Lithium polymer battery chargers for multirotors

This follows on from my post on lithium polymer batteries but I just need to put in the same disclaimer!

I get a lot of questions about multirotor equipment so I thought I’d start creating some posts about that. Please bear in mind that I learn a lot from reading around the blogs and forums and talking to people in RC shops so any inaccuracies are completely due to my own misunderstanding. By reading this you accept that, although I hope it will aid with your own understanding, it is written in good faith and I can’t take responsibility for any damage to yourself or your equipment that occurs through the use of this information. If you see anything that I have got wrong or you feel is dangerous, please contact me through the HexCam website or Facebook page and I will rectify it as soon as possible. Please treat lithium polymer batteries and multirotors with same respect you would give to other electrical and mechanical equipment. Look after them and they will look after you.

You also need to know the following: I am not paid to review or endorse any products but I do have a partnership arrangement with Versadrones to act as their demonstrator and trainer in the UK.

Introduction

Getting the right lithium polymer (lipo) battery charger can be a real minefield. The main reason for this is that often the charger companies only tell half the story. So I will try to clarify it, but am not going to promise anything as I sometimes still get a little confused myself. It is, however, vitally important that you get the right charger for the kind of aerial imagery business you are running. If not, you will find yourself very frustrated. A decent battery charger and power supply can be pretty expensive. But as someone who has a number of “retired” chargers around the office, I would say that you need to see it as an investment. A good charger will maintain battery health, allowing batteries to remain in use for more charge cycles, it can also save you time if you don’t have to be monitoring the charger continuously just to get all your batteries charged in time! If you look across the net you’ll find all kinds of homebuild arangements for providing power. There are ways of doing this, and I have done so myself on occasion. However, I am also aware that for most people the idea of stripping down a server power supply is pretty daunting, so I’m going to focus on professional solutions that might just survive a portable appliance test! If you are up for a bit of homebuilding just have a look around the RC forums. People have built some amazing setups.

What do I need to start charging lipo batteries?

The first thing to say here is that lithium polymer batteries need a specific lipo charger. If you try to charge them with anything else you are risking a fire. The technology (and people’s understanding) has improved over the last few years. If charging indoors, keep the batteries in lipo bags or some kind of metal case.

You are probably going to want two things… a mains operated charging system that can be used for charging back at base and a mobile system while you are out operating. Generally, the charger will be the same but your power supply will change. If you have bought something like a DJI Phantom you will get a neat little all in one plug and charger with it. No problem for small batteries like that so just get on with it and fly! I would still suggest following the 20% rule from my lipo battery post though.

It is recommended that batteries are charged at a 1C charge rate. This means a 3000mAh battery should be charged at a maximum of 3 amps, a 5000mAh battery at 5 amps, etc. Many batteries are now rated for higher charge rates but there is a possibility of battery chemistry damage if high rates are used too regularly. If in doubt, charge at 1C or below. There are ways of doing this efficiently!

The parts of a charge station and lipo battery storage

1. The balance charger
2. The power supply (home)
3. The power supply (field)
4. (optional) Parallel charge board or harness
5. Lipo bag/box

1) The balance charger

I have just sat for about 5 minutes trying to work out how to start this so it may be a little incoherent on first release, I will try to hone it later. Most people find that their first charger is underpowered and wonder why it won’t charge their shiny new 6S lipos in under three hours! When buying a charger, you need to look for several things: input voltage, output voltage, maximum current (amps -A) and very importantly maximum power output in watts (W). I’m going to start by considering charging a single battery on a single channel charger as it is where most people start. Something like the imax B6. When you look at the box below, you think great; it’ll charge up to 5A, so my 6S 5000mAh battery will charge fully in an hour no problem… it even says so on the box… Right?

iMax B6… my first charger.

Wrong. It is true that a 5A charge would give a 1C charge rate for a 5000mAh battery (5A = 5000mA). However, what this doesn’t take into account is the battery voltage or total wattage of the charger. The charger has an input voltage of 11-18V and a total power of 50 watts (not found anywhere on the box).

In this example, I am using an 18V power supply capable of delivering 540W. As a result it can potentially power more than 10 of these B6 chargers at a time.

As you can see from the next photo, the charger will only charge at 2.2A (less than 0.5C). The reason for this is that we are trying to charge a 22.9V battery at a maximum of 50W.

The iMax B6 trying to charge a 6S 5000mAh lipo.

Here is a simple equation: W = A x V. By rearranging that we get: A = W/V.  So 50 divided by 22.9 is…. 2.18A. So basically, this charger is maxed out as it has hit the maximum 50 watt limit. At this rate the battery will take about 2 hours to charge. The B6 also has a 240 minute timeout set as standard (a good safety feature) so it may time out before it charges.

We can quickly work out the maximum battery size that will actually charge at 5A on this charger using the same equation as before.

Rearranging it again we get: V = W/A

So the maximum voltage that will charge at 5A is 50 divided by 10: 10 volts.

Remember a 2S lipo has nominal voltage of 7.4V and a 3S is 11.1V. As a result, this charger would charge a 2S lipo at 5A but as soon as you get to 3S or more, the wattage will max out. This charger does have its uses; I now use it to charge my 2S and 3S AV receiver and LCD batteries as it is a good lightweight charger and frees up my main charger for the big lipos.

This next charger, the Hobbyking Eco 6-10 is very similar to the B6 in most ways. In fact, the user interface on a lot of these chargers is identical. You do have to manually choose a normal charge or balance charge.

