Battery capacity

[Boldly]

I have removed both leisure batteries (105ah lead acid)and subjected them to a 2 amp load for 20 hours (spare bulb just happened to be exactly 2 amps). I forgot to take the starting voltage of battery no 1 but no 2 was 12.9v and the solar controller said the batteries were full so assume the same.
Quite remarkably they both dropped by a very similar amount 12.35v and 12.34v.
I think that this means they are both about 56% full. So does this mean 56% of 105 amp hours?
Or does it mean that the 40 amps used was 44% of capacity and they are both now only 91ah batteries.
They were cheap and are 2 1/2 years old.

 

[oldiesontour]

If the batteries are connected together then the power used will come equally from both as will the voltage you are reading so effectively can be seen as one battery of nominal 12v 210ah...(Assuming both batteries are in the same electrical/chemical/mechanical condition)
Voltage is a guide to state of charge but battery needs to be disconnected from charge and load for a while before testing

 

[Boldly]

Batteries were removed from van and discharged and checked separately as it says in op.

 

[oldiesontour]

Misunderstood question power used was % of available AH, you've lost around 14ah capacity by your measurement in 21/2 years I would expect a bigger loss in.capacity

 

[AdrianChen]

Broadly I think you're correct, and they have lost some capacity over the 2.5 years.

40 amps used has reduced the state of charge from 100 to around 75% (On my chart) so total capacity would be 157AH or 78AH each (ish)

I would say that the starting voltage of 12.9amps is probably not the resting voltage of the battery though as it seems a little high, best left disconnected from charge/load for a couple of hours at least to allow time for any "surface charge" to dissipate.

Using a 2 amp load will give a slightly higher capacity compared to the battery label because for the label the batteries are probably rated at C20 ie 1/20th =10.5amp. The higher the load the less capacity a battery has, so 2 amps will "overstate" the result a little compared to how the battery was specified.

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[autorouter]

Or does it mean that the 40 amps used was 44% of capacity and they are both now only 91ah batteries.

Yes it looks like they are now both only 91Ah capacity. However that was measured using 2A, so at 91Ah that means would last for 45.5 hours until totally flat. So you are discharging at the 45-hour rate, not the standard 20-hour rate. So the 91Ah figure will be a bit optimistic for the 20-hour rate, which might be more like 85Ah. Batteries are normally specced at the 20-hour rate for comparison purposes.

That's a reduction of 20Ah from the nominal 105Ah, so just less than 20% loss. Is that OK? Well, if you were hiring that motorhome out, your customers would expect that if the loss was anything worse than 20% you would replace them. Since you're the owner, it's up to you when you replace them. Depends entirely on how you use them, and how much of a problem it causes. Another consideration is that the test is not very accurate anyway, so the loss might really be between 10% and 30%.

 

[68c]

Get a Battery Load Tester.

A typical one. NOT THE ONE I HAVE.

KOCAN 150W 20A Adjustable Constant Current Electronic Load 2.4inch TFT Color Display 4 Working Modes USB Lithium Battery Capacity Monitor Tester
£33.99
+£4.49 delivery
Amazon.co.uk
Tester · Battery Capacity · Constant Current

Can be used on all sorts of batteries, just set your cut-off voltage and the Constant Current ( amperage) you want to draw. Typically 10% of the Capacity so 10Amps for a 100AH leisure Battery.

 

[Boldly]

Yes it looks like they are now both only 91Ah capacity. However that was measured using 2A, so at 91Ah that means would last for 45.5 hours until totally flat. So you are discharging at the 45-hour rate, not the standard 20-hour rate. So the 91Ah figure will be a bit optimistic for the 20-hour rate, which might be more like 85Ah. Batteries are normally specced at the 20-hour rate for comparison purposes.

That's a reduction of 20Ah from the nominal 105Ah, so just less than 20% loss. Is that OK? Well, if you were hiring that motorhome out, your customers would expect that if the loss was anything worse than 20% you would replace them. Since you're the owner, it's up to you when you replace them. Depends entirely on how you use them, and how much of a problem it causes. Another consideration is that the test is not very accurate anyway, so the loss might really be between 10% and 30%.

Would you consider a 20% loss in 2 1/2 years normal?
Is it possible to speculate further depreciation with any accuracy?
Anyone know something I may have that takes 5 amps?

 

[Richard n Ann]

Would you consider a 20% loss in 2 1/2 years normal?
Is it possible to speculate further depreciation with any accuracy?
Anyone know something I may have that takes 5 amps?

Why are you so bothered? Are the batteries going flat?

I've got a good battery monitor and I can see the battery discharging and charging back up every day, when the battery starts getting tired I will see it. The daily use is a capacity test.
It's just over 5 years old now and there's no noticeable drop in capacity.

 

[Boldly]

Why are you so bothered? Are the batteries going flat?

