Many rec.boaters know Jim Corenman, who has been sporadically active on
rec.boats when he is in the US. Mostly Jim is off with his wife Sue cruising
in the South Pacific on their Schumaker 50, Heart of Gold. Jim is an active
ham, and I have been maintain
ing a continuing discussion with him via AMTOR. One of the long-running topics
has been the use of gell batteries on cruising boats. Jim has summarized the
discussion, and I am posting his summary here. If anyone, who is a ham, wants
to respond directly
If you are not yet registered, you will get instructions. If any non hams want
to send mail to Jim, send it to me and I'll forward it.
Cal 40 Illusion
Battery Comments... part 1/2
Got all of your battery notes off W7DCR a couple of nights ago, and
thanks for your comments. Thanks also to Beau and Chuck for their
input, much appreciated.
Feel free to edit any of this stuff as a follow-up posting ... and
would-be cruisers (with ham licenses) that want to write directly.
I've tried to consoldate all my thoughts and responses so far, and
my appologies for re-running a bunch of basic stuff for the sake of
The motivation for using gels is no acid, higher output voltage,
good charge rate, and the fact that they seem happy spending their whole
life between 50 and 75% charged, without the wet cell's need for period
full charge/equalization cycles to avoid sufation.
The disadvantage is higher cost, and a marked sensitivity to abuse.
The two things that are *** gels seem to be long-term overcharging,
and leaving a battery at a low state of charge (e.g less than half charge
for more than a day or two).
Chuck mentioned that the batteries can't be allowed to build enough
pressure to cause venting... I think it's a lot more critical than that,
and I have always understood that gels don't want to gas AT ALL, never
mind the pressure. Once the battery is fully charged, any additional
current will start to decompose the electrolyte and cause damage. There
is a critical voltage, the so-called float voltage, that is high enough
to allow the battery to eventually reach full charge, yet not high enough
to allow it to continue to draw current once charged.
This float voltage is temperature dependant, and for long-term float
(e.g more than 7 days) requires careful regulation and temperature
compensation. I have to agree with you, Stan, that it's a lot easier
(and safer) to simply disconnect the charger once the battery is charged.
The temperature vs. Float voltage curve that I have (from Sonnenschein)
is as follows:
10C (50F) 14.16
20C (68F) 13.80
30C (86F) 13.50
40C (104F) 13.30
50C (122F) 13.20
The critera on boost/acceptance voltage is now obvious, thanks to Stan.
The actual voltage is unimportant (within limits), because whatever it
is, you have to reduce it before you get to the point where you would
have to worry about what it is.
When you switch to float doesn't really matter either, as long as the
battery is not yet close to full charge, and the 10% (0.1C) rule is as
good as any. The usual cruising regimine, of pounding the batteries hard
as long as the engine is running, and then quitting when the current
begins to drop, isn't that far from the ideal.
So charging from an engine alternator, assuming a proper regulator set
in the right ballpark, almost certianly can't be the cause of many problems,
because the engine isn't charging for all that long. It's hard enough to
get the batteries to full charge, much less overcharged.
It seems the failures must stem from overcharging over a very long
period (i.e. weeks), or leaving the batteries in a discharged state.
(In a note about storage, Sonnenschein recommends not allowing self-
discharge to take the battery below 50% charge... At 20C, that's 16 months).
Beau mentioned high inverter loads... Good point, but I don't think it's
a factor here. The folks we know that have had battery problems don't have
large AC loads or big inverters. Gels have a much lower internal resistance
than wet cells, and can put out heaps of current.
Part 2 to follow...
Battery Comments... part 2/2
Now, about lifetime... I agree that East Penn's 200-cycle number is
too low. So far we've done 1100+ partial cycles (25% average depth of
discharge), to a remaining capacity of 400 AH, 75% of the original.
Eyeballing the graph in the West catalog, at 75% remaining capacity, we've
used up 3/4 of the rated life, so we should be able to get 1500 partial
cycles by the time we reached 50% capacity, or 375 equivalent full cycles,
remarkably close to Chuck's number.
