General Charging Procedure

Common information for all types of chargers.

  1. Check the electrolyte-levels in all the cells. If these are below the tops of the separators, top up with distilled or deionised water to the tops of the separators. Do not fill to a higher level before charging, but adjust the levels after charging.
  2. If you are using a constant-current charger or a boost-charger, remove the vent-plugs or manifolds before charging. There is no need to remove the vent-plugs or manifolds if you are using a constant-potential or a ‘smart’ charger.
  3. Check that the charger is switched off.
  4. When fitting the charger to the battery, connect the positive lead to the positive terminal and the negative lead to the negative terminal.
  5. Switch on the charger. See below for the correct charging conditions depending on your type of charger.
  6. Stop charging if the battery begins to gas freely (some gassing is normal during the last stages of charging) or if the battery temperature rises above 50°C.
  7. Switch off the charger.
  8. It is good practice to wait for about 20 minutes for the gases to clear before removing the leads from the battery as some chargers remain ‘live’ and can cause a spark.
  9. Check the electrolyte-levels in all the cells and top up if necessary. See Section D.
  10. Refit vent-plugs or manifolds if these have been removed.
  11. Wash the battery with hot water and dry it.
  12. Note. Many customers severely underestimate the amount of time necessary to charge a flat battery. This results in customers returning batteries saying that they have charged the battery but that it is still not holding charge.

Types of Charger

There are many types of charger available; their working principles and the procedure for using these is given below.

Smart Chargers

The latest generation of chargers is able to check the battery condition, and to supply automatically a controlled charge that will charge the battery in the fastest time without damaging it and without overcharging it at the end of the charge. Some ‘smart’ chargers have a special setting for all-calcium batteries and will charge these from flat, which most other chargers are unable to do.

Charging Procedure with ‘Smart’ Chargers
A. Follow the manufacturer’s instructions.
B. These chargers should be able to charge over discharged (below 11.00V) batteries. Note that some have a special setting for all-calcium batteries.

Constant Current Chargers

These maintain a fixed, constant, pre-set current throughout the charging period irrespective of the battery on-charge voltage. Do not charge AGM batteries on a constant current charger.

Charging Procedure with Constant Current Chargers
A. Ideally, charge each battery on a separate charger unit. If this is not possible, charge batteries in series. We do not recommend charging batteries in parallel because it is not possible to control the amount of current passing through each battery. If batteries in different states-of-charge are being charged in series, each battery should be removed as soon as it is charged. (If you wait until the last battery is charged, some of the batteries will be overcharged).
B. Measure the open-circuit voltage of the battery. To obtain a stable voltage, the battery should not have been used or charged for a minimum of 3 hours before checking the voltage.
C. Charge the battery at the recommended charge rate. If you cannot set the recommended rate, extend or reduce the charging time on a pro rata basis. For example, if the recommendation is to charge the battery at 4.0A for 6 hours (24Ah = 4.0 x 6), charge the battery for 12 hours if you can only set the charger at 2.0A (24Ah = 2.0 x 12).
D. Charge the battery for the number of hours shown in the table below depending on the open-circuit voltage.
For example, if the battery has a voltage of 12.16V, charge it for 10 hours at the recommended charge rate.

Above 12.40 4
12.31 – 12.40 6
12.21 – 12.30 8
12.11 – 12.20 10
12.01 – 12.10 12
11.91 – 12.00 14
11.81 – 11.90 16
11.71 – 11.80 18
11.00 – 11.70 20

E. If you are charging a battery below 11.00V (over discharged) that has been in service, a specialised charger capable of providing a very high charging voltage may be necessary, and the recommended current may not be obtainable at first. In this case, monitor the current and adjust as necessary during the charge. If a battery has become over discharged, it will have lost both life and performance because of irreversible sulphation. Charging may reduce further its potential life.

Constant Potential Chargers

These maintain a fixed, constant, pre-set voltage throughout the charging period. The current cannot be set and will fall as the battery state-of-charge increases.

Charging Procedure with Constant Potential and Modified Constant Potential Chargers.

A. These chargers are normally designed to charge one battery at a time.
B. Stop charging when the battery is gassing freely and the battery-voltage shows no increase over a period of at least 2 hours.
C. Note. The majority of constant potential chargers are incapable of charging a severely over discharged (below 11.00V) battery in a realistic period of time. A minimum of 24 hours is normal. It might be impossible to charge an over discharged battery.

Modified Constant Potential Chargers

The majority of commercial chargers, particularly home-chargers, are of this type, and allow neither the voltage nor the current to be preset. Use the same procedure as for Constant Potential Chargers in the paragraph above.

Boost Chargers

These provide a very high initial current, and are used mainly to put some charge into a flat battery when it is needed urgently by the customer. The current falls as the battery state-of-charge increases, and the battery temperature is monitored to make sure it does not overheat.

Charging Procedure with Boost Chargers
A. Boost charging is not recommended except in exceptional circumstances e.g. a stranded customer, as this will reduce battery life, especially if a battery is boost-charged more than once.
B. Never boost-charge any battery that is below 11.00 Volts as it will be too sulphated to accept a charge; scrap the battery or charge normally.
C. Only use a boost-charger that limits the charging voltage to a maximum of 14.2 Volts and that has a temperature monitor.
D. Follow carefully the charger-manufacturer’s instructions.