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Battery Voltage Question

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Battery Voltage Question

 
Old 09-16-2011, 05:40 PM
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Battery Voltage Question

My 2008 C300W4 is presently at the dealer to determine why the battery voltage drops so quickly after parking. They believe that too much current is being drawn from the battery when the key is off. As an example, the battery will read about 12.2 v after 6 hours. After 24 hrs or more, it will read about 12.05v. My experience with car batteries is that a fully charged battery should read 12.6V. When you get down below 12.2v after only 24 hours, something seems wrong. Well, they gave me a 2010 C300W4 loaner, so I measured its battery and I'm getting about the same voltage reading after sitting in my garage as my car, i.e. 12.1-12.2v after several hours. Is this normal for these cars? Any ideas? I've checked my digital voltmeter and it is not in error.
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Old 09-16-2011, 08:16 PM
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Benz rule of thumb for silver/calcium/lead AGM OE battery.

"Battery Voltage and State of Charge:

12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%"

Last edited by Glyn M Ruck; 09-16-2011 at 08:18 PM.
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Old 09-16-2011, 10:19 PM
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Since both my car and the loaner are exhibiting the same type of voltage drop as a function of time, do I assume this is normal for these cars? Does MB undercharge the battery by design or is the key off current draw high in these cars?
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Old 09-17-2011, 01:28 AM
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"11.89V = no charge" seems not right, assuming my C300 and other cars have the same type of battery. As long as my battery is above 11.5V or so, they will all start but a bit weak.

The other way to determine battery is in mormal operation is to measure its standby current. I haven't done on my C300, as the battery is in an inconvenient place. I measured on my 02 QX4 and 00 RX300. Their standby current are around 100mA. Assume battery has capacity of 80Ah. Car of 100mA not in use will have 33 days before battery dischages to not startable.

When I have time I will check how much C300 standby current is. Shouldn't be too far off 100mA.

I noticed that when I use dash center meter to moniter the battery, battery voltage seems to drop faster at the beginning from ~ 12.3V to ~11.8V, then it drops slower to ~11.5V then even slower afterwards. I don't thin the monitor voltage is the same as direct voltage across battery terminals, but it tells me voltage drop is nonlinear.

I kind of think lzajdel's battery is normal.
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Old 09-17-2011, 12:08 PM
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I measured my 03 Honda Pilot battery after sitting for 24 hrs and it measured 12.6v. I wonder if the data Glyn provided is based on a battery with no load, in other words, totally disconnected from any load. I believe that even a small load (100 milliamps) will cause a battery voltage drop of .2-.4 volts. IF you remove the load, the battery will pop back up to 12.6. Therefore if an unconnected battery measures 11.9, when connected to a load, even a minor load, the terminal voltage will drop to probably less than 11.6.
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Old 09-17-2011, 02:38 PM
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I think the OP's battery is likely fine.

The Benz rule of thumb they use in staff training I believe denotes " useable/usefull state of charge for normal performance. Remember even with the CANBUS's asleep there is parasitic current draw.

11.89v is obviously technically not a "flat" battery.

EDIT - you might have a seat module that is not going to sleep as designed or the A/C sniffer fan in the overhead console not hibernating. They have been the most common cause of this.

That said - some batteries that are below par surface charge sufficiently for the regulator to shut down charging but loose that surface charge very quickly when loaded. This is why you must have the battery load tested.

Last edited by Glyn M Ruck; 09-17-2011 at 02:47 PM.
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Old 09-17-2011, 03:47 PM
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Thanks for the info Glyn.
I would imagine a fully charged battery should still start the car after sitting unused for 20-30 days(warm weather). After all systems go to sleep, any idea what the baseline current draw would be?
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Old 09-17-2011, 09:05 PM
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A good quality trickle charger may be the go if leaving a car unused for 30 days.

Emergency vehicles have them with a neat little external socket.
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Old 09-17-2011, 10:43 PM
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Originally Posted by lzajdel2002 View Post
Thanks for the info Glyn.
I would imagine a fully charged battery should still start the car after sitting unused for 20-30 days(warm weather). After all systems go to sleep, any idea what the baseline current draw would be?
It should still start after 30 days. When I had my global job I frequently left my car standing for 30 days.

Baseline draw is dependent on configuration - e.g. with or without Comand but is in the 170 milliamp region - i.e 0.10A to 0.20A range.

