A flat battery is a real pain in the jacksie. I had a Mercedes once with a battery drain problem. Every second morning I was jump-starting. On the upside, the alternator would charge the battery in no time.
Most cars will charge a healthy flat battery after thirty minutes of driving at highway speeds without using electrical components such as lights, HVAC, and wipers.
In this post, we’ll cover battery charging by driving quick tips and the associated risks. We’ll also cover checking your battery health, alternator health, and what you need to know before disconnecting your battery.
Battery Charging Quick Tips
Obviously, a flat battery is inconvenient, and it always happens when you’re pushed for time. I know. I’ve lived the experience for a couple of months. Here are a few helpful tips to get your car to charge up as quickly and safely as possible when jumpstarting from another vehicle.
- Before jump-starting your car, connect the batteries with the jumpers and run the donor car’s engine at 3000 rpm approx. for five to ten minutes. This will help boost your flat battery and lessen the workload for your alternator.
- Before jump-starting, turn off all electrical consumers, especially heated seats, HVAC, rear heated screen, lights, wipers, infotainment systems, etc., on both cars. After jump-starting, leave the vehicles connected and engines running above idle for a few minutes.
- After disconnecting the jumpers, drive your vehicle at highway speeds with all electrical consumers off (or as many as possible). Avoid performing the jump-starting procedure at night, if you can, as you’ll obviously need to use lights when driving.
- When shutting down your vehicle after a thirty-minute drive, your battery should now be charged. However, if the battery or some other component is causing the problem, you may experience the same issue. So if you can, park in a way that allows access to your battery should you need to bum a jump start again.
Risk To Battery Charging By Driving
The risks to using your car to charge a flat battery aren’t great, but they are real. Without causing undue stress, you should know what the possible risks are. It’s always better to use a battery charger to bring your battery up to at least 12.5 volts.
If you need a great little boost starter and charger, check out the little NOCO boost pack. I bought one and was genuinely blown away by how powerful it is. I’ve been a mechanic for over twenty-five years, and this is the fastest charger pack I’ve owned.
Anyway, you can check it out here on the Auto electrical tools page.
Modern cars, as you already know, are jam-packed with electronics. These guys all require a minimum amount of power to operate, and if the battery is not at peak charge, it can cause some functions to go offline temporally.
It’s usually not a big problem. The computer is programmed to only shut down noncritical kits like radios, heated seats, etc. The trouble is, some cars may need a trip to the dealership to recalibrate sensors to restore functionality, even though the battery is back up to full power.
It is possible to calibrate your vehicle yourself, but you’ll need a good scan tool. You can check out the scan tool I use here on the Auto electrical repair tools page. It has close to full dealer-level capability.
Asking your alternator to charge a battery from a flat is a big ask. It’s able to do it, but it’s not designed to do it. When you start your car with a healthy 12.65-volt battery, your alternator goes to work. It provides power for all the electronics on the vehicle.
The car battery, at this stage, has done its job, and it expects the alternator to top it up when it has all other systems up and running.
Asking the alternator to charge a completely flat battery is going to significantly increase its workload. Asking it to do this regularly will shorten its life.
Alternators can be repaired but are often just changed out for a new one. Replacing an alternator will run a few hundred dollars fitted.
Driving a car to charge the battery can put an extra heavy load on the auxiliary belt (drive belt aka fan belt). It’s the belt that drives the alternator. If it’s worn, it could break under these conditions.
The trouble is, the aux belt will often drive other components like power steering, water pumps, and cooling fan. Meaning if it breaks, you’re on a tow truck.
Checking Your Battery Health
If your battery or alternator is causing the flat battery, then jump-starting and driving your car won’t charge it. The jump-starting procedure might put enough power into the battery to get the show on the road, but the engine will shut down after a few miles when the battery is empty to truck time.
Batteries typically last about three to four years, or maybe I’m just unlucky because some people can get five to six years from theirs. How cold your climate is will affect how long your battery lasts. Batteries hate the cold.
Even a healthy, fully charged battery only supplies about 65% of its power at 0 degrees F. Combine that with the fact the engine oil is thicker in colder temps. It’s no surprise most batteries fail in freezing winter conditions.
Anyway, a bad battery can cause other more expensive alternator problems, so it is best to check it. A faulty battery won’t build resistance to charge, and that causes the alternator to work at full capacity for longer.
It’s worth checking for loose or dirty battery connections, especially in older cars. Bad battery terminals will cause you to believe you have a flat battery, so always check them first. Check battery cables are tight but also check the battery ground straps.
You’ll have at least two straps, a strap from the battery to the chassis and another from the chassis to the engine. All must be clean, tight, and in good order.
Two tests are needed to check your battery. The first test is Battery voltage. The battery will need to be fully charged (above 12.5 volts) before we can perform the second test, which is the Cranking test.
Checking Battery Voltage
It’s a simple three-step process, but you will need a voltmeter.
Set the voltmeter to 20v DC.
Connect the red probe to battery positive and the black probe to battery negative.
Read voltage and charge battery if below 12.5 volts.
12.7 – 13.2 volts is 100% charged
12.4 volts is 75% charged
12.2 volts is 50% charged
12.0 volts is 25% charged
0 – 11.9 volts is Discharged (Flat)
A local parts store will run a battery health test for free, or you can buy a handy handheld battery tester that measures CCA (Cold Cranking Amps). The higher the amps, the stronger the battery.
