What to expect from my batteries

[cpedw]

"I would expect a rested ( ie no load for 24 hours ) volts of about 12 .2 when 50% discharged"

Does it matter if a battery drops below 12.2 when actually in use, say running a fridge?

Even from 12.7 mine drop to 12.4 when the 40w fridge is on which gives a very low "range of use" :

12.7 = 12.4 fridge on
12.6 = 12.3
12.5 = 12.2
12.4 resting =12.1 fridge on = > 50% discharged?

The voltage when current is flowing (e.g. fridge on) is reduced because of the battery's internal resistance. From your numbers, the 40W fridge is taking a bit over 3A at 12V so the 0.3V drop measured at the battery is due to 0.3/3 = about 0.1 ohm internal resistance.

It's hard enough to infer SOC from rested battery voltage. It's next to impossible when the battery is delivering current.

Derek

 

[Daydream believer]

By the way, I've found it difficult to get insurance cover for single handed trips longer than 18 hours. May I ask who you insure with?

GJW.
Just south of La Rochelle ( in case i overshoot) to the Elbe. Unlimited inc night sailing. At night excess goes up to £250-00. But November -April any further south than Brest then I have to have a crew. Which is odd because I have to be laid up 4 months of the year, although the months are not specified, which also strikes me as odd, but i did not want to push it once confirmed it was "any 4 months".
I have been south of Brest without a crew & they agreed, at no charge after I phoned up & asked. - (The wife decided she had had enough & went home!!!So I got to do some decent sailing : )

 

[rszemeti]

Once a year, I give mine a discharge test, battery by battery, using a 4 car headlight bulbs ... it draws around 20 Amps and providing at the roughly 50% mark the lights a bright enough and the voltage looks decent, I call it a pass and leave them in for another year ... as I discovered last year ... 3 of the "house" batteries failed to last 20 minutes ... the new engine battery (only a year old) died in less than 15 minutes ...

I find a decent load of 20 amps or so to be easier to deal with, as it takes a few hours rather than a few days to come to a conclusion.

 

[BabaYaga]

the new engine battery (only a year old) died in less than 15 minutes ...

Any clues as to why that happened? (Still amazed by the short life span of batteries mentioned in this thread...)

 

[ditchcrawler]

I had two Numax sealed 110ah batteries as my domestics and I changed them after 9 seasons although they seemed fine but I was going on a trip to Holland so I bought the same again to be on the safe side. My Red Flash 1100 starter battery I changed after 8 seasons. Some of the battery experiences above seem to indicate something not right with the charging regimes or the set ups. My charging is Sterling alternator manage system and Sterling mains charger with a small solar panel.

 

[Plum]

Any clues as to why that happened? (Still amazed by the short life span of batteries mentioned in this thread...)

Not everyone gets a short life. My last two batteries that I replaced were 8 years old. I currently have two that are still going strong at 9 years. All engine start batteries, not deep cycle.

Www.solocoastalsailing.co.uk

 

[lw395]

Just checked after 21.5 hours at 2 amps & they read 11.73 volts. .....

If they had a capacity of 180Ah between them, 2A would be a bit more than the 'hundred hour rate', or C/100, it would be C/90.
21.5 hours at 2A is 43 Ah or about 25% discharged, 75% charged, if the capacity is still 180Ah.
Here are some graphs:
View attachment 84137
sorry it's not inserting the graph, you'll have to go to the link or google a similar graph, there's various flavours on the web!
from
How to tell if battery is fully charged?

That suggests you should be seeing more like 12.6V
Bearing in mind 'lead acid' batteries are not all identical, the above is only a guide, but it's looking like you have lost a lot of capacity.
2A=~ C/20, implying C=~ about 40Ah, and 43Ah being around 75% of your capacity is much nearer the ball park.
But these curves are for healthy, new batteries, so it won't be exact.
So it's suggesting your batteries are no longer new.

But, the catch is, were they really fully charged at the start of the test?
It's better to plot some data points along the way to get a better fit to a curve, the 12.77V initial on load voltage doesn't pin things down very much. IMHO you really need to check the discharge voltage 15 minutes in, then at least every few hours. It's possible the 12.77 was false being measured too early and things are not as bad as they look, in terms of the batteries, but the charging side needs some attention?

Also meter errors and all that.

HTH?

 

[VicS]

If they had a capacity of 180Ah between them, 2A would be a bit more than the 'hundred hour rate', or C/100, it would be C/90.
21.5 hours at 2A is 43 Ah or about 25% discharged, 75% charged, if the capacity is still 180Ah.
Here are some graphs:
View attachment 84137
sorry it's not inserting the graph, you'll have to go to the link or google a similar graph, there's various flavours on the web!
from
How to tell if battery is fully charged?

Thanks for posting the link to these graphs. Don't know why you could not insert the image into your post