How long will it take to charge my car battery?


How long will it take to charge my car battery?

Jeremy Horne

If the lights on a car have been on all night, and the car fails to start, the likely cause is a discharged battery.  A number of factors enter into the time to re-charge a car battery.  It is useful to be familiar with the basic structure of a car battery to better understand what affects a battery’s re-charge time and be mindful of how to care for the battery. 

The following is a very simplistic explanation of the battery’s basic construction. A number of parallel lead plates in two sets are submersed in a sulfuric acid bath.  Over this bath are six (usually) cells into which distilled water is poured to top off the electrolyte.  There is a positive set and a negative one.  The plates are kept apart from each other by separators.  One set of plates contains a red lead paste – lead oxide and the other a porous lead.  Charging and recharging the battery causes the plates to expand and contract, and after a while the lead falls from the plates and collects at the bottom of the battery. 

There are marginal cases, where a battery is on the verge of being non-functional.  Battery tests are done with a hydrometer measuring specific gravity of the electrolyte in the battery, and a voltmeter.  The lower the hydrometer reading, is the more discharged the battery will be.  First, allow the battery to sit for 12 or more hours, or remove the"surface charge" by turning the healight on for several minutes.  A fully charged battery should have the hydrometer reading about 1.265 at 80 degrees F.  At the same time the hydrometer test is run, connect the voltmeter.  A fully charged battery should read about 12.6 volts.

Now, we have some main factors affecting re-charge time.


It can be expected that the older the battery  the worse condition it will be in.  Even if it is stored correctly, deterioration of materials will affect its ability to be re-charged.

Past Usage

If the battery has been used a lot (read, re-charged many times), it will not be as rechargeable (although imperceptively so) the next go-around.


An abused battery is one that is less re-chargeable.  If the battery has been allowed to be discharged completely many times, it life will be shortened.  Only deep-cycle batteries should be allowed to be discharged completely.  If a battery has been banged about or otherwise abused, do not expect as long a life as one that has been treated right.

Length and extent of discharging

Obviously, the longer the battery has been discharging, as when the headlights have been left on, the more it will have to be re-charged.  The greater load there on the battery (lights, radio, etc.), the more power will have been drained. If the terminals of a battery are corroded, the charging system will not be as efficient.  The charge indicator light often doesn’t warn you when the battery is not being charged properly by the alternator or generator.  Other parts of the electrical system, such as a faulty diode or regulator may lessen charging efficiency.  Before putting a battery back on line, ensure the car’s charging system is in good order.


If the battery has been stored on the floor, there will be more of a current draw from the battery than if it were stored up off the ground.  If the batery has been in a cold place, it will be harder to use.  Heat enhaces current flow.  Trickle charges of 2 amps or less with a cut-off switch on the charger that is activated upon a full charge help maintain the condition of the battery.  If the battery is exposed to dirty conditions and the owner allows dust to get into the cells containing the electrolyte, the re-chargeability will be compromised.   It is a good idea to use distilled water when topping off the cells.  Tap water often contains minerals, which can be deposited on the lead plates, thus shorting them out.

How long, then, does it take to recharge a battery?  One needs to know how to calculate power usage.  The following applies to newer batteries.  Be mindful that the conditions discussed above affect recharging time, and often that time is longer.  Also, the more times a battery has been discharged, the less re-chargeability it will have.

Batteries are rated in amp hours.  Look on the top or side to determine this figure.  Often, it will be in the 40-70 amp hour range.  Over an hour’s worth of time, there is a number of amperes (the number of electrons flowing from the battery to the fixture using the energy – lights, horn, etc.).  So, to determine the amp hours used, add up the amps used by each fixture (lights usually use eight per bulb).  Then, multiply by the number of hours (including fractions of an hour) the appliances have been on to get the total amp hour usage.  This will approximate the amount of current that has been drained from the battery.  Take this sum of usage and subtract it from the amp hour rating of the battery, shown on the top.  The result will be the number of amp hours left.  Keep in mind that this is only a "thumb in the wind" calculation, as the longer a battery is discharging the faster the discharge will be toward the end of its being used up.  The relationship between discharge rate and capacity in amp hours is nonlinear. 

However, you may simply want to get the battery recharged and won’t want to do all the calculations.  There are three basic ways.  Note that  you won’t have to charge the battery fully just to get the car started; the car’s charging system should do the rest of that charging.   First, if you do not need to use the car for several hours, take off the cables, and attach a battery charger to the battery.  Charge at a tickle charge at 2 amps.  Remember the amp hours.  For a 50 amp hour battery and one that is completely dead, it will take 2 amps x 25 hours to get a full charge.  This stresses the battery the least.  The second and quicker (although more stressful to the battery) is to re-charge at 10 amps, or five times as fast.  For that 50 amp hour battery, count on five hours.  The third way is to charge the battery enough to start the car let the charging system do the rest. This can be done by the battery charger (10 amps), a "hot start" device (using about 300 amps or more – very stressful), or jump starting  with another car.  A starter often uses 100-100 amps of cranking power (depending upon the car model), the most used by any part in the car.  However, for a well-tuned car, you’ll need to run the starter for less than 15 seconds. 

Aside from all the above, you should consult an expert in a car’s charging system for specific problems before proceding where the is doubt.


Clive Bird

It depends on the conditions in which the battery has been stowed or used. If there is a problem with the alternator of the vehicle, a battery may require bench charging quite often. If the battery has been stowed away in proper conditions (electrolyte drained, battery stored in clean dry and ventilated area), it might require around 24 hours with electrolyte refilled properly for the battery to be completely charged up. If the battery is not in use for a long time and fluids still retained, it may require topping up and 16 hours for a full recharge. You may refer the parts and accessories page for deatials on maintenance and illustrations


It depends on how long it has been dead. When my car was inactive for a year it took a LONG time and I had to replace the battery twice because it sat so long. However, if you came out this morning and it was dead to do the lights being left on etc. I’d say a 1 hour to be safe. You don’t want it to die again when you reach your destination, so the longer the better!

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