There's a BIG difference in what a block heater and a battery tender do !
Block heater, warms up the engine,oil and coolant
Battery tender, trys to keep battery charged
If my BX23S had to stay outside ,it would have both a block heater AND a battery warmer. Kinda obvious how/why a block heater works but people forget that cold batteries lose a LOT of power. You need a GREAT battery to supply LOTS of power to both spin the engine AND heat the glowplugs. batteries work best(supply rated power) at 'normal room temperatures', so +20*C NOT -15*C .I had to add a 2nd battery to the forklift as it used to sit outside all winter. Could take an hour to start below 5*C,get it to 10*C and 1st time, everytime.