Adding a Third Battery

[Cheap Guy]

I hope this isn't a common question - here's the deal. Last year I finally got to fixing a lot of things on my boat, including taking the dorm fridge out I used for over 15 years and replacing it with a proper AC/DC Isotherm fridge, upgrading the sound system and also adding the ability to use a small TV. In short, I want to anchor out more this year but don't want to worry about running the batteries down. What would be the simplest/best way of adding a third battery?

 

[bobeast]

I guess it depends on your current setup and how much room you have. My 310 came with 2 "house batteries" on one bank and one battery on the other. The house bank starts the genie and the starboard engine and runs †he cabin. The other bank starts the port engine and runs the helm electronics.

I had enough room in my bilge to add a 3rd battery to the house bank which I wired in parallel with the other two.
I upgraded to AGM so now my house bank has 330 Ah which is more than enough for a weekend at anchor. Of course I don't blast my stereo because...human, so my biggest drain is the fridge.

Consider updating all of your lights with LED bulbs. That will have a significant impact. Also I never run my windlass without the engines running, as that is a huge draw.

Another power saver is to have a decent ice-chest on board filled with ice, for beverages and such. The fewer times you have to open the fridge door, the less often the compressor will have to run.

I plan on installing 600 watts of solar panels on top of the canvas, at which point I could probably go for weeks without starting the genie.

If you need to know if your setup will meet your needs, consider marina-anchoring over a weekend. That is to say, starting from fully charged batteries, disconnect the shore-power and go on about your business. At the end of your "trip" and before plugging shore-power again, try to start your engine(s). Getting "stranded" in your slip next to shore power is much nicer than getting stranded out on the water because you miscalculated.

 

[Cheap Guy]

I guess it depends on your current setup and how much room you have. My 310 came with 2 "house batteries" on one bank and one battery on the other. The house bank starts the genie and the starboard engine and runs †he cabin. The other bank starts the port engine and runs the helm electronics.

I had enough room in my bilge to add a 3rd battery to the house bank which I wired in parallel with the other two.
I upgraded to AGM so now my house bank has 330 Ah which is more than enough for a weekend at anchor. Of course I don't blast my stereo because...human, so my biggest drain is the fridge.

Consider updating all of your lights with LED bulbs. That will have a significant impact. Also I never run my windlass without the engines running, as that is a huge draw.

Another power saver is to have a decent ice-chest on board filled with ice, for beverages and such. The fewer times you have to open the fridge door, the less often the compressor will have to run.

I plan on installing 600 watts of solar panels on top of the canvas, at which point I could probably go for weeks without starting the genie.

If you need to know if your setup will meet your needs, consider marina-anchoring over a weekend. That is to say, starting from fully charged batteries, disconnect the shore-power and go on about your business. At the end of your "trip" and before plugging shore-power again, try to start your engine(s). Getting "stranded" in your slip next to shore power is much nicer than getting stranded out on the water because you miscalculated.

Thanks for the great advice. Right now I have one starting battery and one deep cycle battery, with a typical battery switch. There’s plenty of room in the bilge to add a properly mounted third battery - I was wondering if I needed to add another battery switch or just run batteries on one connection and a single on the other, if that makes sense.

The “shore anchoring“ is a great idea! I’ve already replaced every bulb with LEDs and I usually carry a small ice chest on the longer trips, but that’s because I was using a dorm fridge to replace the Norcold that burned out a couple of decades ago. I also carry a fully charged small jump box in case I accidentally run the battery down. I also plan on adding a windlass this summer, which obviously won’t be used unless the engine is running.

 

[bobeast]

Thanks for the great advice. Right now I have one starting battery and one deep cycle battery, with a typical battery switch. There’s plenty of room in the bilge to add a properly mounted third battery - I was wondering if I needed to add another battery switch or just run batteries on one connection and a single on the other, if that makes sense.

The “shore anchoring“ is a great idea! I’ve already replaced every bulb with LEDs and I usually carry a small ice chest on the longer trips, but that’s because I was using a dorm fridge to replace the Norcold that burned out a couple of decades ago. I also carry a fully charged small jump box in case I accidentally run the battery down. I also plan on adding a windlass this summer, which obviously won’t be used unless the engine is running.

Sounds like you are off to a great start. Adding an additional battery in parallel shouldn't require any other changes as long as you don't go crazy with additional loads. The idea here is to last longer with your current typical load, not to add additional load, as may be the temptation.

That said, if your on-board battery charger is also circa 1989, I would highly recommend upgrading it to a modern smart charger. Your batteries will thank you for it. I'm a fan of Victron and Pronautic chargers.

One other point of which you may already be aware. It's not a great idea to mix new with very old batteries or batteries with differing chemistries. Ideally your two batteries in parallel should be of the same vintage, size and chemistry.