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How to Install a Dual Battery Bank on a Boat

In this video I add a second battery to my boat, The Green Machine. Having dual batteries and battery selector switch will allow me to make sure I can always start the boat, even if one battery is run down by the lights and fish finder.

How to Install a Dual Battery Bank on a Boat – Video Transcript

Hey there, Dangar Stu here. Today's video is on installing a dual battery setup in a boat and is proudly sponsored by MarineEngine.com.

I'm finally getting around to putting a second battery in the green machine so we're going to start from the situation where we've already got one battery and we're going to be adding a second. To begin with I'll just do a really quick drawing on the board of what I'm planning to do. I know you're not all a fan of the chalkboard but, bear with me. So here's the basic idea when you have two batteries a battery switch and a single outboard motor these two symbols here are representing our batteries and the positive side of each battery is coming into the switch. The switch then has a positive that goes to the outboard the negatives on the two batteries are bridged together with a wire and then the negative from one battery comes to the outboard. So whichever battery we've set our switch to is the battery that the outboard will use to start and it's also the battery that the outboard will be charging.

I'll show you the actual switch I'm gonna be using in a second, but the basic idea is that it has an input number one, and an input number two. So it's battery one, battery two, and then a common output. The idea with the switch then is that make sure number one can be sent to the common output battery number two can be sent to the common output or they can both be sent to the common output in parallel when you've got both batteries in parallel you still have 12 volts but you've got the accumulative apps of both batteries. The great thing about that is both batteries can be a little low and you can actually have the combined power of them to start the boat. I'm gonna go this way with the green machine so I think it gives you the most versatility you can start on either battery independently you can put them together to give you the most current draw for starting the motor, you can charge them independently, you can charge them together, you can switch them off entirely.

It's a pretty good way to go. What I do need to have to think about though is how I want to connect the other devices in the boat such as the navigation lights, the spot lights, the sound, and the bilge pump. All that kind of stuff. So we'll have a talk about it. Next, before we have a chat about the options I'll just show you how I've currently got this set up and the limitations it kind of imposes on me, which is, I've got my bilge pump down the back here. Obviously, sort of devices at the front, and it's all powered by this single sort of twenty amp wire with a fuse holder here. In addition to what I consider this sort of house circuit. I've then got this starter circuit coming to the same battery. So I've got to confess that when I started wiring this boat, I didn't really have two batteries in mind. I've always only had a single battery in it and yeah, it's been problematic at times but I don't go offshore very often or anything like that. If I do it's using a different boat a bigger boat. and it's been pretty good.

I have had a few instances recently where this battery has gone flat though, and it is a bit of a hassle. You've got to pull it out of the boat, I don't have Power at the water so I'm have to pull the battery out, Lug it to the shop, put it on a charge and put it back in. And it's something I can live without. There are two solutions to that problem. One, is to put a solar cell on it and the other is to have a dual battery set up. My plan down the track is to do both so, pretty soon after this I'll also do a video on charging both these batteries from a solar cell as well as the motor. Now the problem I have is ideally, I'd like be able to switch the battery switch to off but still have the bilge pump connected because I don't want that to ever be off. The standard way around that is that you wire your bilge pump to one of the batteries it has its own fuse, it's independent of everything else, comes to the battery switch here then when I turn this switch to off your navigation lights go off your fish finder goes off the power to the outboard goes off but the wires going straight to the battery for the bilge pump still have power.

To me that's the ideal setup. With the way this boat's wired to moment, that house circuit runs up to the front of the boat where I've got the switch panel for the bilge pump it then runs to the back and goes to the bilge pump. so it's actually quite a bit of work for me to separate the bilge pump from the nav lights etc.One day, when I got loads of time and energy I will separate the bilge pump than the other wiring so you can pretty much guarantee that's never going to happen. So now I've said what I would do ideally, now I've got to decide what I'm gonna do tonight. Really I've got two choices, take those two wires for the House circuit and connect them directly to a battery. Or, take those two wires and connect them to the battery switch. If I connect them to the battery switch and turn the battery switch off, then the bilge pump won't work. If I connect them directly to the battery then I can leave the lights on by accident turn this off and the lights will stay on. I think the answer to this question really lies in realistically, how many use this switch.

