From: John Tserkezis 
Reply-To: Fidonet AVtech Echo 

David Drummond wrote:

  (for the benefit of Bob as well)

>  BL>  Remember to use thick wire...

> Indeed, but how thick is thick?

  Thicker than what they recommend.

>  BL>  Jeeze! You don't muck around. I'm suspicious of an
"intelligent"
>  BL> device. Actually, it's only as smart as the dickhead who designed it.

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

  Everything uses a PIC nowadays.  They're cheap.

>  BL> Those words you quote sound like they were written by a salesman...
>  BL> why on earth would they keep track of ampere-hours in and out? Are
>  BL> your cells lead-acid? 

> They're gel cells.

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

>  BL> If they're NiCad it makes a sort-of sense, but even then it's pretty 
>  BL> silly.

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

> It uses PWM to adjust the effective charge rate.

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

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

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

> yes?

  Less of a problem with NiCads, but they don't like to use nicads anymore, 
cadmium and landfill and all that.  If you overcharge a lead-acid, it dries 
out.  You WILL kill lead acids by far quicker if you discharge one or more, but 
not all of them.  Half the cells will be grossly discharged, the other half 
grossly overcharged.

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

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

>  BL> they will fizz more gas, 

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

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

>  BL> but at the end of the charge, the bottom three cells will be fully 
>  BL> charged. 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
be 2.5V on 
>  BL> hard charge, but it slowly falls to 2.25V just sitting there (it leaks 
>  BL> internally) and on use to 2.1V. A flat battery is 2.0V but it
varies with 
>  BL> temperature. 

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

  That affects the charge voltage.  It varies depending on temperature.  You 
don't normally see it on chargers, because they don't care.  On a solar system, 
you're trying to get every bit of energy into and out of the batteries.

>  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....

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

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

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

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

>  BL> If it worries you, swap the top three and bottom three cells
around every 
>  BL> six months.

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

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

-- 
       -o)
       /\\    Message void if penguin violated
      _\_V    Don't mess with the penguin

Linux Registered User # 302622                         http://counter.li.org>
Fido: 3:712/610  BBS/FAX: +61-2-9716-8310  Internet: jt{at}techniciansyndrome.org
--- ifmail v.2.15