Sorry! Another battery question......

[slipknot]

Bought new (to me) 10mtr sailboat boat last year which has been unused for circa 5 years. Has both domestic and engine batteries which after a top up of distilled water and a charge up seem to start the engine fine (volvo 2030) and run the domestics without problem.
Having said that I think it would be wise to replace at least the engine battery. Would this be ok or is there likely to be any issue whereby the new powerfull battery, may be effected by the potentally weaker older battery.
Should I replace only 1 or both ???

 

[VicS]

You dont say how your batteries are connected and used.

1, 2, Both, off switch.

Diode split charging
VSR split charging

fancy dual output gizmo.

One dedicated for domestics and one for engine starting

Or what

Generally do not connect a very old battery in parallel with a new battery

Replacing engine start but giving the domestic battery the benefit of the doubt for the time being, provided they can be charged independently, sounds like a good idea but after 5 years of neglect it not likely to be much good

 

[john_morris_uk]

I would be very suspicious of the capacity of the batteries after this time - especially if you don't know how they have been cared for.

You could take them off and do a capacity check. If the one battery is really just for engine starting ONLY then borrow one of the 'drop test' gadgets that measure the voltage when starter type currents are drawn. The battery has to be fully charged first, but this will give you some idea about its condition.

For the domestic battery you need to check its actual capacity at a drain of a few amps. I have used a car headlamp unit (or two) for this purpose. Charge the battery up until the SG (specific gravity) says its full (if you can achieve this you're somewhere...) Then connect it up to the headlamp unit(s) and monitor the voltage and SG. Amps times hours until its flat gives you the capacity of the battery at that current discharge rate. You might want to only half discharge it you don't want to run the risk of ruining the thing totally. If you do flatten it completely then charge it up STRAIGHT AWAY - leaving it fully discharged will definitely wreck it. Bear in mind that most authorities reckon that 50% of the capacity is all you should use if you want to have maximum battery life.