Every battery has two electrodes: the anode and the cathode. In a standard nonrechargeable chemical battery, a chemical reaction oxidizes the anode (typically zinc metal powder), causing free electrons to flow through the electrolyte (ionizing solution) and out the negative terminal. The process is sustained by closing the circuit to the positive terminal, creating a continuous positive charge to the cathode (typically magnesium dioxide powder).
A rechargeable NiMh battery is one that can store energy if the electron flow is reversed, because the oxidizing chemical reaction can be reversed by the application of electricity. This is, of course, an oversimplification – a detailed description of the chemistry involved in batteries, especially rechargeables, would vary depending on the type of battery and is well beyond the scope of this article.
There are lots of rechargeable battery types – standard lead acid car batteries were some of the first commercialized – but most of what you see in today’s electronic devices are either Nickel-Cadmium rechargeable battery(NiCad), Nicked-Metal Hydride (NiMH), or Lithium-ion (Li-ion). If you understand these three battery types, you’ll understand 99 percent of the rechargeable batteries you’re likely to use from day to day.
All rechargeables can be measured by their energy density, the ratio of how much energy they can hold per unit of weight. Different battery types have different energy densities, but density can also be affected by improvements in manufacturing and design.
With every charge, deposits form inside the electrolyte that inhibit the ability of the lithium ions to move between the anode and cathode, and the total charge capacity goes down. This gets worse if the battery is stored fully charged or in high temperatures. The best practice is to leave your laptop only half-charged if you’re not going to use it for a long time.
Though they may be the best of the bunch, 18650 batteries still have their issues. You’ve no doubt heard about laptop or cell phone battery fires in recent years. Li-ion batteries can get very hot, and if there’s a failure in the separator inside the battery and the venting organic electrolyte ignites from a spark or heat, it can quickly catch the cells on fire.