Actually, this is basically the same situation that exists by default on many of today’s complex, computer controlled vehicles that have a wide range of on-board electronic devices, many of which consume electric power even when the vehicle is not running. So the short answer is ‘Yes, you can leave the Battery Tender Plus (BT Plus) battery charger connected to a battery even when it is being used to provide power to another appliance. However, this is definitely not a simple yes or no type of question. There are definitely some things to consider that may limit this type of usage. Think about how consumer electronic products are used and consequently, how they are designed. To keep the price low and competitive, it is important to limit the battery charger applications to a realistic, manageable portion from the vast number of ways in which a battery charger may be used. The simplest applications involve charging and maintaining vehicle engine start batteries. The advantage in this application is that there is almost always another, larger source of electric power to charge the battery while the vehicle is running. So the off-line engine start battery charger need not normally supply the full amount of charge stored by the battery. One common exception would be when the vehicle battery is drained because lights were left on. The point is that the engine start battery charger is not normally expected to deliver its maximum charge current for extremely long, extended periods of time.