Understanding the specifications of different types of batteries

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

27 October 2024

Mike Musau (‘Double-M’) has been shopping around for a battery for his power back-up system. He writes, “I inherited a power back-up unit from my employer after they upgraded to a larger one. Our technical guy told me that it would work just fine but I have to replace the battery for best performance because the existing ones are too old. He suggested I go for the modern Lithium type but I am finding them too expensive… Are they worth the extra cost?”

I asked ‘Double-M’ to check the specifications of the old battery and said it 12V and 200Ah. The Ah (ampere-hour) rating tells us about the energy storage capacity of the battery. To get the value, we multiply the Ah by the voltage; in Double-M’s case, this comes to 2,400volt-ampere-hours. Volt-ampere-hours is equivalent to watt-hours, so this battery can store up to 2,400Wh which is equal to 2.4kWh.

Now 2.4kWh is a lot of electricity; it is about 72 units in a month. At current rates, this about Sh2,300-worth of electricity from Kenya Power. Most households consume less than Sh1,000-worth of power (below 30kWh monthly), so the 2.4kWh should be able to run the average house for two and a half days of black-out!

Unfortunately, not all the 2.4kWh are available for use. Ordinary lead-acid batteries should never be fully discharged. Doing that causes permanent damage and the battery is no longer able to receive charge. Power back-up and solar electricity systems use ‘deep-cycle’ batteries which can be safely discharged to about 50 per cent of maximum capacity.

But for longest service life, most manufacturers recommend that they are not discharged below 60 percent of full charge. This means that Double-M’s the realistic available storage is slightly under 1kWh.

The advantage of lithium-ion batteries is that they can be discharged to almost empty – most have a minimum charge limit of 5 per cent. In other words, a 2.4kWh lithium battery will have about 2.28kWh of available storage. In short, Double-M needs only a 100Ah lithium-ion get the same useable storage as the 200Ah lead acid type.

Now a 100Ah lithium-ion costs about the same price as a 200Ah lead-acid battery – about Sh35,000. But, the life-time of the lithium type is about four times that of the lead-acid. This is the main reason why lithium-ion batteries are becoming more and more popular.