The following will hopefully give you all the basic info you will need to choose the best charger and power supply for your needs. I will try to keep all the info timeless and generic, but like everyone I do have my opinions. I will also only be focusing on lithium type chargers because they are the most in demand right now.
How to choose a charger
I see 3 ways of going about choosing a charger
Requirements of your charger
I see the following list (in order of importance) as what most people consider important when choosing a charger.
Most people are primarily driven by price (yes there are those that place quality or specs above price but not many). Maybe they believe that all chargers are created equal, maybe they really are strapped for cash or maybe they just don't care. In the end most people will buy the absolute cheapest charger that will physically do the job. Sadly sometimes this focus on price actually drives them into buying a charger that is not capable of performing the job they bought it to perform.
My suggestion is to buy what you actually need, period. If you need to spend less, then try to find the middle ground between extravagant and cheap. And if you want to go above and beyond your actual needs, then power to you but don't come up short on this one or you will likely be buying again.
This one seems very important to some and a total non-issue to others. Many pilots downright refuse to buy anything from overseas (US pilots of course) and some refuse to purchase from anywhere other than the local hobby shop. Although this is not a bad thing, it can severely limit your choices.
My suggestion is buy the charger you want and not limit yourself to one of the three models your LHS carries. Also don't be afraid of ordering out of country. This allows you to not only save money but it also opens up many more choices.
Honestly specs, and features to some extent, should really be driving your charger selection. A charger is a tool and nothing more. So in basic terms, it needs to accomplish the tasks you require of it.
Ok now that you know that specs are important, how do you know what specs you need? This is where charger selection starts to confuse people. Lets start with the easy specs.
Battery Type: This one is simple, your charger needs to support the type of battery you plan to charger with it. These days people need lithium based chargers for charging LiPo, LiFe and LiLo packs. Some may need Nixx or Pb charging capabilities but they usually have an older charger that can do it, so it may not be a requirement for the new charger.
Cell Count: This one is likewise simple, your charger needs to support the cell counts of your packs. This includes both the max and the min cell count, as the tiny 1s packs are becoming more and more popular.
On a side note, multi-port chargers are becoming more and more popular. These chargers are essentially 2 or more chargers in a single enclosure and allow for each port to be configured differently. There are currently several 2-port chargers as well as a few 4-port chargers on the market. So this should also be taken into account when shopping.
Min/Max Charge Rate: Now we get to one of the confusing parts of choosing a charger. Your new charger needs to support both the min and max charge rates you require. The minimum charge rate is simple, do you have any batteries that require a very low charge rate? If so then make sure your charger supports it. The maximum charge rate is not as simple. Some could care less because they don't mind charging packs for 2-3 hours and for these people most any charger will do, but most people these days are looking for faster charging. To understand what type co charge rate a charger will support, you first have to know what charge rate you require and then you have to do some math to figure out if a specific charger can achieve it, given its wattage limitations. And that leads me to the next point ...
Max Output Wattage: This is the number that many people overlook and then ask why they are not getting the charge rate promised by the charger. Turns out that one number controls exactly what a charger will do in terms of output and that is a charger's rated wattage. See the Understanding wattage in relationship to charging article for the full story but basically the total power a charger can output is limited to its maximum wattage specification and not the maximum charge rate often shown in big bold letters on the front of the box.
Input Voltage Requirements: There are 2 types of chargers out there, those with built-in power supplies and those that require an external power supply.
The AC/DC chargers seem like a great solution with everything being in a single enclosure, but these are almost always severely limited in output. They also cost quite a bit more than the same model that lacks the built in power supply, enough in fact to make them unattractive. They really only offer added convenience to those with basic/small charging needs.
The other 95% of the chargers sold today require an external power supply. This may not seem to be that important but with 1000W chargers on the market now, you very well could require a power supply that costs more than the charger. In short you need to take the cost of a properly matched power supply into account when choosing a charger. See the right side of this page for choosing a power supply.
My suggestion is to be informed. Make sure you understand your actual needs from a charger and then make sure the charger you choose can cover those needs. Download the manual and read over the specs very closely. For example many times the manual will give you a table of outputs to help you understand its limitations.
As chargers get more and more sophisticated, they begin to stand apart in terms of features. Of course your new charger needs to have the basic features to make it useful and safe including a built-in balancer to keep lithium packs happy and safe, all the accessories to support the types of batteries you want to charge, and a useful monitor or status mode for checking packs. But don't stop there. Maybe you could use the ability to charge with the pressing of a single button, or check a pack's internal resistance in order to monitor packs and track their health, or even make a foam cutter out of your charger. These are just a few of the features available on today's chargers, so make sure you research all the features of many chargers before you settle on one.
My suggestion is to again be informed. Download the manual for each charger you are interested in and study it. Find out if the charger offers the features you require and if not, keep looking.
