Beyond that distinction, my general understanding is that the cabin DC items (lighting, televisions, stereo, refrigeration) draw from the port battery and that the starboard battery supplies start power to the generator. After that you have to decipher the electronic schematics which are engineering drawings in the manual, spread over about 8 pages and difficult to follow as uplinks and general logical layout aren't easy to follow (it would help if it was color coded and had some kind of block diagram of major segments).
It's not clear at all to me where navigation lighting, cockpit lighting, helm accessories or other dash power is derived from. I have a vague understanding that it's split between both batteries.
Obviously good DC power is critical on a cabin cruiser -- at a minimum, you need to be able to start the engine. And with all the accessories on board capable of running off DC power, monitoring battery status becomes kind of important. We have the advantage of a 5 kw generator, but obviously in many cases we don't want to run it or run it all the time.
I've been looking into adding DC power monitoring. One basic solution is adding an ammeter to monitor instantaneous DC loads. Knowing the general battery Ah capacity you can guesstimate runtimes off straight DC power by knowing your load. This is a crude method, though, so I've been looking at more sophisticated state of charge monitors that will tell me how much life the battery has left as well.
Most SoC systems and all ammeters require a shunt, an electronic component that produces a tiny bypass voltage the meter actually monitors. The shunt needs to be inline with a cable from the battery (most use the negative side). This is all fine until you look at the cabling associated with a shunt.
Shunts are basically bare, surface-mounted terminal blocks meant to be mounted on a panel with the pass-through power and monitoring leads attached. The battery cables, however, are "optimal length" for the existing install and don't include the extra length for shunt mounting and require you to make up or buy the second leg of the cable if your mounting location isn't right near the DC bus.
Of course on my boat, there's not really space close to the bus for this or enough extra battery cable length to locate it where there is is space without making up and running a new cable. At $12/ft for 4/0 tinned cable, plus connectors and paying someone to crimp them for me (my hydraulic crimper won't do larger than #0), this gets expensive not to mention frustration inducing.
I'm actually surprised nobody has made up either an inline shunt that could be mounted on the battery side. No mobile DC system built *without* a shunt will easily accommodate one without substantial rewiring.
There are two interesting alternatives, though. The first is a very slick Maretron system which uses a ferric ring called a Hall effect transducer that just slips over the battery cable (no splicing or re-cabling required). The downside is that it's an expensive system, probably running in excess of $1600 for the required components and cabling, although it is NMEA2000 compliant, meaning it can talk to other boat systems -- which I don't have anyway.
The next I just found out about, the Balmar SmartGauge. This requires no shunt or transducer, just leads to the battery posts. Nobody seems to know exactly how it works, but my condensed wisdom is that it probably samples voltage and pulses the batteries for internal resistance and has an algorithm for determining battery state of charge from models of these values. It doesn't show instantaneous amperage, but nearly everyone says its as or more accurate than a shunt SoC meter and is self-calibrating/learning over time, including as the batteries age.
This one is likely to be the winner for ease of installation and accuracy of the most important information -- how much battery is left, and the bonus is that it can monitor two battery banks. Everyone else's requires multiple meters, multiple shunts, etc.
It's an open question whether I'll actually "get to it" this season. Replacing the batteries with proper Group 31 AGM cells is a $1200 check and I also will probably need to replace the cabin television with a digital broadcast model (my family insists), and I want to replace all the lighting with LED bulbs, which will definitely cut power consumption on batteries.
For now we'll just have to be kind of cautious about DC power consumption and be slightly more aggressive with generator usage, perhaps running it at anchor more often or for longer periods at night. And we'll have shore power at the slip to run the battery charger to keep the batteries optimally charged when start out.
In an ideal world, much of this would already be in place and/or automated, including automatic generator start so you'd never have to think about it. When the batteries hit 60% charge, the generator would kick in and run until the batteries were charged. I don't think that's warranted for our use, but definitely something on a wish list.
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