Different Ah in a series...

[wazza]

Can I put different Ah batteries in a series.?
I was thinking 2 x 70Ah and 1 x 115Ah as I have enough room for the 115 in a different area of the boat. So I thought it’d be nice to have a little extra power for the house bank..
What does the panel think.?

 

[pvb]

I think you meant to say "in parallel".

It works OK, although lots of people caution against it. The important thing is to ensure the batteries are the same chemistry (ie wet lead-acid, "sealed" lead-acid, AGM, etc) and ideally similar ages.

 

[VicS]

^^^ what he says

Two batteries in series would give you 24 volts !

 

[lpdsn]

Should be OK. The bigger battery will end up working harder and I guess the main downside will be in the charging regime where voltage will still be being provided for the 115Ah battery when the other two are already charged, so there's a risk of minor overcharging. Keep an eye on the water levels in the batteries though and it should be manageable.

PS Like everyone else I've assumed you meant in parallel.

 

[pvb]

...I guess the main downside will be in the charging regime where voltage will still be being provided for the 115Ah battery when the other two are already charged, so there's a risk of minor overcharging.

Larger batteries accept current more readily, so the net result is that all the batteries will reach full charge at about the same time.

 

[VicS]

A better way of connecting two batteries in parallel but see http://www.smartgauge.co.uk/batt_con.html for advice on connecting more than two in parallel

Long cables , unless you make them very heavy , connecting the additional 115 Ah battery would tend to make it do less than its fair share of the work, and charge more slowly, due to their resistance

 

[William_H]

Yes batteries in parallel oK however it would be desirable to have an easy way to disconnect any one battery or 2 batteries to enable you to occasionally check performance of each battery. ie from fully charged see how long you can run domestics on one battery then compare with the other batteries each run alone. If you leave them always paralleled one can drop dead and not be noticed or even suck power form good ones. olewill

 

[wazza]

Thank you all.. yes indeed ‘in parallel’ is what I meant.

 

[lpdsn]

Larger batteries accept current more readily, so the net result is that all the batteries will reach full charge at about the same time.

Had in mind that the smaller batteries would be subject to a voltage that causes gassing for longer than they would if just there on their own.

Whilst a larger battery would accept current more readily I don't accept your theory that all batteries charge in exactly the same time regardless of size.

For the OP it just means being a bit dilligent in checking the water levels until he's happy he understands what's happening in his set up.

 

[halcyon]

Had in mind that the smaller batteries would be subject to a voltage that causes gassing for longer than they would if just there on their own.

Whilst a larger battery would accept current more readily I don't accept your theory that all batteries charge in exactly the same time regardless of size.

For the OP it just means being a bit dilligent in checking the water levels until he's happy he understands what's happening in his set up.

The voltage you see during charge is generated by the battery not the charge source, it is due to chemical conversion. Though with a high wattage charger you can max out the conversion rate and produce high charge voltage, this produces the values that show the higher the current the shorter the charge time the lower the recharge capacity.

Slow is best, 10 hour for flat battery, so 5 hours for 50% discharge.

Brian

 

[pvb]

Whilst a larger battery would accept current more readily I don't accept your theory that all batteries charge in exactly the same time regardless of size.

If you consider the situation when discharging, rather than charging, do you believe that the smaller batteries go flat before the larger battery?

 

[lpdsn]

If you consider the situation when discharging, rather than charging, do you believe that the smaller batteries go flat before the larger battery?

What's discharge got to do with it? I only made a comment on the charging regime.

 

[pvb]

What's discharge got to do with it? I only made a comment on the charging regime.

I was suggesting a similar question. You said you didn't accept that different capacity batteries would charge in the same time. So I wondered whether your belief extended to the opposite situation - discharge.

 

[lpdsn]

I was suggesting a similar question. You said you didn't accept that different capacity batteries would charge in the same time. So I wondered whether your belief extended to the opposite situation - discharge.

There's not a parallel between discharge and recharge in this context, so discharge behaviour isn't relevant.

