Battery Draining Gremlin

[rollo_cd26]

I've been chasing a battery drain issue on Mirabile (CD-26D, hull #78, 1988) for two years. I have two Deka marine 12V batteries connected to a 35 Watt Sunwise PV panel through a charge controller (unknown brand, came with the boat, 5 years ago). I reserve Battery #1 for starting the Westerbeke W10-two diesel engine. At roughly two week intervals I find previously fully charged Battery #1 too low to start the diesel (the interval now is getting shorter). Reading the posts for "Battery Drain" topic I've learned that the charge controller should have a diode that prevents the panel from pulling current at night. Readings on the controller input side with the panel disconnected showed 2.51 volts (same as on output side). Is this an indication that the diode is weak/failed and is the source of my low battery issue? The panel on sunny days puts out ~18 volts. Battery #1 has charged and tested good off the boat several times. I have no leaks, and I have turned the bilge pump from auto to OFF in an effort to find the cause. I have no other devices charging or ON when this occurs. What do the you folks think of the PV charger controller diode as the culprit? Are there any definitive tests I can do to confirm this beside the one described?

Thanks,

[sgbernd]

My fist place to look is the battery. When they get old, they won't hold a charge. Some people like to run them to failure which, while economical, means that they will leave you stranded by definition. So if they are older than 3- 4 years, replace them. If they are somewhere in between (say 2yrs), most auto shops can test them by putting them under a load and watching the rate of voltage drop. Usually they will do it for free in hopes of selling you a replacement.

If the batteries are fresh or tested and found good, you have a leak. The battery selector switch should isolate them while you are not on the boat (assuming you leave it in the off position). Thus, the leak must be bypassing the selector switch. This means bilge pump, solar charger, something plugged into the cigarette lighter, etc. If the bilge pump, you may have a leak (for example, the stuffing gland) causing the bilge pump to cycle on and drain the battery. The solar charger could also have died, and it is easy to disconnect it and see if things improve.

The electrical systems on these old boats are pretty basic and generally easy to trouble shoot.

I replace my main batteries every 36 months and every other battery powered device aboard (flash light, emergency GPS, clock, etc.). I hear the AGM last longer but the principle is the same.

Steve Bernd

[Paul D.]

My Trojan T-105's lasted ten years. Our current set I may replace earlier as I cheaped out and bought a one year-old set in excellent condition for half price. I think good deep cycles of any type should last more than a few years if they are being charged properly, sized to the electrical use so that they normally only get discharged to ~50% and maintained with a trickle charge during winter storage.

I would likely refer to Calder or Don Casey to get the procedure right, but before I would replace the batteries I would check with a multimeter any circuits bypassing the battery switch for drain as well as the normal "Off" set up for when the boat is left for longer periods.

Good luck! These sorts of things qualify as 'deck leaks' to me and are often the hardest to really get to the long term solution, but are very satisfying when you do.

[Tom Keevil]

Don’t just replace your batteries without testing them. Our last set of Trojans were replaced after 10 years. Three years seems very short to me. You can test them yourself by watchng what happens to the voltage when you crank the engine with the fuel shut off, so it won’t start. If it only drops a small bit and stabilizes, your batteries are fine. If it keeps dropping, new ones are in order.

[Keith]

my AGMs are going on 8 years. They were both strong last season and check out fine when I closed up last month

Keith

[Maine Sail]

#1 Charge both batteries to 100% SOC preferably in parallel / BOTH so they are at the same SOC.

#2 Stop charging & disconnect the batteries from the vessel, charger & each other

#3 Let them sit for a week or so (depends upon temp) then test the open circuit resting voltage of each battery.

#4 With this information we will know if you just have a bad battery or a parasitic draw.

FWIW East Penn / Deka makes three grades of "Marine" 12V batteries, Starting, Dual-Purpose and "Deep Cycle". They are sometimes mislabeled by distributors.