NiMH vs Lithium Ion: Which battery is better for electric toothbrushes?

Rechargeable batteries in electric toothbrushes these days are either NiMH or Lithium-ion cells. What are the differences? Is one better than the other?

This question is important because nearly all powered toothbrushes have build-in batteries, you are not supposed to change. In order to be able to enjoy a long life, it is important to know how to treat your battery and what to avoid.

  NiMH Lithium ion
Energy density 140 – 300 Wh/L 250 – 676 W·h/L
Cycle durability 180 – 2000 400 – 1200
Price lower higher

Nickel-metal or NiMH Battery

NiMh batteries have a higher energy density compared to nickel–cadmium (NiCd) type . This is the amount of energy it can store in relation to its size. And with toothbrush handles size does matter a lot more than with i.e. a power drill.

All battery types self-discharge over time. That means they use power even when not used. NiMH cells loose loose the most energy the first day after charging. It slows down after that and the amount of power it is loosing over time depends a lot on temperature. Higher temperature means it will discharge faster. Another factor that plays a roll here is age. Older batteries discharge faster than newer ones.

No matter how “full” a NiMH cell is, it is able to power your toothbrush with nearly the same voltage. You will not notice any difference in brushing speed until the battery is almost dead.

When it comes to security, NiMH batteries have very small risk of bursting when overcharged. Overcharging will cause hydrogen gas to build up within the battery. In order to protect the cells from rupturing, the have a safety valve to allow the gas to safely escape.

Depending on the brand a Nickel-metal cell can be charged 180 to 2000 times. When it not longer can hold the charge, it is recyclable. The chemicals used are just mildly toxic which makes the battery kind of environmentally friendly.

The often feared memory effect is negligible if you treat your battery right.. It is true that partial discharge may lead to memory effect. Fortunately it can be treated by charging  and discharging the battery completely a few times. It is not a very practical solution though if you consider  a charging time of around 12 hours some electric toothbrushes require. If you want to do a few cycles you would not be able to use your brush for several days. Lithion-ion cells do not have this problem.

Lithium ion or Li-ion Battery

Lithium ion batteries  have a higher energy density than NiMH batteries. Smaller sized batteries allow to build slimmer handles. Philips is using this kind of battery in all their newer electric toothbrushes.

Another advantage is the faster charging. There is no need to wait for 12 hours before you can use your new toothbrush.

The self-discharge rate of Lithium-ion cells is much lower than the NiMH kind. Additionally there is no memory-effect. The need to discharge and charge them completely is not necessary to prolong the battery’s life. On the contrary, draining a Lithion-ion battery completely is not advisable. It will ruin it. The build-in computer should that takes care it does not happen.

One disadvantage of this battery type beside the higher price is a slight safety risk. In contrary to other types the electrolyte used is flammable and the whole battery is under pressure. Faulty manufacturing can lead to fires and resulted in recalls in the past. You may have followed the issues Samsung had with their S7 batteries.

A lot of toothbrush owners complain that their toothbrush dies shortly after the warranty of two years end. That is no surprise if you know, that Lithium ion cells age. The average life span is two to three years.

The chemicals used are lightly toxic. If the battery or your toothbrush handle no longer works, you should take care of safe disposal.

Sources:

http://www.powerstream.com/Storage.htm
http://batteryuniversity.com/

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