Battery Equivalents and Replacements

How To Charge a Car Battery

The dead car battery may be a rather annoying experience, especially when we are in hurry. But, it can happen to anyone, even with modern cars that regularly do self-tests of most systems, and warn the users if something is out of required range.

If the car battery is discharged enough, it is unable to crank the engine, but that doesn't mean that the battery is "dead" and that it must be replaced with a new one.

Updated: January 14, 2022.

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Check The Car and The Battery

So, if You have issues with the cranking, check your car and the battery itself:

- check if some lights were turned for a longer period of time while the engine was turned off.

- check for multimedia devices - if they are connected directly to the car battery and were turned on, their load can discharge the battery even if the sound volume was very low or even mute. If this is the reason for the dead battery and it is repeating regularly, consider reconnecting the multimedia system, or getting another multimedia system, or use a dedicated car audio battery.

- check cigarette lighter port - most cars turn this port off when the car key/card is removed, but some cars have this port connected directly to the battery.

- security systems of modern cars have a very low drain current, but if the car was not driven for a really long period of time, even such a small load can cause issues even with the best car batteries.

- check the battery terminals and cables - if there is corrosion, clean the battery terminals and cables.

Also, it is a good idea to check the battery voltage using a voltmeter or universal multimeter.

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So, take a voltmeter or multimeter and check the battery's voltage while the engine is OFF. If the voltage is:

- 12.7-12.8V or more: the battery is practically fully charged and if the car will not start, there is some other issue involved,

- 12.4-12.7V: both flooded and AGM batteries feature at least 50% SoC and should easily crank the engine. If the car will not start, there is probably some other issue involved with the car.

- 12.0-12.4V: both flooded and AGM batteries are at least at 25% SoC (State of Charge) and if they can't crank the engine, start the car using a car battery jump starter or using jump cables and another (functioning) car.

After the engine is started, drive the car for at least 30 minutes with as many devices as possible turned off. Longer drives will allow the alternator to recharge the battery almost completely, of course, but 30 minutes are generally considered as a minimum for a good alternator to recharge the battery enough so that the battery is able to crank the engine on its own, next time the user tries to do so.

If the battery dies again in the next few days, take the car to the workshop and let them check the causes of the dead battery.

- 12.0V or less: the battery is seriously discharged, with even AGM batteries being below 25% SoC. While it is possible to jump-start a car even with such discharged battery, in order to charge it, the alternator and the charging controller must work heavily to charge it - and this may cause damage to the alternator and/or charge controller.

Note: It is up to a user what to do in such situation and it is his/her own responsibility. Personally, the author of this article drove several different cars after being jump-started with the battery almost completely discharged. But, that was MY responsibility, not yours. Whatever You do, it is your own responsibility.

broken car

If the battery voltage is below 12V, it is a good practice to recharge the battery using an advanced, microprocessor-controlled, 8-15 Amps, AGM/Gel-Cell/Flooded battery charger and recharge the battery completely - actual Amps of the battery charger depends on the car's battery capacity.

Such battery chargers analyze the battery first and adjust the charging process according to the battery's condition. Also, if the battery requires desulphation, the battery charger will try to desulphate the battery plates and after the battery is fully charged, it will even try to equalize the cells.

Advanced battery chargers are also capable of detecting dead cells - dead-cell means that the battery is unusable and that it has to be replaced.

Note: some cars don't allow the batteries to be charged while being connected to the car due to possible issues with the onboard electronics. Also, some cars don't allow the users to disconnect the battery from the car - such activity may trigger a security system to block the car.

If You plan to charge the car battery, please, check the car's documentation regarding battery charging using a dedicated car battery charger.

How to Charge a Car Battery With Another Car

A discharged car battery can be recharged using another car's charging system, just be sure that the discharged battery in the first car is at most the size of the healthy battery in the second car.

So, place both cars next to each other and let the second car's engine run idle.

Connect both batteries with thick jump start cables - first, connect positive terminals and only then negative terminals.

jump start cables

Let the engine work for 5 minutes - the discharged battery will receive some charge (so-called "surface charge") that would be probably enough for one or two cranks without additional help.

However, the goal here is to fully or almost fully recharge the discharged battery.

After five minutes of charging, try to start the first car's engine - if it is successful, remove the jump start cables by removing the negative terminals cables first and only then cables that were connected to the positive terminals.

If the engine doesn't start, let the second car charge the first car's battery for five more minutes and then try to start the first car's engine. If it starts, remove the jump start cables.

If not, there might be some other, more serious issue, not just discharged battery - even when the battery has one cell dead (short-circuit cell, for example), the second car's battery and charging system should be able to start the first car's engine.

So, if You are unable to start the first car's engine even after 10 or so minutes of charging, with the second car's battery still being connected, remove the cables and tow the car into the repair shop to check what's wrong with the car.

Note: keeping the second car's electric system (a healthy one) connected to the electric system of the car with electric issues (first car), may cause issues or even damage to the healthy battery, alternator, voltage controller, or even electronics of the second car - rarely, but it is also possible. So, if You are going to connect two cars with jump-start cables (or by any other way), You are doing it on your own responsibility.

If the first car's engine successfully starts, and in most cases, it does start, drive the car for at least 30 more minutes to recharge the battery as much as possible.

Also, if the car has a Stop&Start system, be sure to turn it off until the battery is fully charged. During charging, if possible, turn off any multimedia device, lights (if legally allowed), and any other electric load that is not absolutely required.

And let the engine rev at higher RPMs than usually - it will charge the battery faster.

After recharging the battery for 30-40 minutes, the battery has enough charge to crank the engine easily.

But, one should try to crank the engine the next day - if the engine starts, the battery is probably good to go for some time more; if it doesn't, do yourself a favor, replace the battery in a certified car shop and let them check the car's electric system for any other possible issue - better safe than sorry.