|Re: Ok, the hunt is on... Nimh charger solution for N-cell batteries in the USA|
Message #4 Posted by Dieter on 8 June 2011, 3:19 p.m.,
in response to message #1 by gene wright
I must say, I prefer the AA-N-adapter idea. Here is a picture of a homebrew adapter, made of a piece of aluminium tube:
Please note: the shown (disposable) Duracell batteries are not rechargeable and are only there to demonstrate the different sizes. ;-)
However, if N-batteries are charged in AA-chargers, there are a few precautions that have to be considered:
- The key issue is the so called relative charging current. In numbers, that's simply the actual charging current divided by the battery's capacity. For instance, 100 mA for a 500 mAh N-size battery are 0,2 C. Regular NiMH batteries usually are charged with rates between 0,1 and 0,5 C. For a 500-mAh-battery this translates to a charging current of 50...250 mA.
- The charging time is roughly 1.2 divided by the relative charging current. For instance, 100 mA and a 500 mAh battery give 0,2 C, so the charging time for a completely empty battery is about 1,2 / 0,2 = six hours.
- Faster charging means more stress for the NiMH-battery, unless it is not specifically built for it, and definitely requires some kind of charge control. In other words: a special charger that is able to detect when the battery has been completely charged and then turns the charger off.
- The essential point here is the much, much higher capacity of AA-sized batteries compared to the N-versions we use here. Their capacity is roughly five times as high as in the N case, which means that the relative charging current is five times as high as well. In other words: if an AA-N-adapter is used to charge an N-battery in an AA-charger with, say, 700 mA current, this would mean a rate of 1,4 C or less than 1 hour. Which is more than a battery like the GP50NH will take without the risk of damage.
- So, the solution is simple: yes, use an AA-N-adapter to charge our N-batteries in a regular AA-charger, but be sure to use one with sufficiently low charging current. One may think of a good old simple-charger from the Eighties when regular NiCd AA-cells had the same capacity as N-NiMH-cells today. For instance, I got this old Panasonic charger for 4x AA with 50 mA charging current, which is perfectly fine for this case, giving a relaxed charging time of 12 hours. Some of today's higher end chargers may be set to currents as low as 100..200 mA while providing exact charge control (current, voltage, temperature, battery-full-detection). This would be perfect.