Proper Charging
The second key component to proper use of NiMh batteries is the proper use of a good charger.
Using a cheap “rapid” charger will not fully charge the cells, will overheat them and shorten their life.
Modern, smart chargers are readily available, easily affordable and will charge the batteries at the correct rate to fully charge them, while avoiding over-heating.
They will then switch to a trickle charge mode to keep them at peak capacity.
So what is a “proper charge rate?” Most battery manufacturers recommend a charge rate that is between 0.5-1.0c. That is to say, the charge rate should be one half to full capacity of the battery.
So, if a battery is rated at 2000 mAh, the recommended charge rate would be 1000 mAh, and max charge rate 2000 mAh. A full charge will take from 1-2 hours at those rates, respectively.
If you have the time and want to extend the life of the cell, charge at .25c. Sanyo recommends a charge rate between 300-500 mAh for their Eneloops (rated at 2000 mAh). Charging at those rates will give you somewhere between 500-1000 cycles, according to Sanyo. The other batteries I tested have similar ratings.
Personally, I like chargers from BTI, Maha or Ansmann; especially the ones that have selectable “soft” charge rates. I buy enough batteries that I can spend 4 hours charging them, so I charge at 500 mAh. Once the batteries are well charged, it’s important to know how to utilize them properly. And that brings us to…
Proper Cycling of Batteries
To ensure good results (“good” defined as the mic not dying mid-service), it’s important to use the batteries properly. Fully charged batteries should always go straight from the charger to the mic. Once the charger is empty, it should be re-filled with another set of batteries.
When the service or event is ended, remove the batteries from the mics, and charge. If you have enough charging bays, you can simply alternate from one set to another. However you do it, you always want to go from charger to mic.
The reason is that NiMh batteries will self-discharge over a period of 30-60 days. So while you may not lose a lot of capacity from Sunday to Saturday, you’ll be down 10-20% or so. Why push it? Go straight from charger to mic, and charge immediately afterward. Keep a set on the charger all week and you’ll always have fully charged batteries to work with.
Some are concerned about the “memory effect,” the loss of capacity that happens when NiCd cells are recharged before being fully depleted. NiMh cells have no significant memory effect, so charge them when you’re done using them. Don’t stretch them farther than needed.
Good chargers include a refresh cycle that will fully discharge each cell, then fully charge it again. It’s a good practice to do this every 3-4 months. This procedure will prolong the battery’s life and ensure top performance.