Battery voltage and charging
Graham Reynolds | 16th Aug 2023 | 5 minutes to read
Car battery voltage is something that seems simple to assess, but the precise voltage may be different than you’d expect, and proper measurement needs to be taken at the right time and with the right tools to be accurate.
Learning how to correctly assess your battery’s voltage is a necessary part of properly charging it and different types of batteries will have specific nuances to consider.
With help from our friends at Mechanic.com.au, we’ll look at how different car batteries hold charge, as well as assessing voltage and charging methods:
How Many Volts Is a Car Battery?
Many people believe car batteries hold 12 volts. However, if a battery's open-circuit voltage measures only 12 volts, that battery is significantly discharged.
In practice, a car battery has six cells, each of which has a typical resting voltage of 2.1 volts. Therefore, since the cells are connected in series, the total rest voltage of a fully charged battery should be at least 12.6 to 12.8 volts.
The above resting voltage value is known as the open-circuit voltage. However, this value could be misleading or dead wrong if the measurement is taken immediately after the vehicle is switched off. Thus, to get a reasonably accurate test result, flooded lead-acid batteries must rest for at least 8 hours. Preferably, though, the battery must rest for 24 hours before you take a voltage reading with a multimeter.
This is because lead-acid car batteries can “recover” after use, meaning that a battery’s resting voltage can increase immediately after use. For example, the voltage of a lead-acid car battery that contains calcium can rise by between 5% and 8% after the vehicle is switched off.
Such momentary battery voltage increases can be cancelled out by briefly discharging the battery. However, there is no accurate way of telling just how much the battery should be discharged to remove the effect of the momentary battery voltage increase.
Things are a little different with AGM (Absorbent Glass Mat) batteries. The minimum rest voltage of an AGM battery is 12.8 volts. If this voltage drops down to 12.6 volts, the battery is only 75% charged. If it drops down to 12.3 volts, the battery is only 50% charged.
Note that when an AGM battery’s resting voltage is at or below 11.80 volts, the battery is effectively flat.
How To Charge a Lead-Acid Car Battery
The charging of LSI batteries has become a rather exact science because different types of LSI batteries require different charging profiles. In simple terms, a charging profile consists of three phases, each of which is described below.
Constant Current Charge
In this phase, a controlled current replaces almost all the car battery's lost capacity, which can take between 5 hours and 8 hours. This time is determined by the battery's state of charge, ambient temperature, the temperature of the battery, and the strength of the charging current. Most modern car battery chargers will charge a car battery only up to about 70% of its capacity.
Topping Off Charge
This phase can take an additional 5 to 7 hours and uses a reduced current to charge the battery up to its full capacity.
Float Charge
Most modern battery chargers can supply a current that is smaller than 1 ampere to maintain the battery’s full charge for indefinite periods. However, the float current is not supplied all the time. Any high-quality battery charger detects when the battery's nominal voltage drops for any reason. The battery charger will then apply a float current as required to restore and maintain the battery’s full capacity.
The charging time for a lead-acid battery can be reduced to about 8 hours by using a higher current during the first phase of charging. However, doing this will reduce the battery’s useful service life.
So, what does this mean for the average car owner who needs to charge their car battery? It means you should only use a high-quality car battery charger that is recommended for your specific type of LSI battery.
How Long Should I Charge a Car Battery?
Different batteries require different charging times. The type of battery, its age, states of charge and health; the ambient temperature, and the capabilities of the charging equipment all affect charging times.
The details of how each variable affects charging times are highly technical, but the fortunate thing is that most modern battery chargers are highly sophisticated. Modern battery chargers detect not only a battery’s overall state of health, but also the type of car battery installed and automatically apply an appropriate charging strategy.
If you use the correct or recommended car battery charger, the table below provides some rough estimates on how long it should take to charge a car battery.
| Open Circuit Voltage | Estimated Charging Time in Hours |
|---|---|
| Higher than 12.4 volts | 4 hours |
| From 12.31 volts to 12.40 volts | 6 hours |
| From 12.21 volts to 12.30 volts | 8 hours |
| From 12.11 volts to 12.20 volts | 10 hours |
| From 12.01volts to 12.10 volts | 12 hours |
| From 11.91 volts to 12.00 volts | 14 hours |
| From 11.81 volts to 11.90 volts | 16 hours |
| From 11.71 volts to 11.80 volts | 18 hours |
| From 11.00 volts to 11.70 volts | 20 hours |
Note that this table is based on charging currents that do not exceed 10% of a battery's capacity. For instance, a 60-ampere battery requires 6-ampere charging current, while an 80-ampere requires an 8-ampere current.
Repco Batteries & Battery Accessories
Whether you need a multimeter for testing your battery or a full replacement battery for a failed unit, Repco has you covered. Use our online Rego Search to check the fitment for your vehicle or head in-store to get help from our friendly staff.
