Pulsating power drain

If the battery is good
You have a parisitic draw somewhere:

Switch your Digital Multimeter to DC AMPs. Amps is usually indicated by an "A" on the Multimeter Switch. AC is usually shown as a "~" symbol and DC shown as a "-" symbol. You usually have to move the Multimeter positive lead to a separate socket on the Multimeter. Sometimes there are 2 sockets, a high range and a low range. Always test on the highest setting first. For example: high setting on your multimeter may be 10 Amp. Test on the 10 Amp setting first, then if the current drawn is less than your Multimeter Low setting, move to that setting and keep testing. In my example my Multimeter low setting is 0.3 Amps. Also indicated as 300mA (mA x 1000 = A).

WARNING! Once the multimeter is on Amps do not connect it directly across the battery and do not hit the starter button while testing for current Amps. This will cause the internal fuse in the multimeter to blow! A multimeter set on current is a very low resistance, almost a short circuit and will draw as much current as your battery will supply till something melts. Always plug the Multimeter leads back to volts when you have finished testing to avoid blowing the fuse next time you use your multimeter.

Voltage ReadingTo test for battery drain: Switch everything off on the car. Disconnect just one battery lead. For example disconnect the Positive Battery Lead. Set your Multimeter to Amps as described above. Connect the Positive Multimeter Lead to the Battery Positive terminal. Make sure the Positive Lead you removed from the battery does not touch anything grounded, like the car frame etc…. Connect the Negative Lead from the Multimeter to the Positive Lead you removed from the Battery. You should now see current drain measured in Amps. Move to the lower Amp setting on your multimeter if the current is lower than the setting on the Multimeter Low setting. Start to unplug the wires or fuses around your car and see if the current reading goes to zero. This will point you in the direction of the current thief. You can convert to Power measured in Watts by multiplying it by the Battery Voltage. Power = Volts x Amps 4.2Watts or (12Volts x 0.35Amps).

You need to run a parasitic drain test with a multimeter. Disconnect the vehicle battery positive terminal and connect the multimeter in series between the battery positive terminal and lead. the multimeter should be set to display amps. With all systems turned off, the keys out of the ignition and all doors closed, alarm off and any hood/bonnet switch depressed then after 20 minutes the amp reading should be around 0.03 or less. If you have more than 0.05 then you may have a parasitic drain, causing a n accelerated loss in battery power. In this case, using the same setup then pull fuses one by one from the fusebox until your amps drop down to the normal range. The fuse you pull which causes the range to drop to normal represents the system which is at fault.
The most common causes of parasitic drain from my experience tend to nearly always be failed back up batteries in alarm siren modules or badly wired or faulty stereos. Those are the two systems I'd pay particular attention to.
Of course it may be a simple human fault like you left your trunk/boot light or interior light on.

Get a multimeter and set to amps. Disconnect the ground cable at the battery. Connect your multimeter between the neg battery post and the negative cable. What is the amp draw? It should be close to zero.

Then one at time you pull each fuse and check the display on the multimeter. If it drops then that circuit is drawling power.

On my SUV it was the factory amp for the stereo system.

Hi. Best way to find it is old fashioned leg work and an amp meter. You need a cheap multi meter with a 10 amp range. turn everything including ignition off and connect multi meter. You disconnect neg terminal and put amp/multimeter in line between the negative wire and battery. A good reading drain will be around 0.3 amps.
if it is higher than about 0.5 amps remove then refit one fuse at a time until the drain disappears, That's the guilty circuit. Jon

Here are the general instructions to find a battery parasitic drain, it is different for each problem and there is no magic bullet fix.

Instructions

• Charge your battery if necessary. Dead batteries (and even weak ones) don't give accurate results on a draw test. The dome light is a good indicator of battery strength. If the light is weak, pulsating or refuses to turn on, you'll need to charge the battery.
  
