2000 blazer lt, battery wasn't charging.so replaced alternator and battery itran fine and alterator gauge was up around 14 volts. parked it for 3 days and went to start it and battery was dead. thought maybe command start was sucking power so turned the swith to off under the dash.trickle charged battery to full power went for a drive and whithin 20 mins "err" on factory radio display came on again voltage gauge read normal ...14 volts. put the battery charger on again and it was very weak. what could be sucking the power from the battery?

 

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You need to determine which circuit is drawing current. Welcome to electrical gremlins! Have you recently added any wiring for a stereo or lighting?

 

The easiest way to do this is with an AMP meter and pulling fuses. Put an AMP meter in line with the battery with the key off to see what if any current draw you have. If you see a pretty substantial draw, pull and replace fuses one at a time until you see a drop.

 

All the computer vehicles have a key-off parasitic battery current drain and could be froma sticking relay, hood light not turning off, a module is not shutting down,cell phone charger, ABS module, rear window defogger , DVD player, computer sensors, clock, radio or a shorted diode in the alternator .The normal parasitic drain is usually50 ma after the car has been shut off for an hour or so. Howeverit can be as much as300 ma or more.You can use a good qualitymultimeter and see how much battery drain the system has.

I keep a Deltran 800 ma Battery Tenderoncars that are not driven very much.Two days ago I found that the18 month old AUTOLITE battery on The Blazer was dead due tothe normal 50 ma battery drain after not being driven for two weeks.I installed a new AC Delco 75-6yr battery on The Blazer and attached theBattery Tender. The new battery hasbeencharging for 24 hours and the battery is still not fully charged due to the battery drain and a low initial charge. New batteries come with a low initial charge and should be charged up either by driving or using a charger of some sort before putting them into service.

 

Testing for current draw is not as simple as it may seem on computer controlled vehicles. To properly do this you need to fabricate two 12 AWG jumper wires, each about 6" long with aligator clips on each end. Disconnect the negative batterycable and attached the two jumpers in line with the battery negative terminal and the battery cable clamp. (That returns the ECM system to operating mode after disconnecting the negative battery cable). Then clip your ammeter (or multimeter) clips to each side of the two center aligator clips of the 6" jumpers you made. (That places the ammeter in parallel with the negative battery post and the negative batter cable) Now you are ready to test for current drain.

Test:
1.) Ignition key in lock position. 2.) Turn ignition key to run position. 3.) Turn ignition key back to Lock position and wait one minute. (That allows thecapicators to charge up). 4.) Disconnect the jumper wire clips from each other whilekeeping the ammeter clips attached to each jumper wire clip.(That now places the ammeter in series with the negative battery terminal and negative battery cable) 5.Read amp draw on the ammeter to see if the drain exceeds 20 - 50 ma. If it exceeds that amount,start pulling fuses until the drain drops below 20 ma. Then you have isolated the circuit that is source of the battery drain.

 

What Duntov is describing is creating a DC Shunt. CLICK HERE for a quick DIY article I found that details this which keeps things in proportion. You could also grab some current sensing resistors and create your own shunt which would then be read using the mV scale on your meter. Just using wire may not yeild the proper proportion of resistance for the comparison to be meaningful.

For current limits up to 45A, I use two Ohmite 15FR010 current sensing resistors (5W 0.010Ohm 1%) in parallel. With this setup, I use my multimeter in the mV scale and a 200mV reading means that 40A is being drawn through the system (Current = Volts / Resistance).

I will say that if you start off with the key off, amperage levels should not exceed that of a regular meter. Just be sure to have your meter in the higher amp mode (0-10A or whatever your meters full Amp measuring scale is). If you have an auto-scaling meter, this test becomes easier. If all that you have is a meter with a mA scale, then you will need to create a DC shunt.

 

hey guy's thanks for all your help! after all your help my bud and i went through the fuses and sh it and found that the 6 pack cd player was empty but still pushed in trying to go and load a cd ....it was like the energizer bunny kept goin and goin...............we took the 6 pack and drank it nooooo we ejected it and every thing was good volts were where they should be and no "err" after 20 or 30 mins of cruisin. we werewondering if that would be factory 6 pack? its located in the console/armrest in front seat? why would it kill the battery?

 

Sounds like the factory cd changer to me. I had one in my old Bravada and it was weird for sure. I had the cartridge get stuck in it more than once. And yes, if it gets jammed or for whatever reason it thinks it needs to find that 7th cd in the 6 cd cartridge, it can drain the battery because it has uninterrupted power so that you can remove the cartridge with the key off (or atleast that was the way mine was).