Testing batteries

[Nicho]

We have two 120 amp/hr batteries (presumably connected in parallel) feeding our 12v domestic items, and I would like To test them. Do I have to disconnect each battery to test it individually, or can a test be done with it still attached to the other battery. No doubt a daft question, so forgive my total lack of electrical knowledge. From the picture, can it be determined if they are wet cell batteries please?  Somewhat embarrassed to have to ask this!

[Yngmar]

You can test the bank together or separately. Seeing as there's some corrosion on at least one battery post, you should probably remove them and re-install with some grease applied to the posts, so you might as well test them separately.

The label (cleverly always on the sides where you can't see it), should tell you all about the batteries, but it looks like those two plastic covers are intended to lift off and depending on what you find underneath you will know if they're FLA types (with screw caps) or sealed (with valves).

[Nicho]

Thanks very much. I hadn't noticed the corrosion so will take the leads off, test independently and grease the terminals. The boat is relatively new to us, so still learning!

[Lyra]

Let's start with the type of batteries - I think they are wet since non wet sealed batteries (AGM, GEL etc) typically have a flat top with no visual way to open them.
I could not identify what this black round on top is - if it is some kind of a "window" these are probably "maintenance free" sealed wet batteries.
If you can you easily pry out the two rectangular tabs, these are regular wet batteries and  this will expose the electrolyte openings and you should be able to see the top of the plates.
If these tabs seem to be "glued" or when removed no opening exposed  then again - probably wet sealed type.

Testing - You could test while the batteries are in parallel, but they should anyhow be disconnected from all circuits, so disconnecting the link between them as well is no big deal. If any of them is in poor condition it will affect the second so in any case, whether tested together or separately both will have to be replaced as batteries in a bank must be identical.
Charge them fully.
Disconnect them and wait for several hours (24 hours is best but practically 3-4 hours will also work).
Check voltage - if it is 12.6-12.8 then they are good, at least in terms of being able to be fully charged. Any voltage below that means that they cannot be fully charged (or lost charge very fast during the 24 hours). every 0.2V lower means roughly 25% of charge lost.
Then, I would check voltage when they are connected to loads - turn on the fridge (I assume it consumes 4-5A, if you have a clamp meter you can check exact current) -  voltage on the battery terminals should drop by no more than 0.1V.

My experience is that when the house batteries get old and charge less you notice it without doing too many tests, so if you did not notice any major change over time then they are probably OK and if you did they should probably be replaced. Nevertheless the above tests may give you a more "scientific" answer.

[Nicho]

Thanks Lyra, very helpful. We bought the boat last October, but with winter weather and now lockdown we have hardly seen it!  The batteries will be charged fully due to the smart charger being permanently on, which will have to be switched off for the test. I have a battery tester which applies a hefty load to the battery and displays whether the battery is good, fair or otherwise. The reason for the test is that the charger cooling fan cuts in quite a bit when the fridge is running, so it was suggested perhaps the batteries are past their sell by date. Thanks again.

[Symphony]

Think you will have a voltmeter on your panel so you can check the resting voltage. Disconnect the shorepower for a few days to check the level of self discharge. If after a week or so it is still in the 12.6-12.8v range then that is encouraging. Worth fitting a battery monitor which will enable you to keep a closer check on your usage and condition of the batteries.

[Klausen]

I would not grease the posts, no grease can conduct electricity. Clean them and put some grease only to the outside.
Greetings
Klaus

[Nicho]

Think you will have a voltmeter on your panel so you can check the resting voltage. Disconnect the shorepower for a few days to check the level of self discharge. If after a week or so it is still in the 12.6-12.8v range then that is encouraging. Worth fitting a battery monitor which will enable you to keep a closer check on your usage and condition of the batteries.

Thanks. The voltmeters read 13.5, but that is with the smart charger on. I will switch everything off and check again after a few hours.

[fjoe]

It is a flooded battery (standard), no VRLA battery (AGM or Gel).
The black round part in the lid is either a charge eye (green: fully charged, black: discharged, bright yellowish: low acid level) or an acid level indicator (black: acid level ok, bright yellowish: acid level too low). It is measuring only in one of the 6 cells. If another cell has a damage it will not indicate it.
Due to physical reasons VRLA batteries cannot use such a device.

[Nicho]

It is a flooded battery (standard), no VRLA battery (AGM or Gel).
The black round part in the lid is either a charge eye (green: fully charged, black: discharged, bright yellowish: low acid level) or an acid level indicator (black: acid level ok, bright yellowish: acid level too low). It is measuring only in one of the 6 cells. If another cell has a damage it will not indicate it.
Due to physical reasons VRLA batteries cannot use such a device.

Thanks, very helpful - we are down on the boat this week, so I'll be checking all that.