they are started to be charged both voltages will increase together. As far as the charged point of voltage are the same, say 14. 0 volt, they will reach this together. So one should not be charged before the other. And if I should continue, when they both reach the charged point they have the capacity of their own to serve, say 90 Ah and 60 Ah.

 

The only restriction could be this, if one of the batteries is old and cannot be charged up to 14 volt, then things will not be as it should.

 

What do you think?

[Guest Surfer]

Wouldn't two 60 amp batteries weighmore than a single 120 amp battery. In another post itwas pointed out that two different 60 amp batteries may not equate to 120 as their true capaity may be different even though they are both 60 amp.

[beejay]

Is there an estimation of how long further it gasses? I may add a kind of timer.

 

Use AGM batteries. They don't gas in the conventional sense and are approved for use in aircraft because of this. They can be turned upside down without spillage. They have low self discharge rate, withstand poor maintenance better and last twice as long as a standard SLA battery.

[harry.m1byt]

Please somebody explain the point that I cannot see.

 

If two batteries are connected in parallel whether they are the same capacity or not, they should come to the same valtage after a while.

Then, when they are started to be charged both voltages will increase together. As far as the charged point of voltage are the same, say 14. 0 volt, they will reach this together. So one should not be charged before the other. And if I should continue, when they both reach the charged point they have the capacity of their own to serve, say 90 Ah and 60 Ah.

 

The only restriction could be this, if one of the batteries is old and cannot be charged up to 14 volt, then things will not be as it should.

 

What do you think?

 

I agree completely, until someone offers me a logical explanation as to why not. I don't usually buy magazines, so I don't have any access to the article to read the 'logical' explanation. I'm an electrical engineer, rather than an expert on the way lead acid batteries might behave - but to me logically they should both take what ever charge might be needed to bring them individually to a fully charged state and deliver their entire charge to what ever load might be connected. All assuming both batteries are in a reasonable condition.

[aaman]

It's not the voltage, it's the amperage (electrical capacity}.

 

AN ANALOGY

 

Imagine you have two barrels, one of ten gallons capacity and one of five gallons capacity.

 

Stand them on the floor side by side then join them together with a piece of pipe fitted anywhere into the side of each barrel. They are joined in parallel, as would be the two batteries in question.

 

If you now add water to the barrels then the smaller one will overflow before the larger one is full. Ergo, you won't be able to get 15 gallons of water into your two barrels.

 

Ten gallons maximum. It works exactly the same with batteries. Except that the water levels in your barrels are visible to the naked eye whereas electricity being much less tangible isn't as obvious and the concept needs to be imagined.

 

Now substitute barrel capacity for battery capacity.

 

Voltage doesn't enter into the equation, it's only a method of expressing the "pressure" of the electricity, which in this scenario is meaningless. Notice I didn't specify how to add the water to the barrels. You could do it a cupful at a time or use a hosepipe it doesn't matter to the end result, the water pressure has no effect on the outcome, just as the electric pressure (voltage) has no effect on the amperage of two batteries.

 

I can't explain this any simpler.

Edited April 30, 2009 by aaman

[thelonegroover]

I think aaman has explained very well.

 

If you have two different capacity batteries to charge, they must be done separately or the larger capacity one will never get a full charge. You can then connect them in parallel to use them. Their voltages will eventually even out.

[doganboncu]

It's not the voltage, it's the amperage (electrical capacity}.

 

AN ANALOGY

 

Imagine you have two barrels, one of ten gallons capacity and one of five gallons capacity.

 

Stand them on the floor side by side then join them together with a piece of pipe fitted anywhere into the side of each barrel. They are joined in parallel, as would be the two batteries in question.

 

If you now add water to the barrels then the smaller one will overflow before the larger one is full. Ergo, you won't be able to get 15 gallons of water into your two barrels.

 

Ten gallons maximum. It works exactly the same with batteries. Except that the water levels in your barrels are visible to the naked eye whereas electricity being much less tangible isn't as obvious and the concept needs to be imagined.

 

Now substitute barrel capacity for battery capacity.

 

Voltage doesn't enter into the equation, it's only a method of expressing the "pressure" of the electricity, which in this scenario is meaningless. Notice I didn't specify how to add the water to the barrels. You could do it a cupful at a time or use a hosepipe it doesn't matter to the end result, the water pressure has no effect on the outcome, just as the electric pressure (voltage) has no effect on the amperage of two batteries.

 

I can't explain this any simpler.

 

 

I completaly agree with the example, except the shape of the barrels.

 

Please check the figure below for what I mean.

 

I think the blue one is the correct shape. ..

 

Regards

Edited April 30, 2009 by doganboncu

[harry.m1byt]

I think aaman has explained very well.

 

If you have two different capacity batteries to charge, they must be done separately or the larger capacity one will never get a full charge. You can then connect them in parallel to use them. Their voltages will eventually even out.

 

Except it is wrong. ....

 

Imagine the same two linked barrels and yes the capacity of the barrels is the amp/hour rating. Both barrels exactly the same height, height being the voltage. Height providing pressure if water and voltage is just pressure anyway.

