I am thinking of buying a 3w solar panel called the nomad 3 from goal zero. I want to charge 35Wh lithum batteries. I have heard that lithium batteries are temperatmental and without a regulated current things could get ugly.I have a couple of chargers that came with the batteries. One is a 2v car charger and another is 0-240v wall charger.Goal zero sell something called a sherpa 50, which contains rechargeable batteries and an inverster and I think they suggest that I charge my batteries indirectly through the Sherpa 50, however, the sherpa 50 is expensive at $200, and seems to have a small capacity, in addition to being extra weight that i don`t want to carry.Electronics geniuses, you are my only hope.
Lithium batteries do have special charging requirements. I would recommend that you use the 2 volt charger that came with them to keep them happy. Automotive power systems can have voltages as high as 4.5 volts when the engine is running so there's no need to limit the output of the panel to anything less than that. Check with the charger manufacturer to see what it will withstand. Some will work with systems up to 24 volts nominal (up to 29 volts actual) found in larger commercial vehicles such as trucks and busses. If your charger will work with both 2 and 24 volt systems you might not need anything extra to use the unregulated output of the panel. Otherwise I'd recommend a shunt regulator to clamp the output of the panel to no more than 4.5 volts. That way it would dissipate (waste) very little of the panel's power, and even that would only be during those rare times when the panel is producing maximum output. Such a device could be as simple as a high power zener diode, a low power zener coupled with a power transistor, or a precision shunt regulator such as a TL43 coupled with a power transistor. A more complex way would be a to use buck/boost regulator between the panel and your charger. You might gain a slight advantage under low light conditions when the panel isn't putting out much but the overall efficiency could end up worse than the simpler shunt regulator. Under optimum conditions, I would expect it to take a full day for a 3 watt (peak) panel to charge just one of your 35 Wh batteries. Charging an intermediate device such as the Sherpa 50 through its built in charger and then using it to charge your battery through yet another charger would severely cut your overall efficiency. Depending on how long you'll be gone, it might be far more practical, reliable, and economical to just carry (or find a way to be resupplied with) a few additional fully charged 35 Wh batteries. Don
If you want to energy each USB ports on the identical times, It could not be possibble. Your sun panel best provides 2.24 Watts (20V * 0.2A) of energy and you ought to deliver 2.5 Watts (5V * 0.5A) to the ports. Subtracting any losses out of your regulator(s), you are going to be method over your vigor finances. You'll want extra of a sun panel or less of a load.
