How do I check my Battery? - PeachParts Mercedes-Benz Forum

Joseph_H · #1 11-09-2003, 01:39 AM

I need to make sure my battery is in top shape, how do I check it? Without taking it a pro, could I perform certain things at home to see if my battery is in great functional shape? Reason for this is that I need to eliminate it from being a cause for my cold start problem that I get once in a while whenever the car sets for few days without operation. I am looking at other things to solve this cold start issue as well. Thanks.

Jason Beal · #2 11-09-2003, 01:40 PM

Take it in to any auto parts store and they'll check it for free.

haasman · #3 11-10-2003, 12:44 AM

Probably more than you would ever want to know .....

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How To Use a Hydrometer

A hydrometer is an inexpensive a float-type device used to measure the concentration of sulfuric acid (Specific Gravity) of battery electrolyte ("battery acid"). From this reading you can easily and accurately determine a non-sealed battery's State-of-Charge. A hydrometer is a glass barrel or plastic container with a rubber nozzle on one end and a soft rubber bulb on the other. Inside the barrel or container, there is a float and calibrated graduations used for the Specific Gravity measurement. The following is a list of instructions on how to correctly use a battery hydrometer:

BATTERY HYDROMETERS

[Source: Popular Mechanics]

If the battery has been charged within the last four hours, remove the Surface Charge. If the battery has been discharged within the last 15 minutes, wait for at least 15 minutes before testing it.

While holding a clean hydrometer vertically and wearing glasses, squeeze the rubber bulb, insert the nozzle into the electrolyte in the cell, and release the bulb. The electrolyte will be sucked up into the barrel or container allowing the float to ride freely. Start with the cell that is closest to the POSITIVE (+) terminal.

Squeeze the rubber bulb to release the electrolyte back into the battery's cell.

To increase the accuracy of the measurement, in the same cell, repeat this process several times so the float will reach the same temperature as the electrolyte. If you are measuring a large battery, stratification can occur when the more concentrated electrolyte settles to the bottom. If you notice a difference in the readings between the top and bottom of the cell, average the two readings.

At eye level and with the float steady, read the Specific Gravity at the point the surface of the electrolyte crosses the float markings. The Specific Gravity reading should be between 1.100 and 1.300.

Release the electrolyte back into the cell from which it was taken and record the reading. Be sure to avoid spillage.

If the hydrometer is not temperature compensating, measure the electrolyte temperature. If the electrolyte temperature is not 80° F (26.7° C), then compensate using the Temperature Compensation Table and examples in Section 4.4.

Repeat the process for each individual cell. The Specific Gravity reading should not have a difference of more that 30 "points" (.030) between the lowest and highest reading or 10 "points" (.010) below the battery manufacturer's recommended temperature value with the battery fully charged. If so, try and equalize the the battery by following the battery manufacturer's procedures. If equalizing does not help, replace the battery.

You can determine the battery's State-of-Charge by taking the average of the temperature compensated cell readings.

Throughly rinse the hydrometer with water after using it.

Haasman

Benz300 · #4 11-10-2003, 10:58 AM

hehe. Haasman, couldn't you have made life easy and suggested a DMM instead ? but then again life's not easy now is it

Good post on the hydrometer. something good for the knowledge base !

Joseph_H · #5 11-10-2003, 01:17 PM

Would a DVM be sufficient enough? For example, I measure across the -/+ when cold and off and i get ~12.37 VDC and when engine on i get about 14.35 VDC. Would be an indication of battery state? Would the Hydrometer test described above show something the DVM doesn't? of course I could just easily take the battery to Sears or auto parts shop for a thorough test as well, but wanted to see if i could do the testing in the convince on my garage and without having to take the battery out etc...

haasman · #6 11-10-2003, 01:50 PM

Yes, of course, a DVM is sufficient or a Sear's load test.

The reason I bought a hydrometer years ago was when I was wrestling with a battery for a diesel. Start, no start, start, no start problems. Battery intially tested well but turned out had a "surface charge". Having no idea what that really meant someone suggested I invest in the hydrometer. It instantly revealed that the battery was toast.

Not too expensive and definitely tells the chemical side of what's up with your battery.

Haasman

Mike Murrell · #7 11-13-2003, 08:06 PM

Hydrometers are useless on sealed batteries which most seem to be these days.

Checking with a VOM set to the 20 scale is interesting, but I've seen more than one read the recommended 12.60 or better and go dead the next day.

When it's done....it's done.

Metalman5 · #8 02-23-2005, 05:49 AM

I remember from testing other cars, using a DVM to test is O.K. if you are able (with a friend) to test under load. When testing, no load, 12.5v +/- .5 is O.K. Have someone test under the load of starting. It should not drop below 11.25v. If it does, the battery is in sad shape, you can try charging it, but chances are that unless you have had it drained fairly well recently, it is toast.