Running out of battery. Can we mix sizes?

JumbleDuck said:
One dud cell does indeed screw up a battery, but that's because it's in series. Banks of batteries in parallel are used all over the place without problems.
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That's certainly true and, to be honest, I can't find fault with pvb's comments either. But I do fall back on Nigel Calder's recommendations though; not to use as a trump card in the discussion, but because that is certainly how I understand conventional wisdom saw it.
It will be interesting to see if we get a definitive answer.
 
Agree with pvb, no such thing as two batteries with exactly the same capacity.

They will supply and accept charge based on their own capacity.
 
neil1967 said:
I see where you're coming from, but surely different capacity batteries, even of the same type, will have different discharge characteristics - I don't suppose it is linear, so the fact that you might have discharged each battery by 10% AH (for the sake of argument), I'm not convinced that they would be at the same voltage - but I could be wrong!
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If they are the same type, it's no different from wondering if the left hand two third of each plate will discharge differently from the right hand third.

Tesla cars use many thousands of Li-ion batteries wired with parallel groups in series.
 
Rigger Mortice said:
That's certainly true and, to be honest, I can't find fault with pvb's comments either. But I do fall back on Nigel Calder's recommendations though; not to use as a trump card in the discussion, but because that is certainly how I understand conventional wisdom saw it.
It will be interesting to see if we get a definitive answer.
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You have to remember that Calder is in many ways only a journalist, making money out of re-presenting information. That information isn't always correct. As for the definitive answer, it's that batteries in a bank should be of the same type (ie flooded, maintenance-free, AGM, etc) but they needn't be of the same capacity
 
Camelia said:
I agree with Rigger and Calder. All batteries should be the same capacity.
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All batteries IN ANY ONE BANK, should be a similar capacity. If you have to mix battery sizes treat each as a separate bank. So there is no reason at all why you can't have a 45ah engine start battery, and 120 ah and 75 ah domestic batteries - providing they are charged separately and used separately. Otherwise the larger of the "twinned" batteries in the domestic bank will never get fully charged. More importantly it's very unwise to mix gel, sealed and open batteries or to have a mix of antimony or Ca hardened plates in your batteries. All these require totally different charging regimes.

I've been mixing capacities successfully for about 20 years.
However, if you really have totally discharged your present batteries, I'd suggest you start with replacing them, whilst one cannot be certain they are NU, the only way to check is to have them out and fully tested on the bench.

PS Many on board regulators, in modern alternators, appear to make such "smart" chargers as the Adverc or Sterling redundant.
The one I've just fitted starts the charge @ 15.1v (measured @ input to the splitting diode) drops off to 14.4v in bulk phase and down to 13.5 in float. I've disconnected the Adverc as a result after 21 years of using them.
 
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charles_reed said:
All batteries IN ANY ONE BANK, should be a similar capacity. If you have to mix battery sizes treat each as a separate bank. So there is no reason at all why you can't have a 45ah engine start battery, and 120 ah and 75 ah domestic batteries - providing they are charged separately and used separately. Otherwise the larger of the "twinned" batteries in the domestic bank will never get fully charged. More importantly it's very unwise to mix gel, sealed and open batteries or to have a mix of antimony or Ca hardened plates in your batteries. All these require totally different charging regimes.
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How does the larger battery fail to charge when it is driven to the terminal voltage that successfully charged the smaller battery?
 
charles_reed said:
All batteries IN ANY ONE BANK, should be a similar capacity. If you have to mix battery sizes treat each as a separate bank. So there is no reason at all why you can't have a 45ah engine start battery, and 120 ah and 75 ah domestic batteries - providing they are charged separately and used separately. Otherwise the larger of the "twinned" batteries in the domestic bank will never get fully charged.
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That's untrue, for the reasons I've given already. The batteries will be subject to the same charging voltage and will accept current according to their ability to absorb it, until both reach full charge.

charles_reed said:
More importantly it's very unwise to mix gel, sealed and open batteries or to have a mix of antimony or Ca hardened plates in your batteries. All these require totally different charging regimes.
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That bit is true, as I've said already.
 
NPMR said:
We have 3 x switches, one for each battery ( and a single black 'return/negative). When the engine is running/

charging I put all 3 batteries to 'on'. When sailing, we now take the engine battery out of the mix (to stop it running down). When I tested yesterday, Domestic battery No3 gave nothing (literally 0 volts) until Domestic Battery No2 was switched on. I am told that No3 is a shunt or standby battery to add to No2 if it needs help.
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If one of the house batteries is dead the other can't be far behind, it's always the way :rolleyes: . So replace them both with 100 AH batteries wired together in parallel which will give you at 50% about 100 amp hours before you need to recharge them. That should be a couple of nights light use before reaching the 50% mark regarded as a sensible limit before excessively shortening the life of your batteries.

