Battery Question
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There is a much greater electrical draw on the battery when the car is running. So the alternator inputs more of a charge into it than the battery tender needs to.
Pro
Joined: Aug 2010
Posts: 545
Likes: 4
From: Calgary Alberta
Voltage should be higher with the battery minder attached. 2.2 volts per cell with 6 cells in the battery. Fully charged the battery should be 13.2. The battery minder on you should see in the 13 volt range.
The specs below are for an Optima battery but they should be similar for most automotive batteries. If your tender is not reaching at least 13.2 there is either something wrong with your battery or battery tender.
Recommended Charging Information:
Alternator: 13.65 to 15.0 volts
Battery Charger (Constant Voltage): 13.8 to 15.0 volts; 10 amps maximum; 6-12 hours approximate
Float Charge: 13.2 to 13.8 volts; 1 amp maximum; (indefinite time at lower voltages)
The specs below are for an Optima battery but they should be similar for most automotive batteries. If your tender is not reaching at least 13.2 there is either something wrong with your battery or battery tender.
Recommended Charging Information:
Alternator: 13.65 to 15.0 volts
Battery Charger (Constant Voltage): 13.8 to 15.0 volts; 10 amps maximum; 6-12 hours approximate
Float Charge: 13.2 to 13.8 volts; 1 amp maximum; (indefinite time at lower voltages)
Race Director
Joined: Aug 2003
Posts: 13,421
Likes: 8
From: Lake Mary Florida
Obviously there are smarter people here than those who designed and built the vehicle and have much better answers to the OPs question. So I'll let them answer it.
Last edited by Jet-Jock; Sep 28, 2010 at 12:44 PM. Reason: there is one in front of you and one behind you everywhere you go in life
Drifting
Joined: Jul 2007
Posts: 1,365
Likes: 1,818
From owner's manual: 2-83
When the key is in ON with the engine not running, the voltmeter shows the voltage output of your battery. When the engine is running, it shows the voltage output of the charging system.
The reading will change as the rate of charge changes (with engine speed, for example), but if the voltmeter reads at 9 volts or below, your instrument panel cluster and other systems may shut down. The Driver Information Center (DIC) will read LOW VOLTAGE when your vehicle is at 10 volts or below. Have it checked right away. Driving with the voltmeter reading at 10 volts or below could drain your battery and disable your vehicle.
When the key is in ON with the engine not running, the voltmeter shows the voltage output of your battery. When the engine is running, it shows the voltage output of the charging system.
The reading will change as the rate of charge changes (with engine speed, for example), but if the voltmeter reads at 9 volts or below, your instrument panel cluster and other systems may shut down. The Driver Information Center (DIC) will read LOW VOLTAGE when your vehicle is at 10 volts or below. Have it checked right away. Driving with the voltmeter reading at 10 volts or below could drain your battery and disable your vehicle.
1) Read the voltage from the dash gauges with the key on and the engine off.
2) Disconnect the neg battery cable. Using a digital voltmeter (DMM) read the voltage directly across the battery posts.
Significant difference, with the voltage reading of 2 being higher than 1.
With the key on and engine off there are many different electrical circuits in the C5 "LOADING" the battery. Also the location of the ammeter function in the electrical system. All this causes the voltage being reported by the dash gauges to be LOWER than actual battery voltage.
Last edited by 8VETTE7; Sep 28, 2010 at 01:22 AM.
Pro
Joined: Aug 2010
Posts: 545
Likes: 4
From: Calgary Alberta
I have to disagree with the above statement that the manual makes. Try this test for yourself:
1) Read the voltage from the dash gauges with the key on and the engine off.
2) Disconnect the neg battery cable. Using a digital voltmeter (DMM) read the voltage directly across the battery posts.
Significant difference, with the voltage reading of 2 being higher than 1.
With the key on and engine off there are many different electrical circuits in the C5 "LOADING" the battery. Also the location of the ammeter function in the electrical system. All this causes the voltage being reported by the dash gauges to be LOWER than actual battery voltage.
1) Read the voltage from the dash gauges with the key on and the engine off.
2) Disconnect the neg battery cable. Using a digital voltmeter (DMM) read the voltage directly across the battery posts.
Significant difference, with the voltage reading of 2 being higher than 1.
With the key on and engine off there are many different electrical circuits in the C5 "LOADING" the battery. Also the location of the ammeter function in the electrical system. All this causes the voltage being reported by the dash gauges to be LOWER than actual battery voltage.
Drifting
Joined: Jul 2007
Posts: 1,365
Likes: 1,818
The test you describe behaves exactly as one would expect. Typically any power source will experience a drop in voltage when loaded. The exception being constant voltage power supplies. A CV power supply such as a MIG welder has an assortment of capacitors meant to maintain the voltage at a desired level. Not so with a chemical storage battery. The OP's problem sounds to me like he has an older battery or a battery tender that does not work properely. Another possibility is the multimeter being used does not read the same as the in vehicle voltage gauge.
IMHO it is easier to use the unloaded battery voltage as a basis for determining the state of health of the battery vs having to remember what the day one loaded controlled test voltage/conditions were.
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