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mikejulietgolf avatar image
mikejulietgolf asked

Gel batteries behaving strangely, any ideas?

Apologies for the length of this post but I wanted to include what I feel is relevant information.

I have a bank of 3 x BAT412121104 gel batteries on my sail boat, they are linked up to a 12/40 Centaur Charger which is left on while we’re away from the boat. I use a ODELCO DCC 4000 battery monitor to see what the batteries are up to. When the batteries and charger were installed 3 years ago I recalibrated the battery monitor to 390 amp/hours.

When the batteries 2018 were new the battery monitor was reading 376 amp/hours, after a year (2019) this was 378-384 amp/hours but the next year (2020) around 365 amp/hrs. This year (2021) they seemed to deteriorate quickly down to 270 amp/hrs.

One weekend while moored for 48 hours the voltage got down to 12 volts and we had to charge with the engine. While on the mooring we were using about 2 amp hours for the fridge and a bit of led lighting at night. After that episode the batteries were still reading 270 amp/hours after charging.

Last weekend after anchoring for 24 hours the windlass dragged the voltage way down causing the nav instrument to turn off, the engine was running but that didn’t seem to have any impact.

But after recharging the batteries were reading 309 amp/hours and each time they are charged (I turn the charger off overnight due to the constant noise) they seem to gain 10 – 20 amp/hours and when we left the boat were reading 351 amp/hours and 12.77 volts after being off the charger over night. The people who fitted the batteries said it was to be expected that the batteries would loose 30% of their capacity after 3 years, which was a bit disappointing as I was expecting a significantly longer service life than that.

Sorry for the long story but can anyone give me a clue as to what’s going on? Do I need to replace the batteries or have they staged a miraculous recovery?

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markus avatar image
markus answered ·

Hi,

the best way to look how good your batteries still are, is to do a capacity test.

For this, you have to fully charge your batteries, let them rest for a few hours and then discharge them again with a constant and known load. Ideally C20 or C10 (depends on how the Ah rating of your batteries is specified) until end of discharge Voltage is reached.

Gel batteries are very sensitive to over charging or too high charge rates. You have to make sure, they are charged with the correct Voltage. I have seen some gel batteries die even sooner than in 3 years, because they had been charged with 14.7V instead of 14.4V

Also discharging them too quickly is a problem. Windlass can be a very heavy load and may exceed the current, the batteries can well deliver. (Just guessing, I don't know the exact specs of your windlass)

BR


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mikejulietgolf avatar image
mikejulietgolf answered ·

The windlass is 1000 watts and was running for less than 5 minutes. It’s never been a problem before. The Centaur Charger is on the gel battery setting.

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markus avatar image markus ♦♦ commented ·

"It’s never been a problem before."

Yes - but with batteries degrading, such high loads (1000W/12V=83Amps) can get a problem.

I suspect your batteries are dead. To proof that, do a capacity check.

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mikejulietgolf answered ·

@Markus

Thanks for the response.

“For this, you have to fully charge your batteries, let them rest for a few hours and then discharge them again with a constant and known load. Ideally C20 or C10 (depends on how the Ah rating of your batteries is specified) until end of discharge Voltage is reached.”

not sure what C20 or C10 means. What do I need to look for as the Ahr rating, where do I find the ”the end of discharge voltage“ ?

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markus avatar image markus ♦♦ commented ·

C20 means: Capacity in Ah divided by 20 = discharge current in Amps.

Also called the 20hr discharge rate

See here for end of discharge Voltage: https://www.victronenergy.com/upload/documents/Datasheet-GEL-and-AGM-Batteries-EN.pdf

BR

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markus avatar image markus ♦♦ markus ♦♦ commented ·
It might not jump in your face directly when you read the datasheet.

End of discharge Voltage is 10.8V ;o)

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JohnC avatar image
JohnC answered ·

Hi @MikeJulietGolf

I'm going to suggest that your batts may still be ok, and that your monitor is deceiving you. I don't know that device. but anything that only counts and shows Ah isn't good enough to rely upon.

I don't really want to delve here too much, but one example is that an Ah in is different to an Ah out, in real energy terms, by virtue of having different Voltages. Does it compensate for the speed of discharge too?

It's useful as a rough guide only, and can't provide anything worth using for assessing battery health.


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mikejulietgolf avatar image
mikejulietgolf answered ·

@JohnC

thanks for the response. I’ve always wondered what how the amp were being calculated I use it as it’s the default display it also does Volts to 2 decimal places that I’ve checked with a multi meter and it’s pretty accurate. Am I right in thinking if I’ve got 3 x 130 amp hour batteries at 12.8 volts that I’ve got 390 amp hour, or do I need to go through the the capacity test routin?

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JohnC avatar image JohnC ♦ commented ·

@MikeJulietGolf

Bear in mind that 390Ah is under lab conditions (temp is important too). Then to achieve that they draw them down to a V you can't live with. If you look at the battery specs it may be quoted at C20, then the figures will be different at C10 or C100. So it's really just a reference point.

Given your batts are 3x parallel (edited), if there's deterioration then it will likely show as a Temp variation between them as resistance usually varies. If, after a strong charge session, you can detect an obvious T-diff with your fingers, then they're likely on the way out. If not, roll on..

Make sure they're charged fully regularly, by checking how much charge they can accept at say Float V. The 'Tail', and it should taper off to a point where it won't go lower in a reasonable time. What your monitor's Ah reading will show at this point I can't say, but if it isn't near the same every time, then it's probably just showing you how inaccurate counting Ah alone can be..

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