Given good connections between the batteries of a bank and no large currents which might lead to a non-trivial voltage drop across the cables connecting them then during discharge the batteries are going to be at the same voltage regardless of size. They're also going to have more or less the same voltage applied to them during charging once the charging currents reduce after the bulk charge.

In any event, my earlier post in this thread gave the advice for the OP to check the water levels of the smaller batteries more frequently until he's satisfied it's not necessary. Are you arguing that was bad advice?

 

[pvb]

There's not a parallel between discharge and recharge in this context, so discharge behaviour isn't relevant.

Given good connections between the batteries of a bank and no large currents which might lead to a non-trivial voltage drop across the cables connecting them then during discharge the batteries are going to be at the same voltage regardless of size. They're also going to have more or less the same voltage applied to them during charging once the charging currents reduce after the bulk charge.

In any event, my earlier post in this thread gave the advice for the OP to check the water levels of the smaller batteries more frequently until he's satisfied it's not necessary. Are you arguing that was bad advice?

There is indeed a parallel; if you think (as I do) that there's no possibility of the smaller batteries going flat under discharge before the larger battery, then I think that demonstrates that batteries in a bank deliver, and accept, current according to their capacities. As a result, under charge, the smaller batteries aren't going to suffer from overcharging. As for checking water levels, that's a generally accepted regular practice for those who still use wet lead-acid batteries.

 

[lpdsn]

There is indeed a parallel; if you think (as I do) that there's no possibility of the smaller batteries going flat under discharge before the larger battery, then I think that demonstrates that batteries in a bank deliver, and accept, current according to their capacities. As a result, under charge, the smaller batteries aren't going to suffer from overcharging. As for checking water levels, that's a generally accepted regular practice for those who still use wet lead-acid batteries.

I'm surprised this one is going over the top of your head. It isn't a parallel with the discharge position. Trying to draw that analogy is just confusing the issue.

When chargers are doing equalisation they keep the batteries at a high voltage (let's say 14.8V for typical wet acid). That period is larger for a larger bank. Having smaller batteries in the bank means they are kept at 14.8V longer than would otherwise be necessary for themselves, so a little bit more time spent gassing. That's all.

As for checking water levels, the whole point is check the levels of the smaller batteries more frequently until the time between necessary top ups is understood. So not as simple as check or don't check as your last response implies, but the planned period of checking.

Maybe the discussion is just getting boring. It is for me. The OP will be fine - just check the water levels.

 

[pvb]

I thought it was going over your head! Having large and small batteries in one bank won't be a problem for the OP.

 

[lpdsn]

I thought it was going over your head! Having large and small batteries in one bank won't be a problem for the OP.

Not at all. Let's agree to disagree.

And I do agree with you that the OP shouldn't have an issues.

 

[LittleSister]

The voltage you see during charge is generated by the battery not the charge source, it is due to chemical conversion.

Is that right?

So if I run down my battery to the point where it's showing, say, 11v open circuit, and even less when I put a load on it, how does it magically generate 14+v when I connect it to a spinning alternator?

My understanding is that there is indeed a chemical conversion going on, but during charging it's the electrical power applied to the battery (generated by something else) that's converting the chemicals back to their charged state. (Only in discharge mode are the chemicals 'self-converting'.)

 

[halcyon]

Is that right?

So if I run down my battery to the point where it's showing, say, 11v open circuit, and even less when I put a load on it, how does it magically generate 14+v when I connect it to a spinning alternator?

My understanding is that there is indeed a chemical conversion going on, but during charging it's the electrical power applied to the battery (generated by something else) that's converting the chemicals back to their charged state. (Only in discharge mode are the chemicals 'self-converting'.)

Because it takes a voltage to generate the conversion, the alternator only limits the max voltage. In your case above assuming 10 hour recharge, so 300 amp hour battery bank @ 30 amp, you will see initially 13.0, rising to 14 + after 10 hours.

Brian