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Old 06-24-2005, 05:03 PM
Diesel's Avatar
Diesel Diesel is offline
MC Devotee
Join Date: Aug 2004
Boat: 95 Prostar 205
Location: Tulsa
Posts: 1,623
Since I have posted a few times on the topic I have had quite a few requests and PMs regarding dual battery setups and isolation. As a result I have composed the following guide to help you wire up a dual battery system using a heavy duty isolation relay. I have been using dual battery relays for over a decade in all kind of vehicles and I have found the relay to be the most effective and trouble free method yet (and yes I have tried just about every other method!). This is a very straight forward, easy installation that just about anybody should be able to accomplish in a few hours. Please use this at your own risk and please understand the following information has been obtained solely from my own personal experiences. If you have any questions or need help please ask. Also if I left anything out please let me know. With that said here we go…….

The easiest way to add an additional battery to your boat is to simply hard wire it in parallel. To connect the batteries in parallel you simply wire the negative post of battery 1 to the negative post of battery 2 and the positive terminal of battery 1 to the positive terminal of battery 2. In this configuration the two batteries act as one large 12volt battery and you get double the capacity. There are a few problems with this method. First, when not in use the batteries will continually charge and discharge into each other eventually ruining both batteries over time (I am speaking from experience here!). Second, since the batteries are permanently connected you run the risk of running down both batteries while the engine is not running. Yes, you get double capacity but after a day of running the stereo at the beach you still risk having two dead batteries and not enough juice to get back to the dock. The solution: a dual battery relay.

The two batteries are still wired in parallel but a relay is inserted in the positive cable to isolate the batteries when the ignition is OFF or in the ACC position. When wired properly the relay provides complete automatic control of the battery bank by simply using the ignition key. With the key in the OFF position or in the ACC position the relay is not energized and the positive circuit between the batteries is open. There is nothing to remember, switch, and no fear of accidentally disconnecting the main battery with the engine running potentially harming the charging system. In theory it works the same as an on/off Perko switch but instead of manually turning the switch the ignition key acts as the switch. Once installed you can forget about your batteries and enjoy the water! I like this because I do not have to remind or provide specific instruction to my wife, father, friend, etc….. when they drive my boat.

Below is a schematic that details the connections required to install an isolation relay. I have used a Borg Warner R3098 for many years in both my boat and jeep. The BW R3098 is identical to Stinger’s SR200 and can be found at most local auto parts stores. It was used in late 80s early 90s GM C/K3500 trucks for battery isolation and was mfg specifically for this purpose. It has a continuous duty rating of 200 amps and a peak of 650 amps and usually costs around $30. In its natural state (not energized) the relay is open which provides isolation. If your alternator has the potential to output more than 200AMPS you will need to find a different solution. Currently the largest alternator MC has offered from the factory has been the 05 and later models at 90amps.

Start by hard wiring the negative terminals between the two batteries using 2 Gauge cable (negative to negative). If possible try to keep the batteries as close together to save on battery cable expense. I have found most auto part stores have pre-made battery cables on the shelf in various lengths that work very well if the batteries are fairly close to each other. On marine Optimas (as well as any other dual post marine battery) I prefer to uset the 3/8 stud on top or on standard Optimas you can use the side posts. This will keep the aux negative connection out of the way of the MC factory top post connectors (engine) and make it easy to unhook the system during storage.

The positive connections are also wired positive to positive but a Borg Warner R3098 is used to open the connection to isolate the batteries. The relay has for posts on the top, two 5/16 suds (large) and two #10 studs (small). The 5/16 studs are connected to the positive terminals of the two batteries. It does not matter which way they are connected. Most BW relays do not come with 5/16 nuts so you will have to find a couple. I prefer Nyloc stainless steel 5/16 – 18 NC nuts to prevent corrosion. My local auto parts store stocks these as well and are in blister packs out on the floor. Here is a pic of the relay:

The small #10 studs are used to energize (close) the circuit. One small stud is wired to a ground and the other is wired to the ignition circuit of the boat. Again it does not matter which stud is used for the ground and which is used for the ignition circuit. For the ground I usually just use the closest battery and use a large ring terminal to hook it up. The other small terminal will be connected to the ignition source.

The easiest way to find the ignition circuit is to gain access to the back of the key. In all the MC boats I have done the ignition circuit has been a purple colored wire. Keep in mind there are many other purple wires in the harness so it is best to use some type of circuit tester or Fluke. If your batteries are in the back of the boat and you are confident in finding the ignition circuit you can open the harness near the engine to find the ignition circuit to save the aggravation of running a wire all the way from the dash. By far the easiest way is to locate the ignition terminal on the back of the key and run a wire from the dash to the remaining small terminal on the relay. Once wired up you can test the relay with a Fluke or you can just listen for an audible “click” from the relay when the key is turned to the ON position. If using a Fluke to test it should only show continuity (closed circuit) when in the ON position. In ACC and OFF the circuit should show no continuity and should be open. If using the your ear to test, the relay should click once when the ignition is turned to the ON position and click again when brought back to the off position. It should not click when the key is turned to the ACC position.

The only thing left to do is to relocate all the accessory components to the auxiliary battery. This usually can be accomplished by moving only the amplifier battery cables to the aux battery. In most systems I have not had to rewire the head units to the aux battery. If you are using external amplifiers the amp draw from the head unit is so small that it would take a long time to draw down the main battery. If you do run into a situation where your head unit is draining the main battery I would start by relocating the constant lead of the head unit to the aux battery and leave the ignition wire and ground hooked up to the factory wiring. The ignition circuit on the head unit should only pull a few mA which should not drain the main battery.

Diagram 1:

05 ProStar 209, Navy Flake, Slate Gray, MCX 1:1

Last edited by Diesel; 06-24-2005 at 05:09 PM.
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