Battery voltages

[MikeB_374236518]

What is the battery voltage for a fully charged battery and one when it needs charging.

I took the voltage of a battery after fully charging 13.4v, left it for 24hrs and checked again 12.54v. I then ran two 50w bulbs for 2hrs and the voltage was 12.18v. The bulbs where still shining brightly. Checked 24hrs later and the voltage was 12.29. Is this a good battery?

 

[antrimboy]

Those voltages look good to me. Check it again in about one week, unless you need to use it before that, and if it's still over or close to the 12V., it should be fine.

 

[rob_jax]

Mike,

I'm not sure.

Assuming you have a standard lead-acid battery - that will have a no load voltage of 12.6 volts. Some modified chemistry batteries (notable AGM sealed leisure batteries) have a no load voltage of 12.9 volts.

But going with 12.6 volts for 100%, 75% charge is around 12.4 volts, 50% around 12.2 volts, 25% is around 12.1 volts, and less than 12 volts is empty... Note that these are readings for the battery under no-load conditions.

Two 50 watt bulbs for 2 hours is 2 x 50 x 2 or 200 Watt Hours - which when divided by 12 volts is 16+ AH

So your battery went from 12.54 volts - that's around 90% charged (as expected if you used a caravan charger) - to 12.29 volts - that's around 65% charged.

And finally 16 AH is equal to (90 - 65) or 25% of the battery capacity - or 65 AH.

So if your battery is around 65Ah - then that's perfect, but if it's a 110AH battery then it has a few problems, and these might be rectified by using a "proper" multi-stage charger...

Robert

 

[EmmanuelMuldoon]

Here are some figures I wrote down ages ago that came from someone who seemed to know what they were on about.

After charging, leave to stand for 30min.

If it then measures 12.8V its "full"

if 12.4V it's 75% full

if 12.2V it's 50%

if 12.0 it's 25%

all measured off load.

An alternative approach is to actually measure the capacity. This is what I do for a car or leisure battery.

I connect a load of about 35W, measure & note down the voltage & current.

Leave it on & measure again every hour or so.

When it gets down to 10.4V it's flat.

If it's managed, say, to average 3A for 30 hrs then it's capacity if 90Ah.

Obviously I don't get up all through the night!, but if I have started one morning & it's still going ok late evening & the next morning, you can extrapolate the readings. Plot them out on a graph for more accuracy.

 

[rob_jax]

Just remember that you need to measure the voltage "off-load" - otherwise there's no point in doing such a test...

Robert

 

[MikeB_374236518]

Thanks everyone for your replies.

I should have given the reason I posted this this question in my first post. I purchased a 110 elecsol as it was given a good write up in one of the mags. The battery arrive with a lot of the electrolight missing having been tipped up. I was sent another one which arrived with a little electrolite missing. The ecosol people said if I toped up the original battery It would most probably be OK, the previous results are from this battery.

The following is the results of the same test on the second

battery. After charge 13.21v, 24hrs later 12.67v, after load 12.27v and 24hrs later 12.38v.

When I was looking for the name of the battery I found the warranty card, which I should have read as it states a full battery is 12.8v, 50% discharged 12.4v and fully discharged 10,7v.

Further comments will be appreciated.

 

[rob_jax]

Hi Mike,

Forget the "after charge" readings - they just tell you the memory of the charging process as it drops away to the expect "off-load" readings.

Elecsol batteries are one of the makes that use a modified chemistry - so yes, that would have been useful info up front - and 100% charge is around 12.8 volts.

If your battery is only charging to 12.67 volts, then your charger isn't fully charging the battery - only to around 85% - which sounds right for a caravan charger, but not optimum.

Robert

 

[MikeB_374236518]

Robert,

Sorry to go on about this but I can't get my head around this stuff called electricity. I know some people find it simple but I simply can't understand it. I started this thread to find out if my two batteries are faulty and should be returned to the supplier. Sorry to take so long to get to this point but not understanding the subject I don't know what information you need.

I charged both batteries on a bench with a Halfords fully automatic battery charger bought for this purpose as I wanted them fully charged. All voltage readings are off load.

Your post 26th. You said two 50w bulbs for two hrs = 16+ ah, what voltage loss does this relate to. Also you said a 65ah battery is OK but 110ah has a few problems.

What would you do in my situation?

 

[rob_jax]

Hi Mike,

You obviously feel that there is something wrong with the battery or you wouldn't have posted. And from the information, it's possible to estimate how well the batteries are working.

For the top 75% of the battery's capacity - the no-load voltage drops away linearly with use. Use an amount of power and the voltage drops a bit, use exactly the same amount again and the voltage drops by the same amount.

So that's like looking at people walking down a smooth hill - you can see if someone is half-way up - and every time they walk down 10 metres, that's always the same percentage of the total hill.

So if we think of the expected no load voltage from an Elecsol battery as the top of the hill - 12.8 volts, and the 12 volt reading at 25% is close to the bottom of the hill.

From those two measurements we can figure out that your "fully charged" reading of 12.67 volts is quite near the top - but not as close as it should be (Battery charger - later)

Likewise after running the two light bulbs for a couple of hours - we can work out how much power has been used up - in Amp Hours.

