Parasitic draw

[max from alaska]

I lose about 1V over 2 weeks on both house and the engine battery (but not the thruster battery) when my boat is on a trailer, with rotary switches for the 2 house, 1 engine, and 1 thruster battery turned off. Does anyone have insight to a possible cause? The bilge pumps are direct wired to the thruster battery per the wiring diagram so that can’t be the cause. I suspect the monoxide detector is direct wired to the house battery but I can’t imagine it can draw the batteries down this rapidly. During storage the solar panel should trickle charge but this draw happens in spite of the solar panel (note that it’s often cloudy and the solar panel can’t provide much charge). With the rotary switches off, what could be causing this?

 

[Jared Dunning]

A few common culprits:

• CO/Smoke Detector – Almost always wired direct to the house bank for safety. They typically draw ~20–40 mA, and that alone can explain the drop you’re seeing.
• Stereo memory / electronics – Many stereos, chartplotters, or NMEA devices have a “keep-alive” memory circuit that stays powered if they’re wired before the battery switch.
• Battery monitor – If you’ve got something like a Victron or Balmar monitor on the bank, those always sip a little current.
• Solar regulator – Charge controllers themselves use a little power even when the panel isn’t putting much out (like on cloudy days).

The reason your thruster battery isn’t dropping is because nothing is tied to it except the bilge pumps, and if they’re not cycling, that battery basically sits idle. If you want to confirm, put a multimeter in amps mode between the negative post and cable with the rotary switch off. If you see ~30–50 mA or more, that’s your drain. Pulling fuses one by one will tell you exactly what’s drawing. Even a tiny draw (tens of milliamps) will add up over a couple weeks, especially if your solar panel isn’t keeping up during cloudy weather. A 1-volt drop in that time isn’t uncommon.

 

[max from alaska]

Thanks Jared. This is what I need to start the troubleshooting. The amp info especially will allow me to better interpret what I read with the meter.