[Soekris] voltage questions

[der Mouse]

[top-posting damage repaired manually -dM]
>> I have a solar site I want to convert over to 24v for effeciency
>> reasons. Soekris 4801 spec says it can only take 6v-20v [...]
> Easy, just get two 12v batteries, they don't need to be big ones.
> Gel cells would do.  Connect to your new 24V system and tap one of
> the 12v batteries for your computer.

This will work at first, but I wouldn't depend on it for more than
minutes without consulting a tech who knows the battery technology in
question.

This is because one of the batteries will be carrying more current than
the other (because one of them has the Soekris in parallel with it).
This means that one will soon be charged more than the other, and,
depending on the battery technology, this can affect the voltage the
Soekris gets and may even end up damaging one or the other battery,
possibly severely.

Also, if the Soekris is pulling (say) 1A, then you've got *something*
dropping 12V at 1A, dissipating 12W, and given how the circuit is wired,
it's got to be one of the batteries.  Can the battery stand dissipating
12W in the long term?  Again, I'd want to check with someone who really
knows the battery technology before depending on this.

Given how wide a range of voltages the Soekris can take, I'd be tempted
to just measure its min and max current draw and try to find a resistor
value that will keep it in its voltage range spec for all of them.
Then get a nice hefty resistor of that resistance, rated something like
1.5x-2x the highest power dissipation in that range, and stick it in
series with the supply.  Crude but, perhaps, effective.  (Sometimes,
the simpler the better.)

If you really want to be safe you might find a zener diode rated for
something near the high end of the Soekris's range - 17V, 18V,
something like that - and stick it across the Soekris, just in case the
thing pulls very little current at some point during startup - though I
imagine Soren can tell you whether that'd be appropriate.  (If you
can't find a high-enough-power diode, you can use a nice hefty bipolar
transistor as the power pass element, so the zener current is divided
by (one more than) the transistor's gain.  Use two in a Darlington pair
if necessary.)

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