Battery Care: Tips for Checking and Maintenance (Articles)

No car battery is completely wear-free. Therefore, every battery will eventually become weak enough that its performance is no longer sufficient for a reliable engine start, even if it is always operated under optimal conditions.
The time it takes to reach this stage largely depends on the vehicle user. If the following instructions are followed, the battery will last a long time (within the limits of its design and quality).

The electrochemical processes in a lead-acid battery are somewhat complex and will only be touched upon here in the context of practical use with car batteries.
A good description of the details can be found, for example, at:
http://www.microcharge.de/index.php?option=com_content&view=article&id=51&Itemid=56



Features of a car battery (starter battery)

During a cold start in winter, a healthy TDI engine requires the battery to withstand loads of approximately 5 seconds of pre-glow with 60 amps and 2 seconds of cranking with 300 amps. Adding these together, this example represents about 900 amp-seconds, or 0.25 amp-hours, which is approximately 0.3% of the nominal capacity of an 80 Ah battery.
The key factor when starting the engine is not the capacity, but the low internal resistance of the battery. Only this allows the starter motor to be supplied with hundreds of amperes without the battery voltage dropping so much that the engine fails to start due to insufficient speed!
The measurement for internal resistance in typical battery specifications is the cold temperature test current.

During discharge, lead sulfate forms on (and in) the battery plates, creating an insulating layer that inhibits the flow of current. Therefore, partially charged batteries provide less starting current than fully charged batteries.
During the charging process, lead sulfate is typically converted back into lead (on the negative plate) and lead dioxide (on the positive plate).

However, lead sulfate tends to form crystals, which become larger the deeper the battery is discharged and the longer it remains discharged. Once these crystals reach a certain size, they are no longer converted back into lead or lead dioxide during recharging, but instead form electrically inactive material within the battery cells. This resulting loss of capacity can be recognized, among other things, by the fact that the acid density no longer reaches the maximum value of a new battery (1.28 kg/l) even after long charging times.
The layer of crystals that forms on the battery plates constantly hinders the flow of current during charging and discharging: the internal resistance increases, the starting power decreases, and the battery charges more slowly than a new battery.
Batteries that are so severely damaged are referred to as (partially)sulfated .

And that's precisely where the weakness of starter batteries specialized for short, high discharge currents lies: they are quite sensitive to deep discharges (e.g., due to a long-running parking light, hidden power consumption from faulty electronics, etc.), and they respond to such treatments with sulfation.
Example: Parking lights with a total power consumption of only 10 watts (2 lamps at 5 watts each) draw approximately 0.8 amps from the battery. However, after a night with 12 hours of parking lights, over 10 amp-hours are consumed, which corresponds to about 13% of the nominal capacity of an 80 amp-hour battery, or approximately 45 cold starts in winter!

Modern batteries are "cycle-stable" to a certain extent, typically up to about 20% depth of discharge; therefore, a fully charged 80 Ah battery will not be significantly damaged by being left with the parking lights on for one night.
With 12 hours of continuous lighting (6 lamps at 5 watts = 2.5 amps), consuming approximately 30 Ah, or even with 2 nights of parking lights (approximately 20 Ah), even an 80Ah battery will be discharged beyond the specified 20% for cycle-resistant batteries, and the risk of permanent sulfation cannot be ruled out.

Common starter batteries lose approximately 1% of their capacity per day due to self-discharge , which means that a fully charged, new battery will be practically discharged after about 3 months of inactivity without maintenance - and will already be significantly sulfated at that point!

Batteries also sulfate even when they are predominantly charged with too low a voltage, which means that a portion of their capacity is constantly unavailable. In this process, a corresponding amount of battery acid remains bound in lead sulfate, which cannot be broken down even if the battery is eventually charged with the optimal voltage (see above).

In addition to the sulfate issue, deep discharges (e.g., a forgotten light left on, listening to music until the system shuts down due to voltage drop) can cause parts of the "electrically active material" to be forced out of the plates and into the sludge chamber of the cells, where they are irretrievably lost for energy storage.

