Make sure all electrical equipment is turned off (lighting devices, heater fan, windshield wipers, etc.).
Make sure that there are no special warnings on the booster battery that prohibit such operations.
Make sure the booster battery has the same voltage as yours.
If the battery used as an auxiliary battery is a battery installed in another vehicle, the vehicles must not touch each other. Make sure that the gearshift lever is in the "Neutral" position (or "Park" in the case of an automatic transmission).
"Lighting up" from another car will help solve the problem only temporarily. You should find out the reason that led to the battery discharge. There can be three such reasons:
1. The battery may have been discharged from repeated or prolonged attempts to start the engine, or because the exterior lights were left on.
2. The battery charging system is faulty (the generator drive belt is loose or broken, or the generator wiring is faulty, or the generator itself is faulty).
3. The battery is faulty (low density of electrolyte or damaged plates).
(The original article is available on the website AUDIMANUAL.RU)