How do I balance an 8s 400mah NiMH pack?

[dannstrait]

So I have a Triton Jr. charger and I want to perform a forming/balancing charge on my 9.6v 400mah NiMH (2/3aaa) pack. The lowest current my charger can deliver is 100mah (.1amp increments only). Since this is .25C, I'm at a loss of how to safely balance this pack without badly overcharging the cells (16 hrs x .25C = :poof:).

Most people are charging cells of a much larger capacity, so I haven't found too many people with this problem. However, I did come across this hobby charger (106B+) that allows charging in .05a increments (50mah minimum) that could potentially solve my problem. It also claims to have a smart balancing feature that doesn't take 16 hours (read page 20 of the manual).

Are there any hobby chargers out there that perform a break-in charge on battery packs the same way the Maha C9000 performs break-in on individual cells (simply enter pack capacity and exact .1C charge rate is calculated)? Furthermore, are there any hobby chargers out there that allow you charge at less than 50mah and/or in 10mah increments?

I'd prefer not to buy another charger so I was hoping there was some other recommended procedure I can perform with my current hardware.

 

[Mr Happy]

If you want to do a break-in charge on a 400 mAh pack you need a charge rate of 40 mA (0.1C). You don't really need a proper charger for this if you are handy with home brew stuff.

For instance, you could find a typical wall wart or power brick that might for instance have an output of 16 V or 20 V DC (you want a higher voltage above 16 V for good current regulation). Assume your pack when fully charged will be at about 12 V so calculate a current limiting resistor like this:

R = (16 V − 12 V) / 0.04 A = 100 ohms

Therefore you could connect a 16 V power supply in series with the current limiting resistor and use it to charge your pack for 16 hours (get the polarity right!).

To discharge the pack you can find a 12 V 80 mA bulb (a 12 V 1 W bulb should do it) and watch the discharge voltage. Stop the discharge when the pack reaches 1 V per cell, which is 8 V for a 9.6 V pack.

 

[dannstrait]

Thanks Mr. Happy - you're a life-saver.

I'm still a newb though and just need a little more explanation.

First Question: I can only find a 12V wall-wart charger. How do I use the formula you provided below with this power supply? Is the voltage too low? Do you always have to use more voltage than the battery pack being charged?

R = (12 V - 12 V) / 0.04 A = 0 ohms ???

or

R = 12 V / 0.04 A = 300 ohms ???

Second Question: Is it possible to use the suggested resistor setup with my charger? Can I just set the Triton Jr. to output 0.1 A and then still use a resistor to limit the current to 0.04 A? How do I determine what voltage the charger puts out?

 

[clintb]

Mr Happy has given sound advice, and on the cheap, for a proper balancing charge. I want to give you a link that expands on the iCharger forming charge method. Link

 

[Mr Happy]

First question: You do always need more voltage than the pack being charged. Each cell will reach about 1.5 V when fully charged, so an eight cell pack will reach 8 x 1.5 = 12 V. You need more than 12 V to push the forming current through the pack, and a good bit more than 12 V to make sure the resistor provides good current regulation.

Second question: No, this is not possible. The Triton Jr. will do everything possible to make the current equal to 0.1 A. If you put a resistor in the way it will just push harder to overcome the resistance.