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13.5 volts at idle is a good rule of thumb. You are also going to want to know how much current (amps) it is capable of putting out. Your run of the mill ammeter is only going to be good for about 10-15 amps and does need to be hooked up in series. This means disconnect one terminal of the battery and hook one lead of the ammeter to the battery cable and the other lead to the battery post. This will cause the current to flow THROUGH your meter and that is why it is only good for 10-15 amps, passed that it will pop the fuse in it and if there were no fuse it would turn your meter into a bubbling pile of plastic. To measure the available amperage of the charging system you need an inductive ammeter. I have an adapter for my Fluke meter that works great. If you can get your hands on one you will need to clamp it around the battery positive and turn on absolutely every electrical consumer in the car with the car running. You should see somewhere in the area of 80-100 amps depending upon the load and which alternator you have in that car.
Now, if you are going through batteries at an alarming rate I would suggest that you have a draw on the system. The glove box light on the 850 is notorious for staying on and draining the battery. The 850's have a very high parasitic load anyhow. On the older cars you typically would see something in the low teens, 10-15 milliamps, and every now and then something around 20 or so. All in all not too bad, but when the 850 hit the scene in '93 we started seeing stuff up closer to 30. If you can keep it under 30 and drive it regularily you will not have a problem. If you have a light staying on somewhere you will see something like 300-400+. Seeing as how we are talking about milliamps here you can use a regular ammeter and just hook it up in series, remember 100 milliamps is still only .1 amps. The old down and dirty way to check this stuff was to simply hook a test light up in series with the battery, just like you would the ammeter. Problem is that if it lights the light you still have no idea of how much current you are pulling. You can do this real quick and if the bulb does not light or is so dull you can barely see it you will PROBABLY be ok, but if you want to know for sure you will need to put an ammeter on it.
I would start with a check with the ammeter first because that is the most likely problem from your description and then proceed with the other checks.
Good Luck Mark
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