You also reported the no-load voltage after that experiment - and that tells us another point on the hill. The voltage dropped from 12.67 to 12.38 volts - a loss of 0.29 volts

A drop from 12.8 volts to 12.0 - a loss of 0.8 volts is 75% of the hill - so your 0.29 volts is 27% (75% x 0.29 / 0.8)

Battery capacity is measured in Amp Hours - a battery that can supply 1 amp for 110 hours would have a capacity of 110 Amp Hours - or 5 amps for 22 hours would be the same...

From your experiment we know what percentage of the "battery capacity hill" has been used (27%) - and we know the power that equates to in Amp Hours. So it's easy to calculate that the storage potential of the battery to be around 65 AH

Your battery should have a capacity of 110 AH - but it only seems to have a power of 65 AH - which is rather less than it should be - only a bit more than 1/2 in fact.

Why should that be?

1. You bought a fully automatic battery charger (Halfords) which is good - but you need to make sure you switched the charger to the leisure battery mode - you need to look at the manual

Otherwise the battery charger will try to charge the battery as if it is a normal car battery - which tops out at 12.6 volts.

The leisure battery mode increases the battery charging voltage by only 0.3 volts - but this is enough to completely charge the battery.

2. The battery isn't holding a complete charge because it has been allowed to stand in a discharged (or part charged) state.

Batteries run down by themselves - so leave a battery to itself for a few months - and it'll discharge itself. So your leisure battery needs a regular charge during the months it lies unused.

If you don't then the battery will lose capacity - a chemical reaction occurs in the battery called "irreversible Sulfation" and this robs your battery of capacity.

And this would seem to have happened - at least partially.

So keep the battery charged - and your charger may have a third mode to remove "irreversible Sulfation" - check the manual, useful if it does.

What to do?

Check the charger manual for different modes of use - and try recharging the battery using the special leisure battery mode.

If the charger doesn't have these other modes - then it's a dedicated charger for a car battery - not ideal, but still capable of charging the battery to close to it's peak (around 90% - so your battery will never actually have a capacity of 110 AH - but only around 95 AH)

Robert

 

[waffler]

Don't forget that air temperature affects voltage. Colder days lower volts. During charging the cells get hot and so can give a false high. I've noticed a 0.2 volt difference between evening and morning due I think to temperature.

 

[rob_jax]

Hi Chris,

There is a very significant temperature dependent current coefficient - lead acid batteries lose a significant amount of capacity as the temperature drops.

However there is almost no temperature coefficient - it's around -4 milliVolts per degree C

Robert

 

[EmmanuelMuldoon]

Think there are two of us with the "handle" Chris on here!!

I last posted on the 26th, but another Chris posted on the 29th!!

Moderators???

 

[MikeB_374236518]

Robert

Thanks for your reply, I understand your hill analogy but it's the maths I have trouble with. The reason I am skeptical about the batteries is that both of them had electrolight spilt in transit.

The blurb on the front of the charger states that it is suitable for all petrol and diesel cars and deep cycle leisure batteries. It has two switches one labeled battery type, standard or sealed and the other vehicle type, bike or other. The switches are set to standard and other. I wonder if the charger can fully charge my batteries.

On monday I will take one of my batteries to the local garage to see if they can fully charge it.

Thanks again for your help.

 

[rob_jax]

The Elecsol batteries aren't sealed, but they use the specialist chemistry of a sealed battery.

So I'd suggest charging them using the "sealed" setting - and see if that charges them fully

Robert

 

[MikeB_374236518]

Robert,

I tried to Fully charge the battery to the recommended 12.8v using the sealed option as you suggested but it only reached 12.67. So I will report back when I've had a word with my local garage. It could be my chargers not up to the job!!!!

 

[MikeB_374236518]

Robert,

Still confused. I got the battery back from the garage yesterday and the said it was OK as the put it on their machine that checks each sell. I checked the voltage to-day 12.57v. According to Elecsol the fully charged rate should be 12.8.

Should I just accept that the full charged rate is 12.67v that my charger charges to and get on with things I understand?

 

[rob_jax]

Hi Mike,

Their testing has obviously discharged the battery a little bit - as you would expect.

I'd probably continue using it - I think Elecsol have a sensible warranty period so if it does fail earlier than expected, that shouldn't be a problem.

And maybe we should be looking at the voltmeter - is this an external meter (make, model, rough idea of price paid?) or one provided with the caravan? It could be that your voltmeter needs calibrating...

Robert

 

[MikeB_374236518]

Hi Mike,

Their testing has obviously discharged the battery a little bit - as you would expect.

I'd probably continue using it - I think Elecsol have a sensible warranty period so if it does fail earlier than expected, that shouldn't be a problem.

And maybe we should be looking at the voltmeter - is this an external meter (make, model, rough idea of price paid?) or one provided with the caravan? It could be that your voltmeter needs calibrating...

Robert

Robert,

There is a five year warranty on the batteries so I think I will wait and see what happens.

My brain is beginning to ache so I shall go and lie down in a darkened room.

Thanks for all your help, I think I understand batteries a bit better now!!!!

 

[MikeB_374236518]

Robert,

There is a five year warranty on the batteries so I think I will wait and see what happens.

My brain is beginning to ache so I shall go and lie down in a darkened room.

Thanks for all your help, I think I understand batteries a bit better now!!!!

I posted this as a comment by mistake and you might have missed it. Thanks again.