Lucas Tanure wrote:
> Thank you very much Russel. Yes, I'm just a  beginner.
>
> It's Hobby. Its a 4 Series battery , each one is 1.2v, NiCd ,  AA Type,
> 2100MAh. Rechargeable
>   
2100mAH AA sound more like NiMH. Which is slightly higher volts. The 
discharge curve of NiCd and NiMH are similar but not the same.

1st two hits on Bing (Google has decided it's clever to simulate 
Bookmarks and show me places I have been before, or my own sites).
http://shdesigns.org/batts/battcyc.html
http://www.thomasdistributing.com/techfacts3.htm

Detecting 20% capacity left by voltage could be hard as you are likely 
on flat part of curve. Detecting  nearly charged or nearly flat by 
voltage is fairly reliable.

The Curve moves up and down somewhat according to current draw, so you 
need to know current.
http://www.thomasdistributing.com/images/discharge.gif

So as Veronica Merryfield said you need to count. That is detect the 
first sharp drop from fully charged and count Time x Current (= charge 
or coulombs). The system can be self training by detecting the end point 
where you leave the flat part. Also each cycle the capacity is slightly 
less. If you charge before it's flat you have to detect that too.

You can measure current on bench once if you think it doesn't change and 
use a constant, or use 1/4 of an LM3900 and very small series resistor 
on supply from battery terminal with LM3900 as differential amp that 
gives 1V at max charge, 2V at no current and 3V at discharge current 
etc.. or if charge current can be high and discharge is much lower you 
can offset to higher volts.

The battery pack will vary from nearly 6V on charge to nearly 4V when 
almost empty. So you may want a LDO regulator or some other PSU circuit 
to drive the PIC. It certainly needs a more stable voltage for the A/D 
reference. You might want to divide battery volts by 2 using resistors 
to get a 2V to 3V range for ADC.

I'd not run a PIC directly connected to 4 x NiMH. I'd have at least a 
series diode (not Schottky) or some kind of  regulator. 3 x NiMH is fine 
for direct connection. You may "get away" with 4 x NiMH direct to VDD & 
VSS of PIC, but fully charged, on 1/100C trickle or near charged on 
1/10C or just after charge the battery pack does exceed max ratings of 
many 5V chips (4.75 to 5.25).  I just checked a probably 1/2 charged 
pack here, the Battery pack volts rises to 5.8 on charge within 2 
minutes, on disconnecting it typically drops instantly to 5.3V and then 
5.2V within a minute (a 1/2 charged pack). Sticking 4 x AA fresh 
Alkaline in the holder gives 6.4V,. Can the user swap the cells?  (End 
point on Alkaline is lower than NiMH, maybe 0.9V vs 1V)



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