Charging Batteries with Solar Cells

erco

Lots of posts on this simple-sounding subject got me thinking. It's·quick & easy·to insert a blocking diode between the battery and solar cell to avoid losing battery power when the panel voltage drops. Using a Schottky diode (0.3V voltage drop) is more efficient than a standard silicon·diode (0.7 V),·but·in this·efficiency game, losing that 0.3 V is still quite·wasteful over a long period.

A·better solution·might be to use a·low-current circuit to actually switch the connection directly with no diode losses. Latching relays (as used in battery-powered home thermostats) only require brief power to open or close the relay. A simple comparator IC could control the relay, based on battery and solar panel voltage. Taking the concept further,·multiple relay/comparator circuits could switch several solar panels in series/parallel to make best use of any available sunlight throughout the day.

Two AA batteries in a house thermostat last for years, so·that's an established precedent. The·monitoring/switching circuit·should be powered by the charged battery, obviously. As long as·it consumes less power than·a diode would·waste, it's a net gain. Seems straightforward, perhaps systems like this already·exist somewhere. Anybody·know of·this approach?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·"If you build it, they will come."

K2

Another idea, borrowed from synchronous switchers, is to use a low voltage power MOSFET in place of the relay. The ON resistance of the average power MOSFET, nowadays, approximates a copper wire. The gate, of course, draws nothing in a static situation. The only engineering to be done is pretty much the same as with your solution...a low-power comparator or two would be need to determine when to toggle the MOSFET on and off.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"Let's not bicker and argue about who killed who."

Beau Schwabe

Even better if you could find a latching relay, that way your not wasting any current holding the relay in one position or the other.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Tracy Allen

Hi Erco,

It is different if you are talking about big panels and batteries, but if it is going to be a low-power system, then a low-dropout, low quiescent current regulator can work well. It has to be one that that does not pass current in the reverse direction when the input voltage drops below the battery voltage. The PBB137 that I mentioned in another thread passes only 10µA or less in the reverse direction, however, it is not a meant as solar charger because of its 2.6 Volt dropout. I've use a Linear Tech voltage regulator LT1129-adj for a solar float charger, because it too only passes a few µA in reverse when the sun goes down, and the dropout is only 0.3 Volts.

That said, I don't think there will be that much energy gained or lost by using a Schottky blocking diode in series with a solar panel. The current delivered to the battery is going to be about the same with and without the diode except at a certain dusk-like light level. A solar panel with good illumination, and operating into a load sufficiently below its open circuit voltage, acts more like a current source rather than like a voltage source. It would be different with a MPPT controller, but that is not the topic here I think.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com

Martin Hodge

I believe something like this is what K2 is talking about.

http://www.solarbotics.net/library/circuits/misc_switching.html

Leon

Energy harvesting is getting very popular. Microchip has this new kit for experimenting with the technique:

www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2042&param=en548091

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Leon Heller
Amateur radio callsign: G1HSM

rahinajoseph

Hi..

Using solar power to produce electricity is not the same as using solar to produce heat. Solar thermal principles are applied to produce hot fluids or air. Photovoltaic principles are used to produce electricity. A solar panel (PV panel) is made of the natural element, silicon, which becomes charged electrically when subjected to sun light.This electrical charge is consolidated in the PV panel and directed to the output terminals to produce low voltage (Direct Current) - usually 6 to 24 volts. The most common output is intended for nominal 12 volts, with an effective output usually up to 17 volts. A 12 volt nominal output is the reference voltage, but the operating voltage can be 17 volts or higher much like your car alternator charges your 12 volt battery at well over 12 volts.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
solar LED light

Post Edited (rahinajoseph) : 7/22/2010 9:06:19 AM GMT

Tracy Allen

Thanks for that link Leon.

I'm looking at the Linear Tech LTC3108, to capture electricity from waste heat on a stove chimney, to run an XBee. That chip is part of their energy harvesting product line. It is optimized for a TEG (thermoelectric generator) but the chip will also work with single solar cells. Erco said "solar cell" not "solar panel", so maybe this is on topic. There is no place for a blocking diode when working with a single solar cell, which has an open circuit voltage of only about 1/2 Volt. The harvester chip uses a transformer and mosfet switches to step up (gradually) inputs as low as 20mV to system power as high as 5V.

Energy harvesting is different from maximum power transfer in the usual sense. It is optimized to function at low levels, and it has enough to run the system and to charge whatever storage device is present, the excess reverts to being waste.

I was always amazed by solar calculators. Such a small solar panel to run it, even under indoor light.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com