K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: Jul 7, 2010)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

Reconditioning/Equalizing your Mercedes Battery

Xantrex 40amp Charger

Reconditioning Your Battery
The process has two main names; "reconditioning" or "equalization". Both terms are used to describe the process but I like reconditioning better since it more describes the process. However, either can be used.

My battery was replaced about 1-1/2 yrs ago and I've not checked it until the other day. As an excuse, it's in the trunk buried under the carpet and in a plastic 'box'. Not necessarily a good excuse but it's true since I hardly ever think about it. When I checked it, the plates on all cells were uncovered by about 50%. So I added distilled water, and the hydrometer showed that the electrolyte was that of "water". The charge was almost non-existant as you would think. So here's how it was brought back by reconditioning.

Reconditioning batteries refers to a method of charging deep cycle wet-cell batteries and is intended to restore battery capacity, revive battery efficiency, and extend battery life. The process involves periodic application of a controlled overcharge cycle to batteries. This type of charge cycle requires that certain procedures and precautions be followed.

Why is this needed?
While a battery is being discharged, sulfuric acid in the electrolyte reacts with the lead plates in a chemical reaction that produces electricity and lead sulfate. When the battery is recharged, electricity flows back into the battery and causes a reverse chemical reaction that turns the lead sulfate back into lead and sulfuric acid. However, with each discharge and recharge cycle, a small amount of lead sulfate will remain on the plates. Using a three-stage charger, the amount of residual sulfate left on the plates will be less but some will still accumulate with each discharge and recharge cycle.

If this sulfate is left in place for very long, it will harden or crystallize and eventually reduce the battery’s capacity, increase its internal resistance and destroy the battery’s ability to produce an adequate amount of power. When this occurs, even an equalize charge can not remove the sulfate and the battery becomes useless except as a recyclable item. (Recycling batteries is recommended to reclaim the lead and prevent contamination of the environment.) Over time, the electrolyte tends to stratify into layers of acid and water with higher concentrations of acid near the bottom of each cell and more diluted electrolyte near the top. This causes uneven specific gravity within a cell and further reduces its capacity and efficiency.

The Equalizing Process
An equalizing charge is a controlled overcharge cycle that performs several actions within the battery and provides certain benefits. During equalization, the voltage is raised to approximately about 16.5 volts for a 12-volt battery. The current output of the charger should be limited to about 5% of the battery's capacity. For example, a 200amp hour battery should be allowed to accept no more than about 10 amps of current. However, a large car battery of 850amps (a typical Mercedes battery), requires at least 10 to 20 amps. This is problematic with some chargers since this level exceeds the available current from the charger. So smaller currents MUST be employed. The good news is that this will help prevent overheating but will take considerably longer to complete the process. My charger (Xantrex 40A) has a "recondition" mode where it will produce a constant 15amps while in this mode.

The equalize cycle is timed to be between 4 and 8 hours depending on the features of the charging source, but the cycle can always be terminated early if necessary. This elevated voltage results in a vigorous charging action to take place within each cell that has several effects on the battery. First, much of the residual sulfate is forced to recombine with the electrolyte in the form of sulfuric acid. Crystallized sulfate that will not recombine is broken loose from the plates and falls harmlessly to the bottom of the battery. Deep cycle batteries have additional space beneath the plates intended to collect this material. This action cleans the plates exposing fresh lead to the electrolyte and restores battery capacity. The vigorous bubbling action that occurs during equalization stirs up the electrolyte and restores it to a consistent mixture of acid and water. The equalizing process also causes all cells in a battery to reach their maximum idle potential of 2.1 volts.

Specific Gravity Levels vs Battery Charge States
Charged State Specific Gravity Voltage
@ 80F *
100% 1.265 - 1.275 12.65 Ideal but hard to attain due to lead sulfate buildup
75% 1.225 - 1.235 12.42 Most batteries can attain this
50% 1.190 - 1.200 12.21 Condition of most batteries
25% 1.155 - 1.165 12.00 Condition of my battery!
0% 1.120 - 1.130 below 11.9 After adding distilled water to cover plates
* The measured voltage is "open-circuit" w/ no battery loads

How to do a Battery Reconditioning
The info here shows how to go about doing this and is very general. As was noted, the process depends greatly on your battery charger's capability. It should be able to produce at least 10amps, preferably 20amps. Based on this the following procedure shows the major steps necessary to complete the process.

WARNING: Explosion hazard; Battery reconditioning feature works only on flooded lead-acid batteries. Do not attempt to recondition sealed lead-acid batteries. During the battery reconditioning process, the battery generates potentially flammable gases. Follow all the battery safety precautions listed here. Ventilate the area around the battery thoroughly and ensure that there are no sources of flame or sparks in the vicinity.
Temp Comp Hydormete
1) Disconnect all loads from the battery by removing battery cables or by opening a disconnect switch.
2) Connect the red positive (+) clip of the cables to the positive (+) terminal of the engine battery.
3) Connect the black negative (–) clip of the cables to the negative (–) battery terminal.
4) Apply AC power to the charger, and turn the AC power ON/OFF switch to ON (if it has one).
5) Force the charger into the Equalize or Recondition Mode. If it's a manual charger, set it for 10amps.
6) Monitor the specific gravity of each cell of the battery during reconditioning with a battery hydrometer as shown in the picture. Reconditioning is complete when the specific gravity of each cell of the battery remains constant or attains at least 75% or better. The table above shows specific gravity vs percentage of battery charge as well as the typical voltage that you can measure. When the battery reaches the target specific gravity, you can terminate the process.
7) Turn the AC power ON/OFF switch to OFF.
8) Remove the black negative (–) clip and the red positive (+) clip from the vehicle’s battery terminals.

You should now have a recondtioned battery ready for the task of providing power to your Mercedes.

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