I've got a good battery monitor and I can see the battery discharging and charging back up every day, when the battery starts getting tired I will see it. The daily use is a capacity test.
It's just over 5 years old now and there's no noticeable drop in capacity.

View attachment 956051

The reason I wanted to know is that we only have cheap domestic Curry's fridge bought seven years ago. Whilst on the Costa del sol and Costa del la luz last week was getting strange readings. Fully charged one minute then 70% the next, then fully charged again.
Was trying to work out whether the fridge was kaput or just struggling with the 36° heat or if the batteries were shot. Sounds like probably a combination of all three!

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[autorouter]

Typically 10% of the Capacity so 10Amps for a 100AH leisure Battery.

If you get one of these battery testers, those settings would give you the 10-hour rate. Usually the 20-hour rate is used on the battery label as the nominal capacity. Sometimes the 5-hour rate and/or the 100-hour rate is also given. I've never seen a '10-hour rate' spec.

I would set the amps to 5%, and set the cutoff voltage to about the 50% level, to avoid over-discharging the battery. That will give you the 20-hour rate.

 

[Richard n Ann]

The reason I wanted to know is that we only have cheap domestic Curry's fridge bought seven years ago. Whilst on the Costa del sol and Costa del la luz last week was getting strange readings. Fully charged one minute then 70% the next, then fully charged again.
Was trying to work out whether the fridge was kaput or just struggling with the 36° heat or if the batteries were shot. Sounds like probably a combination of all three!

What were the strange readings taken off? Cheap battery monitors aren't accurate because they don't use a shunt. Even the more expensive ones aren't perfect. Mine for instance doesn't know the battery is a gel type so it says the battery is fully charged before it actually is.

 

[Boldly]

Readings were from epever solar controller.

 

[autorouter]

Fully charged one minute then 70% the next, then fully charged again.

How are you measuring the percentage charge level? Is it a voltage reading or an amp-hour reading with a shunt/battery monitor? And presumably if you have a domestic fridge it is running from an inverter. How big is the inverter,and are you running other things from it?

Edit: I see you are using the solar controller, which presumably works from the voltage level. That's not a very good indication of charge level if there is any charging or heavy loads. The 'resting voltage', with no loads or charging for an hour or so, will give you a reasonable indication of charge level, but voltage while charging or powering an inverter is often very misleading.

 

[Boldly]

Even with a shunt you can only measure what goes into and out of the batteries. If your batteries are not the capacity you think they are then you have no way of knowing how much power is left.

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[Lenny HB]

Would you consider a 20% loss in 2 1/2 years normal?
Is it possible to speculate further depreciation with any accuracy?
Anyone know something I may have that takes 5 amps?

20% loss is way higher than normal and it is probable worse than that as you had a very light load test.

For lead ebattery's 2.5% per year is normal loss.

Even with a shunt you can only measure what goes into and out of the batteries. If your batteries are not the capacity you think they are then you have no way of knowing how much power is left.

Providing you set the shunt up correctly it will be fine.
When I had lead batteries I adjusted the capacity on the shunt yearly to allow for natural battery capacity loss.

 

[Boldly]

How are you measuring the percentage charge level? Is it a voltage reading or an amp-hour reading with a shunt/battery monitor? And presumably if you have a domestic fridge it is running from an inverter. How big is the inverter,and are you running other things from it?

Edit: I see you are using the solar controller, which presumably works from the voltage level. That's not a very good indication of charge level if there is any charging or heavy loads. The 'resting voltage', with no loads or charging for an hour or so, will give you a reasonable indication of charge level, but voltage while charging or powering an inverter is often very misleading.

That is why I removed and tested the batteries separately in the house. I must confess to being ignorant of the 20 hour rule so it was not as scientific as I thought.
The solar controller has a default temperature setting of 25° and I think that may have a bearing on what was happening. The fridge should have been replaced already but we could not agree on which one.
I think with a change of fridge the batteries will probably last for another year given how and when we use the van.
Maybe by then lithium will be a more affordable option.

 

[Boldly]

20% loss is way higher than normal and it is probable worse than that as you had a very light load test.

For lead ebattery's 2.5% per year is normal loss

Thanks that was what I wanted to know.

 

[headlight]

The reason I wanted to know is that we only have cheap domestic Curry's fridge bought seven years ago. Whilst on the Costa del sol and Costa del la luz last week was getting strange readings. Fully charged one minute then 70% the next, then fully charged again.
Was trying to work out whether the fridge was kaput or just struggling with the 36° heat or if the batteries were shot. Sounds like probably a combination of all three!

Was that because the fridge was cycling on and off ?
20% is a fair loss of capacity especially as the test load was only 2 amps so could be nearer 30%.
Also in my experience batteries lose capacity faster and faster as they age rather than a constant capacity loss each month/year.