Its not the battery, if it was the problem it couldnt have started. The oil is so cold that its a mission to move it. Its the same with cars, start one at -25 ull see that even though ur battery is in perfect condition, it won start easily:)
Its normal with these temperatures, don’t forget that it is much harder for a diesel to start in low temperatures because the combustion is started from very high temperatures- because there is no spark plug in a diesel to help ignite it so it has to do it on itself 😊
