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Slow charging vs fast charging vs smart charging; does it matter what charger I use?

Last Updated: Sep 18, 2012 04:16PM PDT

The overall consensus is that nearly all NiMH chargers will work with nearly all NiMH batteries, but that poorly built fast chargers are likely to diminish the overall longevity/life of the battery (the number of complete charge cycles).

There are two main issues associated with battery charger quality:

  • The manner in which energy is delivered to the battery.
  • The method used to determine when charging has been completed.

In regards to charge energy delivery, there are two approaches: "slow charging" and "fast charging".

In slow charging, energy is delivered at a low constant rate that does not require charge endpoint determination. These are also known as "trickle chargers". They are not the "best" chargers, but they are reliable, simple, and safe. Their main disadvantage is that they can take a very long time to charge a battery (8 to 12 hours). Slow trickle chargers can also diminish battery life if batteries are simply left in the charger indefinitely (ie, more than a week at a time without also using the battery).

Fast chargers are much more complex (and thus more expensive), since they can deliver energy at a much greater rate than the battery can withstand. For a fast charger to be "good", it must perform two tasks:

  • It must deliver charge current without surges, ripples, or spikes.
  • It must reliably determine when the battery is full and shut off the charge current.

Failure in achieving either of these aspects will definitely shorten NiMH battery life (the number of recharge cycles and the max energy that can be stored per cycle). Specifically, the current delivered must be matched against the battery capacity. Also, if there are surges, ripples, or spikes, the battery discharge characteristics can be adversely affected. This in turn can result in a Box running either way too hot or way too cold, depending on the type of battery damage sustained. Furthermore, if the charge endpoint is not determined correctly, the battery can be overcharged, resulting it its getting very hot (too hot to touch) which will also permanently damage the battery. Such issues are more noticeable with LSD rechargeable batteries than with HSD rechargeables.

Unfortunately for fast charger manufacturers, determining when a NiMH battery is "full" is far from easy. The battery itself provides little indication of its internal state. There are three approaches generally taken:

  • Use a timer to cut off current after a given interval.
  • Look for a specific sequence of battery voltage changes.
  • Check the battery temperature and stop charging when the battery is hot.

Each of these approaches has significant problems and is likely to fail if used exclusively. A good charger will need to test for at least two, if not all three, of these conditions to function reliably. It is in this respect that good chargers really stand out from their cheaper cousins—they preserve battery life by not accidentally over-charging the battery. Many fast chargers on the market simply do not test for charge endpoint conditions very well or very reliably.

Magic-Flight ships smart chargers bundled with the Box kit—not fast chargers. The primary reason for this choice is safety: anyone can use the bundled charger without having to understand any specifics about the rechargeable batteries. These stock chargers are convenient insofar as they are compact (they fit in the tin) and are known to be a good reliable option, not because they are the fastest possible means for charging a battery.

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