"The charging voltage is in the range of 1.4-1.6 V/cell. A fully charged cell measures 1.35-1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0-1.1 V/cell (further discharge may cause permanent damage). In general, a constant-voltage charging method cannot be used for automatic charging. When fast-charging, it is advisable to charge the NiMH batteries with a smart battery charger to avoid overcharging, which can damage batteries and cause dangerous conditions. A Ni-Cad charger should not be used as an automatic substitute for a NiMH charger."
A good safety feature of a custom built charger is to use a resettable fuse in series with the battery, particularly of the bimetallic strip type. This fuse will open if either the current or the temperature goes too high.
I bought a "(3.6V) PowerFilm™ flexible solar module" from the Edmundscientific site. It is possibly the best way to charge a battery because it's automatically the right voltage in the solar panel to the battery. And so all you have to do is attach 2 batteries in series with wire. And you've got yourself a charger that can't break and can work on camping trips, and in any weather to recharge batteries during the day to use in LED headlamps. And just buy extra batteries or chargers if you use a lot of batteries while camping.
Or you could just use the solar panel charger at your home in a window.
But you have to watch the voltage of your charge with a voltmeter to not over charge the batteries.(see the above optimum voltage).
This is some product info: This is the instructions link
"These PowerFilm™ flexible solar modules can be used alone to direct-power your small DC project (anything that runs off 3.6V or less and requires 100mA of current or less) - or, they can be used in your battery-charger project to store energy for later use.
BATTERY-CHARGING TIPS
For battery charging, the voltage of your solar panel MUST be higher than the voltage of the battery that you want to charge. We recommend using the following guidelines:· at least 2.0 volts to charge one 1.2 volt battery
· at least 3.6 volts to charge two 1.2 volt batteries wired in series
· at least 4.8 volts to charge three 1.2 volt batteries wired in series
· at least 6.0 volts to charge four 1.2 volt batteries wired in series.
Multiple panels can be wired together to increase voltage or current, and the panels can be cut (following directions!) for your special project.
Each module is encapsulated in 3 mil clear polyester film.
Specs:· Output 100mA @ 3.6V
· Open-circuit voltage: 4.8V
· Total size: 2.9" X 5.9"
· Total solar cell size: 2.4" X 5.9"
· Total thickness: 0.22mm (8 mil)
· Weight: a few grams"
"Operating voltage 3.6VDC Operating current 100 mA Typical Voc 4.8V Typical Isc 130 mA Total size 74mm x 150mm (2.9 x 5.9 inches) Aperture size 60mm x 150mm (2.4 x 5.9 inches) Thickness 0.2mm (8mil) Weight 3.1g (0.1oz)"
And from the panasonic NiMH spec sheet it says that the maximum voltage that 1.2 V nominal voltage batteries can handle is 1.8 V which is 5.4 volts in a series of 3 batteries(always charge in series). Which the solar panel will never be able to exceed. in other words when the battery is fully charged the solar panel will give out less current, until it is only a small trickle charge. And put simply that means that you can leave it unattennded and it will take care of itself without breaking ruining the battery. As long as you have a diode to keep the battery from discharging when the solar panel is in the dark.
OR GET:AA foldable solar charger if you want a plug and play operation, with no electrical knowledge needed. Even though it costs 3 times as much, for the same parts.
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