Battery question, running vs. overnight. - PeachParts Mercedes-Benz Forum

toomany MBZ · #1 02-08-2007, 06:41 PM

The 126 reads 12.4 volts after sitting all night. Reads 13.9 when running. Question then, what is a normal read this time of year?

#2 02-08-2007, 06:49 PM

Those readings sound normal to me.

kerry · #3 02-08-2007, 06:49 PM

Seem like normal readings to me. You're reading the voltage from the alternator when it's running.

patbob · #4 02-08-2007, 06:55 PM

Quote:

Originally Posted by toomany MBZ

The 126 reads 12.4 volts after sitting all night. Reads 13.9 when running. Question then, what is a normal read this time of year?

AOK.

No-load terminal voltage for a car battery is 13.2V, which you'll only see for a short while after stopping charging and disconnecting everything. Are typically charged around 14V. With a slight load (i.e. clock & battery leakage current), 12.4V is about right for sitting overnight. Might see a few tenths higher in summer due to warmer.

toomany MBZ · #5 02-08-2007, 07:36 PM

Thanks folks. All readings at battery. The ole girl doesn't like to start in cold weather. Ground battery to near death. Needed a jump the other day, still reluctant, yet fired. Leaned on co-workers, have since purchased a portable jump starter, it's in the trunk. Checked CD, same reading, that one starts no problem. Will not replace battery at this time... Food for thought, on said booster pak, is a 110AC outlet. I'm thinking about using a timer, plug in block heater, set for seven hours to come on. A friend with elec. background gives a thumbs down, as the inverter is rather inefficent , and the BH will draw so much power as to render useless, a dead booster results, leaving me at square one. I leave work early in the morning. Trying to get an idea as to eaiser starting. Afraid there are compression issues with the 126, all of 180,000 miles. Have replaced all GP's, only one had a problem. Use 30 second method to start. Will try double GP cycle. Currently have 10% kero, 45 cetane, with PowerService (grey) around 8 oz. in tank.

jnc19610 · #6 02-08-2007, 08:17 PM

I consider those voltages a little low. I worked in the battery industry for 15 year.

12.71 was the minimum terminal voltage spec to ship a 'new' battery.

I consider 12.6 fully charged. 12.4 as 75% charged. Anything lower in the winter time is bad news. 12.2 is 50% charged.

I like to see 13.8 as a minimum voltage when running the car.

On a 6 amp battery charger, I consider them fully charged when they are 15.5 volts.

Sounds to me like your battery is a little old, or a little undercharged.

Try 'topping' it off with a 6 amp charger for a couple hours, then see if the alternator can keep it topped off.

Jeremy5848 · #7 02-08-2007, 09:34 PM

I agree with jnc19610; a fresh battery ought to float at 12.6 Volts or more (newer = higher). A battery that floats at 12.4 Volts sounds like one that is getting a little old or is not being charged well. Older batteries also don't stand the cold as well, probably why yours is not performing well. If you don't have access to electricity for a charger and/or a block heater, then a new battery is recommended.

One way to stretch the life of an older battery is to put a trickle charger on it overnight. You don't need more than a couple of amps to keep a battery topped off if it is fairly well charged to begin with. The waste heat from the charging process also keeps the battery warmer that it would otherwise be. Warmth does wonders for the chemical process that creates electricity from lead and acid.

Sitting for several days between uses is also not good.

Another possibility is that your alternator isn't keeping up with the demands made on it in the winter. Your reading of 13.9 Volts is fine but the alternator needs time to pump energy into the battery. You may be driving with lights, heater, etc., on and sitting in traffic with the engine idling. In such a case, it would be possible for the battery to slowly be drawn down without you realizing it. If you drive on the highway for your daily journey, you are better off.

Running a block heater with a battery and inverter can be done but it is an expensive process in the sense that you will need a large (100 Amp-Hour) deep-cycle battery separate from the car's battery. Running a block heater for a couple of hours will pretty much drain such a battery so you will have to have a means of recharging the battery overnight. I would look into a fuel-burning recirculating coolant heater first (admittedly they are expensive).