CHARGERS

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THANKS TO GREEN INDUSTRIES  FOR SOME OF THE INFORMATION HERE. 
http://www.greenbatteries.com/nimh-nicd-battery-chargers/


RAPID CHARGER AND A FAST CHARGER
There is no standard in the industry, so manufacturers can use the terms in different ways.  The selling point is the amount of time it takes to charge a battery is dependent on the capacity of the battery being charged and the MAH the charger can produce.  In other words, politicians, strobe makers, and charger manufacturers all make up their own statistics since no one really cares about checking on them... then there is me...

HOW LONG WILL IT TAKE  TO CHARGE BATTERIES?
Simple math - Simply divide the capacity of the battery by the charge rate of the charger, then increase the amount of time by about 20% to allow for a certain amount of inefficiency.  As an example, a battery with a capacity of 1600 mAh will require about 4 hours to be fully charged by a charger with a charge rate of 500 mA.
 

HOW TO DAMAGE A BATTERY
The most common cause of premature battery failure is overcharging.  The type of chargers most likely to cause overcharging are the 5 or 8 hour so-called  “rapid chargers” that they really don’t have a charge control mechanism.  These are chargers that are not smart and simply deliver a charge for a period of time rather than reading the battery.

If the  charge cycle is interrupted part way through the charge and re-established the cycle starts over and you cook the battery.  The easiest way to avoid these scenarios is to use a smart charger, a charger with microprocessor control. 

WHAT IS A TRICKLE CHARGER?
A trickle charge is a charge rate that is high enough to keep a battery fully charged, but low enough to avoid overcharging. Maintenance charge is another way to describe trickle charge.

Although most  manufacturers do not recommend that you leave a battery in the charger for long periods of time, many people leave their batteries in the charger on trickle charge for days or weeks to keep their batteries "ready to use". If you know the rate of trickle charge that your charger puts out and it is around one tenth of the battery capacity or less, then you should be alright if you are just going to do this occasionally. Generally speaking, though you do not want to leave a battery charger plugged in unattended for long periods of time.

IS TRICKLE CHARGING HARMFUL TO BATTERIES?
Many battery manufacturers do not recommend long term ( months at a time) trickle charging.  If trickle charging is used then the charge rate should be very low or only intermittent.  The best smart chargers will only send an occasional pulse charge to the battery once it is charged.  

DOES RAPID CHARGING REDUCE THE LIFE OF BATTERIES?
Using a properly designed smart charger, most NiMH batteries can be recharged in about an hour without any damage or significant reduction in their life. However, NiMH batteries must only be rapid charged with a charger specifically designed for charging NiMH batteries.  

WHAT’S THE DIFFERENCE BETWEEN A NIMH BATTERY CHARGER AND A NICD BATTERY CHARGER.
Many of the inexpensive NiMH battery chargers are simply NiCd chargers that have been modified slightly.  Typically a 5 hour NiCd charger has a switch that allows the charge time to be increased from five hours to eight hours. Thus a 5 hour NiCd charger becomes an 8 hour NiMh charger.  Pass on this approach

NiMH smart chargers have actually been designed to detect when a NiMH battery is fully charged and then shut off or go into a trickle charge mode.  Because of the more complex circuitry, this type of charger costs more to make, but should lead to greater battery lifeSome of these chargers only cost slightly more that the "dumb" chargers.  We strongly recommend investing in a smart charger for your NiMH or NiCd batteries.

WHICH ARE BETTER, NICD OR NIMH BATTERIES?
This really depends on what you are going to use them for exactly. NiCD batteries are commonly used for power tools and in that capacity they are in many ways superior to NiMH batteries.
For high drain digital devices where weight is of primary importance, NiMH batteries are the best choice. NiMH batteries are also considered an environmentally friendly battery chemistry. NiCD's are toxic and recycling them is mandatory.

WHAT MAKES A CHARGER A “SMART CHARGER”?
Any charger that uses a computer chip to control various aspects of the charging process can be considered a smart charger. Technically even a charger that can detect and adjust the charge rate based on the battery inserted into the charge station can be considered a smart charger, but anything that is either manual (steady charge rate as long as it is plugged in) or uses a timer to manage the charging process, we do not consider a true smart charger. There are even various levels of smart chargers. 

Different features that work together, sometimes in mysterious ways because there are just so many variables with batteries and chargers. In order for us to consider a battery charger a smart charger it needs to have a common charging feature known as negative delta V. Negative delta V  is basically a technical method for a charger to know when a battery has reached its charge capacity and then shut the charging off, or sometimes change to trickle charge mode. 

Other features that contribute to a battery chargers “ mart" status are: battery rescue (implemented in various ways to attempt to "jump start" an overly discharged battery - i.e. less than 1.0 or 0.9 volts - so that it will take a charge), temperature sensors, discharge and conditioning features, battery test features and even timers to limit the total length of the charge so even if you leave it plugged in, it turns itself off after a preset time. 

Remember, all manufacturers consider their chargers "smart" with any or all of these features and they are not all the same!? Hey, neither are we for that matter... but we get to evaluate them all.