Although my idea on tapping one battery on the assumption that there can never be battery mismatch if the bilge pump generally never runs, in practice, as Byron, for one, points out, there are likely to be more items than the bilge pump that need 12 volts.
This battery equaliser sounds like the cats meow for mixed volts, the only thing might be to rotate the batteries once in a while, if the 12 v always comes from the 'end' battery. Unless, again, the actual scenario is that the pump or other equipment generally never actually runs.
There is a cost to balance out, too. How does the cost of this unit stack up versus a minimal dedicated battery and trickle charge solar panel? And if the boat and 12 volt useage is high, such as daily, then once again, it depends on the usage compared to charging rate available.
In my opinion, if the situation is just to have the bilge pump standing by in case of unexpected leaks, and it never normally runs, then there seems little point in spending money on wiring up something high tech. If a leak does occur causing the bilge pump to run, surely the least of the problems will be propulsion battery discharge imbalance.
John
2f. Re: 48V to 12V
Posted by: "chris Baker" chris@currentsunshine.com creativeengineeringaust
Date: Sun Feb 13, 2011 12:25 pm ((PST))
Another option for supply 12v is tap one of the 12v batteries of the
series (and don't worry Denny there's more...) and use a DC-DC
battery equaliser.
This is a special DC-DC converter that converts at a 4 to1 ratio,
rather than 48 to 12. The 48v side of the converter is connected to
the 48v pack, and the 12v positive lead is connected to the 12v tap
(which needs to be the battery whose negative pole is the 48v
negative.)
This equaliser sits there minding its own business until it sees a
mismatch between the 12v and 48v. That is, if the 12 v side has a
dropped to lower than 1/4 of the 48v side it will direct power to the
12v battery until it comes up to be equal again.
The cool thing about this is that you can have large transient loads,
such as a 12v anchor winch, because its pulling the current out of
the battery but which would otherwise be far too much current to be
supplied direct by a dc converter. Then the equaliser can take its
time about balancing them again.
Another thing I like about this is that it is bidirectional. If the
12v side is high, the current will flow to the 48v side until it is
equalised. The point of this in this discussion is that it has been
mentioned that a dc converter does draw some small amount of power
all the time, even if it has no work to do. So the option of adding
a small 12v solar panel to account for this load is still possible.
And it will keep the whole pack topped up, not just the 12v tap.
You can get these from Solar Converters at
http://www.solarconverters.com/eqlist.html
I've order one of these but it hasn't arrived yet and so I can't
offer any experience of its performance.
Cheers
Chris
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