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| NiMH break-in charging |
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| IanB:
--- Quote from: RoGeorge on December 27, 2022, 05:50:17 pm ---I remember -dV (negative delta V) being the end of charging criteria for CdNi, while for NiMH the charging is considered completed at 0dV (zero delta V). --- End quote --- No, this is not the case. Check the data sheet for the cells linked in the first post: https://www.batteryspace.com/prod-specs/A2700.pdf They show a typical charging strategy of 2100 mA (0.8C) with a −10 mV delta V for termination. The negative delta V happens when the rising temperature at full charge causes a drop in the internal resistance of the cell. At constant current, when the cell EMF reaches a maximum and the internal resistance drops then the cell terminal voltage also drops. The drop in voltage is proportional to the charging current, so a 10 mV drop at 2100 mA would be a 5 mV drop at 1050 mA, or a 2.5 mV drop at 500 mA. Since the OP was attempting to charge at 500 mA and looking for a 3 mV drop, it is no wonder the termination was unreliable. |
| pcm81:
--- Quote from: IanB on December 27, 2022, 06:05:54 pm --- --- Quote from: RoGeorge on December 27, 2022, 05:50:17 pm ---I remember -dV (negative delta V) being the end of charging criteria for CdNi, while for NiMH the charging is considered completed at 0dV (zero delta V). --- End quote --- No, this is not the case. Check the data sheet for the cells linked in the first post: https://www.batteryspace.com/prod-specs/A2700.pdf They show a typical charging strategy of 2100 mA (0.8C) with a −10 mV delta V for termination. The negative delta V happens when the rising temperature at full charge causes a drop in the internal resistance of the cell. At constant current, when the cell EMF reaches a maximum and the internal resistance drops then the cell terminal voltage also drops. The drop in voltage is proportional to the charging current, so a 10 mV drop at 2100 mA would be a 5 mV drop at 1050 mA, or a 2.5 mV drop at 500 mA. Since the OP was attempting to charge at 500 mA and looking for a 3 mV drop, it is no wonder the termination was unreliable. --- End quote --- Thank you! This makes sense. I did not know that -dV is linearly proportional to charge current. |
| pcm81:
Has anyone seen a case where battery temperature goes up but -dV does not get triggered even though it should? I am charging at 1.5A, have -dv set at -3mV and have few cells that terminate due to over-temp, set at 45C rather than due to -dv. Most cells terminate good at those same settings by -dV. |
| NiHaoMike:
Take a look at this project for the ultimate NiMH charger design: http://www.ultrasmartcharger.com/ It also helps to not fully charge (except occasionally for balancing) or fully discharge the cells. That's how Prius batteries regularly last well over 10 years. |
| pcm81:
--- Quote from: NiHaoMike on January 01, 2023, 04:12:00 pm ---Take a look at this project for the ultimate NiMH charger design: http://www.ultrasmartcharger.com/ It also helps to not fully charge (except occasionally for balancing) or fully discharge the cells. That's how Prius batteries regularly last well over 10 years. --- End quote --- I am using Sky RC MC3000, which "should" have good temperature and voltage monitoring. These are brand new cells i am breaking in to capacity match them in a battery assembly. |
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