Qualities of different types of battery chargers

A battery charger can be built using several different types of technique giving different end products. What should be chosen depends on area of use, type of batteries to be charged and power source used. The power source is most of the time mains supply but can also be gen sets, alternators and wind generators. Below is a comparison of properties of the main types of chargers for Lead-Acid batteries.

Transformer charger - unregulated
The unregulated transformer charger is the least complicated and cheapest charger on the market. The charging voltage varies with the input voltage and with the state of the batteries being charged. This means that the charging voltage sometimes becomes so low that no charging takes place and sometimes so high that the batteries will be overcharged, resulting in damaged batteries.

This type of charger should never be without supervision and only for temporary charging. They are mostly used for charging starter batteries that accidentally have been drained.

Transformer charger - regulated
The regulation of these chargers prevents overcharging. They can be connected and left without supervision for extended time. The regulation varies considerably between different brands. Regulator settings that never charge the battery fully are not uncommon. If the supply voltage drops too much, it can be difficult to achieve a fully charged battery.

Regulated transformer chargers are used for leisure batteries, traction batteries and stationary batteries with regulation adapted to the specific type of battery they are intended for.

Simple regulated primary switch-mode chargers
These chargers are designed with the size and weight advantage of the primary switch technique, and are thus light and compact. They will also have the advantage of not being affected by input power variations. In general they have a simple fixed charging characteristic (eg. IU) without the flexibility necessary to charge the batteries 100% full. A charging characteristic with time constants with no means of taking simultaneous consumption into account will make the charger switch off or switch over to trickle (maintenance) charge too soon.

Advanced primary switch-mode chargers
This type of charger is a fully electronic charger with free adjustable charging characteristic that will give a fast efficient and reliable charging, for every type of battery and every mode of usage. Overcharging does not exist and the charger will not be affected by variations in frequency or shape of the input current. Input voltage can also vary within a considerable range and still give correct output voltage and full output current. The output current has a very low ripple (AC component on DC) which means that they can also be used to charge solid electrolyte batteries (Gel-batteries).

Advanced switch-mode chargers have low weight and low volume in comparison to the charge. They are also quiet (no humming) and develop little heat due to high efficiency. This makes them suitable both as free standing and on-board chargers. These chargers are used for all types of batteries.

Battery system and size of charger

Battery capacity
The capacity of the battery is stated in Ah (Ampere hours) and varies with the amount of current (A) that is drawn from the battery. For leisure batteries the 20-hour capacity is mostly used, ie a battery with 80 Ah can during 20 hours give a constant current of 4 A before the voltage has dropped below 1,75 V/Cell. A higher current than 4 A, means that the battery will give less than 80 Ah, and a lower current that 4 A means that the same battery will give more than 80 Ah.

Dimensioning of batteries
The choice of battery capacity is determined by the demand and the time between recharging. The demand of the consumer is stated in Watts. Since the capacity of the batteries is stated in Ah, we have to recalculate the consumption and make into Ah according to the formulas below.

Example: In a 12 volt system there are light bulbs consuming a total of 60 Watts. They are used in total 4 hrs between each charging of the batteries. This means that the light bulbs consume 60/12=5 A (formula 3), which means that the capacity demand will be 5x4=20 Ah (formula 4). The capacity demand of all consumers is calculated and added to give the total demand.

If the total demand has been calculated as 100 Ah, you use a safety factor of 1,3 for Gel batteries and 1,7 for wet batteries. This corresponds to a choice of a 130 Ah Gel battery and a 170 Ah wet battery.

Choice of batter charger size
Most battery manufacturers recommend that their batteries should be charged using a charger that in A will give between 10 to 30% of the battery capacity in Ah. Because of this most regulated battery chargers are designed for battery sizes somewhere in that range. This means that for example a 15 A charger should be used for batteries with a capacity of between 50 and 150 Ah.

For some applications simultaneous consumption very often takes place during charging, by a refrigerator, lights or other electric equipment. This means that the consumers will use current that otherwise would have gone to the battery, and they will thus reduce the effective size of the charger. It is therefore important to consider this consumption when choosing battery charger size in order to get as good charging as possible.