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NiMh Batteries - Any good charging guide out there?

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NiMh Batteries - Any good charging guide out there?

Old 07-31-2023, 02:56 AM
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Default NiMh Batteries - Any good charging guide out there?

Hi,

I am back into the hobby after over 20 years. Back in the days, I used NiCd`s for TX and RX. Charging them with a computer charger was an easy task, giving confidence that the batteries had their capacity and topped up. I have now replaced many of these battery packs with NiMh packs. I have two different computer chargers which support NiMh. It seems to me that these chargers have a hard time to determine when the batteries reach a full charge state. I have a gut feeling that they are never topped fully up when I charge them and cycle mode does not help much either. What is the recommended charging paramenters for the NIMh cells? Any good guide out there?

Thanks
Old 07-31-2023, 04:49 PM
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MrRover75,
I am going to assume you are using new NiMh batteries for your transmitter and the receiver/servo flight pack and not to drive an electric motor for propulsion. Most NiMh battery packs will be either four or five cells in series, similar to the NiCads, however the NiMh batteries have nearly 3 to 4 times greater capacity than the same physical size NiCad battery. An AA size NiMh cell will normally be rated between 1800 and 2200 mah capacity versus 500-600 mah for a AA size NiCad. Also the NiMh cells will peak at a slightly higher voltage, typically somewhere near 1.40 to 1.43 volts per cell versus 1.3 to 1.35 volts for NiCads. The problem with computer chargers is there is no balance port for these packs and the computer therefore assumes (sometimes falsely) that all the cells are good and are charging evenly all the time. If a NiMh pack is being quick-charged, i.e. trying to charge it in less than several hours, it is possible for the voltage to build up to the peak voltage prior to the battery actually attaining its full capacity charge. The computer charger stops charging periodically to measure the pack voltage and then resumes charging if the voltage is not up to the rated full charge voltage. During a quick-charge, it is possible for the battery to temporarily be in an over voltage state during the few milliseconds that the computer pauses charging to take the voltage measurement. Then the computer thinks the battery is fully charged and turns itself off when the battery is actually less than fully charged. Of course, then the voltage will gradually decrease to more nearly reflect the actual charge condition and should be detectable with a volt meter. Occurrence of this false-peak voltage is much less when slow charging. My recommendation is to slow-charge NiMH batteries. Use the old style NiCad chargers to charge the NiMH batteries slowly overnight (15 to 24 hours). Measure the voltage of the fully charged pack as soon as it is disconnected from the charger to make sure it is somewhere near 1.42 volts times the number of cells in the battery pack. This can help indicate if the pack may have a faulty cell. Fresh off the charger, a 5 cell pack should read near or slightly over 7 volts. Then place the fully charged pack on the computer charger and do a discharge test to measure the actual battery mah capacity and compare that to the battery's rating. If it matches within 10%, you are probably good to go. Note that over a period of several hours to a day, many NiMh cells' voltage may drop to about 1.38 volts per cell, so I also suggest taking a voltage reading a couple of hours after taking a battery off charge and see what the long term stabilized voltage will be. A 5 cell battery that measures 7.1 volts fresh off the charger might only read 6.9 volts a few hours later. NiMh batteries should never be discharged to less than 1.10 volts per cell. They are rated at 1.20 volts per cell, so that is why the 5 cell pack is rated at 6.0 volts, even though it will pump out 7 volts when fully charged and drop to 5.5 volts when depleted. Like NiCads, the NiMh cells also slowly self discharge over time, but just not as rapidly as the NiCads do. Therefore, be sure to check the voltage and top off the pack again if it has been more than a couple of days since it was last fully charged.

I hope this helps. Welcome back to the hobby.,
David
Old 08-01-2023, 11:18 AM
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Hi David

Thank you very much for your reply. This clears up a lot of questions. Yes, I have replaced most of my old RX packs with 4 and 5 cells AA NIMH packs. So, to sum up, if the capacity of the battery is known/verified, the voltage will give a good indication of the charge state?
From your answer, I assume that a charge current of 5-10%/C will be best?
What will be a reasonable discharge current? If it is to hard, I guess the voltage will drop to much and make the charger stop to early?
Old 08-01-2023, 06:01 PM
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MrRover75,

Yes, once the battery pack is established as being good and the fully charged voltage is known, then you can use that as a reference to be fairly confident that the pack is fully charged. Know that the voltage/capacity discharge curve is not linear and has roughly the same shape as that of a NiCad battery. Reference the attached graph for typical values at different temperatures. As for the possible discharge current, it could discharge at two C (4 amps) without affecting the voltage very much. It is capable of discharging at much higher rates and that is also why you do not want to drop a fully charged NiMh or NiCad cell in your pocket where it could short out on a key ring and set fire to your pocket. Don't ask me how I know that. Charging at 10% of C will allow for the battery to charge for up to a couple of says before overcharging becomes a problem. Although charging at 1 C is acceptable if you can be sure to take it off charge in an hour, I don't recommend it because the higher the charge rate, the fewer recharge cycles the battery can achieve. With my aging memory, I would forget to take it off charge and this could lead to the battery becoming overcharged and be damaged. At my age, I am beginning to believe in the Here-After... I go into my shop to get something, get distracted and ask myself, "What am I here after?"

Enjoy your flying.
David

Attached Files
File Type: pdf
NiMh voltage-capacity curve.pdf (249.5 KB, 67 views)
Old 08-01-2023, 10:16 PM
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Hi David,

Again, thank you very much for your answer. Sharing your knowledge is greatly appreciated

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