> They're gel cells.

JT> Still lead-acid. They use a gel electrolyte that recomposes the
JT> gas on production. They should never release gas unless they're
JT> abused, or way past their use-by date. That's why even though
JT> they are technically called 'sealed' they should never be
JT> actually sealed.

 ROFL! You can't put the hydrogen genie back in the bottle. What
haoppens, is they allow the hydrogen to escape (hydrogen ions are so
small they leak though steel) and they let the hydroxyl ions oxidise
something and form a solid compund instead of gas. The cells still dry
out, at the same rate as any lead-acid battery similary treated.

 Car batteries get a bad name, but a car treats them cruelly.

JT> They are float charged. That is, the output voltage is fixed
JT> (most of the time anyway. The normal "fast" charge current is
JT> limited, but it doesn't stay at this point unless the batteries
JT> are flat, and it's limited to a very low value when the
JT> batteries are VERY flat).

 The battery voltage *cannot* be fixed, but a variable charge
current *tends* in that direction. When flat the voltage is low and
the source resistance is high. The maximum current has to be limited
to prevent overheating. When charged, the voltage is high and the
source resistance is low, so the charger should monitor voltage (and
temperature) to reduce current to a trickle-charge (normal leakage of the
battery electytrolyte).

JT> It keeps track of the amp/hours going in and out to monitor the
JT> overall capacity more accurately.

 This is bullshit. Any error would be cumulative, and the cell's
capacity varies with both temperature and age. I actually tried
this sort fo thing a few years ago when they first used NiCads to run
electric model planes. The idea was *maximum* charge rate between
flights, but NiCads lose capacity if you keep over-charging them. Once
you use a micro, it's easy enough keep track of ampere-hours. The
catch is that it means fuck-all. I came to the conclusion that it
was worse than useless.

BL> It *will* work with a 6V tap and half the cells run flat. The
BL> six cells in series are never the same anyway, and this can be
BL> a problem with NiCad batteries and the fancy gel-cells, 

> yes?

JT> Less of a problem with NiCads, but they don't like to use
JT> nicads anymore, cadmium and landfill and all that.

 Oversharging is the *main* problem with NiCads. They develop a
memory, the voltage rises, internal resistance rises, and they lose
most of their capacity.

JT> If you overcharge a lead-acid, it dries out. You WILL kill lead
JT> acids by far quicker if you discharge one or more, but not all
JT> of them. Half the cells will be grossly discharged, the other
JT> half grossly overcharged.

 I love your use of technical terms like "gross". David is talking
about 150 AH battteries with an unbalanced load of one lousy amp!
Gross, my arse. As for drying out a lead-acid battery... death is
the fate of all batteries. It is just a question of what happens
first: lack of electrolyte or the lead plates gone spongy and high
resistance. Lead-acid batteries *have* to be overcharged to get the
best life. I quoted the actual voltages in another message.

BL> lead-acid is pretty tolerant of being overcharged.

JT> As above, not a good idea to do on a repeated basis though.

 You don't understand lead-acid. They *should* be overcharged.
Ideally, there is a standby long-term trickle charge. What kills lead
acid is deep cycling... running them flat before charging, or high
intermittent current loads. With David's 150AH batteries, I would 
expect a 0.2 - 0.5A trickle charge. It makes the DVD's 1A pretty
negligible.

> These are not supposed to fizz - they're sealed.

JT> Oh yeah they do. It's classed as abuse- they do release gas.
JT> Actually, it's part of the normal charge cycle. They are
JT> charged to higher than the recommended cycle charge point,
JT> (higher than long term voltage) and kept there for a short
JT> period of time. Then the charge voltage is brought down to the
JT> normal float point.

 That's right. What you call "float" I call trickle... the current
that just maintains the full 13.4V charge.

BL> That's why the *best*way to test a charge battery is with a
BL> hygrometer (the float in a bulb).

> Nowhere to stick the hygometer....

JT> Ahem. This is AVTech. Need I remind you where it needs to be
JT> stuck? 

 It's still the *only* way to be sure of a full charge in lead-acid.
As the cells age, the "lead" is no longer pure and the actual voltage
rises. It's no good monitoring voltage, or ampere-hours in and out...
all you can do it give it a trickle charge and recalibrate *that*
rest voltage as the new "full-charge."

 It was never much of a problem with lead-acid, but Nicads and the
new variants are utter bastards for overcharging.

> These are guaranteed for ten, life expectance of 15....

JT> Tell Bob how much they cost. Let him do the solar economics
JT> math with a life reduction of more than half. I would not
JT> recommend in anything that would reduce their life more than
JT> normal operation. It will cost you dearly. 

 I can't believe two things: firstly that David is using 150AH gel
cells. I *know* what they cost, and my 5-year estimate is pretty
accurate. Rechargable batteries slowly lose their ability to hold a
charge and deliver full current. His 15-years would be like my mother
at 97! Utterly stuffed!

 Secondly, why the hell isn't he using gas bottles for the frig, or
even kerosene? Is he a Green freak?

 Solar power is terrific for hot water, or battery-backup lighting,
maybe even running a TV for a few hours, but when it comes to real
power like a frig or cooking, there is nothing to beat hydrocarbons.
My bushie relations were using kerosene refrigerators 50 years ago!

> The buggers weigh more than 20kg each. I'm not moving them any
> more than I have > to.

JT> Besides, you'll be abusing the top and bottom halves in a
JT> different way every six months now. :-)

 The whole and utter secret of engineering is how much. Anyone can
use aword like "gross" or "abuse" but an engineer puts
an number to
it. Trust me, one lousy amp unbalance does not amount to either of
those words for a 150 AH cell. In fact, if David likes to measure his
fully-charged trickle current, he will find that it is almost that
much anyway (assuming the dickhead who wrote the software had half a
brain, and that is a real ask, in my experience of programmers
Brenton excepted).

Regards,
Bob
      


  

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