Changed out altenater and battery battery is fully charged with car running altenater shows it's a drop in voltage rather than 14-15 volts it should be putting out ,had it tested showed good replaced and got the same thing do you know what would make it discharge instead of charge. when engine is off voltage slowly returns to battery

14 volts charge is correct. Standing voltage on a good fully charge battery is 13.2 volts or so. I check one on charge with a 6 amp charger and it was 14.2 volts. It dropped to 13.8 with charger off.

A fully charged battery should read 12.6 volts or above. When the engine is running, that number should be between 13.7 and 14.7 volts. There could be a charging issue going on if your alternator is going bad, not getting the battery voltage up high enough. If this is the case, your alarm will continue to alert you of a low battery as the voltage drop when the car is on or running will drop the voltage further. I would suggest taking your car to a parts store that does on the car battery load testing. This can find if your alternator is going bad.

That is definitely wrong, not normal. Voltage should stay at 14 volts or near that until the battery is close to fully charged, then will slide down to about 13.5 volts, and stay there while engine is running. Also, a fully charged battery should show about 12.6 volts at a minimum, especially a new battery.
I don't know what is going on with your system. Take it back to Merc- were they aware of how it is acting, voltage-wise? Doesn't sound like it. One small possibility: is your alternator belt not tight enough?

The car battery has 6 cells, each cell holds 2 volts, and connected in series adds to 12 volts. Just like your flashlight, each battery hooked in series results in increased voltage. Its possible one cell is not holding a charge, which slowly drains power from adjacent cells. In battery series connection, if one cell won't hold a charge, electrons will flow to lowest cell voltage in attempt to equalize voltage. So when one cell drops to less than 2 volts, the electrons from other cells will flow to this cell to recharge and bring voltage up to equalize voltage level. To diagnose for dead cell in 6-cell car battery, remove the 6 caps to expose the electrolyte liquid AFTER you have recharged the battery for 20 minutes with 10-amp charger, or until "fully charged" indicator comes "on." With a digital voltage meter set to 5 volts DC reading, connect the negative black probe to negative side of battery and touch the liquid electrolyte with the red positive probe. The voltage meter will display the voltage reading on each cell as you move the red positive probe from one cell to another. Make a note of each voltage reading from each cell. Wait one hour or more, then take another reading of each cell. The cell that has the lowest reading is the one that's draining power from the adjacent cells. A normal battery will show no change in voltage level between the first and second reading an hour later. A defective battery will show a drop in voltage to indicate one or more cell is slowly draining power to gradually render the battery completely dead 36 hours later.

Hi John, The easiest way to check the alternator is to connect a voltage test meter between the positive and negative battery terminals. A fully charged battery should give 12 and a half to twelve and three quarter volts. Have some one start the engine (making sure the vehicle is in neutral or park and with the park brake fully applied). Voltage during start should not drop to lower than eleven volts. With engine running the voltage reading should be around thirteen volts. Turn on the head lights (full beam) and the heated rear window. The voltage should be maintained or increase slightly above the previous reading. If during start up the battery voltage dropped below that stated earlier the battery will probably need replacement. If the voltages remain constant or drop during the engine run, and if voltage drops below pre start conditions, the alternator requires attention. Regards John

quick check is to remove a battery cable while the motor's running. If the motor stays running when you pull off the battery cable, the charging system is functioning properly, if it dies then there is a problem with the charging system. Don't leave it off long, as this test is stressful to the circuit. You can test the voltage regulator by putting a volt meter across the battery posts to verify voltage. A fully charged 12 Volt battery should show slightly above 12 volts DC. With the engine running and the volt meter hooked to the battery, the charging system should be showing around 13.8 volts DC.

Do you have any indication that the battery is not being charged properly, such as slow cranking speed when trying to start?
With the engine not running, the battery (fully charged) should show about 12.5 volts with a voltmeter. With the engine running at normal (not idle) speed, the voltage should be 13.5 - 14.5 volts depending on the state of charge of the battery. At idle, the voltage may drop below 13.5 volts depending on how many accessories you have turned on (headlights, a/c, blower motor, radio, etc). If the 12.8 - 12.9 volts you referenced is at idle with accessories on, but increases when you increase engine speed, you're probably OK.

Sounds like a bad alternator. Drained your battery and when you turned on the lights it took all the power left in your battery. When you drain a battery down that far, they are real hard to jump, and coupled with the alternator, the car is not producing enough current to start and stay running.

I would take the alternator off and take it to oyur local auto parts store to be tested. If bad, replace it and charge the battery fully before starting. Then check voltage at battery with motor running. Should be 13.2 -14 Volts DC. If not, you may have a bad battery cable or blown fusible link. (especially that if the alternator tests good).....Mike

ok when the pcm detects something wrong whit the charging system the pcm show you in your board CHECK GAGES if this is not on, dont worry about

You shoulld expect more than 12.8 volts from the alternator up to nearly 14 volts when charging a low battery. The battery can only show the amount of voltage your alternator is sending to it after being used for a while. I would suggest that you have the battery tested & recharged completely, your voltage will be above 12 volts and if it returns to below that , you will know your alternator is not charging. Also, if you test the output of the alternator and see it is below 12 volts after the battery is fully charged at 12+ volts you will have confimed the problem also.

good luck.