I am having issues with battery discharge over time with a no load condition.  I tested both batteries with a load tester and they tested OK. Battery 1 discharged completely after several weeks of non use when selected on the battery switch.  Battery 2 measured 11 volts after one day when selected. The engine started, but I am concerned that this will only get worse. I placed an amp meter in series with the positive lead and the battery post and the no load current draw was 39ma. I tried isolating each circuit but was unable to find the culprit. What is a nominal current draw with everything off? I'm leaning towards batteries here, but don't like needlessly spending money.

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Your nominal current draw should be zero when everything is off.

  1. New batteries will not help this problem. A load test will not necessarily indicate a cell with a small short.

A couple of things to look for are your battery charger could have a leaky diode or bridge rectifier. You could also have a leaky diode in one of the alternators. Depending on which specific alternator you have, one of the voltage regulators could be not turning off the field current when off.

The only way to find the issue, is to disconnect the culprit device. This is easier said then done. A binary approach is usually best. I would start by disconnecting one of the alternator outputs, check current when off. Trysecond alternator and check current again. Then check the battery charger by disconnecting it and checking current draw again.

You might also have something still on even though you think everything is off. 39 ma is not a lot but it is still 4.68 watts which means it could be a small pilot lamp.

Unfortunately, a problem like this takes time and sweat to find

Do you have a stereo? Like Ron said tracking down the load will take time. The easy solution would be to just turn the batteries off when not using the boat. I'm assuming the boat is not kept in the water.

Check your Stereo System.  The digital clock if equipped will kill a battery in two weeks. (ask me how I know) 

Check all your deck drain hoses. Bilge pumps will kill a battery within a week of regular use during rainy season. (Again, ask me how I know) 

Finally, Add a charger. Not to bandaid the problem but after you find the problem. 


Thanks for the helpful feedback  The boat is left in the water, so my concern is a full bilge and no power to run the pump. I think the alternator is the only thing I did not check. I will go there first and also add a switch for the stereo.  A charger would definitely provide some peace of mind when away for more than a week at a time. 

I would not leave a boat in the water without a charger. Get one big enough to power the bilge pump continuously. 

William Wise said:

Thanks for the helpful feedback  The boat is left in the water, so my concern is a full bilge and no power to run the pump. I think the alternator is the only thing I did not check. I will go there first and also add a switch for the stereo.  A charger would definitely provide some peace of mind when away for more than a week at a time. 

Just a thought, but I like to start in the middle: Keep your DVM reading amps and installed in series as you have done.  Pull each fuse (assuming you have a fuse panel) one by one until the draw stops.  If you have all the fuses out and a draw is still present, then the load exists somewhere between your battery and fuse panel.  From there I would look to see if any circuits have been added that circumvent the fuse panel.  If no "smoking gun" found there, then disconnect the battery completely and take a voltage reading across the terminals.  Record your results.  Come back a day later and repeat the test.  Compare your results.  If the battery is losing voltage with nothing connected to it, then your problem is likely to be the battery itself.  

Are you on a mooring or a dock? and does your boat have a shore power connection? If you are on a mooring, I would invest in a solar charger. If you are on a dock, but don't have a shore power connection, I would install a NOCO GCP1. We just installed one in My Son's Offshore 22 CC. Plug in a heavy duty extension cord and it will keep the battery charged.

Or you could just use this: Requires no hardwiring because it uses the chargers own plug. 

Google: Pro mariner Mfg. # 51310


The NOCO GCP1 requires no hard wiring, just plug and play. They also have the GCP2, which splits into two interior cords.
The big decision is where do you drill the hole.

Be careful where you put that tho. Surveyors will BOUNCE any plug within the engine room. And you will be left with a hole to fill.  Just food for thought for the future. 


The right thing to do is find the source of the drain. As far as reserve capacity for bilge pumps goes, if you consider the average current draw of a bilge pump for a boat that size and the (100 or so) amp-hours that a standard marine deep cycle battery can deliver, the pump could be reliably powered for several days; even a week.  Depending on how often you check in on the boat dictates whether a battery charger is really required for your application. 

Thanks for the feedback. I found the ignition circuit caused the load to go away, but needed some temporary jumper wires to further troubleshoot the issue. It will have to wait until the next trip up there. I'm not going to give up until I find the source. Shore power is an issue so I need to figure this out. 

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