NiMh Charging Circuit questions.

[3v0]

I found this PIC based charging circuit online. The full article is here
NiCd Battery Charger

My question is how can PIC pin 18 get an accurate reading from the battery? The current to the battery is either that for full charge or trickle charge never off.

I will be charging a 750 mAh NiMh pack with 3AAA cells. Instead of the 1R0 5Watt resistor I will use a 16R 1/4 watt which should result in a charge current close to C/10.

3v0

 

[Boncuk]

Hi 3v0,

there is a simpler solution to that.

Use a MAX712. It can charge up to 16 cells at any desired current.

Boncuk

 

[3v0]

Wrong answer. At about $10 each the MAX712 is not a solution.

 

[Boncuk]

Wrong answer. At about $10 each the MAX712 is not a solution.

I'd say wrong address to buy it.

The MAX712CPE costs €3.11 in Germany.

 

[DemonicSandwich]

Pin 18 of a PIC16F711 is an ADC input. The guy is likely using it to watch for the ΔV to see the end of charge.

 

[3v0]

Pin 18 of a PIC16F711 is an ADC input. The guy is likely using it to watch for the ΔV to see the end of charge.

I understand that.

Let me put it another way:
I am curious about the interaction of the voltage from the charger and the battery.
Since the battery voltage is read while being trickle charged what determines the voltage read by the PIC via the voltage divider.

 

[MikeMl]

Are you asking how a voltage divider works?

I dont think that the software reads the battery voltage during trickle charge; only during the initial charge up.

 

[3v0]

Are you asking how a voltage divider works?

I dont think that the software reads the battery voltage during trickle charge; only during the initial charge up.

The voltage divider is standard stuff.

I have been playing with the circuit on a BB. It looks like the 317 is following the battery voltage, maybe just a tad higher. But my old bench supply let out its magic smoke. Need to deal with that.

 

[peufeu]

> Since the battery voltage is read while being trickle charged
> what determines the voltage read by the PIC via the voltage divider.

The key is "trickle" (very low current)

Voltage measured is : real battery voltage + (battery internal resistance * current)

If current is small, no problem. With a pulse charge at 1A you'd need to sample the current when it is not charging...

 

[3v0]

> Since the battery voltage is read while being trickle charged
> what determines the voltage read by the PIC via the voltage divider.

The key is "trickle" (very low current)

Voltage measured is : real battery voltage + (battery internal resistance * current)

If current is small, no problem. With a pulse charge at 1A you'd need to sample the current when it is not charging...

Thanks peuleu .

 

[MikeMl]

A big issue with charging batteries is to limit the initial charging current; both to protect the supply (transformer/rectifier/regulator) as well as to keep the max charging current in-line with the battery maker's specs. Write back if you want to add an electronic current limiter to your charger.

 

[3v0]

Thanks MikeMl, the circuit I am using uses a LM317 to limit current. But I am open to suggestions.

Moved to blog/journal.