I just went thru this myself last month on my Z302. I installed quite a large stereo system in the boat as well as a separate video suite in the mid-cabin so if the kids got bored coving out they could go down and watch a movie. My goal was to completely isolate the house batteries from the starting batteries when discharging yet charge both banks simultaneously. As you I’m sure you know, the typical battery selector switches in boats only charge the selected battery unless “both” is selected which allows them to self level as well which is very hard on batteries. I also installed a 140amp alternator at the same time.
Upon research I found that an “isolator” actually has a 0.7 voltage drop due to scavenging where an “ACR” is simply an electrical relay that causes no measurable drop in voltage. In addition I wanted to change out the conventional battery selector switch with one that did not need to be switched from “bank A” to “bank B” when deciding what batteries to be running or charging. This also allowed me to run two “deep cycle” batteries parallel for the house and two “starting” batteries parallel for the starter.
Blue Sea has the answer in their 7650 Add-A-Battery Kit.
http://bluesea.com/category/1/productline/overview/329 This is a pre-packaged Dual Circuit battery switch and ACR. I paid $97 for the kit on eBay. The switch simply has an ON setting which completely isolates the two banks so there is no way to scavenge power from one or the other sides and a COMBINE which in case of an emergency you still can parallel the two banks for an emergency start. In theory if the start batteries are good, this should never be needed.
Simple installation but I decided to upgrade all the wiring to 0-0/4ga at the time. I installed my new alternator then wired it direct to the starting bank (required if you are using a 3-wire alternator as the Merc brain which is part of the starting circuit is what “excites” the regulator to start the charging). If you convert to a 1-wire alternator you can run it direct to either of the banks as they are designed with a self-exciting regulator. Since my house batteries are taking the beating I had preferred to wire direct to them but I didn’t want to switch to a 1-wire alternator because they don’t come fully alive until approx. 1100-1300 RPM and I do a fair amount of idling around at 750RPM where the 3-wire is fully pumping. Currently your boat has the alternator charging thru a 6-8 gauge wire connected to the starter so the voltage travels there then back thru the cable connected to the starter from the battery. It’s always recommended to connect direct to a bank from the alternator when isolating as the whole purpose of the ACR is to open or close depending on what voltage it sees from the other bank. The ACR simply connects between the two banks with battery cable allowing the charge to flow over to the other or not depending on the need while not allowing the two to “level” when no charge source is present. You will have to put a fuse on both sides of the ACR and I would recommend marine rated breakers instead. I purchased two 150amp breakers for my install but if you are using the factory 65amp Merc. Alternator you would only need 75amp rated ones. You can find them on-line or at any Parts America, O’Reily, Napa, etc..
Hooking up the switch is simple. The main battery cable for the starting bank hooks up on one side and the second battery bank and all the smaller house wires hooks up on the other side. All I have to do is turn the switch to “on” and forget about it.
Another cool deal about the ACR is that it functions the same way with an on-board charger. I installed a 40amp Xantrex charger that is also a battery tender at the same time so I can plug the boat in on the dock and it will charge both banks without the need to split the power and run a lead to each one individually. Even though most chargers are multi-bank capable they typically split their output amongst how ever many leads are in place. With the ACR, the charger is hooked direct to my house batteries pumping out a constant 40 amps and if the start batteries happen to be a little low it will allow power to flow there but typically it will be for a very short time once again allowing the full output to be concentrated on my house bank which takes the largest beating.
This worked out to be a great solution for me. As tested, I can cove out for 6+ hrs (proven last weekend) with the stereo borderline obnoxiously loud (and the kids watched a movie) before the amps finally shut down for lack of voltage (I do have 2 group 31 Interstate batteries on the house circuit). Yes my heart did skip just a touch when it happened as I hadn’t fully drained the house batteries before but went up and typed in my code on the pad and the boat started as if the starting circuit had no idea its sister bank died. Drove the boat 30 minutes back to the dock while continuing to play the stereo and the new alternator was able to put out enough amps to not only keep it playing but also put a charge back into the house bank thru the ACR. Plugged the charger in at the dock and the bank was fully charged in 1hr 40mins. I’d call it a success. We would rarely put that kind of draw on the boat as 4hrs are about all we can stand out in the cove but the weather was perfect and the beer was cold.
I liked it so much I just installed the same kit on my Malibu ski boat to keep the stereo separate from the start battery.
Hope this helps.
Dave
Battery Isolators and ACRs
