Leisure battery capacity testing

May 27, 2020
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Afternoon all

After the previous PV regulator gave up the ghost (wasn't regulating voltage so we had 17V being pumped straight into the battery!) we replaced it with a new MPPT controller.

I'm trying to work out how much damage it has done to the leisure battery. I let the new controller charge up the battery to full, then rested it for 24 hours and have now done a load test to roughly estimate capacity.

I left 72W (6A) running for 5.5 hours - 33A total draw.
On my 110Ah battery that would equate to 30% if it was working at optimum.

I was expecting a new resting voltage of around 12.3/4. However resting voltage is coming out at 12.6V?! I've checked with another multimeter and same result. The only thing connected to the battery now is the alarm, everything else is off.

Am I missing something here or do I have the world's most impressive battery?!

EDIT: I've just checked it again after 15 mins and is now showing 12.65V. 😳
 

JTQ

May 7, 2005
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Am I missing something here or do I have the world's most impressive battery?!
Well, yes to the first point. You are probably using some generic SoC vs voltage table or curve pulled off the internet, not the characteristics specific to your battery.
Modern batteries, decent ones anyway, their construction and alloying in the lead, tend to have quite a bit higher voltages than the batteries of long past.
For example a Varta LDF and its Bosch clones will have a full charge rested voltage up at 12.8 Volts, whereas years back and now with some, they could be as low as 12.65 at 100% SoC.
What was yours, 24 hours after fully recharging it, that gives us a datum to better judge things?

Edit: I suspect you have not done it much damage at all by the high voltage. It could have been a characteristic of the solar controller on initial charging, as a brief conditioning routine.
 
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May 27, 2020
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24 hours after resting it was at 13V. Where would I find a graph for the battery? Its a bog standard 110Ah battery that's at least 18 months old. Putting the model into Google comes up with nothing.

Previous regulator had definitely gone. It wasn't just high voltage during the bulk charging phase, it was all the time. Several months of charging at 16.5-17V (only 1-2A mind). I can tell this from my Bluetooth battery monitor as it has a 1 year history.
 
Nov 11, 2009
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24 hours after resting it was at 13V. Where would I find a graph for the battery? Its a bog standard 110Ah battery that's at least 18 months old. Putting the model into Google comes up with nothing.

Previous regulator had definitely gone. It wasn't just high voltage during the bulk charging phase, it was all the time. Several months of charging at 16.5-17V (only 1-2A mind). I can tell this from my Bluetooth battery monitor as it has a 1 year history.
It’s not easy to obtain the curve for a battery but the most realistic way is to finish charging it. Take it out of circuit completely and then monitor its voltage regularly over a period of time. Then you will see how quickly or slowly it drops its charge voltage.
 
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JTQ

May 7, 2005
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I am confident that 13 Volts indicates it despite the 24 hrs resting, is still carrying a fair bit of "surface charge"?

The battery manufacturer, if known, could furnish you with the correct curves, but possibly would not chose to oblige.
Basically the point I tried to make, is these generic tables are very far from a reliable database to make the sort of judgements you are trying to draw.

My Varta data sheet only gives me the 100% figure, of 12.8 volts, not the part charged values, but in itself proves some generic figures like 12.65, are way off the reality.
 
May 27, 2020
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Clive, my understanding is that that wouldn't give an indication of actual capacity (Ah), as even a abused battery could be brought up to full charge for its remaining capacity?

JTQ - I'll have to see what I can find. I just really wanted to see how much capacity the battery has left and whether its worth investing in a replacement for our off grid journeys this summer (COVID dependent, of course).
 

JTQ

May 7, 2005
1,818
68
19,735
Clive, my understanding is that that wouldn't give an indication of actual capacity (Ah), as even a abused battery could be brought up to full charge for its remaining capacity?
Right.

JTQ - I'll have to see what I can find. I just really wanted to see how much capacity the battery has left and whether its worth investing in a replacement for our off grid journeys this summer (COVID dependent, of course).
Well, I thought it looked promising, even if we can't be specific about how good. I would be keeping my credit card safely tucked away.
 
Nov 11, 2009
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Clive, my understanding is that that wouldn't give an indication of actual capacity (Ah), as even a abused battery could be brought up to full charge for its remaining capacity?

JTQ - I'll have to see what I can find. I just really wanted to see how much capacity the battery has left and whether its worth investing in a replacement for our off grid journeys this summer (COVID dependent, of course).
Yes it could be bought up to charge but voltage drop over time can be a useful health indicator. Or you could bring it up to full charge and then do a load test by connecting a small load 5 amps for say five hours. Disconnect, leave for four hours and measure no load volts. But a trip out after lockdown to see how you get on might be more practical and enjoyable.

The link from Sargent gives further information.

 
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May 27, 2020
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Thank you. The test you mentioned is exactly what I done this morning, hence the post. Think I'll leave the battery at the moment and give it a go over the next few breaks away.

Cheers all 👍🏼
 
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