I use an I-charger 106b, a sophisticated computerized charger which permits charging at 5 amps or greater,,, safely. A good Lipo battery takes about 12 to 15 minutes.
Also, use a Lilon charger for Lilon batteries.
I use separate ac to dc transformer to supply 12 to 15 volts DC. this decision to convert was very easy for me, because I have so much gear. For others maybe not so easy.
Next locomotive for conversion is a Pecos River early model ATSF Hudson.