RC equipment is usually built pretty well but there is still a fairly wide range of quality available. You have to decide whether to spend $30 on a cheap charger with no support and just hope it performs well, or spend much more on a quality charger with good support from the manufacturer. Both have their appeal but once again it is almost always better to find a happy place between the super low end and the ultra high end.
My suggestion is to read. Read the forums, read the reviews, read anything and everything until you have a good understanding of what is good and what is bad in the marketplace. It also never hurts to buy from a reputable dealer, as they should support the charger also.
I hope this gives you a good place to start when choosing a new charger to best suit your needs. Just remember to not make any assumptions when looking at chargers. Keep an open mind, take what most people say with a grain of salt and download the manual so you know the actual specs and features of the charger you are interest in.
Choosing a power supply
Every month that goes by brings a new and more powerful charger to the market. That in itself is a great thing but it is starting to be a challenge to feed these new beasts. As of the writing of this article the highest output charger available is 1000W and requires a 24V input to achieve its full output. But lets look at a more normal mode, one that outputs 360W and can be found in most hobby shops.
Okay so now you know you need a power supply (PS from here on out) to feed your new charger. How do you choose the specs for it? Let me attack this question in 3 steps
Find out what you actually need
This step can be a little confusing because there are several variables that need to be taken into account. Lets start with the charger I mentioned above. The manual states this charger requires a minimum of a 12V 10A PS, but for full output it should be paired with a 15V 350W PS. What does that really mean exactly? It means that depending on what you are charging, you can use a variety of different PS types and sizes. For small packs you can get away with a relatively small PS but for large packs you are going to need some serious power.
This brings me to my first point. You can choose a PS in 2 ways
Doing the math to help decide what voltage and amperage the power supply should provide
Here is where I will cover the math that can help you size a PS for your needs. Even if you just plan to buy the biggest PS available, there are still things to consider.
Take the before mentioned charger. It can output a maximum of 360W but it is not 100% efficient, meaning that it will need more power than it outputs, so we will need to figure in the extra power to find out how much the charger will draw. Most chargers are about 80% efficient.
Input watts = 360W / .8 = 450W
Now lets take that number and calculate some possible PS numbers. We will use the following equation in conjunction with some info we found out above to find these numbers.
Watts = Volts * Amps
We know we need at least 14.5V, lets call it 15V for simplicity, to power the full output of the charger. So how many amps are needed to provide 432W from a 15V PS?
Amps = Watts / Volts = (450W) / (15V) = 30A
One nice thing about basing all these calculations on wattage is that we can adjust the input voltage and see how it effects the needed amperage. It turns out that the charger we have been using as an example will accept any voltage from 11-28V, so lets try some other voltages and see how that effects the amperage required.
Amps = (450W) / (20V) = 22.5A
Amps = (450W) / (24V) = 18.8A
As you can see the higher the voltage the lower the amperage needed to provide the same wattage. This brings me to another point. Chargers are more efficient when the input and output voltages are similar. For example when charging 3s (12.6V) packs, the best input voltage to use is 12V. When charging 6s (25.2V) packs, the best input voltage is 24V. This should be taken into account when choosing the best PS for your needs.
Now lets talk about choosing a PS specifically to power what you charge. In this case the charger plays no part aside its output limitations. Lets take a typical 2200mAh 3s pack and calculate the wattage needed to power the charger in order to charge this pack at 1C.
Watts = 12.6V * 2.2A = 27.7W
Factoring in the efficiency losses of the charger
Watts (input) = 27.7W / .8 = 34.6W
So regardless of the charger, your PS should be rated at a minimum of 34.6W in order to charge a 2200mAh 3s pack at 1C. Now lets calculate how many amps a 12V PS needs to output in order to charge our 3s pack.
Amps = 34.6W / 12V = 2.9A
Now lets look at this from a little more realistic perspective. Say you have 3 different sized helis, a 250, a 450 and a 500. You are planning to buy a new charger that will charge the packs for these helis. More specifically it will need to be big enough to charge the biggest packs, the 500 packs, in what ever manor you choose. So lets assume you will want to charge your 2500mAh 6s packs at 2C. Following the same math as above lets calculate how big of a PS you need. As a note a 15V PS is assumed for several reasons. It falls between the 3s and 6s voltages, meets most chargers minimum voltage requirements for full output and is easy to find.
Charger output for a 6s 2500mAh pack charged at 2C
Watts = 25.2V * 2(2.5A) = 126 W
Factoring in the efficiency losses of the charger
Adjusted watts = 126W / .8 = 157.5W
Now we will find the amps required using a 15V PS.
Amps = 157.5W / 15V = 10.5A
So for these helis, these batteries and these charging habits, a 15V 11A PS will cover all the needs of your charger even if it is capable of 1000W of output.
