Supplying batteries since 1952

About Chargers

A battery’s performance is only as good as its charger. Over fifty per cent of battery problems are caused not by the battery itself, but by wrong or insufficient charging. A proper charging regime is based on battery technology, temperature, state of charge and, equally important, the application. Expand the headers to learn more.


Classic 50 Hz chargers are also referred to as linear or ferroresonant chargers. A transformer and rectifier transform the input AC voltage into a lower voltage DC. Voltage fluctuations at the primary (input) side are passed on to the secondary (output) side, causing a ripple that is detrimental to any VRLA battery.

The efficiency of a 50 Hz charger is about 75%. That is, 25% of the energy used is lost as it is converted into heat.

The input mains voltage is rectified and then converted into a high operating frequency (50kHz) by a high-frequency chopping circuit. This high-frequency input is then passed through a transformer and rectified to the correct DC voltage with only a negligible ripple. Because of this high frequency, the transformation can be done with a small ferrite transformer. This makes HF chargers suitable for all types of lead acid batteries.

High-frequency chargers have an efficiency of over 90%, which makes them very cost effective. Because a HF charger does not have a heavy transformer, its weight is generally a quarter to one fifth of that of a similar conventional charger. This allows for wall mounting, thus increasing floor space and safety.

A charging voltage of 13.8V is sufficient to charge a battery without causing damage by overcharging. This is exactly what simple (and therefore cheap) chargers do. Unfortunately the battery will not attain more than 75% SOC (state of charge).

Microprocessor-controlled chargers control both the current and the voltage they provide when charging a battery. This is done in several steps, taking into account battery condition and often even ambient temperature, thus allowing maximum charge without causing any damage. Temperature-controlled charging is especially important for gel batteries.

All batteries suffer from self-discharge. Owners of caravans and boats are aware of this problem, as are those who rely on vehicles or applications that are used infrequently.

Trickle charging, or float charging, means charging a battery at the same rate as its self-discharging rate, thus maintaining a full capacity battery. The difference between a float charger and a trickle charger is that the float charger has circuitry to prevent overcharging. It senses when the battery voltage is at the appropriate float level and temporarily ceases charging; it maintains the charge current at zero or a very minimal level until it senses that the battery output voltage has fallen, and then resumes charging. It is important to note that the appropriate float voltage depends on the type of battery and the ambient temperature.

Traction batteries are an expensive part of any electrically driven vehicle. Reliable and efficient charging equipment correctly matched to battery capacity and required recharge times are a basic necessity for operational effectiveness and optimized battery life and performance. Two charging technologies can be used to achieve this goal: WoWa chargers and HF chargers.

Conventional WoWa chargers offer a reliable and robust way to charge conventional traction batteries. They are cost-effective charging systems for traction applications where a 12- to 14-hour recharge time is appropriate. Their robustness comes at a cost, however: efficiency is only 75%.

High-frequency chargers use switch mode technology, allowing for a better power factor, increased efficiency and low DC ripple. Reduced energy consumption (15%) and extended battery life (15%) are the result.

Chopper chargers use ferroresonant transformers combined with switching technology at the secondary end. This allows for very high charging rates: a traction battery can be charged in as little as one hour – albeit reducing battery life cycle by 30%.

This solution enables multi-shift operations without spare batteries and battery changing. In terms of safety time and total cost of ownership, the combination of an expensive chopper charger plus one battery pack versus one HF charger and two battery packs is an alternative that should be seriously considered.

The charging voltage of a battery charger is, of course, higher than the nominal voltage of the battery it is designed to charge. It is therefore not advisable to use a charger as a power supply. However, some chargers can be used to provide a stabilized power supply.

VMF battery charger models Medium, Large and XL allow such use.