Am I actually going to get up every day and switch it off. And I think the answer is really no. I think what I also avoid doing is leaving it on both all the time because I think really all you've got then is just a bigger battery. You may as well do away with the switch all Together. What I think I'd like to do Realistically, is leave the switch on one, run the boat as though it is a single battery boat. if ever that battery goes flat I can turn it to two and hopefully start the boat again. What I do need to do is alternate though so I need to kind of go look how about I run on one for a week run on two for another week that way they're both getting used they're both getting charged I'm not letting one just really run flat. What I really need to do in that situation, is say right, every Monday swap from one to two so run on one battery for a week, run on another battery for the next week, and keep going like that. That means that both batteries get topped up periodically. You don't have one just sort of sitting there slowly draining way and not getting charged and you don't have one just getting used all the time charged and wearing out faster than the other. I then got the option, should I accidentally leave lights on overnight or something, that I can then swap to the other battery, start the boat, swap back to the battery's gone flat and start to charge it up. A really good reason to have an off position on the battery switch isn't so much that your batteries don't drain.

For me personally I find that when a boat sinks it's really not that hard to get the motor running again. But, if electricity is flowing through the motor, If you've got full battery voltage coming to the starter motor lots and lots of metal gets eaten away by electrolysis. That does a lot more damage than the salt water does. It's actually the electrolysis or the electricity going through it that really harms the motor. This makes me think that the time I would most likely turn it off easily if I was going away for a long time and if I'm going away for a long time I definitely don't want the bilge pump off. So my thought is to hook the motor up to the battery switch but actually have the entire House circuit connected directly to one of the batteries. If the battery switch is set to one or two then if that circuit that house circuit is connected to one battery it can't drain both batteries. So you know you're always gonna have a backup battery to start the boat. And I think that's the best compromise given the way the boat is wired up now. All right, enough yakking. I'll start doing some wiring now. What'll actually do first is show you where I'm going to put these batteries. Then I'm gonna make up some short leads to connect the batteries to the switch. And then the grounds and the batteries to each other. it's a bit awkward, unfortunately you can see I've got a battery strap there where the first battery goes. and I'm actually going to put the second one in behind it there. So they'll be nice and close to each other. But the second one is gonna be a little bit awkward because it's seen under this box here showing my usual lack of preparation I've just realized actually, I don't have much battery wire here but I'm gonna butcher a few leads and hopefully we'll make do.

This particular wire has nothing on one end and an eight millimeter ring terminal on the other so I'm just gonna put a second eight millimeter ring terminal on this end and I'll use this to bridge the two negatives. I just realized I'm pretty sure we are out of connectors to, so I'm just sort of scraping through the dregs see what I can find. I've found one eight mil connector that's big enough to go on to a battery lead so that's good .I've also found a set of these terminals that go on to the top post of a battery but they will need a crimp straight on to a clamp, straight on to the battery lead itself. The batteries I'm using and most marine batteries you find have traditional top posts like this but they also have these threaded eight mil terminals, so I'll be using a combination of the two. Inside the battery switch you've got all eight mil terminals so I know I've got to deal with that at that end. For no other reason than to give the leads a bit of symmetry from aesthetic point of view really let's be honest is I've got one leg near which has got the top post to an eight mil so I'm gonna make that one positive to the switch.

This other longer lead has an 8 mil and nothing, so I'm going to put one of these top post terminals on the end here and that'll be the positive to the switch from the far battery because it can reach a bit longer, and then on this one here I'm just gonna put the other 8 mil terminal here and this is gonna bridge the two negatives. So I'll make those legs up quickly and then we'll sort of mock them in place. Battery wires are a little bit big for normal wire strippers so I'm just gonna use the razor blades to do this just to complete the Trinity or having run out of everything I haven't got any large heat shrink left either. So normally I'd put the heat shrink on them put the terminal on or in this case I'm just gonna crimp this on and then just put some electrical tape around it with these really big terminals quite often I just use a vice to crimp it so I'll show you that. So all I do is just twist these wires a little bit, get the terminal over the end, and then just crimp the whole thing down in a vise normal small connectors you can just use pliers but these are way too big so I find us the best way I'll just put a bit of tape around this. If you squint you can imagine its heat shrink.

