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9V NiMH charging

koudelad

Hello,

9V batteries have always been a pain for me... Expensive, low capacity, almost every DMM / tester needed them.

I just bought a 9V NiMH battery: https://www.westinghousebattery.com/Ni-MH-Rechargeable/Premium-Series.html . To my surprise, it is not made from 6 smaller cells (which would give ca. 7.2-8.4 V), but 7, which has nominal voltage of 8.4, but when charged, can have almost 10 V. This makes it a true primary 9V battery replacement. I am not sure when these became available, but I remember older 6-cell "9 V" NiMH that were kind of useless, because all appliances thought the battery is discharged, due to the lower nominal voltage.

However, I never really had a charger for this kind of battery. I have heard that cheap chargers only use 12 V DC power supply with in series resistor in series to limit the current to ca 30 mA.

Do you have any suggestion for a charging circuit? I am open to building something, even using an MCU. I would love it to have a small small charging current (20-30 mA) or slightly higher (50 mA), a safety timer and the delta V detection. There is a famous old MAX713 charger, but according to the features, it offers C/4 to 4C charging and C/16 trickle charging, which is too high. Most commercially available hobby chargers also have a minimum current of 100 mA.

Thank you,

David

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  • 0

    I think the circuit below would do it. It's based off of a simpler charger here:  Smart Doorbell System – Part 6 – Chime Module Complete Circuit Design  and based on information from a battery mnfr PDF doc which specifies that it should be OK to charge at 0.1C for at least a day. 

    This circuit charges at 20 mA, and would take 15 hours to fully charge the battery. The battery would need to be manually disconnected within 24 hours. 

    However, there is also a microcontroller interface, so that you could turn it on/off based on a timer if desired (or any other logic circuit that can perform the timing).

    Without the microcontroller, you'd need to remember to disconnect after a day of charging. If you need a PCB for this, let me know (it's fairly easy for me to convert to a PCB, since I've already created the schematic in the same CAD software that would create the PCB).

    There's no guarantee that it will work, but it would be easy to test and then that would provide reasonable confidence with it (I'd trust this more than a cheap charger).

    image

  • 0 in reply to shabaz

    I had some time, to lay out the PCB.  This board is 100 x 60 mm and several different PP3 battery holders will fit. All parts are jellybean. The optional microcontroller is connected to the row of pin-headers on the left side of the photo (without it, the user would need to disconnect after 24 hours as mentioned above).

    image

    This is the schematic:

    image

    I'll wait a day in case anyone spots any errors/major problems, and then I'll generate the Gerber files and place the project on GitHub. There's no guarantee it will work, but if anyone tries the circuit, please report back with any feedback/mods!

  • 0 in reply to shabaz

    Very Nice!  This is something that I would like to look into further in the future.  I really do appreciate your efforts to assist and to promote a true sense of community here on element14!

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