DD>> 1) Installing a second lot of wiring for the 6V;

BL> Remember to use thick wire...

DD> Indeed, but how thick is thick?

 Ahh, Grasshopper. How thick is are two short planks? How thick is a
brick and how long is a piece of string? It depends...

BL> I'm suspicious of an "intelligent" device. Actually, it's only
BL> as smart as the dickhead who designed it.

DD> It uses a PIC - doesn't Brenton work in that line?

 I rest my case.

BL> My guess is that it disconnects if the voltage gets to *high*
BL> (to prevent over-charging), with a limited current that drops
BL> to zero as the battery gets up to 13.8 volts (2.3V per cell).

DD> It uses PWM to adjust the effective charge rate.

 What on earth for? My original guess still applies. The maximum
charge rate is set by the construction fo the battery itself, and
relates to heat. You have to limit the maximum charge or you'll melt
the lead plates inside.

 The other limit is gassing. You don't want to keep pumping current
into the battery once it has reached full-voltage charge because the
electrolyte produces gas (hydrogen/oxygen) which does no one any good.
This means that you need to limit maximum-current when the batttery is
flat, and maximum-voltage when the battery is full. Lead-acid
batteries are quite tolerant of this (including gel cells) but you
would need to be careful of a big battery in an enclosed space.

 The idea of monitoring ampere hours in and out is insane. An error
woudl be cumulative, and you woudl either end up wit ha flat battery
or an over charged battery. It sure sounds like a sales spiel to me.

 Like Brenton, I am not terribly impressed by PICs and micros in
general. Computer-control and intellgient-control sounds great, but it
doesn't really mean a lot. The term fuck-all springs to mind.


BL> but good ole lead-acid is pretty tolerant of being overcharged.
BL> The three non-flat cells *will* be overcharged at the beginning
BL> of the charge cycle, they will fizz more gas, 

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

 No they're not sealed. The gas is hydrogen which leaks through solid
steel, and oxygen which combines with almost everything. What happens
is that they fizz, and eventually dry out. You cna brign a gelcell
back to life by drilling a tiny hole and squirting water in with a
syringe.

BL> You can't over-charge a lead-acid battery. As soon as the
BL> charging current falls, so does the voltage. A single cell can
BL> be 2.5V on hard charge, but it slowly falls to 2.25V just
BL> sitting there (it leaks internally) and on use to 2.1V. A flat
BL> battery is 2.0V but it varies with temperature. 

DD> There is a temperature sensor attached to one cell and wired to
DD> the regulator

 Excellent. It sound like you have a really good charging system.

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

DD> Nowhere to stick the hygometer....

 Why did you choose gel cells? The only advantage is lack of mess.
They lose just as much water as an equivalent lead acid.

BL> Over a few years, the life of your top three cells will be
BL> reduced, but you can only expect 5 years out of a battery
BL> anyway (whether you put water in or not). 

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

 I assume it's a pro-rata guarantee? After 5 years, say? 

 A car is a terrible situation for a battery. It gets very hot,
it is jiggled around all the time, it is asked to deliver 100-amps 
in its least-charged state, and it is generally over-charged in
between. A normal car battery in a fixed installation that does not
exceed 40C with no starter-motor loads would match your 15 years
average life. Even so, 5 years is a realistic expectation, before 
it starts to lose capacity.

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

 I had no idea your installation was so huge! Why didn't you use gas
for the frig?

Regards,
Bob


 
  



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