How long do you need to drive to recharge a car battery?
It's the question everyone asks after a jump-start. The honest answer is that nobody can give you a dependable number of minutes. What the battery makers do explain is what decides it, and what to do instead of guessing.
Is there a reliable drive time?
No. None of the battery makers' technical documents we checked gives a drive time.
Varta (Clarios) says longer drives help: "During longer journeys the battery quickly recharges – even without traveling at high speeds. Even at 2000 rpm, the generator provides the battery with two thirds of its maximum energy" (Varta, correct battery care). Elsewhere Varta says that normally "the alternator should adequately charge the battery in everyday use" (Varta, charging).
Short trips are a different story. Varta: "If the car is used infrequently or only for short distances, the alternator cannot fully charge the battery" (Varta, avoid battery failure).
What decides how long it takes?
Six things, set out in the table: electrical loads, engine speed, smart charging, temperature, how flat the battery is, and its health.
| Factor | Why it matters | Source |
|---|---|---|
| Electrical loads | The alternator runs the car's electronics as well as charging the battery. Varta: if a large number of consumers are active, "little energy remains to charge the battery" | Varta, the car won't start |
| Engine speed | More rpm, more alternator output: two thirds of maximum at 2,000 rpm (Varta). Less at idle is our inference from that | Varta |
| Smart charging | Some cars deliberately lower charging voltage to save fuel. GM's system on its 2005–2014 full-size trucks and SUVs targets 12.6–13.1 V in "Fuel Economy Mode"; switching the headlights on raises the target to 13.9–14.5 V | GM 07-06-03-009D |
| Temperature | "Recharging of the battery is slower due to the cold", and the alternator's maximum output is limited. MINI tells dealers to avoid charging below about 16 °C (60 °F) if possible, because the battery takes up current less readily and charging time "is extended considerably" | Varta, the car won't start; MINI SI M61 01 02 |
| How flat it is | On East Penn's charging chart, a battery at 25% takes about three times as long to fill as one at 75% (our arithmetic from the chart) | East Penn |
| Battery health | "Corrosion and sulfation prevent it from fully charging" as a battery ages; deep discharge "causes permanent damage to the battery cells" | Varta, avoid battery failure; Varta, the car won't start |
GM's system is one example, and its figures apply only to those trucks. But it shows why, on many newer cars, a quick check of the running voltage doesn't tell you how fast the battery is charging (our inference).
Will idling charge the battery?
Our inference: only slowly. If Varta's alternator gives the battery two thirds of its maximum at 2,000 rpm, it gives less at idle, and at idle more of that output goes to the lights, fans and heaters that are running. So idling to recharge a flat battery is slow, and a pattern of short starts and idling can leave it lower than before. We found no battery maker's technical figure for how slow. Never idle a car in a garage or other enclosed space: exhaust contains carbon monoxide.
How long does a battery charger take, for comparison?
About 3 hours (180 minutes) for a half-charged battery on a 10 A charger, on East Penn's chart, and much longer on a small charger. A plug-in charger puts a known current straight into the battery with no other loads competing. East Penn's chart gives approximate times to full charge at 27 °C (80 °F) for a 12 V battery:
| Starting charge (flooded / AGM resting voltage) | 20 A charger | 10 A charger | 2 A charger |
|---|---|---|---|
| 75% (12.4 V / 12.6 V) | 48 min | 90 min | 7.5 h |
| 50% (12.2 V / 12.3 V) | 95 min | 180 min | 15 h |
| 25% (12.0 V / 12.0 V) | 145 min | 280 min | 22.5 h |
| 0% (11.8 V / 11.8 V) | 195 min | 370 min | 30 h |
East Penn adds that "Charging time depends upon battery age, temperature, capacity, and efficiency of charger." Varta says a full charge "is usually achieved by charging overnight" and that a battery should not stay connected to a charger for more than 24 hours. Small, low-current chargers take much longer than the 10 A column: East Penn's 2 A column runs from 7.5 to 30 hours.
Our inference: a drive recharges as quickly as a 10 A charger only if the alternator is sending about 10 A of spare current into the battery the whole time, and the sources above suggest that often isn't the case at low engine speed or with heavy loads on.
