Lead acid batteries
The majority of classic motorcycle batteries are of the conventional lead acid variety. Whilst we think of these as being either 6 or 12v, this is a slight misnomer as the actual voltages are rarely these values if the battery is in good health.
The primary battery voltage is dictated by the chemistry of the lead acid cell. Fully charged and in peak condition, each cell has a voltage across it’s terminals of 2.1v. Motorbike batteries are made up from a combination of such cells connected in series to give the required voltage.
A ‘6v’ battery is comprised of three cells giving a total voltage across it’s terminals of 3 x 2.1v = 6.3 volts. Likewise a ’12v’ battery is made up of six individual cells giving a total of 6 x 2.1v = 12.6 volts.
These voltages are not however fixed and will vary both with the state of charge (or discharge) of the battery, and also depending upon the electrical load placed upon it. The more the battery is used and loses its charge, the lower it’s voltage gradually becomes. Similarly, when an electrical load is connected to the battery this also drags the voltage down.
State of charge
It is therefore possible to determine the approximate state of charge of a motorcycles battery by measuring the voltage across it’s terminals. This is how ‘intelligent’ battery chargers work by constantly measuring the voltage and stopping charging once the desired level has been reached.
The table below shows the approximate voltages that will be measured across the terminals of a lead acid cell for various states of charge. Also shown are the total voltages for 3 cell (6v) and 6 cell (12v) batteries as will be fitted to most classic motorcycles.
Note that the voltages shown are open circuit voltages which means that there is no electrical load placed upon the battery. These readings are therefore taken with all lights etc turned off, and the engine not running.
|State of charge
The voltage applied to a battery is also very important when it comes to maintaining the condition and chemistry of the lead acid cells and ensuring peak capacity. The table below shows the correct charging voltage ranges for individual cells as well as 6 and 12 volt batteries for various different modes of charging.
|State of charging
|Minimum for charging
|2.25 – 2.27v
|6.75 – 6.81v
|13.50 – 13.62v
|2.30 – 2.35v
|6.90 – 7.05v
|13.80 – 14.10v
(risk of gassing)
|2.40 – 2.45v
|7.20 – 7.35v
|14.40 – 14.70v
Fast charging is not recommended for daily on-bike charging as it results in gassing and requires careful monitoring and possibly topping-up of the battery. Some intelligent battery chargers may employ such high charge rates for part of their charging program as this can help to maintain correct battery chemistry. However it is not recommended for normal charging from the bikes on-board electrical system.
The normal charging voltage range is how the majority of battery chargers and a classic bikes on-board charging system will feed charge into the battery. The minimum charging voltage for a lead acid cell is 2.15v; a little more than the cells own voltage of 2.10v.
Lower charging voltages are useful for trickle (maintenance) charging whereby battery condition may be kept topped up whilst constantly connected to an appropriate mains charger when not used for long periods. In this way the battery will always be fully charged and ready for use.
Selecting a replacement battery
There are a number of different types of battery available for use on a classic motorcycle ranging from the traditional ‘wet’ lead-acid unit, to sealed units and on to modern maintenance-free ‘valve-regulated lead-acid’ (VRLA) batteries such as ‘absorbed glass mat’ (AGM) and gel types. There are also the ‘Cyclon’ cells which are also proving very popular.
With so many different types available it can be difficult to select which one is the most appropriate for your classic motorbike, but hopefully my ‘Selecting a replacement battery’ article can offer some useful guidance.[sc:disclaimer]