Charging different Ah batteries.

[Sneaky Pete]

Can anyone answer a question for me, can I mix a 130Ah battery used for powering the lighting instruments etc with a 75Ah one used to start the engine will both be fully charged from engine and/or shore power, will both re-charge to their full capacity each time. I have this impression that both batteries must be of similar Ah rating. Not being electrically minded I am not sure if they are on a split charging system or what.

 

[NB Willawaw]

In theory, it doesn't matter if they are different capacities providing they are the same technology.

It would be a good idea to better understand how they are wired/ switched together though.

 

[pvb]

As long as they're both similar chemistry (eg both wet, both AGM, both sealed, etc), there's no problem - they'll each take whatever charge current they need.

 

[Plevier]

That's OK if it's a single voltage system which will be around 14V.
If you have a smart charger (or smart alternator regulator) going up to 15V or so, and one battery - the starter - is virtually fully charged and the other - the domestic - is very low, the charged one won't like the prolonged high voltage charge. At the least it will gas and lose water and longer term it will shorten its life.

 

[William_H]

2 battery charging

Both the 1,2,both switching and Voltage Sensing Relay type charging arrangements will electrically connect both batteries in parallel.This is very common and usually with different sized batteries without problems. So do not worry. good luck olewill

 

[mawm]

That's OK if it's a single voltage system which will be around 14V.
If you have a smart charger (or smart alternator regulator) going up to 15V or so, and one battery - the starter - is virtually fully charged and the other - the domestic - is very low, the charged one won't like the prolonged high voltage charge. At the least it will gas and lose water and longer term it will shorten its life.

If I connect my shore-powered charger (6A) to the starter battery, when it gets to a certain voltage (13.4?) the VSR kicks in and then 2x 85Ah leisure batteries get charged. The starter battery sits for a prolonged period at this voltage and it gasses and loses water in quite copious amounts. I'm forever mopping and topping up. : (

 

[Paddingtonbear]

There almost daily posts of this nature that bring forward a host of different answers some very informed but others somewhat less so. I have always found the following website very informative and is certainly not lacking in practical expertise! http://www.smartgauge.co.uk/technical1.html .I have no axe to grind nor have I ever used their equipment but Chris Gibson is exceedingly helpful and is prepared to talk you through any problem with absolutely no obligation although he doesn't suffer fools gladly. Sorry about the grumpy smillie it just appeared and I don't seem to be able to get rid of it.

 

[ianj99]

Can anyone answer a question for me, can I mix a 130Ah battery used for powering the lighting instruments etc with a 75Ah one used to start the engine will both be fully charged from engine and/or shore power, will both re-charge to their full capacity each time. I have this impression that both batteries must be of similar Ah rating. Not being electrically minded I am not sure if they are on a split charging system or what.

I've come to the conclusion that the best way is to disconnect the engine battery from the charging circuit after say 30mins. (Starter batteries do not lose much charge during cranking assuming the engine is quick to start so will be topped back up again in a few minutes). If a remote sensed alternator is fitted it will need to sense the baot battery not the engine battery)

This method has two advantages:
No risk of gassing and water loss
The full charger/ alternator output is available for re charging the boat battery once you have disconnected the engine battery and because the charger / alternator is sensing the boat battery, this should not be in danger of overcharging.

 

[pvb]

I've come to the conclusion that the best way is to disconnect the engine battery from the charging circuit after say 30mins. (Starter batteries do not lose much charge during cranking assuming the engine is quick to start so will be topped back up again in a few minutes). If a remote sensed alternator is fitted it will need to sense the baot battery not the engine battery)

This method has two advantages:
No risk of gassing and water loss
The full charger/ alternator output is available for re charging the boat battery once you have disconnected the engine battery and because the charger / alternator is sensing the boat battery, this should not be in danger of overcharging.

I really don't think this is necessary.

