Most NiMH batteries have an average of 2,200mAh. They are also compatible with most devices using AAA or AA batteries. However, these have a higher self-discharge rate, losing about 50% of their charge in the first month. NiMH batteries also have a lower voltage output. Li-ion batteries are more compact and have a lower self-discharge rate. They also have a higher voltage output. But most li-ion options only have a 1,500mAh capacity.