You should leave the cables connected for about 5 minutes with the donor car running to let some charge transfer, then try starting the dead car; if it doesn't start, wait longer (up to 10-15 mins) or increase the donor car's revs, but if it still fails, the battery might be too dead or there's another issue, requiring a mechanic or charger, then drive it for 30+ mins to recharge after success.
You should leave jumper cables connected for around five minutes at first. If your car still doesn't start after five minutes, it may take longer—up to 30 minutes in extremely cold temperatures. If after 30 minutes your car still doesn't start, there's likely more wrong than simply a dead battery.
While it is technically possible to charge your car battery by just idling, it's not very efficient. You'd likely have to leave your car running for hours to even come close to a full charge. This is due to the amount of charge that is initially lost when cranking the engine.
Yes, a battery can be too far gone to be jumped. If it has a dead cell or an internal short, it will suck the life out of the jumper pack before it can operate the starter. If can also be so low on charge that it drains your jumper pack. Here's a very simplified explanation.
The 20/80 charging rule suggests keeping lithium-ion batteries (phones, EVs) between 20% and 80% charge to extend battery health by avoiding stress from full discharges (0%) or full charges (100%), especially the final 20% which is harder on the battery, though modern devices have safeguards and occasional full charges are fine, with 80% often sufficient for daily use.
Stay in the 20-80% Charge Range
For the long-term health of the battery try to keep the phone in the sweet-spot range of 20-80%.
Reduced battery degradation
Charging your EV up to 80% can help your vehicle's battery last longer because the battery finds it easier to charge when it is slightly empty rather than nearly full. Therefore, causing less stress within its charging cycle.
You might need to recharge a battery after leaving the lights on and jump-starting it. It's a common occurrence, especially on vehicles that don't have protective systems that go to 'sleep' after a set time of inactivity. But a jump-start doesn't recharge the battery, so you'll need to run the engine to charge it up.
Signs of a Dead Battery
If a battery is serviced early, reversible sulfation can often be corrected by applying an overcharge to an already fully charged battery in the form of a regulated current of about 200mA. The battery terminal voltage is allowed to rise to between 2.50 and 2.66V/cell (15 and 16V on a 12V mono block) for about 24 hours.
A car battery can drain when the car is off due to parasitic draws (interior lights, radio, faulty relays, aftermarket accessories like dash cams), a failing alternator preventing recharge, loose/corroded connections, or simply an old battery reaching the end of its life, with normal drains from the clock and alarm systems becoming problematic when combined with other issues.
Revving the engine will only increase the voltage by a few hundredths, which won't be enough to revive a dead battery. Most automotive batteries last anywhere between three and seven years, but some practices can reduce their lifespan.
Obvious Signs Your Car Battery Is Dying
A 20-minute drive helps, but it's often not enough to fully recharge a deeply dead battery, though it can restore some charge, especially if driving at highway speeds (55+ MPH) for 30-60 minutes; shorter trips (15 mins) are often insufficient to overcome the drain from starting the car, while a full charge or a battery that's old/damaged needs a proper charger or a very long drive.
To check if your alternator or battery is bad, use a multimeter: a good battery reads ~12.6V (car off), while a running car with a good alternator reads ~14.2-14.7V; if it reads low or doesn't rise, the alternator is likely bad. Alternatively, try a jump-start: if the car runs briefly then dies, it's the alternator; if it starts and stays running, the battery might be weak but the alternator is okay.
If the battery seems reasonably clean, stick it to your tongue. You will feel a slight jolt and metallic taste. You'll probably need to calibrate your tongue by testing a fresh battery and a dead battery to get an idea of what to expect.
Leaving a car battery drained can damage it and make it more vulnerable to extreme cold. A drained car battery can't produce a strong enough reaction to start your engine. A drained car battery can even freeze solid. As a car battery uses its electricity, the liquid electrolyte inside becomes more like water.
Idling can charge a car battery, but it's slow and not nearly as effective as driving. The alternator produces more power at higher RPM, so you only get a partial and gradual recharge at idle. Factors like battery age, weather, and accessory usage affect how much charge you recover.
After jump-starting your car, it is recommended to run your vehicle for at least 30 minutes, as that's how long it takes to charge a dead battery entirely, or at least sufficiently.
The 20/80 battery rule suggests keeping lithium-ion batteries (in phones, EVs, etc.) charged between 20% and 80% for daily use to slow degradation and extend overall lifespan, avoiding the stress and heat from frequent full charges (100%) or deep discharges (near 0%). While not strictly necessary for short-term use, following this guideline minimizes battery strain, leading to better long-term health, though modern devices have built-in optimization features.
So while charging to 100% won't “ruin” your phone, it can make your battery age faster. Luckily, most modern smartphones do have settings now to help you optimize charging, but a good rule of thumb is balancing convenience with constant overcharging.
As a battery reaches its maximum capacity of 100%, its charging speed slows down, because the electrons coming in have to work harder to find space. If you stop at 80%, you're avoiding the slowest rate of charge right at the end.