Car Battery Voltage and State of Charge – Charging Car Battery

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Car Battery Charging



Car Battery Voltage and State of Charge – Charging Car Battery

In this post I would like to share some of my experience with the car battery. In a number of occasions I had my car battery completely flat so I took it out and charged it using standard 3.8A per hour charger.

Even though I took reasonable care to prepare this information your case might be different. Please note all the information is provided is based on my experience and information I picked up from various sources, I am not responsible for any damage or injury caused. Please handle this information with care and check a few other sources to give you well rounded picture.

How to disconnect or connect battery from to the car?

One of the most important things to take into account that lead-acid type car battery is dangerous. Make sure you wear gloves and eye protection. Lead-acid type car battery contains quite some acid in it so if it sprays into you or explodes you could have a permanent injury for the rest of your life.

DO NOT charge the battery that has been frozen because it will explode. Thaw it first.

First and foremost please check your vehicle manual for correct way dealing with the battery as with some cars there might be a specific way of working with the battery and incorrect use may damage the cars electronics etc.

Normal rule is that when taking out the car battery from your car make sure you disconnect (usually black cable) – first and when connecting back connect it – last.

So when disconnecting battery from your car: – (usually black) is first + (usually red) is last, when connecting back: + (usually red) is first – (usually black) is last.

How to find out how much battery charge is left?

You can use one of the chargers which will indicate state of charge. Or you can use voltmeter.

For a 12V car battery any voltage below 12V will usually mean it is completely discharged and if it drops to a certain point you may not be able to charge it again. It is recommended to recharge it once charge level drops below 75%.

12 Volt Car Battery – > @ Ambient Temperature of 70°F or 21.1°C

Charge Levels:

12.643V – 100%
12.443V – 75%
12.233V – 50%
12.053V – 25%
11.883V – 0%

Voltage/Charge level will drop depending on the ambient temperature. Usually by about 0.01V per 10°F or about 5.5°C

How long would it take to charge car battery?

It depends on the capacity of the battery and the output of the charger.

For example a flat completely discharged 70Ah rated car battery with 3.8A output charger would take 18.4 hours to fully charge.

So you divide Ah capacity of the battery by Amper per hour output of the charger. This would give you the rough estimates.

70Ah/3.8 = 18.4

What are the stages of car battery charging? (12V lead-acid type Car Battery)

A lot of chargers, however, will decrease the output based on how much battery is charged and from what I understand that the way it is supposed to be.

For a 12V Car Battery, based on the charger that I have there are 7 stages of charging at what stage it is will depend how discharged your battery is:

1. Reverse polarity protection and diagnosis
2. Recharge/Desulphation (Battery is completely dead) (7.5-10.5V @ 0.8Ah)
3. Start of charging with a high current strength (Battery is discharged)(10.5-12.8V @ 3.8Ah) (Battery Charge: 0% or above, depends on voltage)
4. Charging up to 80% (14.1V @ 3.0Ah)
5. Absorption – Charging up to 100% (14.4V @ 0.8Ah)
6. Trickle charging and monitoring (14.4V @ 0.1Ah)
7. Demand recharging (12.8V @ 0.8Ah)

I hear crackling or sparkling sound noises when charging a car battery what is going on?

Some gases including hydrogen can be released during any stage of the charge. Make sure that you are charging the battery in well ventilated room/area. Hydrogen gas is extremely explosive and any internal/external spark can cause an explosion.

It is very likely that battery is being overcharged and hydrogen gas is being released as a result of that.

If this happens I would personally recommend disconnecting the battery from a charger and leave it for about 12 hours and then check the state of charge to confirm whether it is fully charged or not.

Last Update: 5 April 2015

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