Your battery basically sounds fine and a 3 month old one should not be at the top of your suspect list. If you're losing your radio code (and have not disconnected the battery or run it stone dead) then I would be more suspicious of a poor connection, probably a ground connection and my armchair logic has me guessing it's one of the battery post clamps or between battery negative and main chassis ground point. Voltage drop tests can help point you in the right direction. While you have your nice Fluke meter handy, you might as well check the alternator output. That could well be part of the problem.

As a little background, electrical problems, especially ground problems, can be aggravated by damp weather and initially these problems may seem transient such as after leaving the vehicle parked overnight. Battery output problems can be compounded by cold weather as they will have less cranking ability first thing on a cold day -falsely giving the impression of battery drain for a minimally charged battery. Battery charging problems can be compounded by the added loads and extra idling typical in cold weather. A marginal alternator may have adequate output to keep the battery minimally charged during normal weather, but can't keep up with the demands in cold weather. A minimally charged battery can still measure 12+ volts. A weak/dead battery is often just an indicator of a problem elsewhere.

The 700/900 FAQ has a rather varied set of tests and tips for the charging system. From my notes here's a distillation of what I consider important. Other than for quick and dirty checking, when diagnosing charging system problems always try to begin with the battery fully charged. After charging, if there is still doubt about battery condition then take the fully charged battery to a shop for load testing.

Check Alternator Output:
Warm the engine. Set a minimal electrical load with all accessories and lights off (unless DRL prevents this). Attach your test leads as indicated then take measurements with the engine revved at approx 2000 rpm.
o Measure DC voltage between the alternator B+ terminal and D- ground post. With a minimal electrical load you should get 13.4-14.6 volts (depending on ambient temp and which book you read). Anything below that range typically means worn brushes (likely), a failing regulator/brush pack or worn/grooved alternator slip rings. At normal outdoor temperatures I like to see close to 14.25 volts for a healthy alternator. Anything outside about 13.25-15 volts will very likely cause over/under-charging of the battery depending on electrical load, charging duration and frequency of starts. A high reading almost always indicates a voltage regulator fault. For a quick check you can do this test across the battery terminals, but the readings will be a little lower (typically within .2 volts).
o The most common problem is low alternator output. If the vehicle wiring is in poor shape then first do the voltage drop tests below, otherwise start by suspecting the brushes. Remove the regulator/brush pack from the back of the alternator. The brushes need to extend at least 5 mm out of the holder -add an extra 1-2 mm for an old alternator with very worn slip rings. Worn brushes can be replaced separately (see FAQ), but it's simpler to replace the whole pack especially if you are able to avoid Volvo prices or if you have one of the later more complex design packs. There are a few aftermarket brands -OEM Bosch are generally better and worth the few dollars more. While you have the regulator pack out, check the brush contact area inside the alternator for slip ring wear. If there is any serious grooving (say 1.5 mm or more in depth) then even brushes of proper length may be making intermittent contact -typically you'll see the dash indicator lights flickering on or staying on for a moment or two longer at startup, especially on a cool/moist day. Brush contact can also be aggravated by a worn alternator bushing. In either case, your alternator can often be re-built relatively reasonably at a starter/alternator shop, or you can buy a good used one or a new/re-built (usually on a core exchange basis).

Check Basic Electrical Connections:
Warm the engine. Set a fairly full electrical load (headlights, fan on high). Attach your test leads as indicated then take measurements with the engine revved to approx 2000 rpm.
o Check the voltage drop between the B+ and D+ terminals. It should be less than .5 volts, otherwise there is a wiring problem or a poor connection which you can further isolate as follows:
o Check the voltage drop between the alternator B+ terminal and the battery positive post. It should be less than .2 volts (otherwise check/clean all primary +12 volt wiring connections including the battery post clamp).
o Check the voltage drop between the alternator D- ground post and the battery negative post. Again, it should be less than .2 volts (otherwise check/clean primary grounding including the battery post clamp, battery to chassis, battery to block and block to alternator).
o Check the field signal. Make sure the D+ field wire is connected and not loose. Start the engine keeping it at idle. You should immediately see battery +12 volts between the D+ field terminal and the D- ground post. Otherwise there is a D+ field fault. Fault trace the field wire back to the dash and re-test between there and chassis ground. Note that with or without a signal, the field can energize magnetically at higher revs producing alternator output.

Hopefully these checks will help isolate your problem quickly.
--
Dave -own 940's, prev 740/240/140/120 & quasi-expert only on a good day