If you Google “auto inverter” you’ll find lots of articles that say the less expensive modified sine wave inverters work fine for most electronics, and devices that have AC motors generally run best on pure sine wave inverters.
I’ve been using several inexpensive modified sine wave inverters for years with no problems with all kinds of stuff including switching power supplies for laptop PCs, ham radio transceivers, as well as computer-controlled, AC-powered lipo
battery chargers for drone and RC aircraft
batteries.
I typically charge my Spark and Mavic Pro
batteries in the car with an inexpensive, 100-Watt Harbor Freight inverter, with which I have had no problems. I made an accessory power port plug for my inverter so I can plug it in near the front seats or cargo area of my Outback. Most inverters only come with
battery clips that you clip directly to the car’s
battery, which doesn’t work if you want to drive while charging.
Having said the above, do not charge lipo
batteries, including your
Spark batteries, in the car unattended. There is a risk of fire.