Types of Small Rechargeable Batteries - 18650 Lithium Batteries, AA and AAA NiMH and NiCD Batteries

A small backyard garden can be an electronics-free zone, but with the introduction of many useful tools and gadgets which help and ease many daily tasks, one must know few things about rechargeable batteries often found in devices like solar outdoor lights, automatic watering systems, fountains, various other pumps, timers, etc.

Published: January 9, 2021.


Rechargeable batteries differ in size and shape, capacity, voltage, and chemistry. Never, but really, never:

- physically abuse the batteries,
- throw them in fire,
- charge batteries with chargers designed for charging batteries of different chemistry,
- short circuit the batteries,
- mix batteries of different chemistries in a device,
- replace rechargeable batteries with batteries of different chemistry, etc.

Most common rechargeable batteries that can be found in and around the garden are Nickel Cadmium (NiCd) batteries, Nickel Metal Hydride (NiMH) batteries and lithium (lithium ion, lithium ion polymer, lithium sulfur, lithium phosphate, thin film lithium etc) batteries.

Nickel Cadmium (NiCd) Batteries

NiCd batteries contain cadmium and they are not the most environmental friendly batteries. They tolerate high discharge currents with nominal voltage of 1.2V. To operate for longer period of time, they must be, from time to time, fully discharged and then fully charged, otherwise 'memory effect' can happen which leads to reduced capacity.

NiCd batteries come in various shapes and sizes, but mostly as AA and AAA batteries.

When compared with NiMh and lithium batteries, NiCd batteries tolerate the smallest number of charging cycles, have the smallest capacity (measured in milli-amperes and watt-hours) for given weight, have highest self-discharge rate, have issues with memory effect and pollution.

However, they are cheap and can be found in cheap outdoor lights and other similar devices and cordless tools.

Nickel Metal Hydride (NIMH) Batteries

NiMH batteries don't contain cadmium and other similar pollutants. They are not prone to memory effect and have very small self-discharge rate - newer batteries have capacity loss in the range of 2-3% per month.

NiMH batteries can be optimized for capacity and low discharge currents, for power and high discharge currents or for general applications - in most cases, ordinary NiMH batteries will do their job well in most cases.

Nominal voltage of NiMH is 1.2V, but they should not be charged with chargers designed for NiCd batteries. However, most modern chargers can charge both NiCd and NiMH batteries (chargers detect battery type and charge accordingly).

NiMH batteries cost more than NiCd batteries and less than lithium batteries. Similarly, NiMH batteries have a higher capacity than NiCD batteries and lower than lithium batteries.

Like NiCd batteries, NiMH batteries come in various shapes and sizes, but mostly as AA and AAA batteries.

NiMH batteries are commonly found in devices used in a garden, especially those that are not really cheap.

Lithium Batteries

Lithium batteries are superior to other battery types in terms of capacity and power, they have no memory effect, can be discharged with relatively high currents, they can last for a very long time and they are one of the most expensive types of batteries used today.

Nominal cell voltages vary from type to type, but generally lithium ion battery has 3.6V per cell, lithium ion polymer 3.7V per cell and LiFePO4 around 3.2V per cell.

Lithium batteries are very sensitive to charging conditions and they must be charged with proper chargers. When charged with wrong chargers, these batteries are accident waiting to happen - explosions and fires can occur easily.

Lithium batteries come in various shapes and sizes, like AA and AAA batteries, 18650 lithium batteries and similar.

Lithium batteries are commonly found in high-end devices requiring larger capacities.

Most Common Battery Sizes

As mentioned before, most common rechargeable battery sizes are AA, AAA, 18650, but there are many other sizes as well. Dimensions are given in the following list:

- AAA batteries: diameter x length - 10.5 × 44.5 mm (0.41 × 1.75 inches). Capacities vary from model to model, but generally from 300 mAh, up to, or even more than 1000 mAh. Available in various chemistries.

- AA batteries: diameter x length - 14.5 × 50.5 mm (0.57 × 1.99 inches). Capacities vary from 600 mAh, up to, or even more than 4000 mAh. Note that NiMH AA batteries with capacities of more than 3500 mAh are mostly designed for low currents, while 1000 - 1800 mAh batteries are designed for larger currents.

So, if you need batteries for flashlights or similar low power devices, go for high capacity batteries, but for cordless drills and similar high power units, choose batteries with lower capacity, but capable of producing high currents without adverse effects on the battery's capacity and life.

- C batteries: diameter x length - 26.2 × 50 mm (1.03 × 1.97 inches). Capacities of NiMH batteries go up to 6-8 Ah, while NiCd batteries go up to 3Ah, but can provide, if needed, 100A!

- 18650 batteries: diameter x length - 18.6 × 65.2 mm (0.73 x 2.56 inches). This is one of the most popular dimensions for lithium batteries. Maximum capacities of these batteries is around 3-6 Ah, but their voltage is around 3.7V and that gives them much higher energy density when compared with NiMH batteries. Maximum discharge currents are around 10C (30-40 Amps) and to protect batteries, many models have built-in over-current protection.

These batteries cost more when compared with NiMH batteries and require high-quality chargers, but they store more energy per mass and volume, have a lower self-discharge rate with no memory effect, can provide plenty of energy when slowly discharged, and can withstand high discharge currents. Due to their characteristics, 18650 batteries are often used in various devices and appliances ranging from flashlights, laptops, power tools, R/C toys to various electric vehicles.

Of course, there are other sizes of rechargeable batteries as well, but they are rarely used.

- CR2032 battery is a non-rechargeable 3.0 volts manganese-dioxide lithium battery. It features a very low self-discharge rate and is commonly used as memory backup battery, in remote controls, in toys, ultra-compact flashlights, and similar. It features physical dimensions of (DxH) 20 x 3.2 mm, hence the '2032' as the part of its label.

Long Story Short: There are other batteries that are commonly used in home appliances, garden tools, solar lights, and similar, but these are the most common ones.

When replacing old batteries with new ones, always go for batteries from reputable brands with good reviews - such batteries have been tested in real-life situations by countless users and they have stood the test of time.