>> *BUT* battery suppliers and all who talk at any length about their
>> maintenance make a big deal about the float voltage being correct 
>> to
>> something around +/-0.1 Volt. I really don't want to have to trim 
>> this
>> manually during setup (pot/labour/training/Murphy). And the eg 
>> LM317
>> spec sheet guarantees that you can't get anywhere near the required
>> specs without adjustment.

> How close would you get with a 7805, a zener diode in the GND leg, 
> and a
> series diode to prevent current drain from battery when charging 
> power is
> off? Pretty cheap. More accurate than resistor divider.

A zener wouldn't provide either the initial required accuracy or the 
longer term stability under parameter variations BUT a "precision 
zener" would work just fine. eg a precision shunt regulator such as 
LM385 or a TL431. This would provide the thermal shutdown and absolute 
current limiting of the regulator and the accuracy of the reference. 
The TL431 costs in the $US0.10 - $US0.20 / 1000 and may make a very 
acceptable solution. I'm probably wed by now to my processor 
integrated solution as it is cheap and allows a reasonable amount of 
control BUT I'll do a quick design with TL431 and compare. Unlike uP 
based design you have to have absolute and long term accuracy out of 
the box if you want to avoid calibration. uP solution allows easy auto 
calibration.



        Russell McMahon


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