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Mixing Batteries


SandyD

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Is it OK to mix batteries of different Ah ratings and age? I assume it is always better to replace them all at once but if one has a duff cell can you replace just that one?

 

(the reason I ask is I have an 18Ah battery for running my moth trap but it is getting a little old and does not hold enough power to run the trap, I also have an old 40Ah tractor battery which, whilst it can't start the tractor, still holds a charge and I was wondering if I could use them in parallel to extend the time the moth trap runs. And that got me thinking could I change my batteries in part rather than all at once?)

 

Thanks

 

Sandy

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Personaly, as soon as any of my batteries even hinted at being below parr, I replaced the lot in that bank. In the scheme of things, batteries are a peanuts part of boat ownership, unless they are lithium and I treated them like diesel as a consumable and had no problems then with electrical stuff.

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Yes you can as long as they are in parallel BUT doing so risks an older battery with internal shorts destroying the newer one. That is why it is considered bad practice. In this specific case, I don't see you have anything to lose BUT if the short battery life is caused by internal shorts it is more or less pointless because it will flatten the other battery. If it is loss of capacity because of sulphation then it won't do any harm. It is not a good idea to mix batteries of different types, e.g. open lead acid and AGM, but in this case I don't think it will cause any problems.

  • Greenie 1
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17 minutes ago, Higgs said:

Why not just scrap the 18Ah battery?

Not sure if the old tractor battery will be man enough for the job either - hopefully together they will be enough!

16 minutes ago, Mac of Cygnet said:

This surely must be a first in the thousands of battery topics on CWDF.

Yes, I hunted around but couldn't find anything specific on mixing lead acid batteries of different capacities and levels of health.

 

14 minutes ago, magnetman said:

I think a moth trap would probably benefit from a small LiFePO4 battery.

Yes, that is the usual way of things, but if you have a battery or two hanging around why make the extra expense?

 

5 minutes ago, Tracy D'arth said:

Cruel to moths, why kill them? 

No moths are killed. Identified and released!

 

Thanks Tony, I wil see what happens!

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2 hours ago, SandyD said:

Is it OK to mix batteries of different Ah ratings and age? I assume it is always better to replace them all at once but if one has a duff cell can you replace just that one?

 

 

Short answer: Yes. 

 

Long answer: It depends. 

 

All must be LA batts, no mixing with lithium. And as Tony points out, the other batteries you are not replacing might be close to a cell failing, so watch out for that.

 

 

1 hour ago, Tracy D'arth said:

Cruel to moths, why kill them?  Moth ball will keep them out of the boat, they are entitled to live outside.

 

As I understand it, moth traps are used by mothists to survey moth populations. They are used to collect moths in order to count them, identify them then release them.

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53 minutes ago, SandyD said:

Not sure if the old tractor battery will be man enough for the job either - hopefully together they will be enough!

 

The 18Ah seems to be shot. The 40Ah seems to have some staying power. The 18AH will drag down the 40AH, as the 40Ah will be used by the 18Ah, in a constant battle to charge the 18Ah, wasting the 40Ah battery. Better to take the 18Ah battery and throw it.

 

 

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Just now, Higgs said:

 

The 18Ah seems to be shot. The 40Ah seems to have some staying power. The 18AH will drag down the 40AH, as the 40Ah will be used by the 18Ah, in a constant battle to charge the 18Ah, wasting the 40Ah battery. Better to take the 18Ah battery and throw it.

 

 

 

I don't think this is correct if the 18Ah one is just sulphated. It will reach the same state of charge as the 40Ah one very quickly and then not draw anything from the 40Ah one. You are correct if the 18Ah on has developed an internal short, but we do not know that.

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2 minutes ago, Tony Brooks said:

 

I don't think this is correct if the 18Ah one is just sulphated. It will reach the same state of charge as the 40Ah one very quickly and then not draw anything from the 40Ah one. You are correct if the 18Ah on has developed an internal short, but we do not know that.

