Ni-MH Charging Basics

• The coulometric charging efficiency of NiMH batteries is typically 66%. 1.5C input gets you 1C output.
• The faster you charge the more energy you need to put in. The coulometric charging efficiency drops with a higher C rate.
• The Ni-MH -dV/dt curve is smaller than in NiCd batteries, and it is very temperature dependent.
• Newer Ni-MH batteries can exhibit a -dV/dt earlier in the charge cycle, especially when cold.
• Ni-MH are sensitive to damage on overcharge when the charge rate is greater than C/10.
• The -dV/dt curve is not always easy to detect, slight overcharge is possible.
• At end-of-charge, oxygen starts to form at the electrodes, and is recombined at the catalyst.
• The end-of-charge chemical reaction creates heat, which can be easily detected.
• The dT/dt is a good way to detect end-of-charge during a fast charge.