12.0 volts or below - At 12.0 volts your battery is considered to be fully discharged or 'flat' and should be recharged as soon as possible. The lifespan of your battery will be severely affected if it remains within this voltage range for extended periods of time.
Charging Li batteries at less than 4.2 V/cell is possible but also not recommended. While other battery chemistries do not charge at low voltages, Li batteries will charge but will not reach full charge. An advantage of charging at lower voltages is that cycle life goes up substantially but with much lower capacity.
UNDERCHARGING A LEAD ACID BATTERY
If too low a charge voltage is applied, the current flow will essentially stop before the battery is fully charged. This allows some of the lead sulfate to remain on the electrodes, which will eventually reduce battery capacity.
So, how low are we talking? A typical 12-volt auto battery will have around 12.6 volts when fully charged. It only needs to drop down to around 10.5 volts to be considered fully discharged. If it drops down below that, the battery will become damaged due to excessive sulfation.
A fully charged 12 volt battery should read at 12.6 volts on the multimeter. If the reading is lower than this, you'll want to charge your battery with either a battery charger or by taking your car for a drive.
Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less.
The recommended float voltage of most flooded lead acid batteries is 2.25V to 2.27V/cell. Large stationary batteries at 25°C (77°F) typically float at 2.25V/cell. Manufacturers recommend lowering the float charge when the ambient temperature rises above 29°C (85°F).
A healthy 12 volt battery should maintain a voltage range from 9.6 - 10.5 volts under the load for a good 30 seconds straight.
At 12.4 volts the battery is considered 75% charged and can still ignite your engine. When the battery voltage lowers to 12.2 volts without the engine running, it is considered a bad battery that may start your engine but is no longer capable of holding a charge.
This voltage range means the battery is in good condition for starting the vehicle. If the measured reading is less than 12.2 volts, the battery's resting voltage is weak, which means it most likely needs to be charged or replaced.
Is it better to charge a battery at 2 amps or 10 amps? If you want a faster battery charge rate, go with the 10 amps. Low charging current at 2 amps can be unreliable, but it won't hurt if you have two chargers.
To charge a battery, a charger or your car's alternator needs to raise the voltage across the battery posts. Normal charging voltage needs to remain below about 14.4 volts, which is the level at which gassing of hydrogen and oxygen starts.
Over Discharge /Undervoltage Operation
Similarly, operating at low voltage levels also damages the battery.
To pass a load test, the battery must maintain 9.6 volts at 15 seconds when tested at one-half the CCA rating and 70°F (or above).
Load Testing
As long as battery voltage remains above 9.6 volts, the battery is considered “good.” But if it drops below 9.6 volts by the end of the test, the battery may be “bad,” or the battery may have to be recharged and retested if it was not fully charged prior to the test.
Typically, the voltage of a fully charged, 12-volt deep cycle battery is between 12.8V and 13V. But a dead battery's voltage is below 10 volts.
Yes, a trickle charger can fully charge a battery; it will just take a very long time. Because trickle chargers only emit between 1-3 amps, you can expect to wait days for a fully charged battery. For example, a 1-amp trickle charger will take a full 100 hours to completely charge a dead Battle Born 100 ah Battery.
No, you can't charge a car battery while idling. At best, it'll charge the battery a few amps, but not nearly the hundreds of amps it took to start the engine. After a few days of starting and then idling the engine, you'll steadily deplete the battery instead of charge it.
Usually, the low voltage cutoff (LVC) for a 12V battery is 10.5V. It means the battery will stop running the load when it comes down to 10.5 V.
Attach a battery trickle charger or a computerized smart charger to your old lead acid battery, and allow charging continuously for about a week to 10 days. The extremely slow charging rates dissolve the de-sulphation that kills the battery, and revives it back to being able to hold a usable charge.
Having said that, the absolute lowest level a lead acid battery can be discharged to, UNDER LOAD, is 10.8 volts but this is not recommended (more on this below).
Connect the multimeter to the positive and negative battery terminals. If you don't have a voltage of around 12.6 volts, you may have a bad battery. Now start the car, and look for a revised voltage over 10. If your voltage drops below 5 when the car is running, it is bad and should be replaced right away.
A fully charged car battery should have a voltage of around 12.6 volts. When the battery voltage drops below 12 volts, there is a possibility that it won't have enough power to start the car. It is recommended to have a battery with a voltage of at least 12.4 volts to ensure a reliable start.