Factors to consider when choosing a battery charger

Quality battery chargers today are microprocessor-controlled units designed to charge your batteries based on generated algorithms. Known as smart chargers, what this means is that they will collect information from your rechargeable batteries to determine the correct current and voltage needed to power it. Choosing the right charger will ensure that your battery functions efficiently and safely, and without negative impact to its service life.

Let us take a quick look at some of the things you will probably want to consider when purchasing a battery charger.

Battery chemistry

The first thing to keep in mind is to make sure that your charger is compatible with the chemistry of your battery. Mismatching a smart charger with the wrong battery category or chemistry will fail to charge your batteries and can potentially damage them. Reference your battery labels, packaging, or any documentation included with your battery to determine its category and chemistry first before considering a charger’s rated power and desired features. For example, our household battery charger, the Panasonic Eneloop Pro, which is specifically designed for AA and AAA Ni-MH batteries (and includes them), would not support the charging of AA lithium batteries.

More sophisticated chargers designed for lead-acid batteries, such as those you may have in your car or powersport applications, will usually work with sealed maintenance-free, flooded wet cell, AGM, and VRLA (valve-regulated lead acid batteries) battery models. If your battery is a valve-regulated gel cell, however, it is important to make sure the charger will support it. Not all lead-acid battery chargers will. Documentation and technical specifications for lead-acid and other batteries of larger form factors will often include charging requirements that can point you in the right direction.

Battery capacity

Whether your charger is able to supply enough power to charge your battery in your preferred amount of time is another important consideration, and one that can save you post-purchase disappointments. If you have a battery with a rating of 50 Ah, a 10-amp charger might take roughly 6 hours to charge it. If, on the other hand, your battery’s capacity is rated at 100 Ah, the same charger would take around 11 hours to fully charge it.

You can quickly estimate how long a charger will take to recharge your batteries by knowing just two pieces of information and performing a simple calculation. You will need to know:

1. The amperage rating of the battery charger. You can find it on the charger packaging or on its label; and
   
   
2. The capacity of your battery in ampere hours (Ah). This will be imprinted on the battery package or on the battery itself.
   
   

Your battery’s capacity is how much energy it can deliver over time for its voltage. So, where charging time is a significant factor, you can calculate the charge duration by dividing the battery’s capacity by the charger’s ampere rating.  Then optionally, you can choose to add 10 percent to accommodate a topping charge. Here is what it looks like:

Using the example of the 50 Ah battery, you can easily see how we determined the length of time to charge your battery with a 10-amp charger:

We rounded up the result to 6 hours for good measure and to offset potential disappointment.

To charge this battery in half the time would require a 20-amp charger.  As a rule of thumb, however, you should use a battery charger that is a minimum of 10 percent of your battery’s ampere hour rating, and within 20 percent of its maximum capacity to avoid overcharging. For the 50 Ah battery in the example, you should aim for battery charger that is rated at 5 amps minimum and 10 amps maximum; and for the 100 Ah battery example, choose a charger that is in the range of 10 to 20 amps.

Voltage compatibility

Input voltage: Determining your grid voltage compatibility is the first and more obvious voltage consideration, especially if you are travelling. Mismatching your device voltage to the supplied power grid voltage is an easy way to damage your battery, charger, or any device, and more importantly, it can be hazardous to yourself and others. Many modern battery chargers are available with multiple input voltages that will allow you to operate your battery charger from 110VAC outlets, such as those here in the United States, the standard 220VAC outlet common to European countries, and the 100VAC power outlets that you would find in Japan.

Output voltage: When it comes to the output voltage of the battery charger, it should not exceed the voltage of your battery. For example, it may be fine to charge two 6-volt batteries in a 12-volt charger, but not to use the same 12-volt charger to individually charge a 6-volt battery. Excessive current can overheat your battery and reduce its service life and capacity. Some battery charger models have selectable current outputs. This function allows you to use a larger charger on a smaller battery by reducing the output current to a safe rating. 

A general consensus that is widely supported by studies is that charging your battery to a slightly lower voltage can substantially increase the number of your battery’s charge cycles. For example, regularly charging to 90 percent of the battery voltage can add as much as 50 percent more cycles. This can be particularly helpful with rechargeable lithium batteries. Undercharging Li-ion batteries on a regular basis, and then occasionally charging them to full capacity is widely acknowledged to improve their service life. Battery chargers featuring the ability to manually enter the desired capacity or choosing preset capacity options are available; but with these more sophisticated features, prepare to spend a bit more.  

Battery banks

Many battery chargers provide multiple battery banks or bays to simultaneously charge multiple batteries. Some of these multi-bank chargers are outfitted with a common negative contact. Others offer isolated charging channels for each battery and allow you to independently configure each channel to your needs.  When each bay is able to recognize and respond to its own isolated battery, the benefit to you is that you can independently charge up batteries of varied voltages, sizes, and charge statuses all at the same time. This saves you time and ensures a complete charge.

Other battery charger and charging considerations

• You should avoid battery chargers that do not indicate the end of charge. Excess output current can quickly ruin your battery.
  
  
• You should also avoid the use of trickle chargers that do not have an automated float mode or current control circuitry that guarantee not to overcharge your battery.
  
  
• Batteries should not be left unmonitored or overnight in an unregulated, automatic charger without a shut-off feature.
  
  
• Keep in mind that slower charging favorably impacts battery service life. Fast charging should be considered if it is critical to put your battery back in service quickly.
  
  
• For heavy-duty chargers to replenish power to lead-acid or industrial-grade batteries, you will need to consider other factors before investing in a high-grade smart charger. For example, you might need to think about its resistance to water if it will frequently be used for outdoor or marine applications. You might also want to think about a charger that provides input power. Some battery chargers can supply needed power, which could be potentially relevant to you in the context of a recreational vehicle or other application.

Learn more about rechargeable batteries and battery chargers with these great articles:

Practical guide to charging different types of batteries

Tips for repairing or upgrading battery packs

Smart battery technology and usage tips