I use the same carbon-pile load cell I use for testing batteries, but a field-expedient load tester can be made by wiring 4 headlights in parallel, with both beams burning at once and connect them directly to the battery posts. The idea is to create enough current draw to make the current regulator function long enough to get an accurate adjustment.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!