Choosing a power supply for multiple chargers
Some people like to run multiple chargers off a single larger power supply and that's just fine. This can be done using a power supply with multiple outputs or with a simple parallel cable that allows for multiple charger connections to a single output. The process of choosing a power supply for these needs is the same except you need to figure each charger's requirements individually and then add them all together to find the total requirements. Just remember that different chargers require different voltages, so you will have to find a common voltage that will work for all of them.
A final consideration for the large PSs, APFC
In the world of RC, power factor has never really been an issue until very recently. It turns out there are many complications associated with power today's largest chargers. Sure you need a compatible PS to allow these powerful chargers to work at their full output but what about powering the PS? As it turns out this can be a problem. Take for example a 1200W power supply needed for powering a 1000W charger. If we do the simple math ...
Watts (input) = 1200W (output) / .8 (losses in PS) = 1500W
Amps (input) = 1500W / 115V = 13A
In today's world of 15A house house circuits, most should be able to handle one of these by looking at just the above numbers but in actuality they won't. This is from a hidden electrical concept called power factor or PF. This PF is a function of the design of a PS but is often about the same unless it is corrected. If you want to know the whole story do a search online for Power Factor and do a lot of reading but for those who want to just get some basic info, here is the gist of it. Switching PSs are not purely resistive, so the phase of the AC powering them doesn't line up perfectly. This causes some of the power to be returned to the source and that adds to the actual power needed by the PS. The additive effect increases the current flowing to the PS and as such, that is the current seen by all the components. This includes the AC outlet, the wiring to the outlet and most importantly the circuit breaker. Okay now lets see what that it does to the math ...
The average PS is going to have a PF of about .6. This means that it can only use about 60% of the current carried to the PS. So if the PS needs 12.5A of 115V input, you need to add in 40% more current to get the actual amount seen by the PS.
Amps (input) = Amps (calculated) / .6 (.6 PF losses)
Amps (input) = 12.6A / .6 = 21A
At this point many people just plain don't believe it. They do the initial math and hold to it, ignoring any effects of the so called PF. Well usually about the time these folks start up their new generator and start to charge some big packs, that they find out really fast that this is a real problem and one that they should have considered before they bought what they bought.
The savior in these cases is buying a PS with APFC, or active power factor correction. These PSs have integrated components which continuously monitor the phase differences and adjust as needed in order to achieve a PF of more like .98. In these cases the PF can simply be ignored. The drawback is that these units are few and expensive. The only large models I know of are made by Meanwell and use the RSP model numbering. Many PC power supplies also have APFC but these are only 12V and not as useful as other models.
Compare the options
Now lets looks at some options for powering your charger.
Your car battery
The first and often times most convenient power supply is your car. It has a decently stout 12V power system and can power most any charger with ease. There are some things to consider but for basic charging this will work well for virtually any charger. The things to consider/understand include
Bench top power supplies
These are are a common way to power your charger. The can be bought anywhere from Radio Shack to eBay and come in a wide range of outputs. Some have adjustable voltage and that can be handy for matching the voltage to your charger's needs. As good as these can be there are still shortcomings like cost and size. Some larger bench top models can cost $500 or more. Most are going to cost more than the charger they will power. Be sure to search and read about as many models as you can to see what is out there. Some of the larger output bench top models can be vary large physically and very heavy. These are not something you casually take with you.
So if you want a good looking power supply that offers the simplicity of banana jacks on the front, ease of a 120VAC power cord hanging out the back and a full set of gauges to tell you what is happening, then look around at the bench top models but get ready to spend some big bucks.
Recovered or new equipment switching power supplies
This kind of power supply is becoming more and more popular because of the lower cost and possible high output. These are found in everything from computer servers to medical equipment to RVs. You can buy new models or used system pulls. Either way here comes the shortcomings. Since these units are designed to installed into equipment, they are usually nothing more than square metal boxes with screw terminals or strange plugs on them. So you will very likely have to make the input and output cables and sometimes you will have do some special wiring to make them work. Some employ large, loud fans in order to keep them cool. Many of these are made for commercial use and they don't care one bit if they are quiet or pretty, they just want stuff that works.
Converted computer power supplies
Along the same idea as the equipment power supplies above, you can use a regular, run-of-the-mill computer power supply to power your charger. These can be system pulls or new models. They will be limited to 12V but they can come in models capable of outputting up to 50A or even more. The catch is that a little tinkering is required in order to make one into something usable. The good news is that it is not difficult and most tinker savvy people can do it with ease. Check out my how-to "Converting a PC power supply" or do a Google search for "convert pc power supply" to find lots of other how-tos and info. Just be sure you know what you are getting into before you start.
Like all things information is power. The more you learn before you buy/try, the better your results will be. So be sure to look around at all the options and don't be afraid to do some math to figure out exactly what you need.