A tachometer is a device that measures the speed of rotation of a shaft, usually the rotational speed of an engine. It is measured in revolutions per minute (RPM). Currently most commonly they used digital tachometers, for its greater accuracy.

Within the wide range of batteries on the market we can differentiate two large groups, disposable batteries and rechargeable batteries.
Disposable batteries are those that only allow a single use and when they are exhausted they can not be recharged. These batteries can store their energy for years.

Rechargeable batteries are those that can be recharged once exhausted. This type is losing charge when they are storedaccording to voltage. The most common we can find them of 1.2V and 3.7V

As a last point you have to take into account that lithium batteries, Lipo, Life etc. Due to its composition, specific loaders should be used.

Induction charging is a process by which a device can be a charger without direct contact.

Electrification or charging by induction is carried out when a body with an electric charge approaches another with a neutral charge, which causes a redistribution of charges due to the repulsion between them or when the neutral charge is rearranged upon receiving a nearby body with electric charge.

This reorderingor redistribution causes some areas to be loaded positively or negatively, by loading the device.

We can mostly find this charging system in mobile phones or smart watches which can be charged without connecting them with cable, it is only necessary to leave them in the induction base and the charging procedure will be performed automatically.

It must be said that not all devicesthey are of this technology, but it is possible in many cases to adapt them.