Running out of battery. Can we mix sizes?

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NPMR

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Twice, recently, we have run the 75Ahr batteries flat in our 3-year old Jeanneau S.O.33i on a passage. (one time we flattened ALL 3!)

We have a solar panel for keeping the main domestic battery topped up but having spoken to Jeanneau and others, they all agree, for long term use, we need more amp-power.

Jeanneau suggested short term solution, put another 75Ahr battery in. 'Expert' opinion, put in 100-105Ahr batteries AND a system to beef up output from the alternator.

Long term solution, this winter, will be to add an Adverc and bigger batteries but this means chopping the battery bay about.

However, I could replace the 'spare' or 'shunt' battery with a 100Ahr battery immediately, as it will fit the battery hole.

The question: Can I mix sizes of battery? The suggested mix will be - Engine; 75Ahr, Main Domestic; 75Ahr (as I can't fit a bigger battery in the space yet) and 'Shunt': 100Ahr
 
I've just looked it up in Calder. He insists that it is important to have batteries of both similar type and capacity in a bank.

Why not just put the Adverc in now and see how you get on with that?
 
Camelia said:
I agree with Rigger and Calder. All batteries should be the same capacity.
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This sounds supiciously like folk wisdom to me, though I could be convinced by a goo argument. How would a 110Ah battery know that it was in parallel with another 110Ah battery and not two 55Ah batteries? What about the many, many people who have a small engine start battery and big house batteries?
 
JumbleDuck said:
This sounds supiciously like folk wisdom to me, though I could be convinced by a goo argument. How would a 110Ah battery know that it was in parallel with another 110Ah battery and not two 55Ah batteries? What about the many, many people who have a small engine start battery and big house batteries?
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Are the start battery and house battery connected? How are they charging them?
 
JumbleDuck said:
This sounds supiciously like folk wisdom to me, though I could be convinced by a goo argument. How would a 110Ah battery know that it was in parallel with another 110Ah battery and not two 55Ah batteries? What about the many, many people who have a small engine start battery and big house batteries?
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If you have a 100ah battery parallelled to a 75AH it would effectively be the similar to having a good 100AH battery alongside a clapped out 100AH battery. In the second case the poor battery drags down the good battery; likewise the 75AH battery will drag down the 100ah battery.

It's much the same if you have a dud cell in a battery. One dud cell screws up the rest of the battery.

The starter battery is usually isolated from the main bank with a 1/2/3 switch.

That's how I understand it.
 
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Rigger Mortice said:
If you have a 100ah battery parallelled to a 75AH it would effectively be the similar to having a good 100AH battery alongside a clapped out 100AH battery. In the second case the poor battery drags down the good battery; likewise the 75AH battery will drag down the 100ah battery.

It's much the same if you have a dud cell in a battery. One dud cell screws up the rest of the battery.

That's how I understand it.
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Not at all, you're confusing voltage with capacity. A decent 100Ah battery in parallel with a decent 75Ah battery of the same type will work perfectly well. How could the 75Ah "drag down" the other one? They'll both be at the same voltage.
 
Two fully charged batteries of different capacity should be at the same voltage; however, what happens when you discharge them? Will the lower capacity battery drop to a lower voltage quicker than the higher capacity battery and therefore drag the higher capacity battery down? I would have no problem mixing batteries as a shortish term expedient - perhaps to the end of a season, but longer term it doesn't sound like a terribly good idea.

Neil
 
Camelia said:
Are the start battery and house battery connected? How are they charging them?
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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.
 
neil1967 said:
Two fully charged batteries of different capacity should be at the same voltage; however, what happens when you discharge them? Will the lower capacity battery drop to a lower voltage quicker than the higher capacity battery and therefore drag the higher capacity battery down?
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No it won't - the batteries will supply different proportions of the total discharge current, and will remain at the same voltage.
 
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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Sounds overly complicated, and maybe you need to review the wiring. You'll get better use out of the domestic batteries if they are always connected in parallel.
 
pvb said:
Not at all, you're confusing voltage with capacity. A decent 100Ah battery in parallel with a decent 75Ah battery of the same type will work perfectly well. How could the 75Ah "drag down" the other one? They'll both be at the same voltage.
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I don't think I was confusing the two but, yes, the batteries will be at the same voltage. If one is down the other will effectively be topping it up; and if one is a smaller capacity the time will inevitably come when it has run out of juice before its bigger brother. But I'm happy to be proven wrong. It will be interesting to see what others have to say.
 
Rigger Mortice said:
I don't think I was confusing the two but, yes, the batteries will be at the same voltage. If one is down the other will effectively be topping it up; and if one is a smaller capacity the time will inevitably come when it has run out of juice before its bigger brother.
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The smaller battery will provide a smaller proportion of the discharge current. So the two batteries will drop in voltage at the same rate.

Imagine a 100Ah battery in parallel with a 50Ah battery - the 100Ah battery will provide two-thirds of the discharge current; the 50Ah battery will provide one third.
 
Rigger Mortice said:
If you have a 100ah battery parallelled to a 75AH it would effectively be the similar to having a good 100AH battery alongside a clapped out 100AH battery. In the second case the poor battery drags down the good battery; likewise the 75AH battery will drag down the 100ah battery.

It's much the same if you have a dud cell in a battery. One dud cell screws up the rest of the battery.
Click to expand...

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.
 
neil1967 said:
Two fully charged batteries of different capacity should be at the same voltage; however, what happens when you discharge them? Will the lower capacity battery drop to a lower voltage quicker than the higher capacity battery and therefore drag the higher capacity battery down?
Click to expand...

If they are in parallel the voltages will stay the same. Neither will drag down the other.
 
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!

Neil
 
pvb said:
.. Imagine a 100Ah battery in parallel with a 50Ah battery - the 100Ah battery will provide two-thirds of the discharge current; the 50Ah battery will provide one third.
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What happens with the charging in this situation?

I did feel that this would just be a short term arrangement until winter layup and serious woodwork to get all the batteries bigger boxes and maybe even this is just too much of a problem and we just need to remember battery charging ALL the time? We only have one more 'serious' trip (to the Scillies) to do before lay-up. The rest will be overnighters and day trips closer to home.
 
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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They will be at the same voltage - they're connected in parallel.
 
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