Assaultin' Battery: A New Look at the Switching Process

Its been very quiet on the boating front for a few weeks now. After much action over the Christmas break, the long-suffering wheel bearings finally gave up the ghost, and we came very close to losing a wheel on the way home from the marina after the last trip. Closer inspection revealed that little of the hub and axle was salvageable, and thus the HMS Wee Guy will enjoy a well-earned rest until the funds for new hubs avail themselves.

Personally, however, I’m not much of a fan of rest, and I’m even less a fan of things well-earned. With work having dried up at my temp job, I’m time-rich and currency-poor, and have begun to look for things to do on the boat that are congruent with the aforementioned conditions. 

I had a gander round the boat, and within 8 seconds or so had found enough to keep me busy till at least June.  This included a lot of things that I’d always planned to sort, but had been filed in the non-urgent basket, and I decided that the first of these should be the leisure-battery system.

Initially, the extra battery was just a way of adding ballast, as the front of the boat was still sitting high after the swim-platform addition. Both batteries are located in front of the cabin, directly under where the foredeck begins, with heavy cables running beneath the floor to the engine. They have proved useful in this capacity, and the boat’s static trim has been much improved by their presence there. I plan to add a stereo in the future, plus lights and an electric trolling motor, and so it makes sense to isolate the second battery so that these things can be used without danger of running out of starting power.

Blue to IGN, white to solenoid terminal,  two big ones go to the' + terminals

The basic principle is simple: a standard relay connects and disconnects the batteries, so that they’re connected when the engine’s running (and charging), and disconnected otherwise. To have two batteries cranking the engine is best avoided however, for a number of different reasons, and therein is a design problem. What circuit could be used to connect the batteries when the ignition’s on, but not when the starter’s engaged?

I’ve heard of multiple means of switching them, which all fail this basic requirement. The ignition circuit is commonly used, but this has the batteries connected while starting. The oil pressure sensor can be used to trigger the relay, but an engine can build oil pressure long before it actually fires, and hold oil pressure for up to a minute after its switched off. Therefore the second battery could be engaged while cranking, nullifying the benefit.

I think I may have come up with a solution however. Having repaired a starter motor recently, I remembered a design feature that we could use here. A starter at rest has almost no resistance(voltage drop of 0.3V), and thus the outgoing terminal on the starter solenoid can be used as a ground. When starting however, it becomes +12v, as the solenoid is now providing voltage to that side of the starter. Therefore, if we connect the negative side of the relay trigger to this terminal, and the positive side to the ignition circuit we get a voltage difference of:

12V when IGN on, and starter not cranking  (relay will connect batteries, for charging when started)

0V when IGN on, and starter is cranking (relay will disconnect batteries as long as starter is running, after which returns to above state)

0V when IGN off (relay disconnects batteries, can thus use in isolation when at rest)

This is the best system I’ve come across I think. I’ve wired it all, just need to get some terminals for the house battery and she’s done! I’ll be working through the list over the next few weeks and should be updating again soon, so stay tuned. Happy boating!