I have a LaCrosse BC-700 charger with a 3v 2.8A wall power supply, and I bought a USB adapter cable to power it from a solar panel or from a car USB cigarette lighter adapter. Will it cause any damage to the charger or batteries if I connect it to a USB solar panel or a USB car lighter adapter that has an output of 5V and 2.1A? This is the adapter cable I bought:
USB power adapter cable
Can this be used with a USB port that is not 3V and 2.8A (such as 5V and 2.1A)?
Welcome to BLF
I can’t help ypu here bit sure others will 
so bumped this Independence Day!
Don’t use anything other than the original power supply! I think it is stated in the users guide.
The Charger uses pulses of 2A with varying duty cycle. Voltages above 3V has to be burnt off as heat in the charger (which runs hot enough on its own) and courses it to be extremely hot and burning off its components.
I have the exact same charger. It is a strictly home use charger for me. I find it too bulky to take along anywhere. There are many designed-for-USB chargers around, smaller and lighter too, that you might want to look into.
I wouldn’t use 5V on a 3V rated device…
However you can use a step down DC converter to turn the USB power into 3V output, these boards are very cheap on eBay and works pretty well.