I just received my first "nice" solar charger. Looking at the box, I was a little surprised to read this on the box:
"The less you use the solar panel to recharge the battery, the longer the battery will last"
Looking online I found:
"product's battery life will decrease when the solar panel is used to recharge the device's battery. Extend the life of any internal battery by charging through a wall adapter using provided charging cables."
Is this the norm?
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Honestly, I think only an EE could answer the question and they'd need knowledge of battery chemistry and design, charger design, solar array output voltage, regulation circuitry, etc.
IIUC a properly designed system can tolerate the inherent variability of a solar array, and generally speaking Li-ion cells don't have memory issues that plague some other battery formulas (causing diminishing capacity due to partial charge/discharge cycles) but ideally the Li-ion cells should also be protected and should be connected to smart circuitry that regulates voltage and prevents overcharging.
Loc: Central Illinois near Springfi...
My guess would be that the solar charger has little or no regulation and could overcharge the battery. Most devices have charging regulators and may turn off once the battery is charged. Since I don't know what make of solar charger we're talking about, I can only guess at the reason, but overcharging will shorten the life of a battery. I gather that the solar charger has an internal battery that is used to charge other devices and that the solar panel charges the internal battery. This is convenient, because you don't have to leave the device that you want to charge hooked to the solar charger and laying in the sun. I have one like this that I bought several years ago and have regretted buying ever since. The internal battery is large and heavy, while the solar charger is small and underpowered. I seldom get more than half a charge on the solar cell and need to plug the battery in to a USB charger to get a full charge. The internal battery has to put out 5 Volts even if the device being charged only has a 3 Volt battery because the USB voltage is 5 Volts. The rest of the energy is wasted as heat. I'm really not up on the state of the art solar chargers, but this is an area that will gain importance in a connected world.