What are the Differences Between NiMH and Li-ion Rechargeable Batteries?

NiMH Pros

• High Energy Density: These have an average of 2200mAh (milliampere hours—the amount of current in milliamperes that it can give multiplied by the time in hours). This is greater than the 1500mAh seen in Li-ion batteries. These are the standard numbers in 1.2v NiMHs and 3.7v Li-ions.
• Compatible: If you have 10 gadgets using an NiMH battery, you can use a single pack to power them all—not simultaneously, of course. NiMH uses standard sizes so they are compatible with all devices using sizes such as AAA or AA. Compared to Li-ions, the sizes depend on the manufacturer or model of the device. I did see some AA size Li-ions around the market, so it may be worth checking them out.
• Safe: They have less active materials compared to Li-ion batteries. NiMH can pop if they are overcharged too much or short circuit, but this is nothing compared to a Li-ion, which can potentially blow up!
• Fully Dischargeable: By this, I mean these batteries can be brought down to 0 charge if you can get them to reach that. They will still charge; just don’t let them suffer from reverse polarity, or else they will be damaged.

NiMH Cons

• High Self-Discharge Rate: NiMH lose a large percentage of their charge every month. The number is around 5% in the first week after the charge and about 50% in the first month. There are low self-discharge (LSD) rate types available. They are more reliable than the standard NiMH, but they have lower capacities, usually around 2000mAh.
• Unreliable for Low-Load Devices: You should not use NiMH batteries for devices such as clocks. They will lose charge faster through self-discharge rather than the load. Use alkaline, Li-ion, or lithium batteries instead.
• Low Voltage Output: Each AA cell can only give 1.2v, compared to Li-ion cells that can give 3.7v.
• Long Charging Time: The standard charge time of a NiMH is 10–12 hours. Fast charging these cells can result in damage. Li-ion cells can be charged at around 1–3 hours, depending on capacity.
• Sensitive to Extreme Temperatures: At extreme temperatures, NiMH voltage output will drop. Li-ion batteries can tolerate these temperatures to some extent.