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NiMH Battery Charger FAQ

Frequently Asked Questions about NiMH and NiCD Battery Chargers

[Note: this FAQ mostly addresses questions about chargers designed for NiMH or NiCD cell batteries.  It does not specifically cover lead acid, sealed lead acid (SLA), or lithium ion battery chargers.]

What's the difference between a rapid charger and a fast charger?

Both terms are essentially meaningless.  There is no standard in the industry, so manufacturers can use the terms in different ways.  One of the problems with terms like these is that the amount of time it takes to charge a battery is dependent on the capacity of the battery being charged.  A charger that can charge a standard capacity AAA NiCD battery (180 mAh) in just one hour might take 8 hours to charge a high capacity NiMH (1500 mAh) battery.  It's best to ignore such terms and make a rough calculation of how fast a charger can charge batteries.  (To do this you can use our Battery charge-time calculator.)

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How long will it take a charger to charge batteries?

It's pretty easy to estimate how long it will take.  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.  (1600 mAh/500 mA x120%).  Incidentally, this example would apply to a standard AA NiMH battery and a typical "rapid charger".  Keep in mind that a battery that is only partially discharged will be recharged in less time. 

If this seems too complicated, please use our Battery charge-time calculator.

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Can a battery charger damage a battery (shorten its life or reduce its capacity)?

Yes.  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".  The problem with these chargers is that they really don't have a charge control mechanism.  Most of them are simple designs which charge at their full charge rate for a fixed period of time, typically five or eight hours, and then shut off or switch to a lower "trickle" charge rate.  If they are used properly, these chargers are fine.  If they are used improperly they can shorten a battery's useful life in a couple of ways.

First, suppose fully charged or partially charged batteries are put into the charger.  The charger has no way to sense this, so it will give the batteries the full charge it was designed to deliver.  It is not unusual to put partially charged batteries into a charger since it's pretty easy to mix batteries up and inadvertently put fully charged batteries into a charger.  Do this enough times with one of these battery chargers and the capacity of the battery will start to drop.

Another common situation is for the charge cycle to be interrupted part way through the charge.  The charger is unplugged to see how warm the batteries feel or to use the electrical outlet for something else.   Then the charger is plugged back in.  Unfortunately, this will cause a complete charge cycle to start again, even if the previous charge cycle was almost complete.

The easiest way to avoid these scenarios is to use a smart charger, a charger with microprocessor control. A smart charger can determine when a battery is fully charged and then depending on its design, either shut off entirely or switch to trickle charge. Most of our chargers use microprocessor control. For specific information see our Battery charger comparison table.

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What is trickle charge?

Theoretically 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. Determining the optimum trickle charge rate for a particular battery is a bit tricky to describe but is generally accepted to be around ten percent of the battery capacity - i. e. Sanyo 2500 mAh AA NiMH optimum trickle charge rate is at or below 250mA. One of the reasons it is important for you to understand the optimum trickle charge rate for your charger and batteries is to compensate for the self discharge of NiCD and NiMH batteries. Another reason is because overcharging a battery will definitely reduce its useful life. 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.  They do not apply a continuous low rate of charge.  Some battery resellers state that applying a continuous trickle charge of about 1/10th the battery's capacity is not harmful.  However, we have not seen any battery manufacturer condone the practice.

It is better to fully charge batteries and then store them fully charged in the freezer than to leave them on trickle charge for very extended periods of time.

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Does rapid charging reduce the life of batteries?

Not significantly.  So long as it is done 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.  Chargers designed to rapidly charge NiCd batteries can overcharge NiMH batteries.  While it may be true that rapid charging NiMH batteries can reduce battery life by a small amount (probably less than 10%), this should be more than offset by the inconvenience of always slow charging batteries.

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What's the difference between a NiMH battery charger and a NiCd battery charger.

The biggest differences are in the charge rate (how fast the charger can charge batteries) and the charge control (how the charge determines when to stop the charge). 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.  As we mentioned above, we do not recommend this type of charger design.  While a timer type charger is less expensive to manufacture than a smart charger,  it can lead to overcharging and battery damage if batteries are frequently charged before they have been discharged (that is, the batteries are used for a short time and then fully charged again).

