All About Chargers

Like the "All About Batteries" page, this isn't really going to tell you everything there is to know about all the different chargers available, rather it will focus on the types of battery chargers we come across in normal everyday life. Hopefully it will help you understand the differences between the types of chargers available and help you determine which will best suit your needs.

There are basically 3 types of charger on the market, the cheapest are often called standard or trickle chargers, next are the timer controlled fast chargers and then there are the intelligent or negative delta V (-dV) chargers.

Rough Rule Of Thumb Charge Time Calculation

You can get a rough ideal of the length of time it will take to charge a battery by using a relatively simple formula designed around the value "C" which represents the capacity of the battery to be charged. The basic premise is that the energy you need to put into a battery for standard chargers is 1.4 x C (the capacity of the battery) and 1.2 x C for fast chargers. So this can be shown as:

 Battery capacity (C) = 2200mAh

1.4 x C = 3080mA and 1.2 x C = 2640mA

This represents the energy we need to put into a battery to charge it, so if our Standard Charger has a charge current of 120mA and our fast charger has a charge current of 700mA this gives us:

Standard Charger - 3080 / 120 = 25.6 Hours

Fast Charger - 2640 / 700 = 3.77 Hours

Remember, this is only a rough guide, but it should give you a good idea of how to calculate charge times.

It is also considered reasonably safe and not too damaging to continually charge your batteries at up to 0.1 x C, though I would recommend keeping this to no more than about 0.05 x C if it is for any extended period of time. So for the 2200mAh battery above, that's about 110mA.

Standard Chargers

Standard chargers work by supplying a (fairly) constant charge current to the battery relying on either the relatively low charge current, or the user to ensure the batteries are properly charged. They are usually very basic pieces of equipment and can take several days to charge high capacity batteries but this feature can be what makes them ideal for many applications.

For example, if you only use your batteries at the weekend and one set of batteries will last you for long enough, then you can just put the batteries in the charger when you have finished with them and know they will be ready for the following weekend. Alternatively, you can have 2 sets of batteries and just 1 cheap charger allowing you to be using one set while the other is charging.

The down side is that you will need to use the formula above to know how long the batteries need to be charged for and so also have to remember when you put them in the charger.

Fast Timer Controlled Chargers

These are the most commonly found chargers in the marketplace and often the sort that you find in high street shops at "Bargain" prices, usually bundled with a set of low capacity batteries. Yet despite their popularity, they are the least versatile charger available and the most common cause of consumer dissatisfaction.

They work by providing a relatively high charge current for a fixed period of time then switching to a very low trickle charge current to keep the batteries fully charged. The problem with this is that the timer is based around a specific capacity of battery (often whatever the charger was supplied with), so if it was supplied with 1800mAh batteries and you later want to charge 2600mAh batteries, you will find the charger stops fast charging too early for your new batteries. This leaves you 800mAh short of a full charge for your nice, new expensive batteries and you end up complaining to your supplier that they don't last any longer than the old ones!

An even bigger problem is that because you have a high fast charge current you can now easily over charge the batteries and cause them permanent damage. If you take your 1800mAh batteries and only 50% discharge them, then the charger has no way of knowing and so still applies its high charge current for the preset time and you end up overcharging them by 900mA. Now you batteries should tolerate this for a while, but if this happens regularly then you will reduce the capacity of the batteries much more quickly.

Intelligent or Negative Delta Voltage (-dV) Chargers

These are by far the best option if you can afford it. They are clever enough to know when your batteries are fully charged, so allow you to put your batteries in the charger at any point in the discharge cycle. They are often much faster than either of the other types of chargers and should never overcharge and so damage your batteries, this will help you achieve many more charge cycles and extend the life of the cells.

They work by detecting a particular change in a cells voltage which occurs as the cell reaches its full capacity, at this point the charger will switch to a low trickle charge current which maintains the battery in a fully charged state. The actual charge and monitoring process can be much more complicated than I am describing here, with many of the higher end chargers using some very clever techniques to fast charge the batteries without generating any more heat than they have to and helping to achieve the highest possible capacity with the minimum amount of stress.

Things to look out for!

There are one or two important thing you should look out for when choosing any charger:

1. Some chargers will charge batteries in pairs not individually, this is not ideal as there will always be an imbalance between any 2 batteries. You can minimise the problems this can cause by ensuring you charge 2 batteries of the same make and capacity that are in a similar state of discharge.
2. Some chargers have a single timer or charge control circuit, so you have to put all your batteries in at the same time and let it finish before you can charge any more. This you can't put 2 batteries in first, then another 2 batteries in 20 minutes later and expect the charge control to work properly, this is more often found in the Timer Chargers and the cheaper Intelligent Chargers
3. Batteries can get very hot when charging, this is normal and isn't a problem as long as they don't go over 50°C