Probably a crap stock battery. That us unless you mean you dumped 4 full hicap mags in which case I'll assume 300-400 BB's each mag giving you between about 1200-1600 BB's but it doesn't sound like that's the case. If you're using midcaps that's more or less 400-500 BB's which means either you might have a 800 mAh battery or something and the cold in the garage also affected it.
This battery should work:
http://airsoftgear.ca/index.php?main...roducts_id=687
That is unless it's shooting at a high velocity, it will more or less last you around 1500 shots (generally I equate 1 mAh to 1 shot but it could be more or less depending on climate, conditions, battery quality, and how you use it).
So yeah, give us your battery info (bonus if you can include a pic) and we can try and help you to see if there's something wrong with the battery or it's crap or it's working semi properly. Also note that there are wall chargers that look like "dumb chargers" but are actually smart chargers, the word you'll be looking for is "Negative Delta V detection" or something similar, all you need is some sort of LED to turn on and off that will tell you when the pack is fully charged or not. But then again I personally like the nice $100 chargers with LCD display screens and a tonne of options that most people will never use as well as the ability to charge obscure chemistries like Li2S (Lithium Sulfur), LiFePo4, etc.