Notes from our experienced Pilots on Charging and Checking NiMH Batteries

I wanted to cover a few questions some members have asked about charging and cycling NiMH battery packs.  Some users of these packs are not sure what a safe charge rate should be for a given capacity and what the purpose cycling a battery pack serves.
Unless a NiMH battery is marked otherwise, a reasonable charge rate is around 35 to 40 percent the total capacity.  Double A NiMH cells can vary in capacity from 600 mAh to 2700 mAh, therefore charge rates can vary from around 250 mAh to just over 1.0 amp.  In my opinion, one of the best chargers for the money is the Hobbico Accu-Cycle Elite.  Once the battery type (NiCad/NiMH or Lipo), number of cells and capacity is entered, the charger will choose a safe charge rate.  When the pack reaches the peak voltage, the unit then goes to a very low trickle charge to maintain the voltage.  If you are using a charger which requires you to manually enter the charge rate, simply multiply the capacity of the pack by .35 and you will have a safe charge rate.  2500 mAh x .35 = 875 mAh.  Keep in mind if you do not have a smart charger that automatically turns off when peak voltage or temperature is reached, you must monitor the batteries during the charge.  Turn-off these type of chargers when the pack becomes warm (NOT HOT).  Chargers that stop the charge with a timer are not generally good because they do not have a quantitative way to measure the packs capacity or voltage.  They are estimating the amount of time it will take to charge a pack without regard of packs condition.
NiCad packs are cycled from time to time for the purpose of conditioning the batteries.  When NiCads are not fully discharged between charges, they tend to build-up memory which can limit them reaching full capacity.  Cycling them helps eliminate this memory and allows the pack to charge to full capacity.
NiMH packs are cycled for a different reason.  NiMH packs do not build-up memory and can be topped-off with a charge before each session of flying.  It is a good idea to cycle a new NiMH pack before the first time use to make sure it is reaching full capacity.  During cycling, most chargers discharge the cells to around 1.1 volts.  This means the pack is not 100 percent discharged and still contains several hundred mAh.  This is why a 2500 mAh pack may only indicate 2000 mAh. I will continue to use a pack as long as it cycles to at least 80 percent of its capacity.  Cycling a pack once every few months helps you monitor its ability to reach full capacity.
A NiMH pack will lose much of its charge over several days.  You should charge your packs the same day you plan to use them.  Don't crash an expensive plane because you though you could get one more flight out of a pack indicating a marginal voltage.  Get a good volt meter that can impose a load on the packs and check your batteries often.  A 4.8 volt pack should not be used much below 4.8 volts.  A 6.0 volt pack should not be used much below 6.0 volts.  Also get a smart charger and try to understand what it is telling you about the battery pack.  For example: After flying five to six times on a charge from the previous week, I would expect my 6.0 volt, 2500 mAh pack to need at least 500 to 800 mAh for full capacity.  If my charger indicated a 7.0 volts at 150 mAh, I would suspect something is not right and would cycle the pack.  If the pack did not cycle to at least 2000 mAh, it would be time for a new pack.


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Good article Russ.  The load applied to the batteries by most of the Hobbico and Ace expanded Volt meters is limited to 300 ma.  The Futaba Battery checker has variable loads from 1 amp to 3 amps.  I have two 5 cell packs that I was using in my Extra and using my hobico meter one read 6.16 and the other 6.34.  Using the Futaba the first pack read 4.3 volts using a two amp load and 5.4 volts with a 1 amp load.  Both readings indicate the pack was failing or not charged.  The batteries had been charged overnight the night before.  This pack normally cycles at 1400 mah but cycling after returning home found the pack to have 500 mah capacity and after charging only 1040 mah capacity which means the pack is history.  It has lost 400 mah of capacity since cycling last which was about 3 months ago.  The other pack which is a newer pack cycled its normal 1400 mah so is ok.  I have changed my recommendation regarding battery testing and would only recommend the Futaba tester if you are flying models with more than 4 standard servos and batteries larger than the normal sent with radio systems.  For 6 volt batteries (5 cell) I want a reading of at least 6.3 volts to consider them safe for flying.  


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