![]() ![]() All the usual methods had aspects which made them unsuitable in this application. I built a charging system for NimH in an environment where it was not possible to use any of the more usual methods (negative voltage inflection when charged, increased rate of temperature changed when charged, absolute temperature when charged, timed cutoff). NimH cells have endpoint voltages of around 1.4 - 1.5V, depending on the charge rate. If you cannot do this automatically, then the user should do it with a switch or something. AA should be charged at the highest practical rate. AAA will want to be charged at 350 or 500mA. In order to accommodate both AA and AAA, you will need to select different charge rates. ![]() I will let you search the details yourself as you seem to be quite resourceful and are proceeding on the right track. The charge rate should be at least C/3 in order to generate a strong dV/dt signal. ![]() In other words, the slope of the voltage change is observed during charge at constant current, or the slope of the temperature change is observed during charging at constant current. The charging is terminated using either dV/dt or DT/dt criteria. With NiMH batteries, charging does not rely on voltage thresholds for end of charge. If the capacity is similar to original (or similar to rated) then they are probably OK. To be sure, you can measure the capacity after you fully charge them. It sounds like you completely discharged them, then charged them at 350 mA for less than 2 hours. Other: Which voltage should I consider if I want a single value for NiMH and NiCd chemistries? Have I over-charged them? What series voltage is to be expected to reach when they are almost full of charge? It should be a lower or higher final of charge voltage (3 V)? Then I disconnected the batteries from the charger, waited around 10 minutes, and measured them without load again, and measured 2.77 V. After 25 minutes of charging the voltage was 2.98 V, after 50 minutes of charging they were measuring a total of 3.02 V, and after 80 minutes measuring 3.04 V. When I connected the charger to the batteries in series, the total voltage on them increased to 2.89 V. Then I used this high-side current source, of around 350 mA, to recharge them, in series also (they were with almost the same individual voltages at this point). I waited more minutes again with no load on the AAA batteries, the voltage of them was at 2.55 V after this. Then I disconnected the resistor loads, and the open circuit voltage increased to 1.7 V and it began to increase from mV to mV and reached 2.24 V after some minutes. Then I left this working for 80 minutes, and the voltage after this period was 1.36 V (they did not heat). The voltage measured was 2.2 V with around 440 mA of load connected. Its package shows "Panasonic Eneloop" model with up to 800 mAh.įirst I connected them in series and connected two resistors of 10 Ω in parallel to the series of batteries. Some weeks ago, I bought a new pair of AAA rechargeable batteries, NiMH. ![]()
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