• 2Shut off everything in the car. Pull the keys out of the ignition. Ensure all service lights are off, lighted compartments closed and doors shut. Open the hood, and disconnect the trouble light underneath if your vehicle has one. Wait for 30 minutes before moving on to Step 3. (This enables normal drain from interior components to finish cycling.)
  
• 3Set your battery tester to 10 amps DC. Disconnect your positive battery connection (red cable) and position it safely away from metal components. Connect the meter's positive probe to the battery's positive post while simultaneously holding the negative probe in the air; keep it away from anything metal.
  
• 4Place the negative probe on the end of the removed positive cable to complete the circuit. Yes, you're putting a negative on a positive. Yes, this is ok. If you have a severe drain, you're likely to pop a fuse or two-identifying your problem.
  
• 5Check the meter reading. A normal reading is usually under .035 amps. If you have a minor drain that causes a higher reading, you'll need to check each component.
  
• 6Remove the first fuse in the box, and check to see if the load problem is resolved. If not, replace the fuse and check the next one. Repeat the process with all fuses. If the fuses check out fine, you have a wiring problem.
  
• 7Remove the tester and reconnect the battery. Follow the hot wire to the next connection. Remove the positive cable and connect the positive probe of the tester in its place. Connect the negative probe to a metal object. Check the meter. If the drain isn't resolved, reconnect as normal and follow the hot wire to the next connection. Repeat this step at each connection point until you narrow down the problem.
  

1. Follow these steps.
   Remove the negative side battery cable from the negative battery terminal.
2. Connect the black wire to the com input on the multimeter and the red wire to the 10A or 20A input on the multimeter. the meter needs to be able to read at least a 2 or 3 amps for this test to work. Connecting the red wire to the mA input on the multimeter won't work and could damage the meter.
3. Attach a multimeter(set the dial on the multimeter to measure Amps as per multimeters instructions) between the negative cable and the negative battery post. Wait a few seconds to several minutes for the car to go into sleep mode - i.e. when you make the contact with the ammeter, the cars computer systems "wake up". After a bit of time they will go back to "sleep".
4. If the ammeter is reading over 25-50 milliamps, something is using too much battery power.
5. Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings)last. Perform the same steps for relays found in the fuse panel. Sometimes relay contacts can fail to release causing a drain. Be sure to observe the ammeter after pulling each fuse or relay.
6. Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
7. Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
8. Repeat steps 1&2 to test your repair. The ammeter will tell you exact numbers.
9. You can also try unhooking the big wire from your alternator. The alternator can sometimes have a shorted diode that can cause amps to flow through the alternator's power cable and through the shorted diode and into the case and through the bolts and back to the negative battery terminal. This will drain a battery in a hurry.Make sure to read the ammeter before and after unplugging your Alternator

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.

u must have a short circuit somewhere that is drawing power, and draining your battery...

or you are just leaving something turm=ned on..radio?..lights?..amps?..dome lights?..

1st place to start looking is lights not turning off ie Trunk or glove compartment light. Another would be any aftermarket amplifier or radio, incorrect installation would result in the unit not turning off. If your battery is on its last legs it could happen that it has enough power just after driving to power the engine startup but after a few days will not have enough power due to normal drain from the immobiliser/alarm. To confirm disconect the negative on the battery with the ignition off. Using a multimeter connect the leads between the neg of the battery and the cable that you removed off it. Reading shoul be no more than 0.5 amp. (Multimeter adjusted to measure current and leads connected to the "com" and "10 A" positions.)

First thing I would do is find out if you have a power drain on the battery with everything off. One way to do this is with a multimeter (Select AMPS and put the probes into the right sockets on the multimeter) disconnect the battery positive and put the positive probe on batery positive and the negative probe on the Positive cable you are now connecting the car to the battery through the multimeter. make sure everythings switched off (including all doors closed so interior light is off) read the current and if you have a high reading remove one fuse at a time when the current drops this is the circuit most likely to be flattening your battery! remember that your radio pin may need to be re-entered after battery disconection. Boot lights are another common reason so get a volonteer to be locked in the boot and see if the light goes off
Good luck