Ever since the start of the Covid-19 pandemic, many countries had introduced country wide lockdown to control the spread of the virus. In Malaysia, we have many iterations of lockdown, MCO/CMCO/RMCO/EMCO/Semi EMCO, etc. For the benefit for our non-Malaysian readers, feel free to refer to this Wikipedia article:
The movement control order forces many car owners to leave their car idle for days, weeks or even months. When they start their car after a long idle time, they get a message like below. The first reaction would be, “Do I need to change my battery?”. The purpose of this article is to answer this question and to equip our readers with knowledge on how the electrical system works and to avoid this issue from happening again. We will be focusing on modern BMWs and Minis in this article but the concept should apply to all cars.
The photo above shows the 'Charge Battery' warning on the meter cluster.
The photo above shows the iDrive warning on a modern BMW.
If you need an immediate answer on how you can prevent the messages above, start your car every 2 days and drive around for at least 15-20 minutes to get your battery back to 75% of its capacity.
For those that are planning to see what is going behind the message, warm up your brain because there will be a lot of numbers to crunch in order to understand the theory behind the discharge issue.
We will focus on 2 main parameters which is power supply and discharging.
Power Supply in the car which is the Battery (Energizer) and Alternator (Generator) - See figure 3
Discharge of the battery (Parasitic current draw)
The remainder of this article will be split into 4 parts:
Part 1 - Battery (Energizer)
The battery stores chemical energy which can then be converted into electricity. We will discuss about the following topics:
Battery Capacity
CCA (Cold Crank Amps)
Part 2 - Alternator (Generator)
The alternator is powered by the vehicle's engine and it generates electricity by converting the mechanical energy supplied by the engine into electrical energy. The following topics will be discussed:
Output rating / Capacity
IGR (Intelligent Alternator Control)
Part 3 - Discharging
Parasitic current draw
Part 4 - Table for charging and discharging duration
Part 1 - Battery (Energizer)
Battery Capacity
It is fairly simple to read a battery capacity and it is mainly labelled on the battery itself. As an example, Figure 1 shows an AGM battery with 90 Ah capacity with 900 CCA.
CCA (Cold Crank Amps)
The next thing that we need to understand is the CCA (Cold Cranking Amperes). Why is this important? This is because the capacity of a battery is important in determining the CCA where is the most important to cold start a car.
“CCA is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. Generally speaking, it is easier to start an engine in a warm environment than in a cold one” (1)
So, how do we calculate the CCA of a battery and how much our car engine needs it?
Basically It is Ah * 0.07 = CCA and the factor of 0.07 is the average of the battery type in the industry. (2)
So how do we determined the minimum CCA needed based on the engine capacity?
“The standard recommendation is a battery with at least one Cold Cranking Amp (CCA) for every cubic inch of engine displacement (two for diesels). “ (2)
So For F30 2000 cc would be 122 cubic inch. Meaning minimum CCA needed for 320i would be 122 A. But 320D would be 244 A.
Part 2 - Alternator (Generator)
Alternator Output
Now let's understand what our alternator output or capacity means.
Typically, 320i would be using 170 Amp alternator and 328i would be using 210 Amp. Let's take 170 Amp as the variable to study.
170 Amp means the alternator generates 170 Amp @ 6000 RPM. So during idling, that would the engine runs at around 1000 RPM only and thus the alternator would be generating 28 Amp @ 1000 rpm.
How much electrical energy does the alternator provide to the battery? In the rules of charging, usually it should provide 10% of the battery capacity (again this is critical to register the battery for BMW after replacement in order to make sure the charging rate is a per the battery capacity).
So for example, the battery capacity is 90 Ah, the charging should be 10% of its capacity which is 9 Amp per hour. Also the alternator would change this rate depending on the battery exhaustion like the chart below:
When the battery is at 100% charge level, it only requires 2-9Amp from the alternator. When the battery is drained to 25% charge level, it requires 31 Amp. When it regains about 40% charge, it will require 19 Amp and this will the slowly drop to 13 Amp when it reaches 65% capacity. When the battery charge level increases the power requirements to charge the battery will decrease.
So, if the alternator produces 28 Amp only and the car needs 50 Amp, the battery discharging while stationary would pop up. This message will continue to pop up until the battery is charged to a reasonable level and the alternator can generate enough electricity to both power the car's electrical components and charge the battery at the same time.
IGR (Intelligent Alternator Control)
Before we begin the calculation, let's see what an IGR or Vehicle with Intelligent alternator control does.
“Unlike the normal control of the battery charge, the intelligent alternator control prevents
100 % charging of the battery. The battery charge goes to the range of 75 - 85 % of the maximum possible charge. This means that the battery remains "chargeable" to permit energy recovery in the vehicle overrun phase.” - ISTA
So with these features we can safely assume that the battery is 75% (let's take the worst case) of its capacity all the time even if it is “fully charge” mode. Also this is another reason why you need to register the battery during replacement to the DME - Digital Motor Electronics) in order to perform as per the new battery specification. This is important to allow the DME to know that a new battery had been installed and not overcharge it.
This means that the capacity 90Ah would be 67.5Ah all the time.
Part 3 - Discharge
Parasitic Current Draw
When a car is inactive, there will be a normal parasitic current draw from the radio, clock, anti theft system, comfort access etc. A normal draw would be around 30-50mA per hour. Anything above that would be considered abnormal and that is the sign there is some device that is drawing the current from the battery without notice. Anything that exceeds 100-milliamps indicates an electrical issue that needs to be addressed.
Battery standalone discharge rate
A new AGM typically self-discharges at about 1-2% per month, while an old one may be as much as 2% per week. (4)
Part 4 - Table for charging and discharging duration
So how do we calculate this? Assuming you manage to drive the car around 2150 RPM on average. That would give 61 amp and lets assume car accessories plus engine is taking 30 Amp in total (with no headlight and no accessories running). Below table would use 31Amp per hr as output from alternator straight to battery.
Conclusion
Once the car is not started for 11 days, it takes 65 minutes to reach 75-85% battery capacity with an average of 2150 RPM and minimum accessories turning on while driving. The rest as per the chart.
It takes 32 days to fully drain the battery.
The car can still be started even at 25% of its capacity as is still stores enough power for cranking (based on minimum CCA required). That is after 35 days but this might not be applicable for diesel with the same engine capacity.
Above calculation assumes that there is no abnormal parasitic draw and a healthy battery that has 3-5 ohm resistance (result from battery tester) or <100 cycles.
If let’s say the car is daily driven, it would draw only 1 Ah every 12 hrs of inactive period.
If your car continues to display the "battery discharge while stationary" warning despite daily driving, there are two possibilities of causes this failure:
Battery still healthy (Resistance of 3-5 ohm or <100 cycles) - the IBS cable may be faulty or your car may have an abnormal parasitic current draw of more than 50mA.
Battery cell is getting old (Resistance above 10 ohm). You need to replace your battery.
Check the alternator as a failing alternator will not be able to charge the battery correctly.
Disclaimer - This article is purely based on research and our understanding of formula and concepts from multiple sources. Calculation and formulas used are referenced below. Our intention is to help our readers to understand the reason behind the battery discharge warning and to understand how different electrical power components work in your vehicle. We believe that the article will equip the reader with sufficient knowledge to avoid the unwanted situations. Please do not attempt to drain your battery to the final 25% based on the numbers above and assume it is sufficient to crank your car. Note that every vehicle and battery condition are different. Follow the best practice of keeping your car engine on a regular interval. This keeps not just the battery but all other components in your car in a good running condition.
Thank you for your time for reading this article and hope that this would help our readers make a good judgement and decision"
References