Battery Management

[eje]

My boat has two batteries controlled by an Off/1/2/All switch. I don't have room to add additional batteries without modifying the battery compartment.
I know many reserve one battery for engine starting, but this means I am starting the engine to charge a battery while using only half the available capacity.

How to most manage a system with two batteries?

[Wotname]

Originally Posted by eje

My boat has two batteries controlled by an Off/1/2/All switch. I don't have room to add additional batteries without modifying the battery compartment.
I know many reserve one battery for engine starting, but this means I am starting the engine to charge a battery while using only half the available capacity.

How to most manage a system with two batteries?

You need very little capacity to start the average boat engine, less than 1 Ah in most cases. Of course you need a high cranking current (CCA / MCA) but not much capacity.

[Wotname]

There are several methodologies when using two batteries. Here is a couple.
1 Have a seperate batteries for House and Start
2 Have a main battery for everything and a reserve battery available for when the main battery fails.
3 Have two batteries and use only one at a time e.g. use battery one on odd days and battery two on even days.

Each method has advantages and drawbacks - you have to choose what works for you, your boat and your engine.

What is best for a large long term liveaboard cruising boat is not always the best for a weekend coastal cruiser or a day racer.

[eje]

Thanks, methodologies 1,2 3 seem pretty much all the same to me, use one battery & keep one full.
I use the boat mostly for day sailing & coastal cruising. One battery is more than enough for day sailing, but not for sailing all day/night with the autopilot

[smac999]

Originally Posted by eje

Thanks, methodologies 1,2 3 seem pretty much all the same to me, use one battery & keep one full.
I use the boat mostly for day sailing & coastal cruising. One battery is more than enough for day sailing, but not for sailing all day/night with the autopilot

Then you need to add more batteries. (Or solar) Maybe find a new spot for an engine battery. And use those 2 joined as the house. All 3 don’t have to be in same box. But the joined bank should be.

Most weekend sailers have 5+ batteries.