You need a specialist L-ion charger, to prevent overcharging. My understanding is it's overheating during charging causing an organic electrolyte to overheat that precipitates a fire.
If you're sufficiently worried, remove the battery from the bike and charge it on paving slab or similar.
An electronics engineer friend of mine uses
these for mobile batteries although I don't know if you'd get one large enough for a bike battery.
ps. 'charge rate' in volts is misleading, Amps is what you should be looking at. Think of Voltage like water pressure and Amperage as flow. Charge is measured in Coulombs and an Amp is one Coulomb per Second (Charge rate) Although too high a voltage will still cause you problems. I assume the Voltage warning is because the Li battery doesn't have sufficient internal resistance to regulate itself.