BB marine":1wxjti8p said:
Dan I agree 100% that it is a risk as to where the charging system is wired and which 12V circuit and battery the alternator is regulated by. I have always used the engine battery...
And using the engine battery is the historical norm. But managing risk requires understanding what you (truly)need and answering a lot of "what if?" questions. Consider:
Risk is defined as probability of failure times the resulting consequence. Risk management involves defining an operating context(what's needed), identifying the associated risk(what if), and mitigating the unacceptable risk through design or operational measures.
Recreational boating design tends to ignore the consequence side of the risk equation unless there is a
direct threat to human safety (or catastrophic engine failure). And understandably so. If they find themselves dead in the water the vast majority of recreational boaters can simply call for help and anticipate a reasonably timely response. Another aspect of recreational boat design is to almost completely ignore operator intervention as a means of risk management. Again rightly so. The vast majority of recreational boaters aren't equipped to manage complex systems. As demonstrated by much of the discussion in these forums. So there's the operating context for the recreational boat engine and electrical system. It's no big deal if it won't run and don't count on the crew to keep it running.
Unfortunately risk is difficult to quantify in hard numbers. But we should be able to understand our own operating context. And both reality and perception of risk change with that operating context. My entire boating career has been spent 1) operating charter and commercial offshore fishing boats, 2) blue water sailing, and 3) travelling to remote waters in Alaska. So my operating context is different than the previously mentioned vast majority. Honestly those guys should just ignore my posts when they see them. My input in these forums are typically for those who can't/don't want to rely on timely help or who want to proactively manage and/or repair their onboard systems. Of course simply inquiring minds are always welcome.
To be honest I have never seen a recreational watercraft use one battery start the engine and another battery dedicated to power the ECM...
Nor had I until I joined the world of Ranger Tugs. My immediate inclination was to rewire it "properly", i.e. ECM powered by the starting battery. But before I got around to it I had learned a lot more about my modern recreational diesel engine. With that knowledge and my operating context the greater perceived risk on any given day changes depending on how well I slept. IOW it's a toss-up which way is "better". Maybe I should install a (uninterruptible)switch on the ECM power supply so I can change it based on mood.
The risk is not doing this(powering ECM from starting battery)...
There is no THE risk. It's A risk. Based on operating context.
What is the voltage threshold of the ECM before it switches off? What happens(holistically) if it does? What's the reserve capacity of each battery bank? What loads are on each bank? How much direct control does the operator have on said loads? What is the amp load of the ECM? What if I have multiple failed start attempts? What if the alternator fails?
My boat came to me with the ECM wired to the thruster battery. Based on what I've been able to piece together that was due to the early starting problems with the Mercruiser engines. The ECM low voltage threshold was too high and the batteries supplied by Ranger were too small. So during start the voltage dip caused the ECM to shut down. The early fix was to wire the ECM to the thruster bank(which is typically idle during engine start). Mercruiser eventually updated the ECM to a lower threshold. Despite that when I took over ownership of this boat the only way it would start was with the house/engine parallel switch engaged. I installed properly (over)sized starting battery capacity and haven't had any issues since. But by then I decided that powering the ECM from the thruster battery wasn't necessarily a bad thing. Particularly after I moved the alternator cable off of the house bank to the thruster/ECM bank and wired the ACRs correctly. As long as I don't operate the thrusters or windlass without the engine running it's a pretty robust system. All that said next time I replace the starting batteries(with plenty of reserve capacity) I will likely wire ECM and alternator back over there. Maybe....
Based on this statement
Wolfboy":1wxjti8p said:
After about 3 minutes the first charging relay would combine the house and thrusters, and about 3 minutes after that the second relay would combine the thruster and engine batteries.
He does have ACR's with older specs.
If it's truly taking three minutes for each ACR to close that's well beyond the threshold of either generation. So that fact alone doesn't indicate one or the other. Something else must delaying the batteries reaching the ACR threshold which starts the timed switch. So we don't really know when the timer starts and therefore what the delay is.
