Rechargeable Batteries #1: NiCd vs NiMH

We all like the idea of recharging and reusing batteries.

The first batteries available in domestic sizes were Nickel Cadmium batteries, or NiCads as they became known. These were followed more recently by Nickel Metal Hydride batteries, or NiMH as they became known.

Each of these, at least in principle, can be used to replace the normal alkaline Eveready batteries in normal usage.

How well do they work? What are their pros and cons, and how do they compare with each other?

The first and obvious advantage of the rechargeables is that they can be recharged. They are also much lighter than the alkaline batteries.

On the negative side, the rechargeables do not possess the same energy density as the alkaline batteries. In other words, they go flatter quicker. The NiMH was worse than the NiCAd, as it would self-discharge; in other words, it’d go flat even if it wasn’t being used.

Some of you may have experienced this with mobile phone batteries. I once had an LG mobile phone with a NiMH battery that wouldn’t hold a charge for 12 hours, even if the phone was turned off.

The NiMH had a significant advantage over the NiCADs however – no memory effect. The NiCADs did have a memory effect, which meant that if the battery was not fully discharged before being recharged, it would lose capacity. So if, for example, the battery was only discharged by 10% before being recharged, this would become its new capacity – so when it was only 10% discharged it would be flat.

The advantage that the NiCADS have over the NiMH, however, is that they are capable of delivering higher currents, sometimes required for devices such as battery operated model cars.

Another significant advantage of the rechargeables is  their flat discharge curve. That is, if you were for example using a 1.5V battery, you want it to deliver 1.5V right up until the point where it goes flat, and both the NiMH and NiCADs were good in this regard.

Alkaline batteries, on the other hand, do not have flat curves, and the voltage will gradually drop with time.

The consequence of this is that if for example you were using rechargeables in a torch, the beam would be nice and bright, and then suddenly die when the batteries went flat. With alkaline batteries in the torch, however, the beam would gradually dim with time, and become dimmer and dimmer until it just wouldn’t work at all.

The next level of sophistication was the Li-ion battery, which became the battery of choice for laptops. Stay tuned.

One thought on “Rechargeable Batteries #1: NiCd vs NiMH

Leave a Reply

Your email address will not be published. Required fields are marked *