 

[Plevier]

Starter batteries do not lose much charge during cranking assuming the engine is quick to start so will be topped back up again in a few minutes

The first statement is correct - typically around 1Ah or even less, including heater plugs.
The second statement isn't. Lead acid batteries at high states of charge accept the last bit of charge very slowly and at very low charge efficiency (tending to 0% in fact when fully charged!).
If you have an ordinary alternator at around 14V without a smart regulator jacking it up higher, it's not a good idea to disconnect the starter battery. It won't overcharge and will only take milliamps away from the charge available for the house battery. Undercharging is a more common problem and probably more damaging. Lots of lightly used car and boat batteries tend to settle at something like 85% state of charge - not good for them.
If you have a smart regulator or a multi stage shore power charger, then there is a potential problem as mentioned in earlier posts.

 

[wotayottie]

Can anyone answer a question for me, can I mix a 130Ah battery used for powering the lighting instruments etc with a 75Ah one used to start the engine will both be fully charged from engine and/or shore power, will both re-charge to their full capacity each time.

Likely yes. Only caveat is that they need to be broadly the same sort of construction. Wouldnt mix a gel lead acid with an open lead acid but even then the differences will be minor. The AH capacity is irrelevant.

 

[ianj99]

I really don't think this is necessary.

It isn't 'necessary' but possibly still worthwhile.

My findings are in agreement with Mawm's - post 117.

My Sterling regulator senses the boat battery and therefore cannot maintain the engine battery at the float voltage of 13.6 whilst doing its job and charging the boat battery to 14.9v.

Same applies to my Sterling digital mains charger.

There isn't a problem charging an engine battery to 85% - this has been the case with automotive charging (and most marine) systems for years. The important point is to restore them to 85% quickly - or enough for the next restart which is what happens in practice as batteries age.

Over charging, over discharging and not recharging a flat battery are what are responsible for shortening the life (& capacity) of a battery. As well as leaving one low on electrolyte due to it having been overcharged.

The reason for digital / smart systems is to maximise the capacity and speed up recharging thereby reducing the frequency and duration of charging requirements.

 

[KellysEye]

We charged two 6v Trojan lead acid deep cycle batteries in series and three gel cranking batteries for the engine, generator and anchor winch. We used smart chargers that you can set to identify by number the different types of batteries on the 12 and 240v supplies. So you can mix different types if you want to and different capacities are fine. Obviously you have to take the regulator off the alternator.

 

[Houleaux]

Over charging, over discharging and not recharging a flat battery are what are responsible for shortening the life (& capacity) of a battery. As well as leaving one low on electrolyte due to it having been overcharged.

Slight thread drift, but how easy is it to overcharge batteries? I have an ancient on board charger that puts a steady 3.5 amps into a pair of 110Ah leisure batteries. I only ever use it when I'm on board the boat, but could this do any damage if I left in on too long?

 

[wotayottie]

Slight thread drift, but how easy is it to overcharge batteries? I have an ancient on board charger that puts a steady 3.5 amps into a pair of 110Ah leisure batteries. I only ever use it when I'm on board the boat, but could this do any damage if I left in on too long?

Would be really surprising if you managed to overcharge two big batteries with that tiny charger but if the charger has no upper voltage limit and if you just left it connected long term I suppose its theoretically possible. All that would happen is that the voltage would slowly rise until it got to the level at which gassing would ocurr. You would then gradually gas off all the water and your batteries would dies for lack of it.

Much more likely is that the 3.5 amps is barely keeping up with the natural rate at which your batteries lose charge anyway. But then what do you want such a feeble old charger for? Better to buy yourself a good charger with p[roper controls that allow you to forget it altogether.

 

[Houleaux]

Would be really surprising if you managed to overcharge two big batteries with that tiny charger but if the charger has no upper voltage limit and if you just left it connected long term I suppose its theoretically possible. All that would happen is that the voltage would slowly rise until it got to the level at which gassing would ocurr. You would then gradually gas off all the water and your batteries would dies for lack of it.

Much more likely is that the 3.5 amps is barely keeping up with the natural rate at which your batteries lose charge anyway. But then what do you want such a feeble old charger for? Better to buy yourself a good charger with p[roper controls that allow you to forget it altogether.