You’ll find a battery tester and a voltmeter on the Auto electrical repair tools page and if you need a battery or any parts, check the Amazon link below.Amazon Auto Batteries
Battery Cranking Test (the second test)
In the second test, we’ll use a simple voltmeter to check the voltage on the crank. The graphic below shows the process. The test won’t work unless the battery is fully charged.
Charge battery to 12.65 volts.
Set the voltmeter to 20v DC and use min/max function.
Connect the voltmeter as per the graphic below.
Have the helper crank engine.
Reading on the voltmeter below 9 volts indicates a faulty battery.
Read Before Fitting A Car Battery
Fitting a car battery isn’t difficult; however, most late-model vehicles won’t like having their computers shut down. When power is disconnected, learned values stored in the ECU’s (Computer), KAM (Keep Alive Memory) chips are lost.
This causes lots of issues, some of which can be more irritating than serious. So before changing your battery, know that you may need to perform some recalibration, also known as relearning tasks.
Most cars will relearn themselves after a few days of driving, but some models may require calibrating using a scan tool. See the Mechanics tools page to see the scan tool use.
The kind of problems replacing a battery can cause includes:
- Engine hunting
- Transmission shifting erratically
- Engine lights on
- Air bag lights on
- Abs lights on
- Power window and sunroof issues
- Electric steering warning lights
- Stability control warning lights
- HVAC issues
You can bypass these potential problems by using a KAM tool (battery saver). It’s a simple power lead connected into the 12v port charger or through the data link connector. It powers the car and computer systems while you replace the battery.
The Schumacher memory saver tool is a cinch to use and of good quality. It’s conveniently sold and delivered by Amazon.com.
Notify battery monitor
And finally, you should also know that many late-model vehicles use a dedicated battery control module. Its job is to maintain battery health. The controller adjusts the alternator’s rate of charge based upon criteria such as demand, temperature, state of charge, battery size (known as amp-hour (Ah)), and battery age. Older batteries need a higher rate of charge.
When fitting a new battery, it’s standard procedure to update the control module with the new battery size and battery code. Failing to notify the battery control module (also known as battery monitor) will cause the alternator to overcharge the new battery, which will shorten its life.
Coding isn’t very difficult, but like many tasks on modern motors, it requires a scan tool to input the battery code. Check out the scan tool I use to nail this procedure here on the Mechanics tools page.
If your vehicle has a small sensor fitted to the ground (Negative) battery terminal, then you can bet you have a battery monitoring system.
You might find this post useful. It covers changing a car battery. “How hard to change car battery?”.
Checking Your Alternator Health
A battery’s job is to start the engine, and then it’s the job of the alternator to produce all the amps needed to run all the electrical demands of the car. It must also top up the car battery.
A voltage regulator will sense the battery state of charge and prevent the alternator from overcharging (cooking the battery). Voltage regulators were once a troublesome part of the alternator. Most late model cars now control voltage using computerized control modules (battery monitors aka controllers aka battery control modules).
Many cars will have a battery control module dedicated to power management. It senses low voltages and may shut down noncritical electronics, such as radio, heated seats, etc., in times of battery power shortages.
It’s not uncommon for some car functions to go offline, especially in extreme cold conditions when voltages may be below.
If your alternator is faulty, your car will usually let you know by illuminating the red battery warning light on the dashboard.
Check alternator voltage output
An alternator can be checked with a simple voltmeter. You’ll need a simple voltmeter for these tests. I’ve listed a good meter on the Auto electrical repair tools page.
- Place a probe on the battery terminals in the normal way and note the voltage
- Start the car and let it idle with all consumers off
- A healthy alternator will read about two volts higher approx. then the resting battery voltage
- Readings above 15 volts suggest the alternator is overcharging
- Voltage reading should fall as the car tops off the battery; the time it takes depends on the battery state of charge; about five to ten minutes is normal
- Turning on lights and other consumers should cause the voltage to rise immediately as the alternator reacts to the extra electrical demands
Volt drop test alternator
If you suspect an alternator fault, go ahead, volt drop tests the circuit……still with me? Okay, cool, this stuff is really easy, I promise. You’ll need a voltmeter and the engine running to perform this test.
A voltage drop test simply measures the difference in voltage between the two test probes. A large difference (.2v on the ground side and .3 v on the positive side) means there’s resistance in the wiring or terminals, i.e., the power can’t flow through the circuit with ease.
It’s a great test for locating dirty, corroded, or damaged cables/terminals. Anyway, follow the diagram below to nail the volt drop test like a pro.
Common alternator problems include:
- Faulty alternator overcharging the battery, symptoms include premature battery failure and bulbs blowing
- Faulty alternator undercharging battery, symptoms include premature battery failure, constant or intermittent flat battery.
- Faulty alternator diode, symptoms include draining the battery overnight
- Slipping alternator drive belt, symptoms include high pitched squeal under acceleration, soft battery, and dimming lights, especially in damp conditions
- Faulty harmonic damper, causing lack of belt drive, symptoms of soft battery, and dimming lights, especially in damp conditions
- Faulty alternator bearing, symptoms include a whine or hum noise
Having a workshop manual for your vehicle is always a good plan. They only cost a few dollars but will save you a packet. Good manuals cover repair diagrams, wiring diagrams, system operation overview, troubleshooting sections, fastener torque specs, sequences, etc., all mission-critical info.
Does revving the engine charge the battery? Increasing the car engine rpm will increase the output of the alternator and speed up battery charging time.
You may find the following posts helpful:
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.