At 11:02 07/05/98 +0200, you wrote:
>>>>
Hello All,

I have a project that requires that a PIC be run from batteries. However I
have a few questions, I have been unable to find any information from the
Microchip website, which is quite surprising.

1) Assuming I use NiCad of NiMH batteries with a cell voltage of 1.2v,
should I use 4 (4.8v) or 5 (6.v), bearing in mind the discharge slopes of
each. I would imagine 6 would be my best bet,

<<<<
See below
>>>>

2) As this is a low discharges application, is NiCad or NiMH the most
suitable, or something else?

<<<<
How low is low? 15uA? 0.5uA? I doubt it's that low (even if you put the PIC
to sleep when it's not busy!)
>>>>

3) Should any regulation be used? This is unlikely I know, as batteries
make very nice power supplies, but nevertheless, any thoughts? Battery life
is important, and any form of regulation will draw extra current,

<<<<
I just use a MAX691A
>>>>

4) When the batteries start to go flat, the PIC will stop working(!), most
likely it will malfunction. How should I protect from this? I have
considered using a modified brown out protection circuit, which disables a
latch controlling the power. It is importants that the batteries are not
allowed to recover (if indeed they would at low discharge rates) and start
up the PIC.

<<<<
The MAX691A protects my SRAM chip selects, handles the battery switchover,
is an external watchdog, and generates RESET and RESET\ at a higher voltage
threshold, and has "On battery signal", plus Vlow, and programmable (R
divider) power fail outputs too (wow!) - so will reset the PIC. You could
use a Dallas part if you don't need the extra features of this or other
MAxim chips.
>>>>

Basically I am asking for advice from your experience. Am I on the right
track? Any ideas, pointers to resources would be very welcome.

I'm backing an SRAM (but not the PIC 74A - yet - when I do I'll probably
use a NiCd actually - and am still seeking a decent trickle charge circuit
anyone!) and I have selected a Lithium Thionyl Chloride cell. I'm using the
Sonnenschein one because it has:

A high cell voltage of 3.67V and an operating voltage of 3.60, a wide
temperature range (normally -55 to +85), high energy density, excellent
shelf life reliability, and safety (due to the internal design of the anode
connected to the inner wall of the battery case etc). The one I'm using is
the SL350 which at 100uA (typically more like 0.5uA to 15uA absolute max
for the Mosel SRAM) @ 2.0V will give me 350 hours backup. I'm using 3.6V
cell because at 3.4V (for the current I'm taking capacity drops off
dramatically.

I'd run the PIC at the lowest operating voltage that you can, get the cell
discharge curves from your nearest rep(tile) or better still get him/her to
do the calculations for you. You will undoubtedly need a more powerful
battery than the one  I'm using but I hope the example gives you an
understanding of what's involved.

Hope this helps.
(Fat chance! :-))

Regards

Adrian

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