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 life. Some 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.

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Can I use an older NiCd battery charger to charge NiMH batteries?

The answer to this question depends on the type of NiCd charger.  Depending on the type of NiCd charger you have, the older NiCd charger may undercharge NiMH batteries (most likely), it may overcharge them (less likely), or it may charge NiMH batteries properly (but it's not likely to do so automatically and could take a very long time).  Let's take a look at the three cases.

Many of the older NiCd chargers are the simple timed type charger which will charge batteries for a fixed amount of time and then shut off.  Unfortunately, since NiCd batteries have a much lower capacity than NiMH batteries, the timer is likely to shut off long before the NiMH batteries are fully charged.  This won't harm the batteries, but the NiMH batteries won't be fully charged since the timer will have stopped the charge cycle too soon.

Also common among older NiCd chargers are the so called "overnight" chargers which charge batteries at a low rate as long as the charger is plugged in.  This type of charger can fully charge NiMH batteries, but it might take a very long time to do so.  It's possible that an old NiCd charger could take as long as 48 hours to fully charge new high capacity NiMH batteries! This type of charger is not likely to damage NiMH batteries unless the batteries are left in the charger for weeks at a time, but it may not be very convenient to use. If you have this type of charger you can get an idea of how long you'll need to charge your batteries by using the calculator found above.  

The final possibility is that the older NiCd charger is a rapid charger that will charge NiMH batteries but will not have the necessary circuitry to stop the charge cycle once the NiMH batteries are fully charged.  If the NiCd charger is designed to charge batteries in less than two hours it may be this type.  In this case the risk is that the older charger will overcharge NiMH batteries.  This will be apparent if the batteries get very hot during the charge cycle.  (It is normal for NiMH batteries to get warm as they become fully charged, especially in a rapid charger).  If the NiMH batteries get too hot to handle and stay that way for more than 20 or 30 minutes, then the NiCd charger is most likely overcharging the NiMH batteries and may shorten their life.  You would be most likely to encounter this type of charger if the charger was designed for rapid charging radio control (RC) vehicle batteries.  We would recommend that you not use an NiCD rapid charger to charge NiMH batteries.

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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.

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What is battery conditioning or exercising?

When you intentionally discharge a battery down to a certain minimum voltage and then recharge it this is known as battery conditioning or reconditioning .  It is also sometimes referred to as battery exercise. This is particularly important to reduce what some call the memory effect experienced using NiCD batteries if you habitually do not fully discharge them each time you use them.  For NiCD batteries this must be done periodically, approximately every 10 charge/discharge cycles or so, or the batteries will begin to lose capacity.  For NiMH batteries conditioning is not really needed to reduce any memory effect because that is negligible in this type of battery.  However, reconditioning is very convenient for both NiMH and NiCD batteries because brand new batteries are not charged when you receive them and they must be charged and discharged three to five times before they reach their full capacity.   In addition, occasionally conditioning rechargeable batteries helps to ensure that they give you years or service and save you as much money as possible, before you recycle them and get new ones.

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What is a charge channel or charge circuit?

Battery chargers have one or many charge channels aka charge circuits. Each charge channel can charge one or more than one battery. fro example, it is common for a AA, and AAA battery charger to have four charge stations and two charge channels. This means that each charge channel charges two batteries in the same circuit. This is why you see lots of folks recommend that you keep your batteries in sets to optimize their charging. Mostly, this is recommended because you are probably using a charger that has two batteries in each charge channel, like our TurboCharger 4000 for example.

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What is a charge station?

In a battery charger, the charge station is where you place the battery to recharge it. Many battery chargers have charge stations that accommodate multiple kinds and sizes of batteries. For example, most AA chargers will also accept AAA batteries, and some "universal" chargers will accept other kinds as well in the same charge station. e.g.- AA, AAA, C, and D cells. Still other kinds universal chargers have adapters that come included, or must be obtained separately, to use different kinds and sizes of batteries.

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 "smart" 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...

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