Overcharging (caused by faulty regulators or improperly adjusted temperature compensation, see below) also damages the battery. When the voltage is too high, the cells begin to gas, meaning that water is chemically decomposed and lost, which lowers the acid level. While this can be compensated for by adding distilled water, the gas bubbles that form on the plates can, in turn, force out the active material, thereby reducing the battery's capacity (see above).
In addition, corrosion on the positive battery plates is significantly accelerated, and prolonged overcharging can cause them to dissolve.
Batteries that are off-gassing release hydrogen and oxygen, which can form explosive gas mixtures -> explosion hazard!

Overly high and overly low charging voltages differ by only a few tenths of a volt, and both significantly reduce the battery's lifespan due to the effects mentioned above.

The optimal charging voltage at room temperature is approximately 2.4 volts per cell, which translates to 14.4 volts for a 12V battery system. However, this voltage decreases with the temperature of the battery acid. Therefore, in winter, 14.4V is insufficient for a full charge, while in the peak of summer, this voltage can overcharge the battery!
That is why most alternator regulators have an internal temperature compensation feature that adjusts the output voltage accordingly.
However, the temperature that is typically measured and evaluated is not the (actually crucial) battery temperature, but rather the temperature of the controller itself, which can differ significantly from the battery temperature depending on the situation (for example, when motor heat enters the alternator).


The cables between the alternator (LiMa) and the battery can also cause problems: Their resistance causes voltage drops when current is flowing, meaning that the battery receives less voltage than the alternator is producing. To still achieve an approximate 100% charge for the battery, many alternator regulators actually provide a slightly too high voltage.



What do starter batteries need?

The ideal electrical system for starter batteries primarily includes a powerful alternator with a highly precise temperature compensation system in the regulator, minimal resistance and wasteful cable sizes between the alternator and the battery, no current draw when the engine is off, and a battery maintainer that compensates for self-discharge.

The closer one gets to this ideal in practice, the longer a starter battery will last.
In detail:

1. With the engine off, turn off all electrical consumers.


2. Regularly check the vehicle's electrical system for hidden consumers or leakage currents by disconnecting a battery terminal and placing a current meter (multimeter) between the terminal and the battery terminal.
By interrupting the power supply and then turning it back on, electronic control units can be temporarily switched from standby mode to operating mode.
If the current does not return to non-critical levels within a few minutes (possibly after turning the ignition on and off during the measurement), it is recommended to pull each fuse with a constant voltage individually in order to isolate the faulty component.
Once the source of the hidden power consumption has been identified, one should investigate the normal behavior of the corresponding devices and, if necessary, replace any defective devices.

Constant consumers that, in total, do not exceed the order of magnitude of battery self-discharge of approximately 30 mA (e.g., alarm systems), are practically not critical with regard to longer battery life.
However, the quiescent current generally increases with the equipment installed in the vehicles: nowadays, values of around 100 mA are no longer uncommon, and this does not necessarily indicate a defect.
Such magnitudes can lead to discharge levels below 80% of remaining capacity within just a few days, which, as described above, can indicate the onset of permanent sulfation, even in modern batteries.

The number of days until the remaining battery charge drops below 80% (starting with a fully charged battery!) can be estimated using the following rule of thumb:
Days = Capacity * 8 / Standby Current (with Capacity in Ah, Standby Current in mA)
estimate.
This leads us to the next point:

3. If the vehicle is expected to be stationary for extended periods, to prevent continuous sulfation, the battery should be disconnected, and connected to a trickle charger for more than 3 weeks (as soon as possible after the last trip).


4. Check the charging voltage with the engine running, preferably using a built-in voltmeter, otherwise regularly using a high-resolution multimeter directly at the battery terminals.
TDI-LiMas generally provide enough power even at idle to raise the on-board voltage to approximately 14.5 volts during the afterglow phase, even after a cold start in winter, and to quickly recharge the battery.
A potential full load of the alternator is most likely indicated by a voltage of around 14 volts or less at idle speed. This voltage will immediately increase noticeably if the engine speed is only slightly increased, or if high-power consumers such as rear window heaters, seat heaters, or heating fans are switched off.
Otherwise, the charging voltage should not fluctuate significantly with the load on the vehicle's electrical system!
Exception: Regulators in newer linear alternators have a soft load regulation feature, which prevents abrupt changes in the alternator's drive torque. This results in brief voltage dips (approximately 1 second) when large loads are switched on, which do not indicate a defect.

