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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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    It’s great that you found a true 9V replacement with 7-cell NiMH batteries since older 6-cell versions always had voltage issues. For charging, a proper circuit with delta-V detection and a safety timer would be ideal. You could consider designing a custom circuit using an MCU with a constant current source and termination detection. If you want a simpler approach, a low-current linear regulator with a series resistor can work, but it may not be as efficient or safe in the long run. Have you looked into smart chargers like the La Crosse or Opus models?

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  • 0

    It’s great that you found a true 9V replacement with 7-cell NiMH batteries since older 6-cell versions always had voltage issues. For charging, a proper circuit with delta-V detection and a safety timer would be ideal. You could consider designing a custom circuit using an MCU with a constant current source and termination detection. If you want a simpler approach, a low-current linear regulator with a series resistor can work, but it may not be as efficient or safe in the long run. Have you looked into smart chargers like the La Crosse or Opus models?

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