Can I charge Ni-MH batteries on my bench PSU?

[peps1]

99% I'm able to charge these batteries with my bench supply, but as they have no markings other then 9V 300mAh how would I know what current to charge at? 

[ConKbot]

Since voltage doesnt indicate state of charge on Ni-Cd and Ni-MH batteries you cant use a voltage output power supply to charge them.  What you can do with a bench supply is trickle charge it at 20-30mA (constant current) for 18-24 hours.  This has the benefit of balancing all the internal cells of the 9v stack also.

[peps1]

Thank you! 

And if I wanted to charge two at the same time.......series/parallel/don't?

[SeanB]

Series at 30mA max for 18 hours.

[peps1]

Thank you!

Is the Voltage displayed that's creeping up  on my PSU a indicator how charged the battery is, and if so what number am I looking for?

[bgsteiner]

it should be near 9v but will probably be at 8.7 but may be higher when charging as it will drop under load

[peps1]

oh......its at 9.73v right now! (started at 8.60v 30mins ago)

[Psi]

During charging a NIMH/NICA cell will increase to around 1.5V

So a 9V battery (which normally have 7 cells)  would be 1.5V X 7 = 10.5V
So your 9.73V is fine   it will keep increased up to around 10.5V or so as the battery charges.

As soon as you take it off charge the voltage will fall a little, each fully charged cell will sit around 1.45V or so.
Once you start discharging the cell it will quickly drop to ~1.3V, then slowly discharge to flat at around 1.15V

[pickle9000]

This could be a mini Fundamentals Friday.

[Rick Law]

oh......its at 9.73v right now! (started at 8.60v 30mins ago)

As already said by others, voltage alone does not indicate state of charge.  For NiMH, you can determine how full it is by one of two ways:

(1) Minus delta V (-dV) : most indicative but hard to do.  At the moment it is full, the battery voltage drops slightly thus "minus delta V"  (aka negative delta V).  Depending on battery, the delta could be mere millivolts so it is not easy to detect.

(2) dV/dT starts to decrease.  As you charge, the voltage increase slows.  As it starts to slow down, it indicates the battery is starting to get fully charged.  This one is easier to detect.  But unlike minus delta V, there is no definitive point: to my knowledge, there is no correlation between d2V/dT (d-square-V/dT, ie: rate of rate of change) and how close it really is getting to full other than by experience.

Since both has its draw backs, most good charger are implemented with both ways PLUS timed cut off.  If it fails to catch -dV, it catches dV/dT, if it took too long, it cuts out to prevent battery overcharge and thus preventing battery damage.

The more current you pump in, the more potential to damage the battery and not fully charge it.  Rule of thumb is 1/10c to 1/20c.  C is capacity.  If your total capacity is say 2000mAH (typical AA battery), 1/10 of that is 200mA charge current.  In 10 hours, you fully charge it and you can top it up at 1/20c for another hour or so.  Charge it anywhere between 1/10c to 1/20c,  and cut off to 1/20c when calculated capacity is reached, you are pretty safe and your battery will be charged.

Rick

[peter.mitchell]

oh......its at 9.73v right now! (started at 8.60v 30mins ago)

As already said by others, voltage alone does not indicate state of charge.  For NiMH, you can determine how full it is by one of two ways:

(1) Minus delta V (-dV) : most indicative but hard to do.  At the moment it is full, the battery voltage drops slightly thus "minus delta V"  (aka negative delta V).  Depending on battery, the delta could be mere millivolts so it is not easy to detect.

(2) dV/dT starts to decrease.  As you charge, the voltage increase slows.  As it starts to slow down, it indicates the battery is starting to get fully charged.  This one is easier to detect.  But unlike minus delta V, there is no definitive point: to my knowledge, there is no correlation between d2V/dT (d-square-V/dT, ie: rate of rate of change) and how close it really is getting to full other than by experience.

Since both has its draw backs, most good charger are implemented with both ways PLUS timed cut off.  If it fails to catch -dV, it catches dV/dT, if it took too long, it cuts out to prevent battery overcharge and thus preventing battery damage.

The more current you pump in, the more potential to damage the battery and not fully charge it.  Rule of thumb is 1/10c to 1/20c.  C is capacity.  If your total capacity is say 2000mAH (typical AA battery), 1/10 of that is 200mA charge current.  In 10 hours, you fully charge it and you can top it up at 1/20c for another hour or so.  Charge it anywhere between 1/10c to 1/20c,  and cut off to 1/20c when calculated capacity is reached, you are pretty safe and your battery will be charged.

Rick

There is a third but generally non recommended method, where you measure the temperature, and as the battery approaches charged, the temperature will slowly rise, until it reaches the charged state, where it will dramatically start to heat up, due to the charging energy being dissipated as heat rather than being absorbed by the cell.

On most new batteries, if you have a very good temperature sensor it is actually a fairly effective manner of charging the batteries, however on older batteries, the catalysts in the cell to convert the expelled gas back to H₂O aren't as efficient/strong/good and may over pressure the cell before the temperature sensor ends the charge.