Re: Advice on refridgeration unit please

Bruce in Bangkok wrote:
On Sun, 20 Jul 2008 08:15:36 -0400, jeff <jeff@xxxxxxx> wrote:

Bruce in Bangkok wrote:
On Sat, 19 Jul 2008 22:33:47 -0400, jeff <jeff@xxxxxxx> wrote:

Bruce in Bangkok wrote:
Not to disparage anyone but have a look at the battery manufacturer's
specs and see whether they recommend that high a charging rate. I
suspect you'll see something like 10 - 15% of rated capacity.

That's what Trojan says for their wet cells, though I've done a lot of charging at more like 18-20% will no ill affects. My first set did die after 7 years (they might have gone another year or so if I had tended to them better over a harsh winter), but frankly, they are the cheapest part of the system.

For their gel and AGM batteries, they say 20%
From all I can read the rate of charge is determined more by what it
is doing to the electrolyte then anything else. If you read up on car
alternators you will read many references to "without excess water
loss". So, probably if your batteries aren't getting hot and aren't
gassing excessively the current is not excessive. I think I read
somewhere 120 degrees F was the hottest allowable.

But the point Larry keeps making, and every competent description I
have ever read about lead acid batteries emphasizes is that because
the process is a chemical process it takes some time to accomplish. No
matter how many amps of current you blast in there it still takes time
to convert the plate material back to it's original state.
Its certainly true that there are limits, and its especially true that to achieve 100% takes a long time, but the empirical evidence remains: good wet cells can be charged at about 20% of their capacity over the range of 50% to 80% charge. In practice this means the typical 4 6-Volt bank (450 AmpHours) can be given a useful charge in about an hour.


Try .http://www.batteryfaq.org/ for more information then you ever
wanted to know about batteries.
This says not to go over 25% for wet cells, a rate that Larry seems to say is physically impossible. Sometimes my systems approach this level briefly at startup, but generally I limit it to 20%, or 90 Amps. In practice, most of the charging is between 75 and 85 Amps.


As I read Larry's post he is saying that if you dump a bunch of amps
into the battery only some of them actually charge the battery. the
rest turn into heat which is true.

Of course, its true of all processes. Nothing is 100% efficient. The questions are, how efficient is it, and how quickly can you charge at reasonable efficiency. Ranting in ALL CAPS that its physically impossible because bad math proves it doesn't make Larry an expert.

So here's a quick calc: 90 Amps times 14 Volts times 13% (Trojan says charge efficiency is 87%) yields 164 Watts of waste heat. Certainly in an overly constrained space, this can add up, but its no more than a table lamp or even a TV. Maybe when I have time I'll compute how quickly the mass of the bank will heat up - I wouldn't be surprised if an hour of charging is not really enough to heat to bank to critical levels.

.

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