How to Charge Your Battery

Battery Charging

The purpose of recharging a battery is to restore the full operational condition that the battery is designed to perform.

The recharging process restores the original chemical potential the correct and electrical capacity of the battery. Recharging is the discharging reverse process. Therefore, the more discharged the battery is the more charge is required to achieve full charge state.

Single battery with separate charger


To properly recharge one or more batteries, always follow charger manufacturer directions and safety precautions.

Optimum battery life will be obtained if a “GREEN” hydrometer condition can be maintained, and batteries should not be left in a deeply discharged state. Batteries should be recharged as soon as possible once a “WHITE” hydrometer is observed.
Once the state-of-charge has reached 100% charging should only be continued for long periods at a reduced rate (on-charge voltage of 13.5/13.8 volts) to prevent long term electrolyte loss.

Alternator voltage regulator settings should be in the order of 13.8 – 14.2 volts, the higher setting being preferred. On applications with repetitive deep cycling, a charging voltage of at least 14.8 volts is recommended until full charge is reached however the maximum charge voltage should not exceed 15.8 volts.

Chargers with charge rates up to 50 amperes are generally satisfactory. If the battery becomes hot to touch, or starts to spew electrolyte, charging should be reduced or stopped to allow the battery to cool down before continuing charging at a reduced rate.

Note: Slow charging rates are preferred to fast boost charging.


How to Charge a Flat Battery

A very flat or completely discharged battery may initially only accept a very low charge current (milliamps) which may not be detectable on the charger ammeter.If the open circuit voltage is below 11 volts it may be necessary to override any reverse polarity protection on the charger.

The time required for a battery to accept measurable charge current may be as follows: as a rule of thumb.

Charging Voltage Hours
16.0 V or more up to 4 hours – check every half hour
14.0 V – 15.9 V up to 8 hours – check every half hour
13.9 V or less up to 16 hours – check every half hour


There are two standard methods to recharge two or more batteries with the same charger. The Parallel Charging Connection and the Series Charging Connection.

To charge batteries in parallel:

  1. Connect all the batteries to a busbar, using suitable clamps and adapters.
  2. Connect a voltmeter and the charger to the busbar.
  3. Connect the charger to a power source.
  4. Set the charger to the highest setting.
  5. Adjust the charger so the measured voltage does not exceed 16 volts.
  6. Monitor the batteries hourly for gassing, spewing and temperature warming up. If any of these conditions occur, remove any battery with this condition and test it.
  7. Gently shake the batteries hourly, check the hydrometer for a green dot. When a steady green dot appears, the battery is fully charged.
  8. Turn the charger off.
  9. Disconnect all batteries with a green dot in the hydrometer from the bus bar and load test them.
  10. Continue the recharging process on all remaining batteries with a dark dot until fully charged.

Note: Parallel charging is the preferred charging method


To Charge Batteries in Series

  1. Connect all the batteries, negative terminal to positive terminal, in series, using the required number of booster cables.
  2. Connect the charger suitably rated for series operation (see below) to the remaining positive terminal on the first battery and the remaining negative terminal on the last battery to complete the circuit.
  3. Connect the charger to a power source.
  4. Set the charger to maintain a charging rate of five to ten amps.
  5. Monitor each battery hourly for spewing, gassing and temperature warming up. If any these conditions occur, remove the battery with this condition and test it.
  6. Gently shake the batteries hourly, check the hydrometer for a green dot. When a steady green dot appears a battery is fully charged.
  7. Turn off the charger and disconnect any recharged battery. Remove it from the series and load test it.
  8. Reconnect remaining batteries in series adjust charger voltage as required for remaining batteries and continue charging.

For 2 x 12 volt batteries in series: Charger rated 24 volts
For 3 x 12 volt batteries in series: Charger rated 36 volts
For 4 x 12 volt batteries in series: Charger rated 48 volts


Charging for too long periods can cause excessive gassing. Charging procedures must always be conducted in a well ventilated area.

Charging of batteries in series is seldom used in most repair facilities because it is a multi-battery procedure and is, therefore, not recommended. When removing batteries form series charging, re-adjust the charger to maintain proper charging current rate and voltage output for the remaining batteries.

In a vehicle the charging process follows the same electrochemical behavior but there are some differences: as compared to charging with a battery charger.
The optimal charging voltage of the vehicle voltage regulator is rated between 13,80 to 14,20 volts. If the regulator is set too high, water will be subject to electrolysis. This lowers the electrolyte level over time, and in a maintenance free battery this creates an irreversible damage. If the regulated voltage is set too low, the battery will not be charged properly, shortening the life span of the battery and causing engine starting problems and poor electrical performance.


Conditions causing difficulties in charging a Battery

Short Circuit – Positive and negative plates make contact. A defective porous separator or vibration damage is the usual conditions for the contact. This will cause the battery to self discharge creating high resistance sites or major internal damage if the short circuit is extensive.

Open Circuit – An open circuit is caused by a disconnected cell or post. No current can pass through the battery and there is no voltage at the terminals. The battery does not charge or accept testing. The source of open circuit could cause internal sparks that would result in an explosion in the battery.

Dead Worn Out Cell – The loss of active material from the positive plates creates this condition. The active material lost from the plates fills the sediment chamber or the bottom of the envelope separator and forms a possible connection or short at the bottom of the plates.

Sulphated Battery – A battery that has remained in a discharged state for an extended period of time will cause the lead sulphate present in the positive plate to crystallize and resist a charging current required to that would drive the sulphate ions back into the acid solution.


Temperature Influence vs Battery Power Ability

Of all enviromental factors, the temperature has the greatest effect on battery charge and discharge behavior. This has to do with the temperature-dependent electrochemical reactions occurring at the electrode/electrolyte interface, which may be considered the heart of the battery.

If the temperature decreases, the rate of electrode reactions decreases too. Assume the battery voltage remains constant, the discharging current drops and thus the power output of the battery. The opposite effect occurs if the temperature raises the power output of the battery increases.

Extreme cold or hot temperatures affect battery life when combined with the adverse effects of extended storage or connected to parasitic loads resulting in:

  • Reduced performance – High self-discharge rate – Irreversible damage

In regular operation conditions the cold temperature affects the battery greatly in the ability to provide power and the electrical power required to crank the engine.

The ideal operating temperature of a battery is 27oC. At this temperature, a fully charged battery delivers a full cranking power and the starting system requires about the same amount of cranking power to start the engine.

However, when the temperature drops, the battery cranking power decreases while the engine starting system requires an increased energy to crank the engine.

This is due two factors:

  • The low temperature decreases the electrochemical ionic interaction in the battery.
  • The low temperature increases the oil viscosity in the engine, thus the greater energy to start the engine.



Engine Cranking Requirements

To start a 4-cylinder gasoline engine, you will need approximately 300-500 CCA; 6-cylinder gasoline engine, 500-700 CCA; 8-cylinder gasoline engine, 750-850 CCA; 3-cylinder diesel engine, 600-700 CCA; 4-cylinder diesel engine, 700-800 CCA; and 8-cylinder diesel engine, 800-1200 CCA.

In hot climates, buying batteries with double or triple the cranking amps that exceed your starting requirement can be a WASTE of money. However, in colder climates the higher CCA rating the better, due to increased power required to crank a sluggish engine and the inefficiency of a cold battery.

As batteries age, they are also less capable of producing CCAs.

According to the Battery Council International, diesel engines require 220% to 300% more current than their gasoline counterparts; winter starting requires 140% to 170% more current than the summer.

Comments are closed.