I’m still puzzling about the batteries to use on Dulcinea – the more I search for information the more conflicting views I get! There is a bit of a potential problem with Lithium batteries and the alternators fitted to new engines that produce a lot more power than the old ones. One problem is that the alternators have a fairly low efficiency, meaning that they waste up to half of the power the engine puts in to them as waste heat. They have a built in fan that will keep the alternator cool at high engine revs, but if they are producing a lot of current at mid revs the fan is not very effective, and the alternator can overheat and destroy itself – this is particularly likely to happen with the Lithium batteries as they have a very low internal resistance and so can mop up lots of current. The most common way to avoid this problem is to have a moderate sized lead acid battery as the engine start battery as they have very little work to do and tend to last for many years and charge this directly from the alternator, then use a crafty bit of electronics (called a Battery to Battery charger – B2B, or a DC-DC charger ) to take spare power from the engine battery charging circuits and convert it into a suitable voltage to charge the main domestic battery bank. In this way it doesn’t really matter what the domestic batteries are, as the B2B can adjust its output voltage to suit whatever battery you choose. You can also choose the B2B charger so that it won’t take more than maybe around half of the alternators maximum output, and as the engine starting battery is almost always kept fully charged ( starting an engine takes almost no power, just a high current for a few seconds) you can in that way restrict the power the alternator puts out to safe levels. We have a battery in the bows for the bow thruster ( to keep the wires short) Since the bow thruster is only used momentarily and very rarely that battery doesn’t require much charging current, but does need to be able to supply a heavy current – up to 500 Amps – for typically 10 0r 20 seconds – run it for over 2 minutes and there is a risk it will burn out. The bow battery could be wired in parallel with the main battery as it is now, although that would put a big current through the rather small cables joining the domestic and bow battery – better would be a second small B2B also running off the engine battery. The advantage of the B2B based system is that it then doesn’t really matter what type of batteries the domestic and bow use as long as its suitable for that function, so we can start with eg. lead AGM batteries and later change the domestic to Lithium. From my perspective the big advantage of Lithium is that I can lift them into the boat, whereas lead batteries are just too heavy to manipulate.
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