a bigginer question

System

I probably going to miss some data

looking for solar charging of three AA battries while they are connected and runnig a device

http://www.digi.com/products/wirelessdropinnetworking/sensors/xbee-sensorsspecs.jsp#environmental

is it possible?
can you point me to some devices/documents/examples

thanks

BB.

Re: a bigginer question

It is certainly possible--but you need to get into your needs and the electronics of the devices the AA batteries will power.

First you need to know the load of the device... How many Amps (or mAmps) does it take (peak and average) and how many Amp*Hours per day does it require.

Next, you need to know the voltage range that the device will operate at. As you charge batteries, their voltage may exceed the safe limit of the device.

Type of battery... NiMH batteries are very common, cheap, and have high capacity. But they can be difficult to charge. If you want to charge them fast (C to C/3 --- i.e., a 2,400 mAH battery charged at 2.4 amps to 0.8 amps) get hot and need appropriate charge control that detects heat and "voltage depression" as the batteries approach full charge.

If, you charge at C/10 or less--you can use a simple "trickle" charge type circuit... Just dump the current into the cells and they will probably last 1-3 years.

But, if you are going to do the C/10 route--you may need to look at NiCad--The have less capacity than NiMH (around 0.9 AH for NiCad vs 2.0 AH for AA NiMH). Since you only get 3-6 hours of "full" sun per day--and you want to charge at C/10 or slower--Your bank will ideally need to last at least 3-6 days without sun (to allow a low charge rate and keep the unit running 24 hours per day).

If the device can die after a couple days without sun (in your application) and/or you are willing to replace batteries every year or so--you can even support C/3-C/4 charge rate. The batteries will last as well as in solar powered yard lights (1-3 years or so).

If you do plan on the battery bank being 100% discharged, then you should "match the cell capacity"... Basically with 3 or more cells in an application that takes the battery bank dead--you want all of the cells to die at the same time. Going to zero charge on NiCad (and NiMH to a degree) is not bad. What is bad is when one battery goes dead and the other two still have charge--this cause the dead battery to "reverse charge". Pretty much a death sentence for any rechargeable technology.

-Bill

System2

Re: a bigginer question

thanks alot Bil

I will get into it in a few days but just to have a felling can you point me to some commercial solar charger

for the

"the device can die after a couple days without sun (in myapplication) and/or are willing to replace batteries every year or so--you can even support C/3-C/4 charge rate. The batteries will last as well as in solar powered yard lights (1-3 years or so)."

thanks again

BB.

Re: a bigginer question

Honestly, you probably will be hard pressed to find a commercial charger that can charge your AA batteries in a reasonable time. Most of them take a week or two of full sun to charge a couple AA batteries.

However, you can build a charger yourself pretty easily.

First decide on how much power you will need. Say you are using 3x AA NiCad batteries. Rated at 1.2 volts and 0.9 Amp*Hours (900 mAmp*Hours you may see written too--same thing). Assume the battery needs at least 1.6 volts to charge. And we will aim at C/5 charge current (higher currents and the batteries can overheat). Assume that C/5 charging efficiency is 63%.

Charge current required will be 0.9AH/5=0.180 Amps
Charge Voltage Minimum = 3x 1.6 volts = 4.8 volts minimum

So--to pick a solar panel, you will need a minimum of 6 volts output. 6V*0.18Amps=1.08watts. You can use a 2.16W minimum 12 volt panel too (remember, Vmp--Voltage maximum power is usually higher than "6" or "12" volts--so plug in the actual Vmp for the panel being used).

Next, you will need to install a "blocking diode" in the positive lead of the solar panel (correct polarity) to prevent the panel from discharging your batteries at night. Next, there are several ways of limiting current... but the simplest is to use a resistor.

I would probably pick a 12 volt 5 watt panel, a normal diode and the equation for the resistor would look like:

17 volt panel - 0.7 diode drop - 4.8 volt battery charging = 11.5 volts across diode

V=I*R, R=V/I
R=11.5 volts / 0.180 amps = 64 ohms (560 ohm to 690 ohm will be fine)

Power=V*I
P=11.5 volts * 0.180 amps = 2.07 watts

You will need a resistor(s) rated at over 2 watts of power dissipation.

The same thing above can be done with a 6 volt panel--if you can get one for a good price.

If you are concerned about the battery voltage "running away" during charging--you can add an active circuit to limit or regulate voltage... More complexity--but not very expensive (parts would be $10-$20 USD or so--but you may want help if you are not into electronics).

Obviously, before you connect your device--you will want to make sure that your charger is working correctly (diode installed backwards will prevent charging during the day, solar panel connected backwards to batteries can ruin panel, battery voltage during charging may rise too high for device/load--may need electronics to protect device against over-voltage, etc.).

Can be done--I just don't know your skill level. I have not seen any good quality solar chargers that will do a good job for you if you need to charge the batteries in less than a week (the above is a "waste" of solar power--but it will charge your batteries from dead in two days).

-Bill