Choosing an Automotive Battery Charger

RS Components stocks 133 different chargers for lead-acid batteries, with the most expensive being nearly 10 times the price of the least expensive. How do you choose between the chargers, making sure the chosen device is up to the job, while providing value for money?


Sizing your charger

Firstly, you’ll want to choose a charger that is capable of charging the battery you’ve got; not all the 133 lead acid battery chargers are suitable for every size of lead acid battery. There are many different sizes used today, from the smallest batteries used in toys, lawnmowers and wheelchairs, to a 200Ah monster used to start agricultural machinery. At the lower end, a quad bike might use a 3Ah battery versus 7Ah for a bigger motorcycle. A typical car battery is 40 or 50Ah and batteries for trucks, lorries and buses are usually in the region of 150Ah.

The majority of lead-acid battery chargers have a three-stage charging profile. The first stage charges the battery with a constant current to 80-90% of capacity. When the battery voltage reaches its maximum, the second stage kicks in, using a timer to fill the rest of the battery’s capacity. The third stage reduces the charge to levels that render the battery safe from overcharge (float mode), meaning it can be left plugged into the charger indefinitely without compromising safety.

Chargers with two stages don’t have the float mode, so shouldn’t be left to charge for more than 48 hours. Chargers with only one stage are effectively only doing the float stage. They are limited to use in trickle charging – charging lead acid batteries in storage to compensate for self-discharge and keep them full. They are not suitable for charging flat batteries.


Figure 1: Three-stage charge cycle for Mascot lead-acid battery chargers.


A lead-acid battery charger may be sized appropriately by comparing the charging current with the battery’s capacity. A good rule of thumb is that the charging current at the start of the charging period should not be more than a fifth of the battery capacity, that is, 10A for a 50Ah battery. Some battery manufacturers may allow higher charging current, so check the battery’s documentation. Selecting the charge current appropriate for your battery is a good first step to narrowing down the choices to a manageable level.

Another selection criterion is the output voltage of the charger, which should match the battery’s voltage. For the majority of automotive applications, this is 12V, but for applications outside regular vehicles it could be 6 or 24V.


Typical sizes

For example, at the small end of the range, the ALSC series from Ansmann can charge 2.4 to 24Ah batteries with an output voltage from 2 to 24V. The right voltage is automatically selected when the charger is connected, and charge current is up to 900mA depending on voltage. It can also be used successfully as a trickle charger for seasonal equipment like lawnmowers, which are left unused in the winter.

Figure 2: Ansmann ALSC series chargers can charge batteries up to 24Ah
with a variety of different voltages.


A battery charger for a typical 12V, 40Ah car battery might be this charger from RS, whose charging current is between 2 and 50A for flexibility. Like most chargers, it comes with crocodile clips for attaching to the battery terminals.

Figure 3: This RS lead-acid battery charger is suitable for car batteries.
 Charging current is between 2 and 50A.


At the heavy-duty end of the scale is this dual-channel charger from Mascot, which can charge one 12V battery at 50A or two at 25A. This 4kg charger is designed to be permanently installed in an industrial environment. It accepts European mains voltages only.

Figure 4: This Mascot heavy-duty charger offers charge currents up
to 40A for very large battery installations.


Selection criteria

Once you are sure that you have narrowed down the search to chargers that are capable of charging your particular battery, there are a few other attributes to consider.

An important one is the environment the charger will be used in. For a start, which country will it be in? Most lead-acid battery chargers feature universal input; that is, they accept all the major mains voltages used worldwide, but not all do, so it’s worth checking. If the charger will be plugged directly into the mains, you’ll need to select whether you’re using a UK 3-pin or European 2-pin plug, for example. At the output, most smaller chargers come with crocodile clips, but you might require some other terminal type like Fast-On connectors.

Form factor may be important. Will the charger sit on a lab bench? Will it need to be wall- or panel-mounted? Does the charger have a heat sink or fan in its casing which needs to be exposed to free air flow? Is the size or weight of the charger important and is this charger to be kept at home, or should it be portable, and how much space is available for storing it?

As a final point of comparison, most manufacturers offer a warranty, but the warranty’s length might prove to be the deciding factor between two battery chargers whose specs are otherwise identical.