However, I have had some problems with the auxillary batteries. To explain, over winter I do not 'winterise' but simply check the state for everything from time to time, and guard against a freeze-up (when the weather requires this) by turning on the central heating boiler's pilot light and the water heater's pilot light if a mere 0 degrees C is forecast, actual central heating on (at frost setting) for a sub-zero forecast.
For a start, with the boat merely static on its mooring, recharging of the 'standard' 12V 'start' and 'domestic' batteries became rare... and I found that the meters revealed they were losing charge quite significantly when left alone for more than a couple of weeks - easy enough to rectify, simply hit the switches to recharge them from the 'drive' batteries via the 72:12 DC:DC converter.... for a while...
BUT, they did need more than a few minutes. As previously wired, to leave recharging switched on most of a day (to give healthy recharge) meant leaving the key switch on with the boat unattended, so I did a long overdue rewire so that the relays connecting the batteries (for re-charge) to the output of the converter was powered by that converter - which itself is powered when the main 72V ON switch by the steering position is on.
A long overdue rewire since previously the relays had been powered by 'key on' from the start battery... and when this had been near-drained for the drive motor cooling fan coming on thanks to water in its relay, the problem was the relay wouldn't go over because its power source was what you wanted to recharge!!!
I didn't have to do anything very drastic - there was still some battery power left, and a light tap on the relay case caused to to flick 'on'..... and recharge to take place.
Note that the drain on the 'drive' batteries caused by even six hours of recharging the auxillary batteries was easily replaced by running the generator for a mere ten minutes or so - and the 'drive' batteries showed absolutely no sign of losing charge unless I used them for this recharging process. (Mind you, they do have the 5W solar 'top up' 'on' all the time.)
However, a new problem arose when I started using the 'domestic' battery more for needing to run the full central heating - this battery provides the central heating pump. This, as expected, meant the 'domestic' battery definitely needed a recharge after a night's use. BUT, this started to fail... recharge appeared to be OK but a small further usage meant the 'domestic' battery voltage fell quite rapidly for even slight use.
The 'domestic' battery came new when the conversion was done, so I was rather surprised that I could only deduce it was 'failing' (as conventional lead-acids can) simply for being 'old' - compounded, perhaps, by the fact that it never had very regular re-charging. After further attempts to give it a decent recharge (via the converter) it became clear it certainly had something very wrong since it blew the converter (attempting a longer recharge!!)
For the time being, I managed to get an alternative converter to recharge the 'start' battery quite rapidly - and simply re-wired for this battery to also power the 'domestic' functions, for the time being - all of which has worked perfectly OK - except the the 'start' battery certainly needed a decent recharge after running the central heating pump all night.
However, this sequence of events shook me out of accepting the 'standard' arrangements we had - separate auxillary batteries for 'start' and 'domestic' function charged after use via the converter from the 'drive' batteries. And caused me to think harder...
For a start, what would happen if I left the converter 'on' powering the 'start'/'domestic' function? Answer, the 'start' battery became very thoroughly recharged whilst, in fact, the central heating pump had been driven by the 'drive' batteries (via the converter.) BUT, this was a drain on all six, so the drain per battery was really quite tiny.....
Consideration hit as another eureka moment. I didn't need a 'domestic' battery at all - all those functions could be powered via the converter from the 'drive' pack. The consumption involved I could easily calculate was relatively tiny compared to their normal use driving the boat - no question they would somehow accidentally become discharged in any significant way at all without me noticing - and easily recharged by the generator in very short times.
This thought caused me to wonder if I actually needed a separate 'start' battery - clearly our conventional one needed care to be sure to keep it OK by 'serious' recharging sessions... fom the 'drive' batteries which equally clearly didn't need much care... to be sure they would lose absolutely ideal plate voltage when left alone for two-three weeks... but only fractionally and a very short session of generator (around ten minutes) would have them right back to normal values.
I'm as cynical as anybody about sales' claims for devices, but I think I have seen for my own eyes that Odyssey -type batteries simply do not lose charge if left reasonably near full re-charge... whereas I've also seen that 'standard'-type batteries DO, and by contrast.
Overall, an idea was forming in my mind that (a) at least for all our domestic purposes, I might as well power them via a converter from the drive batteries and (b) we might be better to use one of the 'drive' batteries as our 'start' battery taking advantage of the superb Odyssey properties.
That's enough talk for this posting... I'll write another about the considerable trouble I took rethinking and testing as regards our battery requirements - end conclusion all-but verified is that if you have 'drive' batteries for a conversion, as we have, you do not need any separate/other batteries.
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