Are my batteries knackered?

[ghostlymoron]

Thanks Plevier, that's very useful.
Assuming a new battery, would you expect that figure to be the same as the label says e.g. 85 ahr

 

[Plevier]

One would hope so. However 10-20% down would not surprise me on cheapies where they will have pulled every trick in the book to get best test results.
Of course this crude light bulb test is not going to give you accurate figures anyway. In a lab you would use constant current regulated load in controlled temperature with continuous data logging.
Some claim 110Ah out of a battery weighing less and costing a lot less than a 90Ah battery from a good name. I don't believe it. They rely on nobody checking.
Weight need not be a good guide to cranking performance, but it is a good guide to low rate capacity. There is no substitute for lead in the chemical reactions that produce the electricity!

 

[sailinglegend420]

?...Assuming a new battery, would you expect that figure to be the same as the label says e.g. 85 ahr

New batteries don't reach their rated Ah capacity for 20-50 discharge-recharge cycles.

 

[NickRobinson]

New batteries don't reach their rated Ah capacity for 20-50 discharge-recharge cycles.

Not heard this one before...

On many yachts, that could be year 3/4 when some are already heading for the skip....

 

[sailinglegend420]

Not heard this one before...

On many yachts, that could be year 3/4 when some are already heading for the skip....

Start with this link from East Penn batteries describing their rated Ah capacity - page 2 - "...All ratings are after 15 cycles ..."

edit -add link:

https://www.unavco.org/projects/project-support/polar/support/deka_battery_handbook_2007.pdf

 

[westernman]

The story so far....

Have taken all the advice on board (well most of it) and tried running the fridge from each domestic battery in turn with new cabling direct from the battery to the fridge. The end result was exactly the same. Nasa was reading 12.7v in both cases and when I switched the fridge on the voltage fell away to the fridge cut-out voltage in circa 30 secs. What I didn't do was to measure the voltage at the battery end during the test (not enough meters or hands). I've no idea what reading the Nasa was giving during the test but as soon as the fridge cut out it was back to 12.7v (I can't monitor the fridge voltage and watch the Nasa at the same time!)

My conclusion thus far is that either the batteries are knackered or the Nasa isn't reading correctly, although it reads exactly 13.8v when the 240v charger is running (which is set to deliver 13.8v via the dip switches) and exactly 14.4v when being charger by the alternator, so this seems unlikely. I did put my multimeter across the battery terminals when the fridge wasn't running and this gave 12.4v rather than the 12.7v on the Nasa but I put this down to using a cheapo meter??

The boat is on a swinging mooring so lugging 50kg of batteries into the dinghy isn't something I relish the prospect of so have decided to invest in a decent battery tester.

Any further suggestions if you disagree with my conclusion gratefully received!

I had similar symptoms. On shore power or with the engine running and the batteries charging, the fridge would run fine, but on battery only it would endlessly cycle without cooling anything. It would start up and then almost immediately cut out - however the fan would continue to run for 1/2 a minute or more. Then it would try to start up again and then immediately cut out again.

Potentially the fridge compressor is knackered, or there is an excessive amount of refridgerent in the circuit. Either of which will mean that the compressor has a very hard time starting up and will have an excessive current draw during that time.

In my case the fridge compressor was knackered.

However, you will get the same symptoms with dead batteries or dodgy wiring.

I guess the only reliable diagnosis is to check instaneous current draw of the compressor as it starts up.

 

[Plevier]

Start with this link from East Penn batteries describing their rated Ah capacity - page 2 - "...All ratings are after 15 cycles ..."

No link.
Bit of backside covering there I would say. There can't be any spare in their ratings!
Shouldn't be a significant effect other than in poorly commissioned field filled batteries or after long storage without refreshing. A few % possibly. I would expect factory filled and charged to be pretty much up to the mark.
Certainly not going to be noticeable within the accuracy of a crude light bulb capacity test.

 

[sailinglegend420]

No link.
Bit of backside covering there I would say.......

Not heard this one before...


On many yachts, that could be year 3/4 when some are already heading for the skip....

Start with this link from East Penn batteries describing their rated Ah capacity - page 2 - "...All ratings are after 15 cycles ..."


edit -add link:


https://www.unavco.org/projects/project-support/polar/support/deka_battery_handbook_2007.pdf

And here's another quote from Trojan:

"A new deep-cycle battery will not deliver its full rated capacity. This is normal and should be expected as it takes time for a deep-cycle battery to reach maximum performance or peak capacity. Trojan’s batteries take between 50 – 100 cycles to work up to providing full, peak capacity".

http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf

 

[Plevier]

Start with this link from East Penn batteries describing their rated Ah capacity - page 2 - "...All ratings are after 15 cycles ..."


edit -add link:


https://www.unavco.org/projects/project-support/polar/support/deka_battery_handbook_2007.pdf

And here's another quote from Trojan:

"A new deep-cycle battery will not deliver its full rated capacity. This is normal and should be expected as it takes time for a deep-cycle battery to reach maximum performance or peak capacity. Trojan’s batteries take between 50 – 100 cycles to work up to providing full, peak capacity".

http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf

First link won't work for me.
It will be more noticeable with thick plate stuff like the Trojans, but I don't think the original question was about those types.