>> > http://www.tunecharger.com/
>>
>> Not overly cheap.
>> PIC16F675 based boost converter
>> Circuit on page 10.
>>
>> 
>> http://www.tunecharger.com/2006_t_0001_mtc_3-330.2%20%20reva%20technical%20reference%20manual.pdf.
>
> By a strange coincidence I found the above recently (have you been 
> hacking my gargoyle [tm] searches? :-)

The manual is dangerously naive when discussing battery charging 
needs - both lead acid, which it is mainly intended for, and NiCd and 
NimH. It is dangerously underspecified for any of these tasks. As a 
panel to battery energy transferer it may do a good job.

Their comments on multiple sources in parallel are correct but 
generally do not represent means of using more than one source at once 
which may confuse the technically inexperienced. .

I am uncertain of the merits of their two stage charging process and 
it seems to reduce efficiency. They first use a boost converter to 
charge a capacitor to above battery voltage and then discharge the 
capacitor into the battery using a separate FET switch. I don't see 
that not using the boost converter directly is an advantage. 
Discharging a capacitor into a lower voltage load will necessarily 
cause I^2R losses (even if it's not evident where the R is it still 
exists). They would probably be better off using the second FET as a 
synchronous flyback diode to get better efficiency.

To use this circuit in a larger current / power system you would need 
to ensure diodes and FETs are adequately sized, which is a relatively 
trivial task, and you would need to ensure that the inductor had 
adequate current handling ability. This is liable to be the hardest 
requirement. Many manufacturers of prewound inductors or of cores 
provide sizing information.

I'd suggest that a straight boost converter would work fine in this 
application. Then add software which allows battery voltage and 
perhaps charging current to be measured.

        Russell

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