Next you need some way of monitoring the charging. Lots of options; here are a couple that are regularly named on the forum:

http://www.sterling-power.com/products-pmp.htm

http://www.smartgauge.co.uk/

Once you can see what is going on you can tell when its time to charge the batteries and see how well the solar panel is working too.

Pete
 
pteron said:
How does the larger battery fail to charge when it is driven to the terminal voltage that successfully charged the smaller battery?
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Try it and check the SG reading in the two different batteries.

I made the point that the larger will not be FULLY charged not that it would not be charged at all - hence many smart chargers having a variable timed absorption phase.
 
charles_reed said:
Try it and check the SG reading in the two different batteries.

I made the point that the larger will not be FULLY charged not that it would not be charged at all - hence many smart chargers having a variable timed absorption phase.
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After what time? What if I left them to charge for a week? Do you still say it won't be fully charged?
 
charles_reed said:
Otherwise the larger of the "twinned" batteries in the domestic bank will never get fully charged.
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When you have two electrical components in parallel, all that determines the current through each is the supply voltage. With 13.8V (or whatever) across both batteries there is absolutely nothing which battery A can do to affect battery B.

It's not the case that the alternator supplies a current which the batteries split between them. The alternator supplies a voltage, as a result of which each battery takes what current it likes. If the alternator can supply that, everything is fine. If not the voltage at the alternator will drop, the individual currents will drop and the total current will drop until equilibrium is reached.
 
pvb said:
As I read it, the battery wasn't dead, but the switching system is peculiar.
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The battery state was fine but is not in 'use' by itself - it only comes in when the Domestic battery is switched on. It supports the Domestic battery but only when you switch it 'on'.

This is a standard boat and as far as I can tell is unmodified so is as built by Jeanneau.
 
NPMR said:
The battery state was fine but is not in 'use' by itself - it only comes in when the Domestic battery is switched on. It supports the Domestic battery but only when you switch it 'on'.

This is a standard boat and as far as I can tell is unmodified so is as built by Jeanneau.
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There's no point in having a battery sitting there unconnected, just in case you need it. It's much more efficient to have the domestic batteries permanently connected in parallel. The batteries will last longer, and recharging will be quicker.
 
charles_reed said:
All batteries IN ANY ONE BANK, should be a similar capacity. If you have to mix battery sizes treat each as a separate bank. So there is no reason at all why you can't have a 45ah engine start battery, and 120 ah and 75 ah domestic batteries - providing they are charged separately and used separately. Otherwise the larger of the "twinned" batteries in the domestic bank will never get fully charged.....
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Charles, as others have posted, this is just plainly incorrect! All split charging and battery combiner systems allow multiple banks of different capacities to be mixed and charged successfully. Some would argue that a very much smaller battery may get overcharged but not the larger undercharged. The larger will take longer than the smaller - which is why your SG readings may be different at the point that you decide to stop charging.

Let me offer some feedback from a battery manufacturer - Lifeline - where recent research by them has 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. This goes against most previous guidelines, but Sabre yachts, which use Lifeline AGMs, regularly ship with different sized batteries to be able to get the maximum capacity from all the available space.

See this excellent site for more info:

http://www.morganscloud.com/2011/10/03/mixing-battery-sizes/
 
Not being an electrical engineer, I like to use the 'water tank' analogy. 2 water tanks connected in parallel (at the bottom). Water (charge) added will fill both tanks proportionally to their size - the height of water being analogous to voltage. Conversely, water drawn off will reduce the level in both tanks equally so that the levels (voltage) will remain the same.
So the conclusion I draw is that it's OK to have different capacity batteries in parallel and the capacity will be the sum of the individuals. No idea whether you should mix battery types (I've no analogy for that and can't afford fancy AGMs anyway) but gut feeling would be that it would not be a good thing.
 
ghostlymoron said:
Not being an electrical engineer, I like to use the 'water tank' analogy. 2 water tanks connected in parallel (at the bottom). Water (charge) added will fill both tanks proportionally to their size - the height of water being analogous to voltage. Conversely, water drawn off will reduce the level in both tanks equally so that the levels (voltage) will remain the same.
So the conclusion I draw is that it's OK to have different capacity batteries in parallel and the capacity will be the sum of the individuals. No idea whether you should mix battery types (I've no analogy for that and can't afford fancy AGMs anyway) but gut feeling would be that it would not be a good thing.
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Correct on both counts.
 
Interesting thread.

I must admit that I find the "it's Ok to mix capacities" arguments are more convincing than the others, even though it does seem to be commonly said that the batteries must be the same size.
 
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