Thanks for your reply. I have a BM1 battery monitor so I can see what's going into and out of the batteries together with their voltage at any time. The voltage remains in the 13V + region when connected to the charger - never as high as 14V - so can I assume that it's doing no damage? The rate of charge seems to stay at about the 3.5A no matter how long its been on - when I'm not drawing any power that is. I'd have expected it to fall as the batteries were charged, but then I don't claim to be an expert!

I agree that a new charger would be better but my electrical consumption isn't high enough to justify the cost. This one was fitted to the boat when I bought it and is sufficient for my needs. I'd just like to know if it could do any damage if, by mistake, I left it running for 24 hours or so.

 

[Plevier]

Thanks for your reply. I have a BM1 battery monitor so I can see what's going into and out of the batteries together with their voltage at any time. The voltage remains in the 13V + region when connected to the charger - never as high as 14V - so can I assume that it's doing no damage? The rate of charge seems to stay at about the 3.5A no matter how long its been on - when I'm not drawing any power that is. I'd have expected it to fall as the batteries were charged, but then I don't claim to be an expert!

Something fishy here.
220Ah of capacity should not take anywhere near 3.5A float current at under 14V when fully charged, just a fraction of an amp, maybe 0.1-0.2.
If they were really taking 3.5A they would be gassing a lot.

 

[Mistroma]

The voltage remains in the 13V + region when connected to the charger - never as high as 14V - so can I assume that it's doing no damage? The rate of charge seems to stay at about the 3.5A no matter how long its been on - when I'm not drawing any power that is. I'd have expected it to fall as the batteries were charged, but then I don't claim to be an expert!
...
I'd just like to know if it could do any damage if, by mistake, I left it running for 24 hours or so.

Something fishy here.
220Ah of capacity should not take anywhere near 3.5A float current at under 14V when fully charged, just a fraction of an amp, maybe 0.1-0.2.
If they were really taking 3.5A they would be gassing a lot.

Houleaux said 13V, never as high as 14V. Data for my Trojan T105s says self-discharge approx. 0.6% (varies with temp. of course) and float charge at 13.2V, 1%-3% of C/20.

So for 220Ah batteries, that puts self-discharge around 1.3A and float at 2.2A-6.6A. A steady 3.5A sounds reasonable on that basis. Of course OP hasn't said anything about make and type of battery.

He did mention "if I left it running for 24 hours or so". I can't see a couple of days at 13V, 3.5A float causing any damage to flooded deep-cycle batteries.

Perhaps Houleaux can give more information about the batteries (sealed, etc.)

 

[Houleaux]

Houleaux said 3V, never as high as 14V. Data for my Trojan T105s says self-discharge approx. 0.6% (varies with temp. of course) and float charge at 13.2V, 1%-3% of C/20.

So for 220Ah batteries, that puts self-discharge around 1.3A and float at 2.2A-6.6A. A steady 3.5A sounds reasonable on that basis. Of course OP hasn't said anything about make and type of battery.

He did mention "if I left it running for 24 hours or so". I can't see a couple of days at 13V, 3.5A float causing any damage to flooded deep-cycle batteries.

Perhaps Houleaux can give more information about the batteries (sealed, etc.)

A mixture actually! Two different makes (sorry can't remember what, but budget(ish)). One can be topped up but the other's sealed.

If it helps, the BM1 reads zero amps when there's no load on the batteries (so no allowance for self-discharge) and 3.5A in the same circumstances when the charger's on. I can't remember exactly what the voltage is after a lengthy period, but I think it might be more than 13.2V.

Looking at another post, I'm wondering if I should splash out £13.99 on a new charger! It seems that that one (Lidl) is a bit "smarter" than my old relic is likely to be! But why is it only suitable for batteries of up to 120Ah (sorry - further thread drift!)?

 

[VicS]

Looking at another post, I'm wondering if I should splash out £13.99 on a new charger! It seems that that one (Lidl) is a bit "smarter" than my old relic is likely to be! But why is it only suitable for batteries of up to 120Ah (sorry - further thread drift!)?

At 3.8 amps max it would take an unrealistically long time to recharge anything larger.

120 Ah is pushing it a bit IMO . 60Ah would be a a more sensible limit for it.

OK for just maintenance charging though.