Do you have any after market equipment in the Car? You can have the battery or the Alternator tested at something like Pep Boys. I'm assuming this is not electroic ignition? How long have you had the car? How long have ou had this problem? One easy way to determin if this is a battery or if something is draining the battery is to charge the battery fully. Then, disconnect the battery and let it sit. See if the battery still holds a charge the next day or two. If it does, then you have something draining the battery. If not, then it's the battery. Dry cell or liquid? If liquid, you can check the levels in the battery. If this is a recent problem, then think back to what you may have changed, worked on. If you can determin this, that may lead you to the problem. Lots of things could cause this. Could be aftermarket stuff, could be a short in the wiring, bad ground, could be cables. If the cables are older, could be issues there. Check the connection between the cable and the piece that you connect to the post. If it shows corrosion or wear, it could be that you have fraid contact wires, poor connection to the post etc. You might also check your voltage regulator, if applicable. Check all your lights in the car. Is anything staying on? Seat Belt light, anything? Also, are you using a defroster? If so, sometimes they don't turn off. When they get a bit older, the elemant fails and it just stays on even when it should turn off. I hate electrical problems. Here is a way to try and isolate the problem.
With ignition off take off the negative cable and put a test light in series with the cable. If the tester lights, you have a current draw somewhere. A electronic radio will draw a minimal amout of current to illuminate the light. pull the fuse for the radio. If the light is still on ,continue to remove fuses until the light goes out. Then work from there to trace the current draw. This will narrow your search down.
It's a process of elimination. You may wish to invest in a portable battery charger if you have not already done so. It will cost 30 or 40 bucks but it will save you a great deal of pain over the long haul.
Good luck.
