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Rechargeable NiMH Batteries (AAA, AA, C, D)
and a Solar Battery Charger

Copyright © 2005 and December 20, 2010 by Robert Wayne Atkins, P.E.
All Rights Reserved.

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Introduction

If you are camping, or if the electricity is off while you are at home, then battery operated radios and flashlights are really nice things to have.

If you purchase rechargeable batteries then you could recharge your batteries using the sun if you have a solar battery charger.

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1.2 Volt NiMH (Nickel-Metal Hydride) Rechargeable Batteries
versus 1.5 Volt Alkaline Batteries

For several years a myth has been circulating around the internet that a 1.2 volt battery will not work in a device that specifies a 1.5 volt battery. The reason given is that the 1.2 volt battery does not have as much power as the 1.5 volt battery. Like many other myths, this myth is not true.

Almost all electronic devices that use 1.5 volt alkaline batteries will also work just as well with 1.2 volt rechargeable batteries, unless the device specifically states that rechargeable batteries should not be used in it.

The average 1.5 volt alkaline battery will drop to approximately 1.2 volts after approximately 20% to 30% of its useful life. For the remaining 70% to 80% of its useful life it will be below 1.2 volts until it reaches about 0.8 or 0.9 volts and then it will stop working. At this point it will be drained and it will not yield enough volts to operate the electronic device in which it is installed.

The average 1.2 volt NiMH rechargeable battery starts at approximately 1.3 volts and then drops to approximately 1.2 volts and it remains very close to 1.2 volts for approximately 95% of its useful life. For the last 5% of its useful life it begins to rapidly decline down to about 0.8 or 0.9 volts and then it is considered drained and it needs to be recharged.

In addition, 1.5 volt alkaline batteries do not last as long as 1.2 volt NiMH batteries when put into use. And a 1.5 volt alkaline battery only provides about 9 amps of current at the beginning of its fully charged state whereas a 1.2 volt NiMH battery provides about 10 amps of current at the beginning of its fully charged state.

Therefore, when you consider all the above information, a 1.2 volt NiMH battery produces more voltage than a 1.5 volt alkaline battery for approximately 90% of the useful life of the NiMH battery. And when it is "drained" the NiMH battery can be recharged again and again for somewhere between 100 to 1000 times depending on the quality of the original battery and the application in which it is used.

I have not done any studies on how many times I have been able to recharge a NiMH battery so I do not know if the average manufacturer claim of about 300 to 400 recharges for a NiMH battery is reasonable or if it is overstated.

Whenever possible you should allow your rechargeable batteries to fully discharge inside the device in which they are being used before you recharge those batteries. In other words, wait until the electrical device either stops working or it tells you it is time to replace the batteries. This will mean the batteries are fully discharged and they are now ready to be recharged.

It should also be mentioned that a NiMH battery does not have the "memory" problems of a Ni-Cad (Nickel-Cadmium) battery. The original Ni-Cad batteries had to be charged to their peak potential the very first time they were charged or they would remember the first charge level and you would never be able to charge that specific battery in the future above that original charge level. NiMH batteries do not work this way and if you don't fully charge a NiMH battery then it only impacts that one usage and it has no impact on the future maximum charging potential of that battery.

Most of the time you will probably restore your rechargeable batteries to their maximum power level. However, it is nice to know that if an unexpected emergency occurs and you do not have the time to charge your batteries to a full charge, then it is okay to use those partially charged batteries and it will not impact the future charging potential of those batteries. This means if you desperately needed your batteries during a serious hard times event after the sun has gone down or on a bad weather day, then you could use your batteries in your flashlight or radio even though they were not yet fully charged and you would not be damaging those batteries in any way.

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Solar Battery Chargers

Solar battery chargers have all the following advantages:

1. They are affordable.
   
2. They are entirely self-contained and there is nothing else you need to buy, except for rechargeable batteries.
   
3. They are easy to use and they do not require a background in electronics.
   
4. They are small, lightweight, and portable.
   
5. They may easily be added to an emergency backpack or bug-out-bag. If you also add 12 rechargeable batteries, a L.E.D. flashlight, and a portable world-band radio then you would have light, and entertainment, and access to global news events for many, many years into the future.
   
