The Fascination of Four Wheels
I wrote this article long time ago, when a laptop connected to a vehicle was a stunning and fascinating scene. However, Iím positive that the article is still important today, when automotive electronics is no longer a subject of surprise, and vehicles increasingly look like computers with an engine attached. Still, the storage battery stays the same...Ö
"The Fascination of Four Wheels" is how a polish engineer Voytsekhovsky called a section in his book "Electronic Toys". This is indeed so, but... until all electronic devices in a car work properly. However, when it gets cold outside some people have to begin their morning jump starting their car from a neighborís car. In the mean time, you look at the poor people indulgently and feel happy to have bought a new high capacity battery (no less than 100 ņH). Yet you notice that the car does not start as easily as it used to in summer. Some time passes, and you too have to call the auto service on a freezing morning.
Why!? Many people ask this question. Naturally, there is no definite answer to it. But some aspects of using automotive electronic systems do need a detailed explanation. This primarily applies to alternator (ac generator), along with one of the most whimsical car parts Ė a storage battery.
As known, a modern three-phase car alternator needs external power to start up. The startup circuit includes an HL display, which is usually a LED display or a light bulb on the front panel of your car (see Fig. 1). This is typical of most cars. When the ignition is on, but the engine is not running yet, the generator startup current flows through the following circuit: the positive side of the battery Ė the HL display and shunt paths Ė the alternatorís rotor Ė the voltage regulator Ė ground. The HL display turns on. DD1, DD2, and DD3 rectifiers are blocked by the reverse voltage. The battery powers all electrical circuits in a car.
When the generator "starts up" (see Fig. 2) and the voltage supplied to the DD1 and DD3 anodes rises to an operating level of 14.2 V, the generator startup current will go through the following circuit: the DD3 rectifier Ė the generatorís rotor Ė the voltage regulator Ė ground. The voltage at both ends of the HL display equalizes, and the display goes out, indicating normal operation of the alternator, which will change to the self-oscillation mode and will supply energy to all electrical circuits in a car, along with charging the battery.
Is the induction always correct, and does the unlit bulb mean the normal state of the alternator? In most cases, yes. However, it can happen that the generator bushes came off or the battery lost its capacity, for instance, after repeated undercharging that led to sulfation. In this case the display will be unlit, but the battery wonít charge either Ė the generator terminal voltage only equalized, but has not exceeded the battery voltage. This may happen even with a completely faultless car. For example, suppose you started the car to drive to work on a cold morning. The lights are on and the heating fan is spinning. The seat, windshield, and rear window heating are on too, and, say, as well as the laud music. I bet that when the engine is idling, the cold generator wonít be able to completely provide energy, and most of the load will be put on the battery, which will discharge. Another example: you drove as usual when suddenly the starter wouldnít turn over one morning, and the battery is dead. Meticulous mechanics will find the generator belt slightly loose. What about the display? Everything is seemingly alright, since the display didnít blink.
For the display circuit to function when the battery is uncharged, the terminal voltage of the alternator needs to turn less than the battery voltage at a small value, for example 0.75 V or 2 V. The range depends on the design of the voltage regulator. Therefore, the voltage drop at the alternator to the open circuit voltage (12.6 - 13 volts) will be unnoticed by the display circuit HL. The DD2 rectifiers will close, and DD3 will run to self-oscillation. The alternator will change to the near idle mode. This means that there wonít be any energy from it, and the front panel indication wonít show anything. Yet, nothing too serious will happen if your office is close to your home. When the engine and battery warm up, and RPM gets higher than idle, the alternator will eventually recharge the battery. But only to some extent, depending on your luck. On a cold morning, your car may not start...
So what happens? Thatís where a little theory comes in. Car batteries can be standard (outdated type, not used any longer), low-maintenance, and maintenance-free. The latter donít have removable caps on cells. In reality, the caps are there, hidden inside and sealed to prevent any access to them from outside. Maintenance-free batteries are filled with catalyst to turn the oxyhydrogen mixture into water, which allows keeping the same amount of electrolyte. The appearance of maintenance-free and low-maintenance batteries became possible due to plates made from low-antimony calcium-lead-tin alloys. Maintenance-free and low-maintenance batteries have higher voltage at the end of the charge reaching 16.5 Ė 17 V. Under this voltage, electrolysis begins at the end of the charge, i.e. the electrolyte water actively breaks down into oxygen and hydrogen gases, and the battery is "boiling". Under a regular voltage of car power of 14.15 Ė 14.3 V, water will practically not decompose and you wonít need to top it up (practically). This is the most common type of batteries used today.
And now more about the problem. An undercharged battery loses its capacity very quickly, i.e. its ability to deliver high current on engine startup. It undergoes irreversible changes that are not to be fixed by charging, since a chronically undercharged battery develops sulfation pretty quickly. The lead sulfate crystals build up that donít carry electrical current, which reduces the plate effective area, and the battery loses its capacity.
And whatís exactly an undercharged battery? Letís take a look at the graph (Fig. 3).