Charger safety. Use a charger suited to your battery. East Penn warns that charging an AGM battery on a charger that exceeds 15.4 V, "even one time", may greatly shorten its life, and that you should never charge a frozen battery. Varta warns that overcharging can produce explosive hydrogen: charge in a well-ventilated place, and wear safety glasses and gloves.
What should you do after a jump-start?
- Drive a longer, steady trip if you can, rather than a string of short hops. Switch off comfort loads you don't need, such as seat and steering-wheel heating. Varta says to keep safety systems and the headlights on at dusk.
- Then charge it properly with a plug-in charger, overnight if it's a smaller one.
- Have it tested. East Penn notes that a battery must be charged before a load test is meaningful. Don't leave it flat: Varta says deep discharge "causes permanent damage to the battery cells", that "The longer the battery remains in the deep discharged state, the more serious the damage", and that immediate recharging is essential (Varta, the car won't start).
- Find out why it went flat. A car that sits, a parasitic drain or cold weather are the usual reasons. See also how long a car can sit before the battery dies and why a car battery keeps dying.
If the car still won't start, see car won't start and just clicks.
How can you tell whether driving has recharged the battery?
Measure the resting voltage after the car has sat for a few hours (MINI's service information says a reading is reliable only after at least 4 hours): about 12.6 V means a flooded battery is full, and East Penn recommends a boost charge below 12.4 V. The battery voltage chart explains how to measure it. Varta suggests that if you drive only now and then, a longer journey at least once a month in winter helps the battery fully recharge.
Where Tachwise fits
The Tachwise device is being built to watch the battery's resting voltage and cranking dip while the car is parked, which would show whether your drives are keeping up. It's not ready yet; join the waitlist. The free app beta reads codes and shows the voltage at the port through your own ELM327 dongle, only while it's connected.
Frequently asked questions
Is 30 minutes of driving enough to recharge a battery after a jump-start?
Not reliably. No battery maker's technical guidance we found gives a dependable drive time. Varta says longer journeys recharge a battery quickly, but short or infrequent trips can't fully charge it. Put the battery on a charger afterwards and have it tested.
Will idling the engine charge the battery?
Very slowly at best, in our reading. Varta says the alternator gives the battery two thirds of its maximum at 2,000 rpm, so at idle, with lights, fans and heaters running, little may be left for the battery. Never idle a car in a garage or other enclosed space, because exhaust contains carbon monoxide.
Does driving at higher engine speed charge the battery faster?
Generally yes. Varta says that even at 2,000 rpm the alternator provides the battery with two thirds of its maximum energy, and that longer journeys recharge the battery quickly even without high speeds. How much reaches the battery also depends on the car's electrical loads and charging strategy.
How long does a battery charger take?
It depends on the charger's current and how flat the battery is. East Penn's chart shows a half-charged 12 V battery taking about 180 minutes on a 10 amp charger and about 15 hours on a 2 amp charger, at 27 °C (80 °F), and says time also depends on battery age, temperature and capacity.
Why does my battery keep going flat even though I drive every day?
Short daily trips may not replace what starting and parked loads take out, especially in winter, according to Varta. A parasitic drain or an aging battery can also be the cause, so have the battery and charging system tested.
Sources
- Correct battery care. VARTA Automotive (Clarios). Accessed .
- Avoid battery failure. VARTA Automotive (Clarios). Accessed .
- The car won't start. VARTA Automotive (Clarios). Accessed .
- Charging car batteries. VARTA Automotive (Clarios). Accessed .
- Battery Care & Maintenance (Form 0004). East Penn Manufacturing. Accessed .
- Bulletin 07-06-03-009D: Voltmeter Gauge Fluctuation Due to Regulated Voltage Control System. General Motors (via NHTSA). Accessed .
- SI M61 01 02: Battery Maintenance Requirements: Battery Care. MINI USA (via NHTSA). Accessed .
Hear first when the Tachwise device is ready
Join the free waitlist: no payment, no deposit. Or try the app beta today; it reads your car's codes through your own ELM327 Bluetooth dongle.