6. They will work in a grid-down situation when there is no electricity (as long as the sun is shining).
   

Solar battery chargers have the following characteristics:

1. The sun must be shining for the solar battery charger to work. A solar battery charger will not be useful during a blizzard, or hurricane, or at night. However, sooner or later the sun will shine again and then the solar battery charger will be able to successfully do its job.
   
   
2. A solar battery charger will not recharge batteries as quickly as a standard 120-volt house current battery charger. However, if the electricity if off, and your batteries are completely drained, then you may not object to waiting a little longer to recharge your radio and flashlight batteries. (Note: Individuals who complain about the time it takes to recharge batteries in a solar charger usually want fully recharged batteries in a few hours. They are not content to wait a little longer for the same results. This time issue can be easily resolved by having a spare set of rechargeable batteries already charged and ready to use whenever a set of discharged batteries needs to be put into the solar charger.)
   
   
3. All batteries are not created equal. That is part of the reason for the price difference between batteries. The more power a battery is able to store, the longer it will take to restore the battery to a full charge. Therefore, the time required to recharge a battery will increase as the number of mAH of the battery increases. For example:
   a. It will take approximately 5 hours of direct sunlight to recharge two AA 800 mAH batteries.
   b. It will take approximately 10 hours of direct sunlight to recharge two AA 1600 mAH batteries.
   
   
4. Solar battery chargers are designed to recharge identical batteries of the same exact brand name and of the same exact size at the same time. In other words, you cannot recharge one AA battery at the same time as a AAA battery. Instead you will need to recharge two AA batteries at one time. Or you could recharge two AAA batteries at the same time. Or you could recharge two C batteries at the same time. Or you could recharge two D batteries at the same time. A solar battery charger will recharge all the different size batteries for which it was designed, but you can only use it to recharge one size battery at a time.
   
   
5. Batteries should be recharged at a temperature between 32°F to 113°F (or between 0°C to 45°C). During the winter this may require that you put your solar charger in front of a window that is facing south. Place your solar charger where it will receive full sun and where a shadow from the frame of the window will not fall across the face of the solar panel.
   
   
6. Remove your rechargeable batteries from the solar charger at the end of each day and replace them in the solar charger the next day. Do not leave your batteries in the solar charger overnight. (Note: Some solar chargers have a blocking diode that prevents the batteries from discharging during the night.)
   

This article will review two different but similar solar battery chargers. One charger will recharge two batteries at the same time, and the other charger will recharge four batteries at the same time.

Both chargers share the following characteristics:

1. Each charger will recharge AAA, AA, C, and D batteries.
   
2. Each charger will recharge Ni-Cad (Nickel-Cadmium) and Ni-MH (Nickel-Metal Hydride) batteries.
   
3. They will not recharge a 9 volt battery.
   
4. Each charger has a separate rear support bracket that you pull out from the rear of the charger so you can adjust the angle of the solar panel to match the position of the sun in the sky. However, this bracket only has one position so you will need to be creative in propping up the solar battery charger so its solar panel is at approximately a 90-degree angle to the sun.
   

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Solar Battery Charger for Two Batteries

Eleven-in-One Battery Charger	Solar Battery Charger

Dimensions: 6.75" by 4.5" by 2.125".
Solar Panel Size: 5.25" by 2.125" or 11.156 square inches.
Weight: 12.2 ounces.
Maximum Charging Rate: 167 mAH into two batteries at the same time.
Price: Approximately $20 plus shipping at amazon.com.

This two battery solar charger was originally called the "Solar Eleven-in-One Battery Charger with Meter." However it is currently being sold simply as a "Solar Battery Charger." Even though the name has been changed this charger has the same exact design as the Eleven-in-One charger. Therefore it will also charge the seven "GUM" size battery that are used in Europe. But the current box label does not mention this fact because it is not important in the United States of America.

