Overdischarge protection for a li-ion powered circuit

[peu]

Im using a 12f675 circuit powered by a li ion battery, to protect it from over discharge I want to measure the provided voltage and if its equal or below 3v I want to flash a led.

Li ion batteries have this behaviour:

When they are removed from the battery charger they measure 4.2v for a couple of minutes of use.
Then they settle around 3.7v for almost all the remaining charge, and then when they are almost depleted they fall from 3.7v to 0v. To avoid battery damage, the discharge must stop at around 3v

any idea? I tought than using the comparator alone to measure this would not produce accurate results, but Im not sure.

Thanks in advance

[sayzer]

Hi peu,


What will stop it from being discharged?

In other words, once you detect <= 3.0 Volt, how are you planning to cut the circuit off? Via what?

[peu]

Once the battery discharge phase is detected in the battery (equal or less than 3v supplied) I put the circuit in very low consumption and flash a led to let know the user the battery needs to be replaced.

If a subsequent low battery level is detected, say 2.8V for example, the circuit does not operate at all via BOD or by putting it to sleep.

I just want to let know the user that the rechargeable battery needs to be recharged or replaced


thanks!

[peu]

I could use some help, it seems simple, but I cant find a working solution

Thanks in advance


Pablo

[sayzer]

Did you come up with a concept schematic ?

A diagram etc..?

I am sure there is more then one way to do it. This forum has so many kind members each is thinking in a different way then the others; looking at the same picture and seeing a different part of it.

I suggest that you start from somewhere and the rest will come up.


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[milestag]

Why not use the onboard AtoD convertors? 10-bit precision should be more than is needed.

Are you using a voltage regulator to power the PIC? If it is relatively precise (1% ish) then you should be fine to use that as your reference voltage. Just use a voltage divider to feed the AtoD input.

Otherwise you may need an external voltage reference. 3-pin voltage refs are cheap and plentiful. I'd guess you need around a 2.5V reference.

Set your voltage divider so it doesn't exceed 2.5V when the battery is fully charged. Then just do the math to determine A/D value at 3.0V.

Here's some code for 18F2525 so may need to change register names, etc. But the concept is the same. Note, I only use 8 bits to keep it simple. It's accurate enough for my app:

battery_monitor: '
TRISA.0 = 1 'make AN0 input
ADCON1 = 14 'AN0 = analog in
ADCON0.1 = 1 'start AD
wait_ad:
pause 1
IF ADCON0.1 = 1 then wait_ad
batt_v = ADRESH
ADCON1 = 15 'return pins to digital
TRISA.0 = 0 'output
IF batt_v < 150 then batt_warn