If the charging voltage fluctuates excessively, you should measure the voltage directly at the alternator. If it also fluctuates there (more than in comparable vehicles), the alternator or its regulator is likely defective, the brushes are worn, or something similar.


If the voltage at the alternator is okay, then the fault lies between the alternator and the battery, which leads to the following point.

5. Keep the cable losses as low as possible.
The connections between the LiMa (alternator) and the battery (including the ground cable!) should not be allowed to corrode. Instead, protect them from corrosion with acid-free oil/grease, check that the connectors are securely seated, and tighten any loose screws if necessary.
When working in the engine compartment, avoid sharp bends in the cables that could cause them to break.

Experienced DIY enthusiasts can replace thin wires with thicker ones (a general rule: the cross-sectional area in mm² should be at least 10 times greater than the length in meters). Make sure the screw or crimp connections are secure!

When dealing with used cars, charging and grounding cables with visibly corroded connectors/crimp connections between the alternator (LiMa) and the battery should be replaced with new ones. It's best to immediately apply a corrosion inhibitor to the crimp connections of the new cables to slow down corrosion as much as possible (see above).

The same applies to starter cables, where even minimal contact resistances can lead to significant power losses due to the high currents, unnecessarily straining the battery (a portion of the energy supplied by the battery remains uselessly trapped in the cable; while the starter current is reduced, the engine often needs to be cranked for a longer time due to the lower speed).
Such weaknesses in engines that are difficult to start can often be identified after just 5 to 10 seconds of a starting attempt by looking for warm or hot areas along the cable routing.
If the engine starts too quickly, but there is still suspicion of weaknesses in the starter motor's electrical circuit, you can, for example, open the connector to the fuel injection pump or the common rail pressure sensor and then engage the starter motor to test.
After that, the battery should be recharged (to prevent sulfation), and the error memory should be cleared (to prevent future misdiagnoses and unnecessary repairs).
Alternative without any error entries in the MSG.
Pull the handbrake, put the transmission in neutral. Disconnect the starter motor's control cable (terminal 50), connect a jumper cable thicker than 1 mm² to the starter motor terminal and connect the other end to the positive battery terminal: The starter motor will turn (don't be alarmed by the noise!).


While all the tips mentioned can extend the lifespan of starter batteries, eventually every battery will fail due to age. To avoid being completely surprised by this, you should check the battery periodically.
The acid density, which can be measured with simple devices available from hardware stores or specialist retailers, provides a good indication of the battery's condition.
A brand-new, fully charged battery has an acid density of 1.28 kg/liter in all cells at approximately 20°C (when completely discharged, the acid density is 1.12 kg/liter).
A lower acid density indicates that the battery is either partially discharged or nearing the end of its lifespan. If, when charged to the optimal voltage, all cells again reach a density of 1.28 kg/liter (which may take several hours, so it's best to check after a long drive), then the battery is still in good condition.


A low acid level (in all cells) does not necessarily indicate a real water loss, because the acid level also decreases slightly when the battery is discharged, and increases again when it is charged.
Therefore, distilled or demineralized water should only be added if the acid level is too low, even when the battery is fully charged!
If you add water to a discharged battery, it can overflow during the next charging cycle - and in the process, it can force acid out, which can surprisingly alter the environment around the battery.

If the acid density remains significantly below 1.28 kg/liter even after a long charging period, then the battery is already partially sulfated or otherwise damaged (e.g., due to failed active material that also sulfates over time).

Modern "maintenance-free" batteries are optimized, among other things, for minimal water consumption, so that, with a properly functioning electrical system, water only needs to be refilled very rarely (or never).
As a consequence, the first batteries are already on the market whose cells cannot be opened. Unfortunately, this also eliminates the possibility of measuring the acid density.