This Solar Battery Charger has a meter that shows when you have the solar panel at the optimum angle for recharging your batteries. The meter eliminates the guess work of whether or not you have the solar panel at a good angle. Just keep changing the angle of the solar panel until the meter needle reaches its highest point and then starts going in the opposite direction. The meter will not work until after you have inserted two batteries in the unit. The meter is connected to a circuit from the solar panel through the battery compartment and until you have two batteries inserted inside the unit that circuit is not complete. (Note: If you wish, you can check the meter in about 2 hours when the position of the sun has changed, and reposition the solar panel to continue to use the sun in the most efficient manner. However, the solar charger will still continue to recharge your batteries even if you don't adjust the angle of the solar panel.)

The meter on the Solar Battery Charger will not show you how much power you have in each battery. It will only show you how efficiently you are using the sun to recharge your batteries. The meter will show you the estimated amount of time required to recharge two fully discharged batteries based on the angle of the solar panel in relation to the sun, and the current intensity of the sun. If your batteries are not fully discharged then it will take less time. And as the position of the sun changes in relation to the angle of the solar panel, it will take more time. And if small clouds occasionally block the sun, it will take more time. Therefore, the meter will only give you an approximate amount of time for fully charging each of the different size batteries (AAA, AA, C, and D). The meter is also not calibrated for the different mAH ratings of the batteries that can be purchased so the meter’s only practical value is to help you position the attached solar panel at the optimum angle to the sun.

The two-battery charger has the following advantages:

1. It is more affordable than the four-battery charger.
   
2. It charges two-batteries at one time instead of four. This is an advantage if most of your electronic devices only use two batteries, such as a flashlight or a portable radio. When the batteries are drained you may put them in the charger on the next bright sunny day and you do not have to wait until you have a total of four batteries that need to be recharged.
   
3. It will recharge the seven standard "GUM" size batteries used in Europe.
   
4. It has a solar meter to help you learn the best angle to position the solar panel in relationship to the sun to maximize the sun's energy and minimize the total time required to recharge the batteries.
   

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Solar Battery Charger for Four Batteries and for Some Cell Phones

Dimensions: 7" by 6.5" by 2.125".
Solar Panel Size: 4.5" by 4.125" or 18.563 square inches (a seven volt solar panel).
Weight: 13.25 ounces.
Maximum Charging Rate: 167 mAH into four batteries at the same time.
Price: Approximately $28 plus shipping at amazon.com.

This solar battery charger will recharge four batteries at the same time. It also comes with a special cord that plugs into the side of the charger and the opposite end of that cord has five standard plugs for recharging the batteries in some cell phones.

There are at least ten different size plug connectors that are used on cell phones and other rechargeable electronic devices, such as laptop computers. Consequently there is no guarantee that this unit will be compatible with your cell phone or other rechargeable electronic equipment. As an example, one of the five adapters would fit my old Gateway laptop computer but none of them would fit my newer HP laptop computer.

The solar panel on this four-battery charger is approximately 66% bigger than the solar panel on the two-battery charger. Therefore it can recharge four batteries in almost the same amount of time as the two battery charger.

The four-battery charger contains an internal blocking diode to prevent battery discharge after sunset.

Therefore, if you live in the United States of America or in any country where "GUM" batteries are not used, then the four-battery charger may be a better investment than the two-battery charger even though the four-battery charger does cost about 40% more.

The four-battery charger has the following advantages:

1. It will recharge four identical batteries of the same design and of the same size at one time.
   
2. It contains a blocking diode so you do not have to remove the batteries at night.
   
3. It comes with a cable that has five different size plugs and this may allow you to recharge the batteries in some of your other electrical devices, such as a cell phone.
   

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Suggestions on How to Use a Solar Battery Charger

Batteries are not designed to be left in direct sunlight for an extended period of time. They will overheat and start to leak and you will get some very nasty stuff on the inside of your solar charger. Batteries react to the sun the same way your skin does. If your skin is exposed to direct sunlight for too long you will get a very painful sunburn (or worse). To avoid the sunburn you could sit in the shade. Therefore the simple solution is to put a white cloth over the batteries, between the batteries and the solar panel, which is on a hinge. Then elevate one end of the white cloth with a short stick or piece of plastic to allow the batteries to breathe. (Don't use a piece of metal which could fall into your charger and possibly short it out.) You do not want to create an oven by completely covering the batteries. You only need to provide shade for the batteries so they do not overheat. This is absolutely necessary in the summer. And it is also necessary, even in the winter, if the sun is exceptionally bright.

