SMFC

Rechargeable Batteries, Answers To All Those Questions?

Compiled by Ian Veldman

Something all radio control modellers have in common is rechargeable batteries, and providing the best care for them to ensure a lasting and reliable relationship is something we dream of. So, like Don Quiote, I set out to conker evil and revive my batteries’ life. To an extend, I was fighting windmills. Fortunately I came across a book “Batteries in a portable world” on the internet. This article is not my work, it is my summary of a 130 odd page book, here is the URL: http://www.buchmann.ca/toc.asp.

About Chargers

To a large extent, the performance and longevity of rechargeable batteries depends on the quality of the chargers. Choosing a quality charger makes sense. This is especially true when considering the high cost of battery replacements and the frustration that poorly performing batteries create. In most cases, the extra money invested is returned because the batteries last longer and perform more efficiently.

Uncontrolled over-charge is a problem of some chargers, especially those used to charge nickel-based batteries. High temperature during charge and standby kills batteries. Over-charging occurs when the charger keeps the battery at a temperature that is warm to touch (body temperature) while in ready condition. Some temperature rise cannot be avoided when charging nickel-based batteries. A temperature peak is reached when the battery approaches full charge. The battery should eventually cool to room temperature. Any prolonged trickle charging will damage the battery. This caution applies especially to the NiMH because it cannot absorb overcharge well.

A lithium-based battery should never get warm in a charger. If this happens, the battery is faulty or the charger is not functioning properly. Discontinue using this battery and/or charger.

Slow Charger — Also known as ‘overnight charger’ or ‘normal charger’, the slow-charger applies a fixed charge rate of about 0,1C (one tenth of the rated capacity) for as long as the battery is connected. Typical charge time is 14 to 16 hours. The slow-charger is inexpensive and can be used for NiCd batteries only. With the need to service both NiCd and NiMH, these chargers are being replaced with more advanced units.

If the charge current is set correctly, a battery in a slow-charger remains lukewarm to the touch when fully charged. In this case, the battery does not need to be removed immediately when ready but should not stay in the charger for more than a day. The sooner the battery can be removed after being fully charged, the better it is.

Quick Charger — The so-called quick-charger, or rapid charger, is one of the most popular. It is positioned between the slow-charger and the fast-charger, both in terms of charging time and price. Charging takes 3 to 6 hours and the charge rate is around 0,3C. Charge control is required to terminate the charge when the battery is ready. The well designed quick-charger provides better service to nickel-based batteries than the slow-charger. Batteries last longer if charged with higher currents, provided they remain cool and are not overcharged. The quick-chargers are made to accommodate either nickel-based or lithium-based batteries.

Fast Charger — The fast-charger offers several advantages over the other chargers; the obvious one is shorter charge times. Because of the larger power supply and the more expensive control circuits needed, the fast-charger costs more than slower chargers, but the investment is returned in providing good performing batteries that live longer.

The charge time is based on the charge rate, the battery’s State-of-Charge (SoC), its rating and the chemistry. At a 1C charge rate, an empty NiCd typically charges in a little more than an hour. When a battery is fully charged, some chargers switch to a topping charge mode governed by a timer that completes the charge cycle at a reduced charge current. Once fully charged, the charger switches to trickle charge. This maintenance charge compensates for the self-discharge of the battery.