NiCd Baterry capacity indication

I am currently trying to build my own Baterry charger for NiCd battery. Have few question seeking for help:

Questions:
1.What are the parameter to indicator baterry charge status whether full charge, 70%, 50% charge etc.

2.Waht is the voltage when the cell are full charge? 1.4v ? 1.5v?

3. What are the relation between the voltage level with the capacity/charge status of the battery? are there linear relation?

Knowing that constant current charging with -dV termination are the recommended method. I am trying replace -dV with voltage monitoring, will it work? Any idea are well come.

thanks.
sam

im not sure about the others but the voltage when charged is 1.2 volts
so if you were to use two in series then you would only get 2.4volts not the three that you would get from two standard alkaline batteries

samseng said:

I am currently trying to build my own Baterry charger for NiCd battery. Have few question seeking for help:

Questions:
1.What are the parameter to indicator baterry charge status whether full charge, 70%, 50% charge etc.

2.Waht is the voltage when the cell are full charge? 1.4v ? 1.5v?

3. What are the relation between the voltage level with the capacity/charge status of the battery? are there linear relation?

Knowing that constant current charging with -dV termination are the recommended method. I am trying replace -dV with voltage monitoring, will it work? Any idea are well come.

Click to expand...

No it won't, when it's fully charged the voltage DROPS, and it's that voltage drop you need to monitor. Another method is to measure the temperature of the batteries, when fully charged they start to get much hotter.

Or just do what most chargers do, just charge it for a certain length of time, ignoring any charge that might have been present when you started.

monkeytree said:

im not sure about the others but the voltage when charged is 1.2 volts
so if you were to use two in series then you would only get 2.4volts not the three that you would get from two standard alkaline batteries

Click to expand...

No. Fully charged Ni-Cad or Ni-MH cells are not 1.2V, that is their voltage when driving a heavy load. The voltage changes with charging current and with manufacturer and cannot be used to indicate that the cell is fully charged and the voltage does nor reliably indicate the amount of charge remaining. My AA Ni-Cad and Ni-MH cells measure approx 1.45V in my charger after an overnight charge at a 0.1C rate.

Standard alkaline battery cells do not measure 1.5V. Their voltage quickly drops to 1.2V when loaded then slowly drops more and more.

With the same load, the average voltage of a modern Ni-MH cell is higher and lasts longer than the voltage from a modern same-size alkaline cell.

Check my (very old) battery discharge graphs at . It's quite informative.

Modern Ni-MH cells have much more capacity than old Ni-Cads:

Thanks for the information. Looks like the only way to auto terminate charging is either monitor -dV or DT/dt (absolute time termination not prefer). Is there any simple circuit to implement the -dV without a controller? Or any suggestion on static way of charging termination?
Appreciate your sharing.

For their Ni-MH cells (they don't make Ni-Cads anymore), Energizer recommends that fast chargers (1hour) use a temperature sensor to detect a certain temp rise above the ambient temp as the main method to reduce tne charging current as a cell approaches 90% of being fully charged, then switch to a timed 0.1C charge rate, then switch to a 0.025C for continuous trickle charging.
They recommend that a voltage sensor and timer be used as backups to terminate a charge that isn't switched by the temperature sensor.