This leads a premade lead I found upstairs so that's all good to go. I'm sure they'll just dock it out of my pay or something. There's no great mystery to these top post terminals I'll show you. You just back these nuts off enough slot the wire in and then crank these two little ten mill nuts down to clamp on to it. Inside there you can see it's got a few little extra ridges that just help it sort of clamp on to the wire and stop it being pulled out easily what I'm going to do now is take the terminals off the old battery. I'll actually clean those terminals up. It's a good chance to just do a bit of that sort of maintenance. And then I'll put these cables on and then we'll just have a look how we go for lengths and choose somewhere to mount the battery switch itself .I've just been squeezed in under here drilling and putting the strap down for the battery box or the second box is strapped down to the deck too. I think it's really important to have a battery strapped down on a boat if your don't it goes bouncing everywhere that's a nightmare. I didn't film that bit though because it's really it's just a lot of me being contorted swearing a lot and wishing I was a truck driver. So I'll show you where we're at now. So these are our two batteries we've got this lead here just bridging the two negatives. Then we've got positive on one battery a positive off the other. I've then got the negative from the motor coming to the negative on this battery and then the positive and the negative of the House circuit coming straight to this battery.

I've also connected this house circuit to the battery that'll be closest to the outside here because I may need to change this fuse periodically. oh and I've got to mention these alligator clips are just I've got this battery on a charger at the moment because it's flat hence the motivation for doing this video. what that leaves us with up here is the positive from the outboard motor and then the positive from the two batteries. and these are the three wires that go to our switch. What I'll do next is put the batteries into their boxes strapped on the deck so they're in their final position. And that willlet us know where these three wires reach. Here are my three positives now what I'm thinking the obvious place to put this which is just onto the box here I can drill through and I can mount it here. it's nice and close it's also nice and accessible. What I'm gonna do first though is just make the connections here so battery one and two go to the top and then the common which is the outboard itself goes on the bottom one. So I'll make those connections first. Then I'll fasten it down.

I've got the common fastener at the bottom here so I'm gonna have the two batteries come in the side. And on this particular style of switch anyway these side panels just pull out so you can give yourself access to those without having them accidentally short or trying to turn this bend.It's definitely neater if you can get thinner wires that you can kind of all have coming out the bottom but in this case it's just too heavy gauge of wire to turn that sort of bend. I've got a few choices now where I put it. Part of me likes the idea of it being vertical for drainage another part of me likes the idea of having it down here on the deck it's only an upper deck, the bilge is below it. so that's okay. And this is also timber so it's not going to be as conductive should water allow it to have these positives then touch against this metal which is essentially battery ground. So I think this timber might be a better choice. It's also more out of the way and less likely to get knocked and broken. So this is the final cert I've got the two batteries strapped to the deck here then on the deck next to them there I've got the battery switch I'm not sure I'm going to leave that permanently in that position I would like to mount it vertically somewhere just for the point of view of water not pooling but it is under here out of the rain and wave when it splashes so I'm not too worried.

Next thing I really need to do is neaten up this wiring just heading to the outboard. There's these two cables really but I'm going to leave that for another day all right it's now about ten o'clock on Friday night now I'm done and hopefully the job's done. So time for a bit of testing. I've currently got the switch in the off position so I'll just go and try and crank it and then try our accessories. So cranking does nothing. but stern lighting, etc. still works. As I was explaining before, that's kind of not ideal from the point of view of being out have devices on even though the switch is off but all power is isolated to the motor which is where you'll get that really high current it'll sort of do damage to the motor should the boat sink. and if you do leave lights on you're only going to flatten one of the batteries not both of them so you can always flip the switch start the boat still. so now I'll just set that to battery one, and it cranks just fine. I'll test battery two. And now both. So it looks alright no problems, can't smell smoke. Which is always good. I'm gonna leave it on battery one just to go home now so we're gonna launch this boat and get back to the island.

Before we wrap up about the only other thing I would say I would also do is clearly label your batteries so you know which ones one, which ones two. I think that's just really important information it's easy to forget. In this case I made one the one that is closest to the back of the boat, the one that's most easily accessible. And two is the one behind it. But I'm sure that will fade pretty fast as a memory. So I'll just go on to the battery boxes and label them 1 and 2 so if I'm doing any diagnosis down the track it's really obvious what's going on. Alright, Well, thanks for watching. It's time for me to put the green machine back in the water and head home. This weekend I'm off for a couple of days for a motorcycle trip. So I'll post a few photos on Instagram. All right see ya!