The chargers will work on either two or four identical batteries of exactly the same brand name, of the same design, and of the same size, at the same time. You should not mix batteries of different brands at exactly the same time. And it is not a good practice to try to recharge batteries that are not equally discharged. Therefore, I normally allow my batteries to get very close to a full discharge before I stop using them. Then I can safely place them in the solar charger and each battery will receive an equal charge.

Based on my past experience it would probably be wise to invest in several rechargeable batteries of the same size. This would allow you to always have spare batteries while your discharged batteries are in the solar charger. And there will probably come the day when some emergency situation demands your complete attention for an extended period of time, and you will cook a set of batteries. Therefore it is nice to have spares.

It is also nice to have a spare solar charger in the event the battery chemicals get all over the inside of your charger and it is damaged beyond repair. These solar chargers are probably not outside the budget of most families and most families could probably afford to buy two or more of them. That decision is up to you.

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mAH = MilliAmpere Hours

Volt is a measure of electrical force.
All NiMH batteries are rated at approximately 1.2 volts.

Ampere, or amp or amps, is a measure of electrical current.
One milliampere hour, or mAH, is one-thousandth of an amp delivered for one hour.

The number of mAH varies considerably for different NiMH batteries, even from the same manufacturer, such as Energizer, and for the same size battery, such as AA.

The higher the mAH rating the longer the battery will operate an electrical device, and the longer it will take to recharge that battery once it is discharged.

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Volt - Amp Meter (Multimeter)

If you are interested in an optional multimeter that will provide volt and amp readings on your rechargeable batteries then you might wish to consider the GE Digital Multimeter illustrated in the picture on the right (product number 50953). It can be purchased at most WalMart stores for approximately $17 near their electrical extension cord display. Before purchasing this multimeter I visited a Radio Shack Store, an Ace Hardware Store, a True Value Hardware Store, and a Lowe’s Home Improvement Store and I carefully examined the different multimeters that were available. The GE Digital Multimeter is the one I selected. Many of the other multimeters did not have a 10A setting so they would not yield a reading for the number of amps in a rechargeable battery. The one disadvantage of this multimeter, and all the other multimeters that have a 10A setting, is that they require a 9-volt battery inside to power the multimeter. However, the 9-volt battery is advertised to power the multimeter for between 100 to 200 hours of continuous use. Since the multimeter has an "OFF" setting you will probably eventually replace the 9-volt battery due to its normal "self-discharge" rate instead of wearing the battery out using it to power the multimeter. Let me remind you that this multimeter is an optional piece of equipment and it is not needed for any reason other than to satisfy your curiosity about the voltage and amperage of your batteries. This multimeter will not measure mAH, nor will any other multimeter on the market.

Now let's look at what the volt and amp readings on a rechargeable battery actually tell us.

You will not be able to use a battery Volt meter or a "Battery Tester" to determine if your rechargeable batteries are fully charged. Simple "battery testers" are calibrated for alkaline batteries and they only show if the battery is still "good" or if it needs to be "replaced" based on the number of volts remaining in the battery. This is of no value to you when recharging NiMH batteries because the battery will be very close to 1.2 volts the entire time it is reacquiring its total amperage power.

You should know the original mAH rating of your batteries and this is not easy to find anymore. It used to be printed on the front or the rear of the battery package but the battery manufacturers no longer provide this information for their batteries. The battery manufacturers assume you will be recharging your batteries in their 120-volt house current recharging units and they design some special circuits into their recharging units to determine when the battery is fully charged so they can automatically turn off the charger.

You also will not be able to use an Amp meter to determine if your rechargeable batteries are fully charged. If you have an Amp meter then you will need to set it on its 10A setting to get a reading on your rechargeable batteries. But all the Amp meter will show you is how much current is currently flowing through the battery at one moment in time. It reports AMPS. The power stored in a battery is measured in mAH, or milliampere hours. An Amp meter will not tell you this value.

Therefore you will not be able to use a Volt meter or an Amp meter or a "Battery Tester" to determine if your rechargeable batteries are at their maximum peak potential charge. You will need to use a different method. That different method will be explained next.

