In the past on older cars with 2 - 3 year old batteries, if I left the lights on and drained the battery, I might be able to jump start the battery and recharge it by driving around town. The problem is that the battery would have been weakened and a few months later it would die again. Jump start it again and a few weeks later it would die. Jump start it again and a few days later it would die. Then I would have to Jump start it everyday. This was consistent on at least two cars I drove.

By the way one time when that was happeing I took it to a garage to have them check the battery since it died just a month after jump starting it without me leaving any lights on, I was told the battery was still good. They tested the battery after charging it and it was good, but a few weeks later, it died.

I guess that it is not until the battery is dead and won't even charge that the garage will admit that the battery is dead because they are afraid of being sued for recommending a new battery if all you needed was to recharge it or fill it with water.

The 850 should have daytime running lights so leaving the lights on should not be the problem. But the accessories can drain the battery when the car just sits if it is wired to be on even without the key being inserted. Also after a few years, the water level in the batteries can get low, causing the same problems, just fill the water level back up with distilled water and the battery should live another year. Another thing to think about is temperature, colder temperatures requires more amps to crank the engine. Explains why it is usually winter when you have battery problems.

I think the battery is good for about 48 months if it is not overloaded. Overloading it might shorten its life to 24-36 months. So I don't think you have any electrical problems, I think it was an old battery, overloaded, and short on water that was left sitting for several weeks.