 

But on balance, I can't see what good it would do to hang on to a failing battery in the circuit, if the 40Ah could do the work on its own.

 

 

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Tony's explanation seems correct to me.  I don't think the presence of the sulphated 18Ah battery will result in any damage to the 80Ah one, provided the former is only sulphated, and has no actual dead cells.

 

EDITED TO ADD:

 

Never leave connected any battery that does have shorted cells.  Very dramatic things can happen if to continue to attempt to charge it.

Edited by alan_fincher
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Victron AGM are very good quality. I have a little 7ah 12v block which has lasted ages. 

Have been through other similar looking batteries and they don't last long at all. 

 

More expensive but the Victron gear definitely is up there in terms of quality. 

 

 

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3 hours ago, mrsmelly said:

Personaly, as soon as any of my batteries even hinted at being below parr, I replaced the lot in that bank. In the scheme of things, batteries are a peanuts part of boat ownership, unless they are lithium and I treated them like diesel as a consumable and had no problems then with electrical stuff.

 

You are however loaded, and can afford to throw £500 at a new set of batts at the drop of a hat.

 

The OP on the other hand, appears to be asking how to fix his problem on a budget of zero. 

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24 minutes ago, SandyD said:

I have a small solar set up that charges the battery(s) 

 

So how do you know the battery is being fully charged? If it is not, then apart from it starting the night only partially charged, it will also suffer an accelerated rate of sulphation (loss of storage capacity). Have you ever measured the charging currant and voltage. It all depends upon how small the panel is, but until the start of this month any charge is likely to very limited. 

 

Until we know the size and orientation of the panel etc. my advice will now be, unless you are sure that you more or less fully charge the battery each day, I would suggest there is little point in paralleling both batteries. You may destroy both by sulphation caused by persistent under charging.

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13 minutes ago, Tony Brooks said:

So how do you know the battery is being fully charged?

 

Tony, there is a charge manager thingy with the panel that has an illustration of how full the battery is (I assume measured from how much charge it is taking ?) and a Voltmeter, I have not used the trap unless the battery appears to be full and the voltmeter says well above 13v

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Just now, SandyD said:

 

Tony, there is a charge manager thingy with the panel that has an illustration of how full the battery is (I assume measured from how much charge it is taking ?) and a Voltmeter, I have not used the trap unless the battery appears to be full and the voltmeter says well above 13v

 

I feared that might be the case. Unfortunately, solar controllers tell lies. It will not be much more than about 60% to 80% charged when the voltage in bright sunlight gets to about 14.2 volts on the controller. However, all too many controllers drop the charging voltage to a lower level long before the lead acid batteries are fully charged. 

 

I think the only way for you to infer how well charged the battery is would be to cover the panel an hour before you want to use the battery and rest the battery for that hour, or put the lamp on for several minutes. If the voltage shown is around 12.7 to 12.8 volts. If 12.5 volts it is around half charged and it around 12.2 to 12.3 it is time to recharge for maximum battery life, although the lamp will work for several hours more. The resting the battery off charge for an hour or more or the moderate discharge is vital to get a voltage that reflects the state of charge. Your 13+ volts will drop fairly quickly when you put a load on the battery and then the drop slows down. That is when the voltage reflects the state of charge.

 

I suspect that you are using an LED bulb. These draw very little current, so it might be as well to use a car flasher lamp bulb to take the surface charge off the battery (say 21W or larger).

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How much power does the moth trap use on a daily basis? I would be tempted to bin the lead batteries and put a single small lithium battery on there. If the trap uses more power than you can get from the solar then its a losing battle whatever you do. 

 

 

 

I don't know how long you have been using solar panels but I've had them on my boats for 28 yars and even the more recent high wattage ones do remarkably little in winter months. 

 

Maybe you need a bigger solar panel. 

 

As someone mentioned above your dead battery does seem to coincide with the time of the year when solar was at its worst. 

 

 

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