BoatsExplained.com - Fitting a marine alternator, 2

Engineers seem to offer varied opinions about running the boat's diesel engine in neutral to recharge the batteries. Some (generally the older brigade) give it a vigorous thumbs down because it could glaze the bores. I, on the other hand, argue that it doesn't seem to have done us any harm over the last 10 years and - as a motor-sailer - we regularly make the engine work hard - which probably makes up for it. Our engine is the ubiquitous

BMC 2.2 diesel rated at 50 hp and our static charging has been those 30 minutes once a day, or whenever the batteries seem low, at 1200 rpm. It's interesting to note that the Mastervolt alternator mentioned earlier, is designed to give a big charge at tick-over revs.
Occasionally we left it for several days and even boasted that we had the amp hours to cope. Recent experience has changed our thinking as big batteries, well discharged take an age to pump up again. From a safety angle, a small, fully charged battery is a
better engine cranker than a huge bank only half charged.
Here, we also confess to being mavericks because we leave all three batteries ganged up, rather than leaving one isolated for starting. The reason is that our annual cruise takes in the French canals and plenty of motor sailing. Even on the anchor and in the marina, I like to follow the sensible warship practice of checking everything each day - which includes flashing up the motor - so there's no danger of running out of starter motor power and we're making all the batteries work all the time, which makes them last longer. Another useful purchase has been two jelly-filled 10Ah and 15Ah 12V batteries, originally designed for backing up a domestic burglar alarm system. They cost a mere £5-£10, while a trickle charger (run from the inverter) was just £25. They can also be recharged from the boat's batteries.
As well as providing portable power for a 12V drill and soldering iron around the boat without long cables and subsequent voltage drop, they're a good diagnostic tool. During the Brixham debacle (PB0396, Battery Management), we also had a problem with radar. The power cable has ajoin several metres before the line switch and the usual remote earth. By taking a portable battery to the factoryfitted integral cable, before the join, we fmally got it running, and concluded that the problem had to be somewhere in the cheap and cheerful switch. It turned out to be a small fuse holder shim which had slipped out of place - we soon found it once we knew where to look. That was another lesson expensively learned, so we've now done what we should have done in the first place and fitted proper marine switch panels.
The gel batteries are occasionally used to power a fish fmder and GPS on long dinghy expeditions. If you don't have a 12V supply they're fme for powering GPS and other systems at home.
On passage, one portable battery is kept connected to a small searchlight mounted on the davits to complement and enlarge our stem light. It's useful if we're being menaced by a ship approaching from astern. At anchor, the small batteries power a riding light, which dims by dawn, so I don't have to get up at 0500 to switch it off.
The batteries are then recharged during the day by a 5 watt, free standing solar panel. We have two of these and leave them inside the wheelhouse in the winter to give the batteries a charge during layup. They also power an intruder alarm.

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