Originally Posted by Ovey
Poly - I have a 75Ah leisure battery and I have Nav lights, Anchor light, Chartplotter (all on switches), a bilge pump (on a press+hold on/off/auto switch), VHF radio (running directly from the busbars but using it's front panel on/off/volume knob) and an NMEA2K network (permanantly powered once battery switched on). As you say, it's not an issue while running, and I'd like to think I turn it off when unattended (I'm usually pretty good at being anal in that respect!) but it's in case of the forgetful moment. At most it would be the chartplotter and VHF accidentally left on and the bilge pump in auto while at the beach during the day.
So why do you believe you would be less likely to forget to switch to a single battery than switch off?
If we assume its not raining hard and the boat is not shipping lots of water then the bilge pump isn't running without being on manual.
The VHF uses relatively little when not transmitting. Probably < 0.5A
The chartplotter - a quick look suggested ~ 0.5 A is probably a reasonable guess although a really big one with a sounder etc might be using a bit more when the backlight is on - but if the screen is lit you are more likely to notice it.
Nav/anchor lights won't normally be on when you are at anchor.
I don't think the NMEA network itself will have any appreciable current draw on the timescale we are discussing.
So you are probably using at most 1A at a time. If we can assume that you arrive at the beach with a fully charged battery, and are worried it might drop below 50% you'd expect to be able to sit there for 37 hrs! So not a big worry. Even if you left the nav lights on and the bilge pump running you'd comfortably have 5+ hrs before you were at risk of an issue.
Now people do report issues with battery systems so I'd consider if your investment would be better served:
- making sure you have a good quality battery isolator and it is fitted somewhere that is protected from water ingress.
- having a way to bypass the switch if you need to (they are one of the main points of failure)
- having a way to monitor battery charge and charging (and a routine to monitor it).
- knowing how (and practicing) pull starting the engine (possible on even some of the biggest beasts)
- if its not possible then consider is a small jump start pack in a locker is more flexible.
Now if you were running bait wells or fridges at anchor etc then I'd be looking at a twin battery set up.