Batteries of different capacities wired in parallel

[tgpt21]

I need to replace my house batteries this coming season. At the moment I have two 100 Ah matching batteries but I believe through calculation and experience that this capacity is a bit low. The original battery box(es) have room for two 100 Ah batteries and there is a third after market box which has the second house battery.

My plan was to squeeze in a 110 Ah battery in the original box and then put a battery of the biggest physical size into it (around 140 Ah capacity) and connect them in parallel.

I have researched the pros and cons of connecting batteries of differing capacities. The internet is quite positive and states it is no problem. But Nigel Calder in the Boatowner's Mechanical and Electrical Manual emphatically states that it should not be done.

My own instinct is that it should not be a problem. I am thinking in terms of the water tank analogy of battery charging. The water pressure in two or more tanks will equalise when connected so the voltage will equalise in batteries in parallel.

What is the difference between making the cranking battery and house batteries common when charging? They are clearly of different capacities but I certainly do it as I want to lose the voltage drop caused by the use of diodes.

 

[[70521]]

I too would be interested to hear what the panel thinks.

 

[JumbleDuck]

As long as the batteries are of the same chemical type, it shouldn't make the slightest difference.

 

[danielefua]

My naive thought: assume you have three identical batteries all in parallel. This is equivalent to having one single capacity battery in parallel to one double capacity battery. Isn't it so? That would confirm that the thing you propose is quite normally done.
Unless one of the batteries is old and battered, I guess.

Daniel

 

[JumbleDuck]

My naive thought: assume you have three identical batteries all in parallel. This is equivalent to having one single capacity battery in parallel to one double capacity battery. Isn't it so?

Yup. The don't-mix-capacities business seems to be a myth promoted by Mr Calder.

 

[tgpt21]

I would agree with all of the above. The batteries need to be of the same type (I am going to use wet type) and in the same condition.

To use the water tank analogy you wouldn't want to use a leaky tank with a sound tank nor would you expect a pipe of differing diameter to put the same volume of water into two tanks.

I could go on with the analogy but I've convinced myself. Thanks for the confirmation.

 

[elton]

As long as the batteries are of the same chemical type, it shouldn't make the slightest difference.

I agree. I'd expect the arrangement to work exactly as one battery, with the combined surface area of both sets of lead plates. Provided the electrolytes are reasonable well matched in terms of acidity as well.

 

[lw395]

You are all looking at this very simplistically, in a DC, steady-state way.
How does your 'bucket of water' thinking cope with the response to dirty or pulsed charge or discharge currents?

But when it comes down to numbers, it would be hard to say exactly how 'non optimum' it might be.
Even two identical batteries in parallel can do odd things.

 

[lw395]

From a technical paper....

""""The charge-discharge testing of a 24V battery with two
very dissimilar strings formed by 4 monoblocs
6CP100 (6V/100Ah C10) and 12 cells
2CP550(2V/550Ah C10) showed that the currents
flowing from and to the individual strings are
principally governed by the string impedance and
independent of the installed string capacity.
Such “asymmetrical” batteries can be operated quite
successfully and yield nearly their complete cycle life
potential when deep discharges to 100% d.o.d C3 are
carried out.
If power outages cause only shallow discharges then
the “low” impedance string furnishes an overproportional
share of the Ah discharged and may thus
suffer in terms of achievable cycle life.
However if these shallow discharges are infrequent
(»50 to 100/year) then the ultimate battery life will be
governed mostly by calendar or elapsed service time
considerations and not by exhaustion of its cycle life
capital."""
The background to that is batteries for telephone exchanges/basestations.
http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf
I think there may be more downside to paralleling random batteries than meets the eye, St Calder did not invent this 'sin'.
I suspect the origin of regarding it as bad practice may be something like milk float batteries which get a lot of deep cycles per year.

 

[danielefua]

You are all looking at this very simplistically, in a DC, steady-state way.
How does your 'bucket of water' thinking cope with the response to dirty or pulsed charge or discharge currents?

But when it comes down to numbers, it would be hard to say exactly how 'non optimum' it might be.
Even two identical batteries in parallel can do odd things.

Yes indeed but real (i.e. non-ideal) batteries are quite often put in parallel and nobody argues...

What I think is that the "Nigel Calder myth" could be just a wrong extrapolation from the case of batteries in series. In that case the equal capacity requirement is obviously necessary.

To summarize (if we agree on a unique physical meaning of "capacity"):
1) batteries in parallel: voltage strictly equal, capacity whatever.
2) batteries in series: voltage whatever, capacity strictly equal.

