thefisch

I've read the best way to test a battery is to disconnect it and let it sit overnight before you measure it. I assumed that didn't matter for a car since I can just let the car sit overnight with the hood unlatched to be able to access the battery in the morning without using any power (i.e. opening the door to unlatch the hood). Then I would measure the voltage at the main battery (or the jump points) under the hood in the morning to get an accurate reading.

However, would the measurement be affected by the presence of the additional battery(ies) in the car? My 2011 has the main battery under the hood and an auxiliary battery (for the transmission) behind the dash on the driver side. Later models added another aux battery in the trunk for eco start/stop. I assume all these batteries are in series so how would I know the measurement of just the main battery without disconnecting it. I know if you measure when the car is running you are measuring the voltage of the alternator output, not the battery. Maybe there is a relay somewhere that cuts off the aux batteries...

Disclaimer: Not an engineer (if that wasn't obvious already).

SLpat

Well you lost me with two batteries in the car - I have a 2011 as well and assumed it only had one.

thefisch

https://mbworld.org/forums/e-class-w...placement.html

mleskovar

The batteries are not in series and I'm guessing your thought about being cut out entirely is correct. From my understanding the only time the aux battery comes into use is when the main battery dies completely and you need to shift the trans into neutral for getting the car up on a flatbed or a short push/tow. I've done a lot of battery measurements with this car and have my own conclusions about the charging algorithm. And you're right about not using any services prior to taking a measurement as it definitely changes. I have an external trickle charge port that I use so it's easy for me. I'm curious about what voltages you are getting.

thefisch

My old OEM battery that I just replaced would read between 12.4-12.2v after it was charged up, used and then let sit overnight. I just put in a new bosch battery and it read 12.56v this morning.

Yesterday, I check the voltage while running to check alternator. It was 13.5v at idle and 14.3v at 1,500rpm.

mleskovar

Your old battery was probably sulfated. That's what happens when either you don't drive enough....or, as I believe in the case with our cars..... the charging algorithm 'to save gas' keeps it at a low charge level. The constant low charge level causes/promotes sulfation. Even with a new battery my charge level hovered around 50% (12.24V) no matter how much I drive so I hook up a trickle charger about every third week to top it up. Like you my alternator output is fine

thefisch

Well the car probably did sit for a few months before I bought it so it may been sulfated. Although my ctek charger is supposed to check for that and help restore it. I even ran the battery through a recondition with the charger a few months ago.

My main issue with the old battery was it could not start the car if power was used while the car was off. It would start fine the next time if all you did was turn off car and get out. But if you ran it on battery power for any longer than a few minutes, it didn't have enough guts to recover and start the car. Like if you had the doors open while cleaning or using stereo out in driveway. One time I was testing how close you had to be with the keyfob to open the doors. After several attempts opening/closing doors, horn honking, lights flashing, I could not start the car later. But it would work after charging it and continue to start again and again so I thought all was ok.

CarolinaBluetec

You can't really test an auto battery properly unless you have a load tester.

aaacsdai

Fisch is on point. Battery manufacturers have moved to programmed obsolescence like everything else with any type of warranty, true battery life is based on use, but you should change this out every 48 months if you want to drive rather than walk. A electrical engineer I am not, but my father is, true batteries will sulfate, they have acidity because of the minerals used to move and create ionic bonds - to move ions from positive to negative in order to create energy. The extra battery in the car is for the ECU to maintain its CMOS like all computers. The ECU has a lot more duties than just controlling the transmission, just a note, if your FOB starts giving you issues replace that CR 2025, it is the weak link in the WiFi handshake, not your other batteries, the others trickle up as required by the A/R and setting in the ROM of the ECU.

thefisch

Funny you mention the load test. When I brought my old battery into Pep Boys, they did a load test 'in the back'. Guy came out and said it showed no volts. I thought he was BSing me as that battery had just started my car to go to the store without any drama. I pulled it out in the parking lot after I got there (mind you there were no other MB owners working on their cars out there so it was lonely). He said he would charge it and retest. 20 minutes later he tested it again and said it showed 2 volts and no amps. Had he said it showed low voltage or amps after the test I would have thought that made sense.

This all still seemed strange to me but I got a good deal on a replacement battery so I put it in and went home.

KarlG

This
If a battery shows less than 9V after a load test it "fails" the load test. Yours failed. It was a bad battery. Replacement was the correct move.

starbrite

So, let me add to the confusion. An AGM battery can NOT be load tested.

From MB training manual :


• AGM batteries cannot be tested using previous methods (load and acid density tests)

• Requires the new Midtronics MCR 717 tester and printer

• Tester measures battery conductance by inducing A/C voltage of a given frequency and amplitude on the battery posts and monitoring the current flow in response to it

• Discard the acid density sheet, and enter test code (recorded by tester) on warranty claim forms

• Battery replaced under warranty must have a test printout attached to the R.O.

• Detailed information about testing, and using the MCR717 can be found in WIS
SI54.10-P-0003-01

aaacsdai

Are you in training Starbrite?

starbrite

Oh, no.

thefisch

So this is why one local MB dealer told me they wanted $50 for a battery test while another quote $150 over the phone. Of course the test fee would go towards a new battery if I needed one. Said no thanks, I will just get a battery that costs about as much as your test.

KarlG

Not sure if above relates with previous dealer equipment that was replaced bt different equipment but it is possible to load test AGM batteries. Independent repair facilities do it every day.


AGM Battery Load Testing


Read the label on the side of the AGM battery to find out the ampere rating. The label has the letters CCA, meaning cold cranking amps, followed by a number that relates to the amperes, if the battery is for use in a vehicle. For example, it may have CCA 70 written on the label. Alternatively, it may have the letters Ah, meaning ampere hours, followed by a number, if the battery is used in a golf cart.

