W203 Battery Voltages?
Discussion
Over the last few weeks I've noticed that my car has been taking a little longer to start. Last night the alarm went off - presumably as a result of a voltage drop.
Having cycled through the service menus, I can see that voltage is 12.0/12.1 after the car has been left overnight and 14.5/14.6 when the engine is running.
Does the standing voltage seem a little low?
Based on the above I'd say that the alternator is doing its job but the battery isn't holding its charge - but I'm not sure what these cars normally read - C270 CDI if that makes a difference.
Thanks in advance.
Having cycled through the service menus, I can see that voltage is 12.0/12.1 after the car has been left overnight and 14.5/14.6 when the engine is running.
Does the standing voltage seem a little low?
Based on the above I'd say that the alternator is doing its job but the battery isn't holding its charge - but I'm not sure what these cars normally read - C270 CDI if that makes a difference.
Thanks in advance.
All I can tell you is that on a boat 12V system which mainly use motor alternators is that 14.3-14.5 would be the range of charge when the engine is running, you wouldn't normally discharge a car battery to much as the load when turing the car over is reasonably high then once on it continues to charge.
There is plenty of info out on the web i.e. http://www.batterystuff.com/tutorial_battery.html
The table below is taken from there.
State of Charge Specific Gravity Voltage
12V
100% 1.265 12.7
25% 1.155 12.0
Discharged 1.120 11.9
Your voltage is at the 37.5% level of a ful battery, normally on a boat you wouldn't let it drop below 50% discharge i.e. 12.2 as the engine may not start (that is why you have a dedicated engine start battery).
If the car is discharging to 12.1 overnight then there is something draining the battery, you would assume that the alarm is low current so it is either something else or maybe a cell in the battery has dropped off. You can then them tested and then replace if required, you had better make sure it isn't the battery before spending £150 on a new one!
There is plenty of info out on the web i.e. http://www.batterystuff.com/tutorial_battery.html
The table below is taken from there.
State of Charge Specific Gravity Voltage
12V
100% 1.265 12.7
- 75% 1.225 12.4
25% 1.155 12.0
Discharged 1.120 11.9
Your voltage is at the 37.5% level of a ful battery, normally on a boat you wouldn't let it drop below 50% discharge i.e. 12.2 as the engine may not start (that is why you have a dedicated engine start battery).
If the car is discharging to 12.1 overnight then there is something draining the battery, you would assume that the alarm is low current so it is either something else or maybe a cell in the battery has dropped off. You can then them tested and then replace if required, you had better make sure it isn't the battery before spending £150 on a new one!
Do you have memory seats? If so it may be the power seats draining the battery as the control unit stops turning off.
You can check this by disconnecting the seats and then reconnecting them. If the battery is OK in the morning it is the seat controller(s). There are a variaty of fixes for this.
You can either disconnect by pulling the power lead from the seat (I think from memory it has 3 wires on the connector. Or pull the fuses which are in the dashboard drivers side fuse box. All you need to do is interrupt the power as this turns off the controllers.
If you have a meter you can check for current via the fuse box. If they are working correctly they should drop down after a few minutes after the car is locked.
You can check this by disconnecting the seats and then reconnecting them. If the battery is OK in the morning it is the seat controller(s). There are a variaty of fixes for this.
You can either disconnect by pulling the power lead from the seat (I think from memory it has 3 wires on the connector. Or pull the fuses which are in the dashboard drivers side fuse box. All you need to do is interrupt the power as this turns off the controllers.
If you have a meter you can check for current via the fuse box. If they are working correctly they should drop down after a few minutes after the car is locked.
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