For PS2, PS3, TMCC, and Legacy, there is no risk to the loco if battery is old or absent. There is a small risk of scrambling the chip on a PS1 engine with a bad battery (generally these can be fixed with a re-programming chip). At most shows, you only have the option of testing in conventional mode, but that is usually sufficient. Some PS2 engines with dead batteries may be difficult to get moving as they start in neutral, and the power interruption caused by the direction button on the transformer keeps restarting the engine.
I always test locos at train shows after negotiating a price. (I offer to leave my driver's license with the seller while I am walking off with his loco).
I have found more that one DOA loco, and the seller is always surprised. The sale is usually dead at that point, since the seller is not likely to lower the price by the hundreds of dollars that board replacement might cost.
I never buy a loco if the seller refuses a test run, unless it is a "fire sale" price and I intend to spend the time and money if it needs a repair.
Bob
