Battery problem

[westhinder]

My service batteries had 11.79 V when I checked today. The boat has been ashore since early december and then the batteries were fully charged. They are 2x 165 Ah semi-traction classic lead-acid batteries, bought new less than 2 years ago. I have no idea what has drained the batteries, as all circuits were switched off and the battery switch was also off. The battery monitor, which is wired direct to the batteries, was still indicating 100% charge, so I am pretty certain there was no current going out. The starter battery has been idle for the same time, but was still reading 12.58 V
So what may have happened? Is it just self depletion? Are the batteries knackered?

 

[sarabande]

a typical battery monitor is rated at 4mA.

Assuming 12 weeks out of the water, that's 2000 hours roughly, so the instrument will have extracted 8Ahr. Not enough to reduce the batteries to 11.79V, so a guess is that "fully charged" when you left the boat was not really 100% capacity, but more like 80%.

Self-discharge rate for lead-acid is around 5% of real capacity per month, so an 80% charged 165Ahr battery is really at 132 Ahr. Three months self discharge at 5% per month then knocks that down to around 122Ahr. But that doesn't really explain the open voltage figures.

Any burglar alarm, bilge pump sensor or gas detection device also wired in and live ?


Charge them with a four stage charger for a few days, then see what happens when you apply some test load.

 

[temptress]

Either the service bank is dud or (more likely) you have a power leak. Your battery monitor is most likely measuring AMPs out vs AMPs in. Not 100% accurate and no good if the amps are taken out before the monitor. Too difficult to explain how to search for a leak here. Google the subject - there are pages ad page of explanations...

 

[rivonia]

The simple thing to do when leaving a boat on the hard is to disconnect the negative terminals = No discharge.

 

[VicS]

My service batteries had 11.79 V when I checked today. The boat has been ashore since early december and then the batteries were fully charged. They are 2x 165 Ah semi-traction classic lead-acid batteries, bought new less than 2 years ago. I have no idea what has drained the batteries, as all circuits were switched off and the battery switch was also off. The battery monitor, which is wired direct to the batteries, was still indicating 100% charge, so I am pretty certain there was no current going out. The starter battery has been idle for the same time, but was still reading 12.58 V
So what may have happened? Is it just self depletion? Are the batteries knackered?

Recharge them as Sarabande suggests although not all automatic multistage chargers will recharge from a very low level.

Charge them individually using a charger that will deliver at least 15amps.

Also investigate the cause of them having been discharged. Check to see what may be connected to the positive direct without being switched off by the isolator. Radio/ instrument memory circuits, automatic bilge pump etc.
Make sure you have no connections to/ from anything to the negative terminals other than the battery monitor shunt that will have bypassed the monitor.

They will have suffered from being so deeply discharged but hopefully still enough life and capacity remain to avoid immediate replacement.

 

[boatshiner]

You're probably looking at something like this, which at least can be permanently connected to maintain the batteries, though if connected in series will only charge to the level of the weakest battery.

http://www.vertar.com/ctek/CTEK-battery-charger-car-MXS-10/

 

[westhinder]

Either the service bank is dud or (more likely) you have a power leak. Your battery monitor is most likely measuring AMPs out vs AMPs in. Not 100% accurate and no good if the amps are taken out before the monitor. Too difficult to explain how to search for a leak here. Google the subject - there are pages ad page of explanations...

I think you are right there must be a leak.
The only thing I can imagine might be the memory of the car radio. But can that cause such a drain on the batteries?
All the other appliances, even the bilge pump, are wired through the switch panel and all the swithes were off. I had even swithed off the general swithes for the services batteries and for the start battery

 

[westhinder]

Recharge them as Sarabande suggests although not all automatic multistage chargers will recharge from a very low level.

Charge them individually using a charger that will deliver at least 15amps.

Also investigate the cause of them having been discharged. Check to see what may be connected to the positive direct without being switched off by the isolator. Radio/ instrument memory circuits, automatic bilge pump etc.
Make sure you have no connections to/ from anything to the negative terminals other than the battery monitor shunt that will have bypassed the monitor.

They will have suffered from being so deeply discharged but hopefully still enough life and capacity remain to avoid immediate replacement.

Thanks Vic. As mentioned above, all I can think of is the radio memory. But is it possible that a memory drains two hefty batteries in two and a half months?