Using different reference sources and my own experience, I showed dependence between the battery level, voltage, and electrolyte density of a lead-acid battery. The graph allows you to estimate the electrolyte density by measuring the terminal voltage. It also allows avoiding an unsafe direct contact with electrolyte, which is especially important when using maintenance-free, inaccessible batteries, when it is not possible.
This is what we see on the graph:
- The higher ’ axis shows the open circuit voltage reading of a six-cell car battery, measured after several idle hours, for example in the morning before the engine startup: Uakk.(XX) Volts.
- The lower ’ axis shows the battery level in percentage: Capacity (%).
- The Y axis shows the electrolyte density in grams per cubic centimeter ÍůŠŤųŚŮÍŤť: P (gr/sm2).
The current lead-acid batteries are dry-charged and it seems itís enough to pour electrolyte and voila, itís ready to go! Unfortunately, this is not the case. Such a battery will work, but its capacity will be around 80%, which has been proved many times. To restore a battery to full capacity, run it though a training cycle before use. I will tell more about it below.
I anticipate a comment Ė ďI donít care about losing 20% of the capacity, my battery is 120 Ampere-hour after all. Iím a clever person and bought it oversized. The instruction says 80Ah is more than enough for me, and with 120-20=100Ah itís way over the top.Ē I want to emphasize that the problem is not in the absolute capacity of your battery in Ampere-hour, but in the charge state, which is of fundamental importance for operation of batteries of any capacity.
Letís look at the graph on Fig. 3 again. At the bottom are allowed and forbidden areas of battery operation depending on its discharge level and weather conditions. If the battery works more or less fine in summer even at half charge (up to 50%), then the starter may not even engage after the first freeze, as the peak discharge level in winter is only 75%. If you happen to have a brand new battery of high capacity, an untrained battery (do you still remember about 80%?), the reserve you have in winter is only 5% (80 - 5 = 75%)! If itís slightly colder the next morning, or you went shopping for a long time the previous evening, often stopping and starting the engine, thatís the reason why even a new battery can suddenly let you down. Youíll feel nervous, have to call the service, and will be running late for work.
So what do you need to do? Thatís a rhetorical question. First of all, conscientious dealers must run training cycles before selling new batteries. In any case, here are several simple tips:
- Measure the battery voltage with a simple multimeter, for example, in the morning on a cold and unstarted car. Look at the table (Fig. 3) and determine the battery level. If itís low, consider charging it. Once again I emphasize that the car generator, with its voltage of about 14.2 V, will NEVER be able to charge your battery to 100%. Only standalone chargers can do this, which takes long time, sometimes up to several days!
- Twice a year, in spring and fall, get the mechanic to properly check your battery and apply a topping charge if needed. Ask for a temporary replacement in the mean time.
Some more tips for those who can spend time, has the ability to, and wants to get the battery into shape, regardless whether itís new or used. Just one training cycle is enough both as a preventive training for a used battery in spring and fall, and before using a newly purchased battery:
If needed, top up each battery cell with distilled water and fully charge the battery at a current of about:
The charging ends when the electrolyte density and voltage stay constant for an hour and thereís ample gas release (boiling).
Then discharge the battery to 10.2 V, at the following or lower current:
Keep an eye on the decreasing voltage. At first, it will decrease gradually, but closer to discharging the voltage will drop sharply. You need to stop discharging before the voltage drops below 10.2 V. During discharging you can use a regular headlight bulb (non-gallogen). Remember that the bulb will get very hot and can burn something.
Start charging the battery in no more than 30 minutes. The charging current is the same as in the beginning:
As before, the charging ends when the electrolyte density and voltage stay constant for an hour and thereís ample gas release (boiling).
Then check the electrolyte level in all battery cells and correct its density if needed. The standard is 1.27-1.29 grams per cubic centimeter. The battery is ready to go.
As a side note, the exact value of electrolyte density in the battery is not as important as having the same density in all six cells.
If your battery wonít work at all and is heavily sulfated, you can still cure it. The cure is once again training checkups. After charging, at the end of the cycle:
- Measure the electrolyte density. If itís greater than the standard (1.27 - 1.29), reduce it by topping up distilled water. If the electrolyte density is lower, keep it as is.
- Check the electrolyte level in all cells and top up distilled water if needed.
- Repeat the cycle.
The number of cycles depends on your patience alone. Usually, up to three cycles is enough to restore the correct electrolyte density in all battery cells. At the end of each charging, check the electrolyte level in all battery cells and correct it if needed. The standard density is 1.27 - 1.29 grams per cubic centimeter. The battery is ready to serve.
- Leave the battery uncharged for more than an hour.
- Discharge the battery without monitoring the voltage (for instance, leaving it overnight).
- Charge a cold battery, at temperatures below the freezing point of water.
- Charge a low-maintenance battery without removing the caps.
- Charge a maintenance-free battery unattended, as potential gassing can exceed the electrolyte capability in caps.
- Charge a battery at uncontrolled current, for example at a constant voltage.
You will enjoy a better service life and maximum performance of a newly born battery.