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How to Determine the Total Time Required
to Restore a NiMH Battery to a Full Charge Inside a Solar Charger

Both the two-battery charger and the four-battery charger require approximately the same amount of time to recharge batteries. The two-battery charger will add approximately 167 mAH of current into two batteries in one-hour. The four-battery charger will add approximately 167 mAH of current into four batteries in one-hour. This estimate is based on a bright sunny day with the solar panel aimed directly at the sun at a 90-degree angle to the sun. If the angle is not exactly 90-degrees then the amount of charge declines and the amount of time required to return a battery to a full charge increases. Therefore it is important to have the solar panel pointed directly at the sun. As the position of the sun moves across the sky during the day this means you will need to periodically reposition the solar panel on the charger so that it once again points directly at the sun. If the solar panel is not pointed directly at the sun then both chargers will only add about 125 mAH of current per hour into each of the batteries inside the charger. If clouds occasionally block the sun during the day then the charge rate can decline to about 80 mAH of current per hour into each battery and this means it will take approximately twice as long to bring the batteries back to a full charge.

Therefore to determine a reasonable estimate of how long you should leave the batteries inside the charger in direct sunlight you will need to know the mAH rating of the battery. Then divide the mAH rating by 167 to yield the fastest possible recharge time in full sun. Double that value to yield the time required to recharge your batteries on a partly cloudy day.

A few examples of different battery sizes and different mAH ratings are as follows:

Battery	Rating / 167	Minimum Recharging Time
Size AAA	800 mAH / 167 =	4.8 hours of direct sun
Size AAA	850 mAH / 167 =	5.1 hours of direct sun
Size AA	850 mAH / 167 =	5.1 hours of direct sun
Size AA	1300 mAH / 167 =	7.8 hours of direct sun
Size C	1500 mAH / 167 =	9.0 hours of direct sun
Size C	2500 mAH / 167 =	15.0 hours of direct sun
Size D	2500 mAH / 167 =	15.0 hours of direct sun
Size D	3000 mAH / 167 =	18.0 hours of direct sun

The above estimates would apply to the two-battery charger and the four-battery charger because the solar panel on the four-battery charger is bigger and it can recharge four batteries in approximately the same amount of time that the two-battery charger needs to recharge two batteries. The above estimates are the minimum time required. If it is partly cloudy or if the solar panel is not pointed directly at the sun the entire day then the above time estimates will need to be increased.

You will also notice that the size of the battery, such as AAA or C, is not the important issue. The important issue is the mAH rating of the battery. The mAH rating of the battery is what determines the total time the battery needs to remain inside the solar charger in order to return it to a full charge.

If you don't know the mAH rating of your rechargeable batteries then you will need to determine the optimal amount of time to leave those batteries inside your solar charger. In other words, you will need to conduct a simple experiment.

The experimental method I recommend is based on repeated trials and keeping track of your results from each trial. If you will follow this method you will be able to determine how many hours you should leave each of your different types of batteries inside a solar charger.

(Note: Although you may not be too interested in the following information at this time, it may become very important to you at some time in the future if a serious hard time event impacts your family.)

1. Begin by determining how many good hours of direct sunlight you have each day. This may be as many as ten good hours of direct sunlight in the summer. But in the middle of winter you may only have about six good hours of direct sunlight. Where you actually live will also impact this value. The following test procedure may require that you recharge your batteries over a period of two or three consecutive days in order to get enough direct sunlight hours.
   
   
2. Place the correct number of fully discharged batteries into the solar charger and put the charger in direct sunlight for five hours following the directions mentioned earlier to avoid "melting" your batteries.
   
   
3. After charging the batteries, place two or more of the batteries inside a flashlight or radio or other device based on the number of batteries required to operate that device. Then turn the device on and write down the time of day that you activated the device. Check the device every 15 minutes to see if it is still working. Eventually the device will either stop working or it will become too weak to be of practical use. Write down the time of day again. Compute how many hours the device worked on a five-hour sunlight charge.
   
   
4. Place the correct number of fully discharged batteries in the solar charger and put the charger in direct sunlight for 7.5 hours following the directions mentioned earlier to avoid "melting" your batteries. After charging, remove the batteries and follow the directions in step 3 above and write down how long the device worked on 7.5 hours of sunlight. Since 7.5 hours is 50% more than 5 hours, the actually operating time of the device should be about 50% longer than your first test (or a ratio of approximately 1.50).
   
