6 . RECHARGE DES PETITES BATTERIES

Les chargeurs solaires pour les petites batteries ne sont pas toujours très fiables, et chaque type de batterie présente des particularités dont il faudrait tenir compte. Il nous a semblé intéressant d’engager ce débat en donnant ici le point de vue d’un fabricant, exprimé sur son site INTERNET. http://www.bti.ca/kordesch.htm

Solar Recharging of Batteries- An Idea whose Time has (finally) Come

In the world of rechargeable batteries for devices like lamps, radios, calculators, notebook computers and cellular phones, what could be better than using the sun to keep them charged and ready for use?
It's a compelling concept but unfortunately, because of technical drawbacks not obvious to the general consumer, solar panel (photovoltaic) charging just hasn't been feasible -- until now, that is. Solar recharging is not only technically feasible, but economical, with the reuseable alkaline manganese (RAM™) battery from Battery Technologies Inc.

Let me explain by starting with two typical kinds of rechargeable cells-- nickel-cadmium (NiCad) and lead-acid batteries.
Rechargeable NiCads: heat is the enemy
NiCads power most small photovoltaic devices sold on the market because they can easily be charged in series due to their chemical overcharge cycle. The problem is, they can't withstand the temperatures encountered in full sunlight, usually above 70° C.
Actually, NiCad cells can never be topped up with any charging device above 50 or 60° C. That's because the charging process produces more heat. Sometimes a charging attempt causes a rapid self-discharge due to "heat-avalanche", when all capacity is lost instantly. Even when NiCad cells are stored at room temperature, they self-discharge within weeks.

So it's no surprise that all attempts to merchandise solar-charged consumer devices have produce little more than disappointed customers.
To be honest, this is true for all rechargeable consumer batteries. There is a danger that cells connected in series can be reversed if the device is not shut off when the lowest-capacity cell is exhausted. That polarity reversal damages the chemistry and causes internal corrosion and finally results in leakage.
Though it's a different method, charging NiCads with a bicycle generator while pedaling will work, until you stop and lock up your bike in the sun for the day. All that heat simply discharges them, the real reason why this simple idea has not found favour up to now.
Lead-acid batteries: long inactivity is the problem
Lead-acid batteries are a little better when it comes to temperature self-discharge. But if you forget to charge them for a few months, they most often become irreversibly dead.
Remember those lead-acid battery-powered toys purchased at Christmas? Too often, in a few months, the cells were useless because no one charged them up during the storage period. Obviously, with usage patterns like this, solar charging just wouldn't work.

Enter, the RAM™ cell
RAM batteries change the situation completely. They endure high temperatures without losing capacity and stay charged for months -- even years.
But even RAM cells don't like polarity reversals or overcharging. Like the trick of filling a pyramid stack of champagne glasses from the one at the top, RAM cells like their charging to stop when they're full, with the energy then directed to the cells that need it. RAM cells like to be equalized on charge, so that the cell with the highest voltage is bypassed when full and the lower voltage cells receive more charge.

We've studied the behaviour of RAM cells in series at our Richmond Hill, Ontario, laboratory and have developed simple circuits to avoid all these troubles. We also recommend that cells of different age or use-history are not mixed. Unfortunately, it is true that mix-ups happen with existing, separate chargers, but even they at least equalize the cells because they charge them in parallel.
In solar charging, batteries stay in the devices, ensuring at least a common history. Here's the challenge we faced: Once RAM cells are all topped up and the charger continues to charge, the cells will leak unless provisions are made to divert the surplus current elsewhere.
Fortunately, designing an overflow is both simple and inexpensive with a Zener Diode of the proper voltage and another diode to prevent self-discharge across the solar panel. (In fact, such discharge-preventing diodes are already installed on the panel by the manufacturer.)
Here's where we get a bit technical. The calculation must recognize this diode voltage drop to work properly. For example, take four cells at 1.65 volts each for a total of 6.6 volts. Add 0.8 volts for the discharge-preventing diode, which results in a Zener diode with a range from 7.4 to 7.6 volts. RAM cells tolerate voltages up to 1.7, so the calculation is not too critical. But above 1.75 volts, the RAM cell chemistry creates gas, which in turn will cause leakage after a time.
A more precise method of overflow control is to equip each cell with a red light-emitting diode (LED) in parallel. In this case the overflow starts at 1.6 V with a few milli-amperes (mA) and reaches 70 mA at 1.7 volts.

What emerges is a nearly perfect equalization of uneven capacity cells --with the added bonus that LEDs are great at indicating state of charge during storage. Even then, however, some micro-Amp discharge must be accounted for, unless the LEDs are turned off mechanically when not needed as indicators.
Finally, to avert cell reversal on discharge, we fitted each one with a parallel diode, which starts to conduct current at minus 0.8 volts. This way, the cell can't reverse polarity by more than -0.8 volts, which is harmless. This way, you can walk away and forget a RAM series cell set-up without the fear of battery leakage.

All these inexpensive provisions make the potential for solar charging a successful venture.
If anyone is interested in more information about these and other circuits,I'd be pleased to respond to requests for literature. Perhaps you're interested in circuits for more sophisticated RAM battery arrangements,including: larger solar devices, military transmitters/receivers, emergency circuits or navigational power sources.

I direct you to a recent publication discussing these problems in general, delivered to the 37th Power Sources Conference (June 17-20, 1996) in Cherry Hill, New Jersey, USA. Another publication is scheduled to appear in connection with the Electrochemical Society Meeting in San Antonio, Texas,October 1996.
BTI has literature about small SOLAR powered devices, especially when used in telecommunication equipment (cellular phones). We can also respond to questions about solar economics. If you're wondering about sunshine patterns and intensities around the world, we have gathered some of that.

We can also advise you on the use of mirrors or concentrators for solar devices -- critical because damage can occur to the solar panels by concentrated heating. Also, the temperature coefficient is such that higher temperatures lower the output considerably.

Interestingly, the best results are obtained on a high mountain in winter, not in the desert!

Greetings,

Karl Kordesch Senior Vice President & Research Director