Dead battery mystery

[SvenglishTommy]

I am at a loss to explain a dead start battery situation without resorting to superstition, maybe you guys have some ideas.

I recently sold my old boat, and went with the new owner from it's winter storage to the mooring. Everything was fine, started first time after the whole winter, battery was reading 12.68V after months untouched. Alternator charging both banks as expected etc etc.

I had to go back 5 days later (isolator sitches left on) to help him mount the mast-top bits and bobs, and the start battery was dead as a dodo. Totally flat, I could read about 2 volts at the terminals.

Charged the battery up and left it overnight with isolators off. 12.77 volts the next day. Turned the isolators on and left it again for 2 more days.. 12.77 volts still.

The only thing I can think of is that the engine control panel was left on, but I cannot see how that could kill a 75AH battery so dead in 5 days.

The currently accepted theory is that the boat was angry with me for selling her, and so drained the start battery. Can anyone come up with a better theory?

 

[KRG]

Approx 30 amps available, 72hrs, 0.5 amp/hr approx, 6 watts, quite possible start panel lamps could drain it.

 

[Sailorsam101]

It's the type of battery

The problem i think may be two fold.

Firstly there may be some kind of current draw even with the isolator switch off. I have seen this often. You can either get a volt meter and measure any discharge or disconnect them and see what happens when you reconnect them.

Secondly and what i think has happened is that your engine starter battery is knackered.

Remember that domestic and engine batteries are different types.

Domestic batteries are 'leasuire' batteries while an engine starter battery is a 'deep cycle' battery

A domestic battery can be run dead flat and charged again without any problems or future use.

While a deep cycle battery once flat is only good for the bin. A flat deep cycle battery may charge and show good voltage but as son as you try to hit it with a heavy load it just dies straight away.

I know all of the above Beach I've been there...good luck

 

[KRG]

The problem i think may be two fold.


Remember that domestic and engine batteries are different types.

Sometimes but often not.

Domestic batteries are 'leasuire' batteries while an engine starter battery is a 'deep cycle' battery.

Start battery shouldn't be a true deep cycle battery as they are not designed for very high current draw

A domestic battery can be run dead flat and charged again without any problems or future use.

Not entirely, a traction battery such as fork lift types possibly but not most batteries sold for domestic use.

 

[maxi77]

The problem i think may be two fold.

Firstly there may be some kind of current draw even with the isolator switch off. I have seen this often. You can either get a volt meter and measure any discharge or disconnect them and see what happens when you reconnect them.

Secondly and what i think has happened is that your engine starter battery is knackered.

Remember that domestic and engine batteries are different types.

Domestic batteries are 'leasuire' batteries while an engine starter battery is a 'deep cycle' battery

A domestic battery can be run dead flat and charged again without any problems or future use.

While a deep cycle battery once flat is only good for the bin. A flat deep cycle battery may charge and show good voltage but as son as you try to hit it with a heavy load it just dies straight away.

I know all of the above Beach I've been there...good luck

Sorry but you are wrong, starter batteries are just thast starter batteries designed to dump a lot of amps quickly into the starter motor. Deep cycle batteries are designed to provide long term power such as disabled buggies, golf carts and boat domestic banks. Liesure batteries are hybrids which can do either though are better at the long term supply. Whilst I would tend to agree no battery will fully recover from a discharge to 2 volts many seem to respond well to some nice treatment and give a bit more service.



I would suspect leaving the engine panel on could well have flattened the battery over 5 days. Electrickery tends to obey the laws of physics rather than the spirit world.

 

[SvenglishTommy]

Glad to hear others think the panel theory might be viable, I still think it's a tall order for a panel to drain a 75AH battery to nothing in 5 days. I cant see a panel light pulling 0.5 amps.. but maybe the alarm went off as well and just sucked it flat.

 

[Dave_Seager]

Glad to hear others think the panel theory might be viable, I still think it's a tall order for a panel to drain a 75AH battery to nothing in 5 days. I cant see a panel light pulling 0.5 amps.. but maybe the alarm went off as well and just sucked it flat.

0.5 Amps x 120 hours (5 days) = 60 amp hours. That will not leave much in a 75 A hr battery, especially one that has been used for a while.

 

[Plevier]

It may not just be panel lights - there may be an excitation feed to the alternator via a resistor (e.g recent VPs have this).
Your nominally 75Ah battery is unlikely to be actually delivering that much. Capacity declines with age, and alternator charged engine batteries rarely get over about 80% state of charge unless you have very long engine runs.

 

[pappaecho]

Sounds like sods law to me. I had a 3.2 litre Mercedes, hch worked fine. On the day before the dead battery, I had used the car several times, and it started without problem.
Following day - dead 9.4 volts across the terminals. Replaced battery, no problems or repeat, so not a drain down by a circuit, just a battery failure

 

[William_H]

Dead battery

If the battery is knackered then for sure it will fail again. However if it was just the key switch for the engine left on (guages as well as ign light/ alternator excitation) then you won't have the problem again. or at least the new owner won't. Although as said discharging a battery is not good for it. good luck (to the new owner) olewill

 

[Momac]

Last spring I had the same thing - starter battery okay one weekend and dead the next.
New battery fitted = no problem since.

 

[alahol2]

Last spring I had the same thing - starter battery okay one weekend and dead the next.
New battery fitted = no problem since.

Ditto. Trouble is the 'dead' battery is sitting in the garage 12 months later showing a charge in excess of 12 volts having last been recharged 4 or 5 months ago. It is about 7 years old.