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Preview: Boat Wiring, Part 6 – The Two Battery System

May 12, 2014

After enlightening us all on every aspect of a simple marine electrical system, Don Eley brings it all together by laying out his simple two battery system. Don even adds a new wrinkle — a new and improved switching system that reduces the risk of having two dead batteries.

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Eric Blake: Now that Don has taken us through the components of a simple two-battery system – from the types of batteries, what to know about cables, choosing the right wire gauges, making good connections, and protecting the system with circuits and fuses – now Don is going to bring it all together. He’s going to lay out the system for you and show you the final element of a new and improved switching system.

Don: Sometimes a battery electrical system on board can get a little bit complicated. So what we’re going to do today is we’re going to look at a simple two-battery system, and we’re going to look at a couple of options we have for switching between batteries and keeping batteries charged.

So here’s a battery switch that I think most people are used to where we can switch between one battery and a number two battery, or we can combine them to the ‘all’ position if we need to. This is the back side of that switch. We would hook the positive lead of the number one battery to this terminal. We’d hook the positive lead of the number two battery to that terminal, and then the common here is actually going to be supplying our load, whether that be the starter motor or our house loads. Then, this switch allows us to pick which battery we want to use. The challenge here is to keep the batteries all charged so we don’t wind up with a dead battery at some point and can’t even start the engine.

Here’s a simpler solution to the two-battery switch arrangement. This simply has an ‘on’ and ‘off’ and will turn both batteries on and off simultaneously as well as combine the batteries if we need to. Then, when we go to charge the batteries when the engine is running we have what’s called a combiner, and it will tie those two batteries together for the charge process but always leave the batteries separate from each other when they’re not being charged. That allows us to feed the engine start and the house loads separately without discharging one battery into the other.

We’ve set up a simple system here. I’ve indicated wire routes with just some red lines indicating the positive side of the system. So we’ve got two batteries. One would be for the engine starting. Number two battery here would be for our house loads. Here’s a typical breaker panel that we might have on board with our cabin lights, running lights, horn, and bilge pump. Then we have a battery switch here that’s going to simplify the process. It simply has an ‘on’ and ‘off’. It’s going to turn the number one battery and the number two battery on and off at the same time. So we’ll have our number one battery connected to the number one side here, and the number two battery and load connected to the number two side. Then this combiner arrangement has a solid state device that once the engine starts, this will combine the two batteries for charging yet keep them isolated when we’re not charging.

Alright. So let’s look at the current flow in this particular system. We have our number one battery here for engine starting. It’s going to come through our switch to our starter motor for starting purposes. Our number two battery here, again, is going to come through our switch to our house loads, and we’re not going to discharge one battery into the other. They’re going to remain completely isolated from each other. Now, once the engine starts, we’ll have an alternator here that will produce electricity. It’s going to flow back through the switch up to the number one battery to charge that battery. It’s also going to flow to our combiner here, so we’re going to have the positive terminal of each one of our batteries to our combiner. This is a solid state device that once the charge comes up, it will tie the two batteries together, and in tying the two batteries together then allows us to charge two batteries simultaneously. When we shut the engine off, the alternator stops its output, the combiner then automatically disconnects the two batteries. They stay separated even though they’re both on. We don’t discharge one battery into the other and wind up with a dead battery somehow. If we need to, we can combine the two batteries manually into this manual position here, but for the most part, we’ve solved this battery switching back and forth, one and two, both battery combination that we’re familiar with for the older style switch. We just turn it on. The batteries are charged, and we’ve got a lot less challenges in front of us.

So we’re going to look at a typical 19-foot runabout. This is a pretty simple system that Luke over at Eric Dow Boat Shop put together for us. It has two batteries here. These are AGM, absorbed glass mat batteries. Here’s the positive lead coming off of each battery. It comes up to our switch, and out of the battery switch we’re going to come up to our small, little breaker panel here for our house loads, loads like the VHF radio and the bilge pump. Off the other side of the battery switch, we’re going to wind up going back to the engine for engine starting. Then we have this combiner arrangement, and the way it works is that it comes off the battery switch through each side, which has a set of fuses, to the combiner. So that once the engine starts, we’re going to see that we combine the two batteries through this solid state combiner. And we have some fuses in place there that protects us from any electrical shorts that we might have.

This is a pretty simple system. I like it because it’s fairly fool-proof. You have one battery switch. We keep our batteries separate from each other. The charging is real simple, and it eliminates most of our problems with battery switching.

Eric Blake: We hope this six-part series has taken some of the mystery out of marine electrical systems for you and given you some ideas for how to create a simple two-battery system on your boat.


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