   • If the second time was almost equal to the first time then five hours may have been too long and you will need to experiment with 3 or 4 hours of sunlight.
     
   • If the second time was approximately 50% longer than the first time then proceed to step 5 below.
     
   • If the second time was a lot less than 50% longer than the first time then do the following math:
     
   
   Divide the second time value by the first time value. Then multiply by 5 hours.
   This is the approximate time required for those batteries to receive a full charge.
   For example, if the device worked 12 hours the first time and 15 hours the second time then:
   (15) / (12) = 1.25
   1.25 x 5 = 6.25 hours.
   
   In this situation your rechargeable batteries will be at a full charge in about 6.25 hours.
   This is your final answer and you may stop the test for this type of battery.
   
   
5. Place the correct number of fully discharged batteries in the solar charger and put the charger in direct sunlight for 10 hours. After charging, remove the batteries and follow the directions in step 3 above and write down how long the device works on 10 hours of sunlight. Since 10 hours is 100% more than 5 hours, the actually operating time of the device should be about 100% longer than your first test, or double the total number of operating hours (or a ratio of approximately 2.00) . If the time was 100% longer (approximately 2.00) then proceed to step 6 below. But if the device operating time was a lot less than 100% longer (2.00) then do the following math:
   
   Divide the third time value by the first time value. Then multiply by 5 hours.
   This is the approximate time required for those batteries to receive a full charge.
   For example, if the device worked 12 hours the first time and 21 hours the third time then:
   (21) / (12) = 1.75
   1.75 x 5 = 8.75 hours.
   In this situation your rechargeable batteries will be at a full charge in about 8.75 hours.
   
   You can verify this value of 8.75 hours by doing the following math:
   If the first reading was 12 hours, the second reading was 18 hours, and the third reading was 21 hours, then:
   (21) / (18) = 1.167
   1.167 x 7.5 = 8.75 hours.
   
   In this situation your rechargeable batteries will be at a full charge in about 8.75 hours.
   This is your final answer and you may stop the test for this type of battery.
   
   
6. Continue putting fully discharged batteries in your solar charger and continue to increase the time of each test by 2.5 hours and keep track of the results. When the device operating time does not increase by the same multiple as the test time, then you have reached a full charge on your batteries. Divide the total number of device operating hours on your last test by the total number of device operating hours on your first test to get the multiplier. Then multiply that number by five hours to get the approximate optimal recharging time for that particular type of battery.
   

You will need to repeat the above procedure for each different type of battery and brand name of battery you have to determine the optimal full sun recharging time for each type of battery.

The above procedure and math assumes full sun every day. Therefore you should make a note of the average sun conditions on each day of your tests and keep that information with your test results so you can more easily interpret any differences in your results on future tests if the average daily sun conditions are not the same.

I fully realize the above is a lot of work. And I also know that almost nobody will bother to follow the above procedure. However, if one day you are impacted by a serious hard times event, and you have a limited number of rechargeable batteries and a solar charger, then you will probably be very interested in using the above procedure to get the maximum charge into your batteries each time you recharge them, without damaging those batteries, so you can get the maximum run time out of your flashlights or radios or other battery operated devices.

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Recommended Brand of Rechargeable Batteries

The major battery manufacturers are constantly changing and improving the design of their rechargeable batteries. This is referred to as "continuous improvement." Therefore the rechargeable batteries available for sale this month may be replaced by a battery with a different name in a different package next month. Consequently it is not possible to make a specific battery recommendation that would be useful for any reasonable period of time.

However, my personal experience with a variety of different Rayovac brand rechargeable batteries over a period of several years has been more satisfactory than my experience with either the Energizer or Duracell brand rechargeable batteries. But the battery quality from these major manufactures could change at any time so you will need to do your own research on the quality of the different brands of rechargeable batteries that are available for sale at the time you are ready to make your investment.

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Revision History:
December 20, 2010 - Added more information about rechargeable batteries and another solar charger.
2005 - Created this web page.

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