In scratching for that stat for you I found another rule of thumb from Benz for newer deep charge silver/calcium AGM batteries as follows - take your pick - not consistent.

100% 12.7V
90% 12.5V
80% 12.42V
70% 12.32V
60% 12.20V
50% 12.06V
40% 11.9V
30% 11.75V
20% 11.58V
10% 11.31V
0% 10.5 V
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Old 09-17-2011, 11:11 PM
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Originally Posted by lzajdel2002 View Post
Thanks for the info Glyn.
I would imagine a fully charged battery should still start the car after sitting unused for 20-30 days(warm weather). After all systems go to sleep, any idea what the baseline current draw would be?
Therein lies the problem, or should I say perhaps lies the problem. MB has had plenty of trouble in the past, in particular the early W211, in that the main brain (for lack of a better term) never goes to sleep because it's looking for an option not installed in that particular car. As a result, many batteries were blindly replaced under warranty until somebody finally figured out what was happening which took over a year.

This very thing happened to a friend's 2003 E500 (1st year of that model) and his battery would be flat in 3 days. Finally, word came down from on high (Stuttgart) what was going on and after the solution was applied, no more dead batteries.

I would think your dealer would remember all that but don't count on it what with all the personnel changes in the service departments.
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Old 09-18-2011, 02:40 AM
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Originally Posted by Glyn M Ruck View Post
100% 12.7V
90% 12.5V
80% 12.42V
70% 12.32V
60% 12.20V
50% 12.06V
40% 11.9V
30% 11.75V
20% 11.58V
10% 11.31V
0% 10.5 V
Based on my limited experience, the above chart makes a good sense.
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Old 09-18-2011, 10:29 AM
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Out of interest - a post I made on the W203 forum regarding AGM batteries.

Quote

"I have read many papers on these things & other than no spill/zero maintenance difference between wet cell & AGM the main advantages are:

- Durability - designed for military - can be mounted on their side if required
- Suffer less damage if discharged below 50% - happy as a starter battery with typical 3 to 5% discharge cycles
- Can accommodate a huge number of such charge/discharge cycles
- Don't suffer low electrolyte damage that wet cell batteries do if level is not maintained
- Can be vented safely in closed areas
- Size due to nature of plate packing & material
- Easier to charge/accept charge quickly/low internal resistance
- Can dispense their charge at a higher rate
- Silver/Calcium/Lead plates last longer than lead or lead sponge
- AGM batteries have a very low self-discharge rate (from 1% to 3% per month vs wet cell at up to 4% a week)
- Almost impervious to freezing
- 97% recyclable"

unquote

Last edited by Glyn M Ruck; 09-18-2011 at 10:50 AM.
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Old 09-18-2011, 10:34 AM
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Originally Posted by Carsy View Post
A good quality trickle charger may be the go if leaving a car unused for 30 days.

Emergency vehicles have them with a neat little external socket.
You need to be very careful doing this. The charger must be regulated & shut down and/or cycle.

AGM batteries are easily damaged by overcharging.
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Old 09-18-2011, 11:41 AM
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In my first post, I mentioned that the loaner C300W4 battery voltage behaves about the same as my car as a function of park time. I assume, therefore that the voltage drop to 12.1-12.0 after 24 hours, while not normal for most cars is in fact normal for Benz.
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Old 09-19-2011, 11:26 AM
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That kind of voltage drop is normal as the comparatively crude charging systems in automotive applications don't really deep charge the batteries correctly & in a well controlled fashion. They surface charge quickly & shut down. Thus the battery never attains full charge which would require slow charging over a longer period to achieve a true 100% charge.

Parasitic current draw will knock this surface charge off of an AGM battery very quickly. Then rate of discharge will slow for a given draw.
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Old 08-26-2016, 04:21 PM
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Bumping an old thread.

My x204, basically same car as a w204 also has been outputing only 12.15 V after car has been sitting 24 hours. This is measured with a multimeter at the battery terminals (NOT from dash console).

Car starts and operates totally fine even after multiple short trips. I'm not sure if the Mercedes Benz charging system just maintains at lower SOC rate or that my battery is got to go replaced.

Curious to see if this discussion has progressed over the last 6 years from the original Op discussion.

Thanks,
Wayne
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