Batteries U/S - Quick diagnosis please

[superheat6k]

2 x 125 AH 'Leisure' duty used for Domestic and one engine start. Parallel connected

New August 2011 so 3.5 years old.

On float charge most of the time

Electrolyte levels OK, but not yet checked SG of the cells

Problem

Day off the charger wouldn't turn the engine over (large Volvo Penta)
Assumed a bit low so didn't think anymore about it

Then yesterday with charger on with a good deal of the lights on and whilst testing out the electric loo following a 3 way valve replacement (approx 30amp load) the voltage suddenly collapsed to below 10v.
Repeated this again this morning with the charger off. Lots of lights on again OK for 20 minutes then a sudden collapse of voltage.

Turning off the lights sees the voltage recover quickly.

New battery time ?

The truck starter batteries serving the other engine are probably 8-10 years old and remain absolutely fine, but are only used for engine start and windlass.

 

[William_H]

It seems a strange set up to have proper start battery for one engine and use leisure batteries for the other engine. I would suggest rewire to use the same start battery for both engines.
Firstly make sure all wiring connections are good. When the battery voltage falls just make sure it is low at the actual terminals using a Digital voltmeter. Obviously if battery voltage is OK you have a bad wiring connection.
The leisure batteries should be separated and check them individually. Give them a good charge then apply a know decent load. ie about 10 amps or more. This should give you a decent battery voltage for 5 hrs or so. ie give you 50 amp hours. If not then you have a bad battery. Check the other battery in the same way. it is possible that one is dud and dragging the other down. Although realistically you may be up for 2 new leisure or dep cycle batteries. 3.5 years might be dissapointing but reasonable if they are not kept fully charged. good luck olewill

 

[lw395]

I would check that the charger is actually getting the voltage up over 14V at the batteries, or the current is dropping right off at a float voltage of 13.6 to 13.9

I've seen similar symptoms from duff charging.
Measure it right at the batteries.

 

[Stemar]

I'd start off by cleaning all the contacts & connections from the battery posts to the engine earth & starter and to the domestic distribution panel, then see what happens at the battery, but putting a voltmeter to the posts when the charger is on. I'd also compare what the voltage is at the distribution panel with what it is across the battery terminals when drawing a substantial current.

I'd guess that the batteries haven't been getting charged properly, either because of poor contacts or a duff charger. It can also happen that dirty terminals will work OK for low current draw, but go jigh resistance as soon as a big current is drawn. Cleaning everything will at least give you a chance!