Daniel

 

[tgpt21]

""""The charge-discharge testing of a 24V battery with two
very dissimilar strings formed by 4 monoblocs
6CP100 (6V/100Ah C10) and 12 cells
2CP550(2V/550Ah C10) showed that the currents
flowing from and to the individual strings are
principally governed by the string impedance and
independent of the installed string capacity.
Such “asymmetrical” batteries can be operated quite
successfully and yield nearly their complete cycle life
potential when deep discharges to 100% d.o.d C3 are
carried out.
If power outages cause only shallow discharges then
the “low” impedance string furnishes an overproportional
share of the Ah discharged and may thus
suffer in terms of achievable cycle life.
However if these shallow discharges are infrequent
(»50 to 100/year) then the ultimate battery life will be
governed mostly by calendar or elapsed service time
considerations and not by exhaustion of its cycle life
capital."""
The background to that is batteries for telephone exchanges/basestations.
http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf
I think there may be more downside to paralleling random batteries than meets the eye, St Calder did not invent this 'sin'.
I suspect the origin of regarding it as bad practice may be something like milk float batteries which get a lot of deep cycles per year.

I will take this into consideration once I have worked out what it means. LOL

 

[JumbleDuck]

... the currents
flowing from and to the individual strings are
principally governed by the string impedance and
independent of the installed string capacity.

Which is as one would expect, and is why in an ideal world one equalises the resistances of the wires to the batteries, which although low are of a similar order of magnitude to the internal impedance of the cells.

 

[lw395]

I will take this into consideration once I have worked out what it means. LOL

It means it's not as simple as a bucket of water, clever people have been paid to argue about it.
I can't see it being a problem on the average boat, but some batteries on boats do seem to die very quickly.

 

[tgpt21]

It means it's not as simple as a bucket of water, clever people have been paid to argue about it.
I can't see it being a problem on the average boat, but some batteries on boats do seem to die very quickly.

Yes they do. I have sealed batteries and the alternator charges at just under 15 volts which causes them (I think) to boil and lose electrolyte which has hastened their demise. I will use old fashioned wet batteries and have distilled water on hand to keep them topped up. Also I have fitted a battery monitor and will not let them in future fall below 50% and nowhere near 100%. That's the plan anyway.

Traction batteries would be the answer (and I think that is what is being spoken about in that technical paper) but out of my price range.

 

[sailinglegend420]

Nigel Calder was I believe a mechanical engineer and not an electrical engineer. His "latest" volume is now 9 years old!!! Thinking and equipment changes a lot in 9 years.

Here us a real world example of why different capacities are OK.

Sabre yachts, who use Lifeline AGM batteries, regularly ship with different sizes to be able to get the maximum capacity from all the available space.

Lifeline were concerned by this but their research proved that a bank can be made up of different capacities and that the life of the batteries was not compromised, as long as they are all the same make and the same age. They accepted that his goes against most previous guidelines.

One way of achieving the largest bank for the available space is to change the starter battery for something very much smaller and use the space for another service battery. It is high cold cranking amps (CCA) that are needed to start an engine, not high Ah capacity. A 37Ah Red Flash or Odyssey AGMs can be less than half the size of a conventional starter battery but deliver very higher CCA. Mine has laid untouched on its side in the bilges for 10 years and always started my 56 HP Yanmar.

 

[MM5AHO]

As long as the batteries are of the same chemical type, it shouldn't make the slightest difference.

Why the need to be the same chemically?

 

[lw395]

Nigel Calder was I believe a mechanical engineer and not an electrical engineer. His "latest" volume is now 9 years old!!! Thinking and equipment changes a lot in 9 years.

Here us a real world example of why different capacities are OK.

Sabre yachts, who use Lifeline AGM batteries, regularly ship with different sizes to be able to get the maximum capacity from all the available space.

Lifeline were concerned by this but their research proved that a bank can be made up of different capacities and that the life of the batteries was not compromised, as long as they are all the same make and the same age. They accepted that his goes against most previous guidelines.

.....

Having to have batteries of the same make and age is similarly onerous to having them the same size.
It's a big spend as soon as one fails.
It's more useful to understand when it might matter to break the size, make, age rules.

Especially if you've got a source of used batteries ex basestation but perhaps don't want to entirely rely on them....

In the past I have gone down the route of having more than one 'house' bank, or essentially keeping a separate battery for the fridge and eberbasto.

 

[lw395]

Yes they do. I have sealed batteries and the alternator charges at just under 15 volts which causes them (I think) to boil ....

You are overcharging. Simple as that!

 

[lw395]

Why the need to be the same chemically?

Different chemistry will have different voltage changes when off load and off charge as temperature changes, so current will flow from one to the other.
May not matter in a base station, which is either on load or on charge, may matter in a yacht which is idle 5 days a week?

 

[nigelmercier]

Nigel Calder was I believe a mechanical engineer and not an electrical engineer...

A lot of what he says about electricity makes sense though, I couldn't do without his bible: http://amzn.to/1g7B7gH

I think if this scenario is going to cause a problem would depend on the batteries, and the way they are wired. I would make sure to wire them with the charge/discharge cables at opposite ends, and use large capacity cable for the interconnects.