On Tue, 2005-08-30 at 14:22 -0400, John Nall wrote:
> While I have some confidence in my ability as a programmer, I have no 
> confidence at all in my knowledge of electronics (not that I don't give 
> it my best shot, though).  So would appreciate any comments on a little 
> scheme I am thinking about.  Without going into the messy details, it 
> will involve a PIC which will get no power except from a solar panel.  
> The voltage from the panel will go to a voltage regulator (probably a 
> 7805) to power the PIC.  The idea is that the PIC will start its work 
> when the sun comes up and raises the voltage from the panel high enough 
> to start things.  When the sun is going down, voltage from the panel 
> will continue to drop (and eventually cease altogether) and the PIC will 
> go off.  It is a non-critical application, and so long as I get 4 or 5 
> hours of operation on most days, that will be fine

Once you've decided how to power the PIC, Microchip's app note AN626
might be useful for you. It explains a circuit to charge a 12v SLA,
stopping when the voltage reaches a predefined level and checking it
every second or so afterwards.

Plus, if you've got other electronics running off the battery while it's
charging, the voltage read on the battery terminals will drop a little.
I've read that you're supposed to leave it open-circuit for a minute
before doing any voltage readings with respect to recharging.

I've been thinking of something similar to you, but on a campervan
instead. I've not done it yet, but was intending to have two batteries
in parallel, but only allow one of them to be connected at a time so I
could get accurate open-circuit voltages on the other. And it would
double the overall capacity, of course. I was going to run my charging
circuit off the battery being charged.

Good luck with it,

Tim

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