Battery test

[Halo]

My 110 ah batteries are 8 years old. Yesterday I was down to 12.1 volts after 4 1/2 hours sailing with radio ais instruments C80 screen in use. Voltage measured under load via C80 raymarine system. Only one battery connected at any time. This is causing me to think batteries may be tired.
What is the best way to test the batteries? Is there a reliable way without removing them? Thanks in advance

 

[PaulRainbow]

My 110 ah batteries are 8 years old. Yesterday I was down to 12.1 volts after 4 1/2 hours sailing with radio ais instruments C80 screen in use. Voltage measured under load via C80 raymarine system. Only one battery connected at any time. This is causing me to think batteries may be tired.
What is the best way to test the batteries? Is there a reliable way without removing them? Thanks in advance

That's a totally meaningless test, sorry.

Best way is to fully charge time, disconnect them (just the negative leads will do) and leave them to rest. Note the voltage and apply a known load, a headlamp bulb is a good choice. After one hour, disconnect the load, leave to rest and note the voltage again, repeat for as long as you want, taking care not to deplete the batteries too much. You can then work out how much useable power the batteries hold etc.

If you don't have one, treat yourself to a decent multimeter.

 

[alahol2]

As per Paul's test but put the meter (in current reading mode) in series with the bulb for a short while to get a reading for the current. I have found that 25W bulbs are rarely 25W. I also just read the voltage under load every half hour until down to about 11 volts. Saves waiting for the battery to recover every hour.
Draw a Time v Volts graph and calculate how many AmpHours you've used during the test. It gives a decent feeling for how many AmpHours you can safely extract.

 

[PaulRainbow]

As per Paul's test but put the meter (in current reading mode) in series with the bulb for a short while to get a reading for the current. I have found that 25W bulbs are rarely 25W. I also just read the voltage under load every half hour until down to about 11 volts. Saves waiting for the battery to recover every hour.
Draw a Time v Volts graph and calculate how many AmpHours you've used during the test. It gives a decent feeling for how many AmpHours you can safely extract.

This is a really bad idea. Taking the batteries down to 11 volts is very bad for them and pointless. You'd never look to deplete your batteries to 11V, that's completely flat. As you would only want to deplete the batteries to a max of 50%, you only need to take them down to a rested voltage of about 12.1V. When you reach that figure, multiply the current in amps by the time taken to get there, in hours, and you have your safe max usable Ah. That will also tell you how knackered your batteries are.

 

[alahol2]

You are, of course, correct. When I did it last I was more or less testing to destruction a very old, knackered battery (Originally 100Ah down to about 40Ah capacity).
On a decent battery I'd be taking it down to about 12V under load which should be about 50%SOC.

 

[KompetentKrew]

Voltage measured under load via C80 raymarine system.

Due to the length of wire in between, the voltage at my chartplotter is lower than measured at the battery itself. Can't remember exactly by how much - 0.2v, maybe??

Measure with a voltmeter at the terminals of the battery itself.

 

[William_H]

Yes batteries may be tired. However the reason for replacing them will be clear when they do not do the job you want them to do. ie provide enough power after charge for as long as you need. It sounds like they will still do the job. so leave as is. If you were planning a long voyage then perhaps yes replace them. ol'will

 

[Skylark]

My 110 ah batteries are 8 years old. Yesterday I was down to 12.1 volts after 4 1/2 hours sailing with radio ais instruments C80 screen in use. Voltage measured under load via C80 raymarine system. Only one battery connected at any time. This is causing me to think batteries may be tired.
What is the best way to test the batteries? Is there a reliable way without removing them? Thanks in advance

I’m pretty much in the same position. My 8 year old house bank is 4x110 Ah.

Over the last year or so (lost track of time due to CV restrictions ?), I’ve fitted a 712 battery monitor. A good exercise but it hasn’t really told me anything that I didn’t intuitively know. It’s time to replace my batteries.

There are a few tell-tales. I started recording panel voltage readings (before I fitted the Bluetooth battery monitor) in the log as I noticed volts being consumed quicker than before. When starting the engine after a few hours under sail, the alternator would let me know that it was working harder than normal.

A bit like reefing the main, if you think that your batteries are on the way out, it’s time to bite the bullet and change them.