gellcells on cruising boats (long)

gellcells on cruising boats (long)

Post by Stan Hone » Wed, 02 Nov 1994 14:00:17


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.

Stan Honey
Cal 40 Illusion
San Francisco
wa6iva

Battery Comments... part 1/2
Stan,
   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
completeness.
---
   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...
de KE6RK

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
189,000.
   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
really using?
   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
causes...
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, ...

read more »

 
 
 

gellcells on cruising boats (long)

Post by Rod Mc Inn » Thu, 03 Nov 1994 02:31:52

    <lots of relevent and good stuff deleted>

Quote:
>---
>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.

        A simple AC ammeter would not work as the current pulses are *not*
AC.  Your concern is valid, however measuring the current would require
current probes and an oscilliscope.  You could attempt to measure the
AC voltage, and infer an associated current.  

Quote:
>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, assuming they filter the charge and don't ask the
>battery to do that...

  The worst units are the cheap automotive type chargers, that have no
regulation at all.  Any switcher or SCR style charger will have
regulation, as will the linear charger with a regulated ouput.  A
common type that has no regulation, and hence a heavy pulsed current,
are the ferro resonant chargers.

Quote:
>---
>Enough for now, Please keep me posted and I'll do likewise.
>Thanks again, and all the best


  Rod McInnis

 
 
 

gellcells on cruising boats (long)

Post by Darrell Irvin;685-2277;60-850;;sptekw » Fri, 04 Nov 1994 06:05:39


|>
|>   A simple AC ammeter would not work as the current pulses are *not*
|> AC.  Your concern is valid, however measuring the current would require
|> current probes and an oscilliscope.  You could attempt to measure the
|> AC voltage, and infer an associated current.

I disagree... The correct kind of ampmeter will measure the  current.  The
problem occurs in that the current waveform is not sinusoidal.  Thus a simple
ampmeter which assumes a sine wave will give a bogus reading.  A true RMS meter
will give a good value (assuming the ripple frequency is within its range) but
will give no clue as to waveshape.  The current probe is the best approach
however converting to RMS value without access to modern digital signal processing
scopes is tough for some waveshapes.  The comment about measuring AC voltage
is partially correct.  Due to the nonlinear characteristics of the "battery"
load you can't infer current by looking at the voltage.  You would need to
use a low value series resistance chosen so its drop is unimportant to the
voltage levels and currents being measured ( aprox .01 to .005 ohms) and then
measure the AC waveform across this sense resisistor using a differential plugin
on the scope.  These instruments are expensive, professional ones and hard
for most people to get access to.  I am lucky in that I work for Tektronix
and have access to such instruments.  I have done the type of measurments
you mention.  If you want to get funny looks, drag a $30,000 Lab scope down
to your boat in the marina someday!

|>
|>   The worst units are the cheap automotive type chargers, that have no
|> regulation at all.  Any switcher or SCR style charger will have
|> regulation, as will the linear charger with a regulated ouput.  A
|> common type that has no regulation, and hence a heavy pulsed current,
|> are the ferro resonant chargers.

I agree, although I did have a cheap "SCR" unit.  It failed with a shorted
SCR and boiled a battery dry.  I have since converted to a West Marine
20 amp smart charger.  This is a highfrequency switching architecture
type of charger which uses a microprocessor to control and regulate
the voltage and current over time using the desirable "three step"
charging sequence.  Water usage is minimal (add maybe a couple of tablespoons
of water to each cell every 3 to 5 months, specific gravity is always right
where it should be.  The charger is running at all times the boat is in its
slip.

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gellcells on cruising boats (long)

Post by Craig Junge » Sat, 05 Nov 1994 03:14:37


Quote:
>.......  The charger is running at all times the boat is in its slip.

Is the "smart charger" isolated from the AC line (like, with a
transformer)? Otherwise, of course, you are setting up a fine case
of stray current corrosion for either your boat or someone else's
boat close to your in the marine - or both.

Perhaps the subject of stray current corrosion should be addressed
since most people posting (or lurking) here leave their boats in
marina slips at least part of the year.

                 Craig Jungers  ---  Royal City, WA 99357
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