It seems to me that the right way to look at lifetime is in terms of
total amp-hours available... For an 8D, for example, 225 AH times Chuck's
350 full cycles is 78,750 AH, use them any way you want. Just like an ice
cream cone, if you take little bites, it will last longer. In these terms,
we have gone through approx. 137,500 total AH to date out of an available
As an aside for Chuck, the golf-cart battery characteristic of
maintaining full capacity to the end of life, and then failing completely,
would make me more than a bit nervous... we know some folks that have
experienced that *** characteristic in the islands, and were searching
for batteries in places like Neiafu, Tonga. While it would be nice if gels
maintained full capacity longer, a linear decay is still prefereable to
sudden death, when we're this far from West Marine.
So jumping back one paragraph, a good way to anticipate battery failure
would be to monitor total amp-hours consumed to date... Do any of the fancy
amp-hour meters keep track of that? Why not??? To me, that would be a much
more useful number than all of this hocus-pocus of trying to keep track of
the current amp-hour state, which never works that well because the
batteries rarely reach the full-charge reference point. I KNOW we're
putting in as much as we're taking out, long-term, but how much are we
Beau's point about monitoring voltage and amps is a good one, though,
and may be the key to this whole mystery. I am thinking that the Number
One gel problem is overcharging in float mode, with a variety of possible
1) motoring for days with a grossly mal-adjusted regulator. Some batteries
are located in the engine box (or next to it), and high temps might mean
the uncompensated float voltage is off by a bunch (it wants to be 13.20
at 50C). This is less likely than the following, however:
2) Plugging an AC charger into a dock for weeks, without a properly
set float voltage. Long-term float must be carefully done. The worst will
be an old-style tapering-type charger, which is almost guarenteed to over
charge the batteries in long-term use. Most new charges are properly
regulated, but only a few of the best are temperature compensated
(e.g. West's 20 and 40 amp chargers have an optional temp sense)...
this alone could be a big factor in warm climates.
Another big potential problem with AC units is ripple current. The
voltage presented to the battery must be a pure DC voltage in order to
maintain an accurate float voltage, but meters read average DC volts,
making this a hard problem to detect (see discussion below).
3) Unregulated (or badly regulated) solar or wind chargers, whose
output exceeds the daily consumption. I think many wind generators are
unregulated, since their outputs are too high for a small series
regulator, and may or may not incoporate an internal or shunt regulator.
A typical scenario is to lock the boat up for a few weeks and leave
the wind/solar chargers going with no loads on the batteries.
I think the biggest potential problems are bad AC chargers left plugged in
for long periods (e.g. in New Zealand), and poorly regulated solar and wind
chargers during periods of light battery loads (e.g. when folks are off
the boat). The boats I know with problems all have a couple of solar panels.
I am still looking for more individual experiences to try to fit into
this hypothesis, but my feeling right now is that it doesn't make sense
to invest in gels without an up-to-date electrical system, with proper
regulators on ALL charging sources, a digital monitor to keep track of
volts and amps at the battery, and some way to check battery temperature.
Now for what I understand about ripple current...
An AC charger with inadequate filtering will not present a pure DC
voltage to the battery, but a series of voltage (and current) peaks
coincident with the peak of each AC cycle. The battery will act sort of
like a big capacitor, soaking up the current peaks without showing a
voltage peak, and the charger's regulator circuit will be happy as a
clam as long as the average voltage is maintained at the float level.
The analogy breaks down, however because the capacitor will eventually
turn the current pulses into an increase in voltage, while in the case
of a fully-charged battery, the current peaks are not stored but go into
decomposing the battery.
The only way to catch this problem is to measure the AC component of
the charge current, by putting an AC ammeter between the charger and the
battery to measure ripple current. Milliamps is OK, but amps certianly
isn't. There is no user solution, other than a new charger with proper
filtering and pre-regulation. Sonnenschein has a LOT of discussion of this
problem in their industrial literature.
It would be interesting (and could be very important) to test the marine
chargers to see if any have excessing ripple current incompatable with gels.
The hi-freq units like West should be OK almost by definition, since they
don't operate at 60 Hz, ...
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