9V Ni-Mh battery discharging circuit

[henk.visser@yahoo.co.uk]

Could anyone direct me as to where I can find a circuit diagram to build a 9V Ni-Mh battery discharger. I have about 30 odd batteries which I'm using for my wireless mics and DI boxes. However I don't use them till they're completely flat and would like to discharge them properly before I charge them again. The rating on my batteries are 250mAh 9V Ni-MH which I've already mentioned above.

 

[audioguru]

You should NEVER completely discharge a battery that has more than 1 cell.
The weakest cell will discharge to zero first then it will be charged BACKWARDS by the remaining discharging cells. When a cell is charged backwards then it will probably become shorted.

A "9V" Ni-MH battery probably has 7 cells so stop discharging at 7V.

 

[tcmtech]

Given the ~7 volt suggested lower limit I would just use a 7 volt 5 watt zener diode in series with a 10 ohm resistor or a group of five 7 volt 1 watt zeners with 50 ohm resistors.

Either way you should have more than enough power capacity and still have a safe lower voltage discharge point limiter.

 

[tvtech]

Could anyone direct me as to where I can find a circuit diagram to build a 9V Ni-Mh battery discharger. I have about 30 odd batteries which I'm using for my wireless mics and DI boxes. However I don't use them till they're completely flat and would like to discharge them properly before I charge them again. The rating on my batteries are 250mAh 9V Ni-MH which I've already mentioned above.

Hi henk.visser@yahoo.co.uk

You are probably using Mantech 250 mAH Camelions. Excellent little PP3's if handled and charged correctly. I have a few that are years old and work like new after literally hundreds of charge/discharge cycles I even basically cooked one when experimenting...with Resin and drying times.

To make a mold.

These little batteries are commonly used in Microphones....that take them below 7V.....and stuff them up. See it lots here from Churches and so on.

The little PP3 is the least understood of all Rechargeables..but an absolute treasure once you understand them....and treat them well.

I believe I can help you....seeing as you are here in South Africa and all. PM me. No PM = no help.

Regards,
tvtech

 

[henk.visser@yahoo.co.uk]

Thanx guys quite a bit of insight you've given me. I am aware of the dangers of discharging a battery too much so I had an idea of something that should have a cut out as protection, yet what and how eludes me and if I'd probably sit down and experiment with a couple of different circuits and combine them into giving me what I want I'd get the job done but I'm racing time which I don't really have as this is a solution which was thought to be implemented about a month ago when I purchased a new batch of batteries. I cannot replace all of the batteries every year whilst I'm convinced that some of them are still in a good condition and capable of delivering a couple of hours of work.

 

[tvtech]

Hi Henk

Thanks for the PM. Here is a simple circuit I use that will not allow the PP3 to discharge below 7.5V. Takes a few hours to flatten completely. Please see attachment.

The same circuit can be added as battery protection inside the Microphone by substituting the 82R resistor with the Microphone power + and - leads. Too simple to work...well it doe's. Believe me

Dropout Voltage across the BC327-40 Collector Emitter is around 100mV to 200mV for everything down to around 8.1V., when the little PP3 is basically battling anyway.

Little PP3's don't like being charged at 1C. They also don't like to be discharged at 1C.

A good balance is 0.5C for long life.

They are my Babies.....and I have yet to loose one

Just realized......I sound like a Woman . Women are Wonderful...and good and all and some carry batteries...

Out of here before I get into trouble.....again...My mind never stops working. Unlike me

Regards,
tvtech

Edit: Note Q1 is BC327-40 and R3 is 82R 5W.

 

[henk.visser@yahoo.co.uk]

Thanx a million tvtech this is exactly what I needed and as a bonus I can add a lil protection to the batteries with this circuit.

 

[Norator]

My cells were in series discharged through a constant current sink, the cell voltages were scanned at some suitably fast rate and I used relays to disconnect each cell as it crossed through the minimum safe cell voltage.

To determine the scanning rate you need to know the dV/dT cell voltage change as it crosses through the min safe voltage.

 

[tvtech]

Thanx a million tvtech this is exactly what I needed and as a bonus I can add a lil protection to the batteries with this circuit.

Hi Henk

Glad I could help

Regards,
tvtech