Boat batteries – how are yours connected?

Defining the problem

Most boats have more than one battery, or bank of batteries; one is used for starting the engine, the other to power the boat systems, such as lights, navigation instruments, fridges, invertors and even charge your phones.

When sailing they should be kept separated so that the boats systems don’t run the starter battery down, but they should also be able to be cross connected in the case of an emergency.

They also need charging from the various charging sources, such as the engine alternator, solar, shore power or a wind generator.

This requires a switching arrangement that operates in different modes:

1. Starting: Connects the starter battery to the starter motor.

2. Sailing: Connects the House batteries to the boat systems whilst isolating the starter battery so it doesn’t get run down.

3. Emergency starting: Start the engine from the House Battery if the starter battery is flat.

4. Charging: Charge both batteries from the Alternator when the engine is running. Charge both batteries from Solar or Wind.

The switching requirements of these modes create complexity and there is no common method of wiring batteries and switches causing confusion, misuse and leading to rundown batteries.

Do you know how your battery connections are wired? Our first recommendation is to make a drawing of your boat by tracing the wiring, then you can figure out how to use the switches.

Some common switch arrangements:

Let’s review some common switching arrangements:

A. Single ‘Off - 1 – 2 – Both’ switch. This is probably the most common.

The starter motor and house panel are both connected to the output.

The input can then be selected to be none (off) the Starter (1), House (2) or both batteries.

The problem with this arrangement is it needs to be managed for the different modes, see the following chart.

B. Two ‘Off - 1 – 2 – Both’ switches

The starter motor is connected to the output of switch 1 and the house panel is connected to the output of switch 2.

The input of each load can be selected to be none (off) the Starter (1), House (2) or both batteries.

It’s a slight improvement but also needs to be managed for the different modes, according to the chart.

C. Dual pole; On-off-combine

The switch is really two switches which close simultaneously, keeping the two battery banks isolated, needs less management during operation, except for charging.

Type of switch	Starting	Emergency starting	Sailing	Charging	Comments
Single Off-1-2-Both	1	Both	2	Both	When sailing with the engine off, everyone forgets to switch back to 2 potentially allowing the boats systems to run down the starter battery.
Two Off-1-2-Both	Start switch to 1	Start switch to both	House switch to 2	One switch to both	Same as above.
On-off-combine	On	Combine	On	Combine	Same as above.

The problem with all passive switch arrangement is that to charge both batteries, they need to be connected, Once connected the switching is forgotten about potentially allowing the starter battery to be drained by the house systems.

The Automatic Charging Relay (ACR)

The addition of an active component in the form of an ACR can dramatically simplify the switching arrangements.

An ACR is fitted across the two battery banks and senses the voltage on each of them.

Typically, there is a single cable to the engine that delivers current to the starter and returns current from the alternator, and this is connected through a switch to the starter battery.

In a similar way, a shore power charger or any solar or wind is connected through an appropriately sized fuse or breaker directly to the House Battery.

When the ACR senses 13v or more on either battery (i.e. its charging), it connects them together. It does not connect at less than 13v and locks open if the voltage drops below 9.5v, to prevent a damaged battery drawing too much current.

Several manufacturers make ACR’s in different sizes and they should be rated to the maximum safe charging current, which can be calculated as follows:

Max charging rate (Amps) = 25% x Battery Capacity (Amp/Hrs)

So, with a 75A/h starter and a 100A/h house battery:

25% x 175 A/h = 44 Amps – so a 65 Amp ACR will suffice.

Please note an ACR is not a fuse or breaker so you should typically include a breaker to protect the charging circuit. For example, in the case above I would use a 50A breaker.

One final note, always connect the automatic bilge pump directly to the house battery, not through the switch, so it activates even when the switch is off and does not run down the starter.

In summary, get to know your boats wiring and switching arrangements and if you don’t yet have an ACR get one, they are only a $100 or so and give you peace of mind that you don’t have to think about switch positions when charging, or forgetting to change them potentially draining your batteries..

If you have issues or questions with your boats switching, please feel free to give us a call for some friendly advice.