Besides the acid density (or, in the case of hermetically sealed batteries), the "final leakage current" can be used as an indicator of the chemical aging of the battery. This current is measured just below the gassing voltage (approximately 14.2 - 14.4V at room temperature) and represents the residual charging current that does not decrease further, even after several hours of constant charging.
If the current (in Amperes) is less than 1/200 of the rated capacity (in Ampere-hours), then the battery is still chemically in like-new condition.
Leakage currents exceeding 1/40 of the rated capacity in Ah indicate a partially sulfated battery, which is likely to need replacement soon.
When the final charging current reaches a level between 1/200 and 1/40 of the battery's rated capacity in Ah, the battery is in its normal aging process. For example, in a 60Ah battery, this would correspond to a current range of approximately 0.3 to 1.5 Amperes.

Older batteries, depending on their chemical composition, can start to produce gas even when charged with a voltage that is actually too low. This is also a clear sign of a battery defect ("antimony poisoning").

Sometimes, individual cells age at different rates. Therefore, significant differences in acid density (a difference of approximately 0.03 kg/liter) can indicate an imminent failure, even after a prolonged charging period. This is especially likely in winter, when preheating and cranking the cold engine require the highest power output.
The same applies to significant differences in acid level (greater than 1 cm) without any prior overheating of the battery or other explainable acid loss.
Such conspicuous cell formations may be on the verge of causing an internal short circuit: accumulated plate material builds up in the sludge chamber to such a height that it touches and bridges the bottom edges of the plates.
The cell discharges, its internal resistance increases significantly (as mentioned above, a lead-acid shunt cannot replace a solid piece of cable), and the battery is, at best, still usable for powering a radio.
Batteries with cell shorts are characterized by the gassing of the remaining healthy cells, even with a correct charging voltage, and a reduced voltage of the charged battery that corresponds to the number of cell shorts (typically around 13.5 volts, but approximately 2 volts lower for each cell short).

Before the battery dies completely due to a short circuit, you should start looking for a new battery while you still have options, rather than being forced to rely on the first available (and potentially expensive) offer in an emergency.
When multiple suitable types are available, one should, contrary to common advice, not focus on the capacity (in Ah), but rather on the cold cranking current (in A), in order to best meet the needs of the starting power, as explained above.
However, be careful: there are essentially two measurement methods for this, which can lead to significant differences. The EN value for the same battery is approximately two-thirds higher than the (older) DIN value. With new batteries, only the more impressive EN value is usually specified.

If the new battery is not installed immediately, it should be fully charged as soon as possible after purchase, and at the latest every 2 weeks. Ideally, it should be connected to a trickle charger. Otherwise, you will end up installing a battery that is already partially sulfated, instead of a brand new one!

Alternative: Purchase a "dry-charged" battery with acid in separate containers. These are virtually maintenance-free and have a very long lifespan. Before installation, you only need to fill the acid into the cells (ensuring the same level in each).
After that, the acid should be given a few hours to penetrate the plates, and then the battery should, of course, be fully charged (either through a longer drive or by using a charger). Afterwards, if necessary, adjust the acid level in all cells to the correct level.

Attention:
Ford vehicles use battery systems with a voltage of 14.8V. Consequently, the charging voltage of the alternator (LiMa) is also high.
Those who install standard batteries in such vehicles are often surprised by how quickly the battery "overheats" and becomes damaged...


Modern battery management with onboard network control unit
Current VAG models use an onboard control unit (control unit address 9) for both battery load management and charging. Vehicles with a start-stop system, in particular, put additional strain on the battery. When replacing the battery, it is especially important to choose the correct type (cycle-resistant, AGM). Absorbent Glass Mat (AGM) is a type of lead-acid battery in which the electrolyte is bound in a mat of fiberglass.
Fault codes stored in the body control module can be read using diagnostic tools (e.g., VCDS), and the body control module can also be programmed with the correct battery type using VCDS.


Thank you, Bertil, for your tips!