 

Fill the barrels up and because of the linking pipe (must be at the very base of the barrels) both will fill up to exactly the same levels due to water finding its own level.

Discharge the barrels (or the batteries) and they will both empty at the same rate and be empty at the same time. The right hand pair of barrels in the above link are the correct way to see it.

Edited April 30, 2009 by harry.m1byt

[aaman]

Except it is wrong. ....

 

Imagine the same two linked barrels and yes the capacity of the barrels is the amp/hour rating. Both barrels exactly the same height, height being the voltage. Height providing pressure if water and voltage is just pressure anyway.

 

Fill the barrels up and because of the linking pipe (must be at the very base of the barrels) both will fill up to exactly the same levels due to water finding its own level.

Discharge the barrels (or the batteries) and they will both empty at the same rate and be empty at the same time. The right hand pair of barrels in the above link are the correct way to see it.

 

I can't argue with that.

However I was using water containers as a substitute for batteries in (an obviously futile) attempt to demonstrate the basic premise of my argument. I always found it easier to explain in terms of water and piping as opposed to wires and amperage because water is easier on the eye.

Back to your supposition, if we become pedantic about it, I reckon that somewhere along the line atmospheric pressure on a larger surface area will affect your argument.

 

Sometimes I wonder why I bother, I try to help people to the best of my ability using simplistic and easily imaginable scenarios only to be shot down in flames.

Edited April 30, 2009 by aaman

[aaman]

Except it is wrong. ....

 

Imagine the same two linked barrels and yes the capacity of the barrels is the amp/hour rating. Both barrels exactly the same height, height being the voltage. Height providing pressure if water and voltage is just pressure anyway.

 

Fill the barrels up and because of the linking pipe (must be at the very base of the barrels) both will fill up to exactly the same levels due to water finding its own level.

Discharge the barrels (or the batteries) and they will both empty at the same rate and be empty at the same time. The right hand pair of barrels in the above link are the correct way to see it.

 

Except that if they are both to be empty at the same time then surely one barrel must be draining water at a faster rate than the other.

I can't put this into the simple terms it so obviously requires (let me dwell on it for a while) but it seems to me that your argument means that my argument must hold some water. So to speak.

Edited April 30, 2009 by aaman

[aaman]

Eureka,

 

If the barrels are batteries, then, in order to both be completely flat at the same time one of them (the larger capacity one) must discharge at a faster rate than the other. Utter *****!! This can't happen!

In order for them to both be flat at the same time then neither can be charged above the capacity of the other, and as the smaller one will not charge to the capacity of the larger, then it's the larger that must sacrifice some of it's capacity.

 

GAME OVER,

 

GAME, SET & MATCH

 

CHECKMATE

 

GENIUS IT IS I AM

 

I WIN

 

No accolades by request, just send used notes in a plain brown envelope to the usual address.

 

I'd like to thank the scriptwriter, the director, the producers, the studio who made it all possible, my grandparents, my parents, my wife, the children, the grandchildren, the dogs, the cats, all the pond fish, the budgie, the train driver I've never met, the bus conductor, her on the bakery counter in Morrisons, the postman, the man in the street, some of the people who know me and last but not least MEEEEE.

This award -which I accept most humbly accept on behalf of all those morons out there who wouldn't recognise simple physics if it came up to them and introduced itself - will take pride of place in the darkest recesses of my garage behind my secret stash of porn magazines,which cunningly conceal the the U/V lights needed for the skunk factory.

Edited April 30, 2009 by aaman

[harry.m1byt]

Eureka,

 

If the barrels are batteries, then, in order to both be completely flat at the same time one of them (the larger capacity one) must discharge at a faster rate than the other. Utter *****.

In order for them to both be flat at the same time then neither can be charged above the capacity of the other, and as the smaller one will not charge to the capacity of the larger, then it's the larger that must sacrifice some of it's capacity.

 

aaman - In part you are agreeing with me and argueing against your own conclusions.

 

In discharge, as in charge the batteries will balance each other - each will draw or deliver its entire contents. The current delivered at anyone one time will be more from the larger capacity battery. This is exactly how the two barrels would behave, but the pipework would need to be rearranged to be more like the way a pair of parallel batteries would be connected.

 

Connect a pipe from each barrel, insert a flow meter in each pipe, then at the down stream ends of the two pipe connect to a Y with the bottom of the Y connected to a tap. More water will be measured flowing out of the larger barrel than the smaller one. Now pump water in through the tap to top the barrels up, again more water will flow back up to the larger barrel.

 

Now because the pipes will be the same sizes and offer the same resistance to flow, whilst being emptied or filled they will do so at similar rates initially - however as the level differences between the two become greater - there will start to be some flow from barrel to barrel to compensate for the differences.

[thelonegroover]

Let’s start with two full batteries. Connect these in parallel and the voltages will eventually even out. The ah will be the addition of the two. If one is 110 ah and the other is 85ah, you will have 195 ah.

 

However, when you come to charge them, the 85 ah will be fully charged but the 110 ah one will not. Any surplus charge will simply overcharge the smaller one or be wasted.

 

You can analogise with barrels and things until you’re blue in the face, this is what will happen with batteries.