Generally when a battery keeps dying even after replacement one of two things is the problem. Either the alternator or alternator belt (however, if this was the case, one would think it would die while driving down the road) OR Something is drawing power that shouldn't be, for example, let's say the switch that shuts off the light in the trunk is broken, so the light is staying on all the time and you may not be aware of it, constantly draining the battery. A quick test for the alternator is to visually inspect the belt, check belt tension. Measure battery voltage using a multimeter, start vehicle, measure battery voltage again, it should be higher than when you measured it with the vehicle off, this is a decent indicator that your alternator is working. To test for a drain on the system, first shut everything off. This includes lights, radio, taking out the keys, closing the doors, turning off headlights, heater, etc. To measure the drain, you must remove the positive battery cable from the battery. Connect a multimeter capable of reading amps DC in series between the positive battery post and the positive battery cable. If you have more than half an amp, you probably have a drain on the battery. Now, with the meter still connected in the manner described, you must remove fuses one by one until you determine which circuit the problem is on (excessive current reading on the meter goes to acceptable levels). This website has a very good description with a few pictures and other information about battery drain:
http://flashoffroad.com/electrical/Batteries/BatteryDrain.html
After determining which fuse the drain is on, you must determine what is now not working (hopefully your fusebox has a diagram on the lid, most do) and troubleshoot that circuit.
Need to check the charging system, the alternator.
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