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Old 12-26-12, 06:44 PM   #6
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So, I've been doing some reading on nimh chemistry (which I do know some stuff about, but not charging specifically). I found this specific site quite informative:

How to charge Nickel Metal Hydride Batteries.

The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.

So, in order to keep the system super simple and easy all I need is to provide approximately 1.41V per cell and keep the current below C/10.

Lets start with the C/10 part. C/10 means capacity divided by 10. Most rechargable cells have a capacity rating in mAh or Ah. The batteries I have on hand are shown below. They are rated for 2200 mAh. So, if I keep the charging current below 2200/10 = 220 mA I don't need a charge controller chip. Since the clock should pull so little energy, this slow charge rate should fairly easily be able to keep up. So, whatever solar panel we select it needs to have a max output of 220 mA or less.

On to the voltage side of things. Max charge voltage should be about 1.41V per cell. The alarm clocks I've looked at both use 3 battery cells, so I'm looking for a max charge voltage of 3 x 1.41 = 4.23V.

Unfortunately, you can't just get a solar panel that puts out 4.23V. However, from what I've seen, 5V and 6V panels are readily available at the current levels we're talking about. So, we have to figure out a way to bleed off either .75V from a 5V panel, or 1.75V from a 6V panel.

To get this voltage drop is actually quite easy, and it is going to be done by another component we need for the solar setup anyway. This component is a diode. Its essentially a one way valve (check valve) for electronics. This prevents the solar panel from draining power from the batteries when there is no sun. A diode also has the added side effect of creating a voltage drop across it. It is similar to a plumbing system with a check valve in it. All check valves create a pressure drop across them because they cause a slight restriction. The same thing happens in an electrical circuit. There is always a voltage drop across a diode, and you can get diodes with different voltage drops.

For a 5V panel, this diode will drop the voltage by 0.75V bringing the total voltage down to a max of 4.25V.
BAT48 STMicroelectronics | 497-2512-1-ND | DigiKey

For a 6V panel, this diode will drop the voltage by 1.75V again bringing the voltage down to 4.25V
MUR1100ERLG ON Semiconductor | MUR1100ERLGOSCT-ND | DigiKey

So, I really just have to pick out a alarm clock, grab a solar panel off ebay, and then get a diode that will drop the voltage the correct amount. Really, its a pretty simple setup.
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Last edited by Daox; 12-27-12 at 10:45 AM..
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