How Long to Charge a Car Battery, and How Far to Drive After a Jump Start
6 minute read · Updated · Al Nahda Battery
The short answer. Divide the amp-hours the battery needs back by the charger's amps, then multiply by about 1.2 for losses. A 60 Ah battery at half charge takes roughly 9 hours on a 4 A charger and about 4 hours on a 10 A charger, plus a few hours more for a smart charger to finish. After a jump start, drive for at least 30 to 60 minutes at a steady speed. Idling with the AC on puts back very little, and a battery that was completely flat needs a mains charger to recover fully.
The question usually comes up in one of two places: standing next to a car that has just been jump started, or holding a charger that came with no useful instructions. In both cases the honest answer is longer than most people expect.
Al Nahda driving makes it harder. Most trips are short hops between towers, malls and schools, with a lot of idling at signals and in basement queues, and the AC runs the whole time. That pattern recharges a battery far less than a run down the highway to Sharjah or Jebel Ali.
The rule of thumb
Work out how many amp-hours (Ah) the battery needs back, divide by the charger's output in amps, and multiply by about 1.2. The extra 20 per cent covers the energy lost as heat, and the fact that a lead-acid battery never accepts charge perfectly.
A battery that will not start the car is rarely at zero. More often it is somewhere around half charge, so the table below assumes the charger has to put back half the battery's rated capacity. From completely flat, roughly double the times.
| Battery capacity | 2 A charger | 4 A charger | 10 A charger |
|---|---|---|---|
| 45 Ah | About 14 hours | About 7 hours | About 3 hours |
| 60 Ah | About 18 hours | About 9 hours | About 4 hours |
| 70 Ah | About 21 hours | About 10 to 11 hours | About 4 hours |
| 90 Ah | About 27 hours | About 13 to 14 hours | About 5 to 6 hours |
Worked from half the rated capacity ÷ charger amps × 1.2. These are approximations: a smart charger slows down near the end, so allow a few extra hours for the final top-up, and a cold, old or sulphated battery may take longer or never reach full.
Idling against driving
At idle the alternator turns slowly and produces only part of its rated output. The AC compressor, the blower on high, the radiator fan, the lights and the car's own electronics all take their share first. On a summer evening in a queue on Al Ittihad Road, very little may be left over for the battery.
At a steady road speed the alternator has a comfortable surplus, which is why a proper drive is the advice after any jump start. Even then it is a top-up and not a full recharge. Many newer cars, especially those with start-stop, also manage the alternator deliberately and keep the battery below full to save fuel.
| Situation | Charge going back to the battery | Good enough after a jump start? |
|---|---|---|
| Idling with the AC and fan on high | Very little, sometimes none | No |
| Idling with the AC and lights off | A small amount | No, it takes far too long |
| Stop-start traffic around Al Nahda | Some, interrupted by every stop | Only if it goes on for a long time |
| 30 to 60 minutes at a steady speed | A useful amount | Yes, to keep going; not a full recharge |
| Mains charger overnight | A full charge on a healthy battery | Yes, the best way to recover a flat battery |
Smart chargers and AGM batteries
A smart charger works in stages. It charges hard at first, holds a steady voltage while the battery fills, then drops to a gentle maintenance level and stays there. The last stage is why the time on the charger's display can run beyond the table above.
AGM and EFB batteries, fitted to most start-stop cars, want a charger with a matching mode. AGM batteries are sensitive to overcharging, so an old unregulated charger is a poor choice for them. On many cars with a battery sensor the handbook says to connect the charger to the under-bonnet jump points rather than the battery itself, so the car's battery monitor counts the charge going in.
Charging with a mains charger, step by step
Read the charger's instructions first; where they differ from these steps, follow them.
- Look at the battery. If the case is cracked, swollen or leaking, do not charge it.
- Work in a ventilated space, away from sparks and flames, with the ignition off and the key away from the car.
- Select the mode for the battery type: standard, AGM or EFB, as the battery label says.
- With the charger unplugged, clamp red to the positive (+) terminal or jump post, then black to the negative (−) terminal or the earth point the handbook shows.
- Plug the charger in and switch it on.
- Check now and then that the battery is not hot to the touch and there is no smell of rotten eggs.
- When the charger shows the battery is full or in maintenance mode, switch off and unplug before removing the clamps, black first and then red.
- Let the battery rest for a few hours, then check its voltage or have it tested.
Why a deeply flat battery may not come back
While a lead-acid battery sits discharged, the lead sulphate on its plates slowly hardens into crystals that charging can no longer break down. This is sulphation, and heat speeds it up. A battery left flat for a couple of weeks in a hot basement can lose a large share of its capacity for good.
Some chargers offer a recovery or reconditioning mode. It sometimes helps a battery that has only just been flattened, and rarely rescues one that has been flat for weeks.
| Resting voltage | Approximate state of charge |
|---|---|
| 12.6 V or above | Full |
| About 12.4 V | Around three-quarters |
| About 12.2 V | Around half |
| About 12.0 V | Around a quarter |
| Below 11.9 V | Flat |
Measured after the battery has rested for several hours with no charging or load. Approximate for a standard battery at room temperature; AGM batteries read slightly higher.
When to stop charging and have it tested
Switch the charger off and get a proper test if any of these happen:
- The battery gets hot, hisses, bulges or smells of rotten eggs
- The charger never reaches full or maintenance mode after a long charge, or shows a fault
- The voltage falls back below about 12.4 V within a day of a full charge
- It charges to full but still cannot start the car the next morning
- The battery is more than two to three years old and has gone flat for no clear reason
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