Divide the CCA or Ah by 2 using a calculator. For example, if the label reads CCA 70, divide 70 by 2 to get 35. This is the figure you will get from the load-test meter, if your battery is charged and in good condition.

Turn on your load tester. Attach the alligator clips to the battery terminals; the clip on the end of the red wire goes to the "+" terminal and the clip on the end of the black wire goes to the "-" terminal.

Press the load-test start button. Let the test run for 10 to 15 seconds and then stop the test. Most load testers automatically stop after 15 seconds.

Read the load test meter. Expect it to read the same as the ampere figure you calculated earlier. If the reading is more than 10 percent below the figure you calculated, charge your battery until full. Leave it to stand for a few hours and test again. If the figure is still more than 10 percent below the figure you calculated, you need to consider getting a replacement.



Read more : http://www.ehow.com/how_8525141_test-agm-battery.html



As an aside, the Mercedes Midtronics MCR 717 is just a Mercedes branded Midtronics MCR 700. Not sure if all the stores are similar, but the local Autozone has a Midtronics tester.

Arrie

Man,


You have just listed the MB instruction how to test the battery in the MB way to avoid warranty replacements.


You say the battery cannot be load tested!!!???!!!


Any battery can be load tested. Load test simply is to draw amperage from the battery and monitor the voltage drop. Starter motors work with voltage and they pull amps to turn. If the voltage drops below certain level the starter will not turn. Simple as that.

aaacsdai

Arrie, based on your post, is that battery a 700 Amp battery or just the overpriced prescribed MB dealer battery nomenclature, on my first and last MB Honolulu B-Service 20K miles-4 years, they told me my battery would need to be replaced at my next A service or at 22.5K miles. I am a firm believer in maintenance and that includes the battery, but the (BS) service cost 1500 bucks and I know they did not flush my transmission as requested, nor did they flush my brake system as requested, this is at least 4-5 hours of work-I have done it on Porsche's and Audi's, that use half the fluids, and if you include their 15 point checks of whatever is their 15 point check it is impossible, but they charged me for it, and the cute little nymph who is my concierge for life per her, swore they did, right, the trans and brakes were still covered with road dust. I did not let them even wash her after they got their oily hand prints all over her. At Porsche-Audi we had to wash our hands before and after we entered or worked on the cars-it was professional and the real reason was the guy who washed and polished for clients before they picked up their vehicle was a 6'5" Hawaiian that told us oil is hard to get off the interior and the paint in his not so humble jolly brown giant voice, besides, only the owner should wash his girl(s) before entering and riding her.

starbrite

Maybe I should have said that it cannot be load tested accurately.

Having said that, AGM batteries have lower internal resistance and higher voltage, at a given SOC (state of charge), than traditional lead acid batteries. They require electronic testers that are programmed specifically for them.

Source: Click here

Edit: Also, note that you should NEVER mix battery types in the same vehicle. ie: an AGM and a lead acid battery.

Alex.currie44

This is an interesting discussion as I know for sure my 2009 W211 had the main battery and the aux battery behind the glove box as a I recall.
It was explained to me that the aux actually started the car which I never believed but who knows.
When I got my W212 2011 MY, I thought it too would have the same set up but I couldn't find it so I went over part lists and diagrams with the parts manager at my dealer. We could not find any reference to a second battery which makes me wonder did they use thw W211 system in the W212 when it was introduced and switch the following year?
In any event, testing in a non AMG vehicle is straight forward. Just measure voltage if that is all you want to know. If load capacity is what you want then you need load tester.

mleskovar

It's simpler than that. The SOC (state of charge) is a general health check for the battery. If the voltage is good but it doesn't turn the motor over then there's probably an internal breakdown/short/leak. If the battery won't charge or the voltage stays low (or doesn't hold) the battery is shot. Yes a load test would verify but those simple conditions tell you all you need to know. Flooded cell batteries can be revived with more acid or water but those batteries are becoming history for cars.

thefisch

I checked my voltage this morning (old habits) after it sat overnight. voltage was 12.44. yesterday it was 12.56v before I left for work. If I look at SOC charts, they would tell me I went from about 85% to 75%. On a new battery I would have thought it would have started at 12.7v and stayed there assuming the alternator is doing it's job. Started no problem and didn't notice any slow start. Maybe I am wasting my time with a multimeter.

mleskovar

My experience matches yours. I've been taking SOC measurements for over a year after my battery was replaced and found that the battery stays at about 50% (12.25V) after it bleeds down from 12.65V. My last Benz remained at 12.6V constantly. I read somewhere that the charge rate and level is controlled now to maximize gas mileage and I believe that is the culprit but can't find anything to verify that. Also, I believe the HVAC motor cycling to dry itself after the car is shut down causes the quick drop. If my guesses are right I would say it's false economy for the miniscule mpg savings over a premature battery failure. I like the dry HVAC though as my car never gets the mold smell like the previous ones.

thefisch

I'd prefer the confidence of a higher voltage rating myself over some fraction of mpg. And from what I've read it's not a good idea to keep these batteries at a low % for the long term. Are you doing any supplemental charging with a trickle or other kind?

Arrie

Would like to know what your statement of the need for "electronic testers that are programmed specifically for them" is based on. The link in your post does not work.


AGM battery still is a lead acid battery, it just is built differently but basic function of the battery is the same as traditional flooded lead acid battery.


I don't see any difference in load testing, i.e. you pull load from the battery and monitor voltage drop.