Automating your charging system

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Automating your charging system while protecting the starter battery.

The case for the DC positive distribution Buss.

On most vessels, the alternator is connected directly to the starter of the engine and while this is okay for charging the starter battery, it is not ideal for charging a large house battery bank. Given that the starter battery is never connected to any house loads, it should never get discharged very much, unless of course the engine is hard starting. Therein lies the problem.

How do you wire the charging system to give priority to the house bank battery? I’m sure right now you’re probably thinking it’s simple. Usually, once the engine is started, you parallel the batteries and charge both battery banks at the same time, but every time you parallel your batteries, you drag down your starter battery by connecting it to a larger, often depleted house bank. This is not good for your starter battery. In my mind, the starter battery is sacrosanct; it should never be compromised. So how do you wire the high amperage DC electrical system to do this? First and foremost it has to be simple; it also has to work automatically and has to be able to bypass any of the automated systems in case of failure.

In a modern ocean cruising boat, you have more than one way to charge the batteries. You have of course the alternator but you also have other options such as solar panels, wind generators, water generators. So how do you combine all these charging sources into a simple distribution network to charge both the house and starter batteries? The first thing you do is create a common distribution point where all the charging sources and loads are connected. In our example system diagram (Diagram #1), you will see we have two charging sources, solar and the alternator. You can of course have other charging sources but for simplicity let’s stick to two for the moment.

So what makes up our system? You have an automatic charging relay (ACR); three switches; house, parallel and starter isolation; positive and negative distribution busses; fuses and an amp hour meter shunt. The amp hour meter is crucial so that you can monitor your battery capacity. Nothing should bypass this shunt; all grounds should go to the negative distribution buss and not directly to the battery. Notice also that the alternator is connected directly to the positive distribution buss and not the starter.

So let’s see how it works: Let’s say you have been at anchor for a while and your house bank is down to fifty percent capacity. You start your engine and the voltage at the positive distribution buss begins to rise as the house bank begins to charge. At this point, the starter battery is isolated from the house bank by the automatic charging relay (Diagram #2). Once the house battery voltage reaches 13.5 volts, the automatic charging relay closes and the starter battery is now charged in parallel with the house bank (Diagram #3). If the house bank is large, it might take a while for the automatic charge relay to parallel the batteries.
This of course works both ways. If you are anchoring for the night or sailing and the voltage at the positive distribution buss drops below 12.35 volts, the automatic charging relay opens the starting circuit preventing the starting battery from being discharged. This all takes place without having to turn on or off any switches.

You will notice in diagram 1 that there are only on/off switches used; there are no 1/2/ both switches. This is done for simplicity; each switch has its own function: house battery isolation, parallel and starter isolation. The house battery isolation switch does as the name suggests: isolates the house batteries from the positive distribution buss. With the exception of the bilge pump circuit, everything is electrically isolated when this switch is in the off position. Notice that the house bank’s main fuse is after the switch. Normally, the main fuse is as close as possible to the battery but because the cable going to the isolation switch from the battery is part of the starting circuit when the parallel switch is closed, it does not need to be fused. If the starter battery is too weak to start the engine, the house bank can be paralleled to the engine start battery using the emergency parallel switch to aid in engine starting. Notice that the starter isolation switch is wired in such a way so that, if the start battery is completely dead or shorted it can be isolated and the motor can be started using only the house bank. The parallel switch can also be used if the automatic charging relay fails; you simply parallel the house and starter battery to charge them. You, of course, have to make sure that you un-parallel them when no charging source is available so that you do not bring down your start battery when your house bank is being discharged.

The system works the same way with the solar panel. During the day, when the solar panel voltage rises above 13.5 volts, the starter battery is automatically connected in parallel with the house bank and at night when the voltage drops below 12.35 volts, it is disconnected.

So as you can see, without adding any real complexity, we can wire a boat in such a way as to make it seamless in regards to battery charging. In other words, you do not have to turn battery switches “off” and “on” to charge the different batteries. This system keeps it simple and reliable, and at the same time protects the starter battery from being accidentally discharged, which is what you want in an ocean going vessel.

James Turnbull
Nautek Marine Electrical Inc.
1773 Lillian Rd. Victoria B.C.
Canada. V8S 1L3
(250) 208-9496
Email: Web:

Automatic Charge Relay (ACR)

An ACR is used to combine batteries whenever a charge source is present and isolate them when it is no longer there. It does this by monitoring the voltage at each terminal (dual sensing) and isolating them if the voltage drops below a predetermined set point. This allows the use of a single charging source without the loss associated with an isolator (diodes). ACR’s are available in various capacities ranging from 65 to 750 amps and should be sized to the maximum alternator output. They can be as simple as a voltage sensing on/off relay to ones with remote manual switching and starter isolation (isolates sensitive electronics from voltage drops and spikes during starting).

Amp Hour meter

   An amp hour meter can be seen as a fuel gage for your batteries. It displays voltage, current and amp hours consumed. In order for it to do this, it needs to be able to measure every electron that passes through the batteries. It’s essential that no ground wires bypass the Amp hour meter shunt; they must not be connected directly to the battery but instead to a negative distribution buss installed after the shunt.

The shunt is a temperature stable resistor which puts out millivolts in proportion to the current flowing through it. Most shunts are rated at 50mv@500 amps. In other words, if 500 amps were going through the shunt, you would measure 50mv (millivolts) across it. If 250 amps were going through it, you would measure 25mv. The amp hour meter uses this voltage to determine how much current is being drawn from the batteries. In order for the amp hour meter to calculate the amp hours used, you would need to program it for the type of battery you are using as not all batteries charge and discharge at the same rate. You would also need to input the size of the battery bank in order for it to tell you the percentage of battery capacity remaining.


James Turnbull has always been in love with the sea. Born in Northern England but raised on Vancouver Island British Columbia Canada, he has lived by the ocean for most of his life. In 1992, along with his wife Kathryn, he sailed their beloved 30’ sailboat, Wind Dancer, from British Columbia to Mexico, then across to Hawaii and back to British Columbia, where they now live with their sons, Josh and Ben. He recently finished refurbishing a 42’ sailboat which he and his family will be sailing on to Haida Gwaii this summer. His background is in marine electrical, specializing in system design. He has been involved in some large projects with the most recent being the new Nordhavn 120, where he was responsible for all the Navigation, Communication and Audio visual system design and implementation. His heart however, still lies with Ocean Cruising Sail boats.




Vice Commodore, OCC 
HighcurrentDCDrawing1.1.pdf (310 views, 228.00 KB)
HighcurrentDCDrawing2.1.pdf (320 views, 229.00 KB)
HighcurrentDCDrawing3.1.pdf (300 views, 230.00 KB)

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