3,
Work in progress and notes related to the PPP
How to calculate the Zener diodes?
1. Check the LE Ds? voltage ratings for the visual multimeter
MIN 1.7V, MAX 2.6V (Jameco part # 333219CB for yellow)
2. Select your battery voltage
3. Calculate zener diode values-- d1 is green (turns on first)
z1 = Battery voltage @ 50% capacity - Led min rating (or use solar cell voltage as reference)
z2 = Battery Voltage @ 80% capacity - Led min rating
z3 = Battery voltage - Led min rating
In the case of the 7.2 volt, 70mA battery.
z1 = 4 - 1.8 = 2.2V ==> 1N52221B 2.4V
z2 = 6 - 1.8 = 4.2V ==> 1N5228B 3.9V
z3 = 7.2 - 1.8 = 5.4V ==> 1N5231B 5.1V
+ for reference 1N5230B,4.7V and 1N5225B,3.0V.
How to calculate how much voltage and current to give the battery.
Once again, find the ratings for the battery. Let's sat 7.2V and 70mA.
The current into the battery should not be more than 10% of the amperage rating so as to not overcharge it. The voltage should be above the battery rating but not too much. The batteries could be charge at a rate of up to 10C but in that case proper monitoring of charge state as well as temperature is very important. A charge at less than 0.05C is also harmful long term.
The two solarbotics options are:
8 volts and 44mA (more like: 6.7V and 30mA) OR
6.7V and 31mA ... 6.1V at 23mA (that can be transformed into 3.3v at near 80ma apparently)
Technically, these are both too much for our battery. A battery of at least 230mA is required for safe usage.
One possible scenario
2 X BATTERY,NIMH,AAA,1.2V/700MAH from Jameco
2.25$/battery or on random cheapo site for 1.5$.
1 x SCC 3733? small cells from Solarbotics, hacked to provide 3.3V and 80mA.
Zener diodes become:
Z1 = 3V - 1.6 = 1.4V.... no need for Zener diodes? Can resistors do it only? So we get only the 2.7V diode. We do the rest with resistors.
Advantage: We could pre-charge some of these batteries with a regular charger so that we only Trickle charge them through the PPP.
A similar scenario is 2 Ni MH? AA, 1800mA batteries from Jameco as well. In this case, the solar panel really does a trickle charge.
Question: We actually need 2xAAA batteries for these solar panels. 3x1.2 = 3.6V?
Another possible scenario
Get a 3.6V NiMH battery pack for 10$... also on sale as cordless phone battery packs at places like Radioshack.
electronic goldmine has a cheap one.