A step up from the B6

As you can see, with this charger I put in exactly the same settings and am getting a constant 5A charge. So what is the difference? Well, the big 200W on the front gives a major clue. The battery is currently at 23.75V as you can see if you enlarge the photo. Going back to our equation: A = W/V… 200W divided by 23.75 is 8.4, so theoretically this charger could charge my 6S lipo at up to 8.4A at this stage of the charge cycle.

This charger will give a maximum 10A charge so I set it to that to see what would happen. As you can see from the next photo, The wattage has become the limiting factor as suspected and we are limited to 8.4 amps, which on this battery is a 1.7C charge rate (this battery is rated up to 5C charge).

I set this up for a 10A charge but it is limited by maximum wattage to 8.4A

In the words of the great Professor Brian Cox “…and that’s why I love physics!”

It is completely predictable, if you know your batteries and can use one simple equation you can work out the minimum requirements of your charge system for the batteries you are going to use.

There are variations on these types of charger so that there are multiple chargers in unit. There are a few out there, that are basically four iMax B6 chargers in one box. The key thing to look for there is the wattage again, particularly with respect to the maximum wattage each channel can support.

My current charger that I use both in the workshop and out on projects is the Fusion Emperor L712B Pro. This is a two channel charger capable of 500W per channel.

This charger gets taken everywhere!

As you can see, both channels are plodding away at 5A no problem. There is also no balance charge option on this charger, the balancing is part of every charge (evening the individual cell voltages out so that they all end up as close as possible to 4.2V).  It also gives you % charge and loads of other useful information on other screens.

At 500W (max 20A) per channel, using the equation A = W/A and the information from the bottom channel on the photo:

500/23.94=20.88A, so I could charge this battery at the maximum 20A (a 4C charge rate). Alternatively I could charge 4 of these batteries in parallel at 5A on each channel, meaning I could charge 8 6S 5000mAh batteries in one hour.

Right?

Wrong.

2) The power supply (home)

At this point a new limiting factor has come into play. In order to use the maximum 500W on each channel, I need to be able to use 1000W of power. You may remember, this power supply only provides 540W, so I’m going to be able to use about 270W per channel which means with this power supply I will only be able to charge 4 lipos at the 1C rate. In order to get the full potential from your charger, the power supply you are using must be able to supply at least the same wattage as the charger, so all I need to do to be able to charge my 8 batteries at a time is to buy a minimum 1000W power supply and it is sorted. Right? Pretty much! Just check that the power supply is also able to supply the minimum amps required too. Also, the charger will work most effectively if you are able to supply power at a higher voltage than the batteries you are charging. So to get the most out of my Fusion Pro I need a power supply that will provide a minimum of 40 amps at 24V and a minimum of 1000W total power. There are some people who will realise it isn’t quite that simple, but I’m trying to keep it as simple as possible. Make sure your power supply voltage does not exceed the voltage of your charger or you will blow it up! For example, I wouldn’t use the iMax B6 ona 24V supply as it is only rated up to 18V.

Something like this SkyRC power supply is great as it has fully adjustable voltage and can provide up to 1200W at up to 50A which will easily let me use the full potential of my Emperor Pro. With chargers improving all the time even this power supply may not be enough!

All very well, but what about when you’re out filming and haven’t got an extension lead into a convenient building…

3) The power supply (field)

Up until now I have generally used a direct connection to my car battery to provide power to my charger when out on location with no mains power. I did experiment with an inverter to convert the output to AC and then plug in the 18V supply, but it was inefficient and also a mess of wires to set up. Using your car battery without the car running also runs the risk of draining the battery. Car batteries also aren’t designed to be deep cycled so you may be reducing the life of your battery. So now I am moving towards a system using either a 24V deep cycle leisure battery or two 12V leisure batteries in series to give 24V.

Using deep cycle leisure batteries in Rotarama’s field charging system

Here, in a setup used by my friends at Rotarama, as you can see, we are using two 12V deep cycle leisure batteries in series to give a 24V output.

These batteries have a combined capacity of 220,000mAh. Enough to charge Our 5000mAh batteries 44 times. More actually as you’ll remember, with the 20% rule, we shouldn’t be putting more than 4000mAh in each time anyway.. More than enough for a day’s work!

If you look carefully at this picture, you will see that we currently have two batteries in parallel on the left hand channel and nothing on the right. As we use our batteries in pairs, it is logical that we charge them in parallel pairs too. Parallel charging can be carried out with any batteries of the same voltage. I feel it is best to only charge batteries in parallel if they are the same capacity and state of charge as well. You can see we are using a parallel charge board that will take up to 6 batteries in parallel.

Which neatly leads us on to our penultimate section….

4) (optional) Parallel charge board or harness

Bearing in mind the equation we talked about earlier, it may be possible to get more out of your charger by using parallel charging.

If you look back at my battery post, you’ll remember that putting lithium polymer batteries in parallel is not only possible, but can be very advantageous at times. It works with charge as well as discharge. So if I put my two 5000mAh 6S in parallel and the balance leads also in parallel using the special board (parallel boards for most types of connectors are readily available at most RC shops and online) then I effectively have one 10000mAh 6S2P battery. As a result I can charge the pair at 10A and each battery is getting the equivalent of 5A (back to the 1C charge rate). If I added a third battery I would create a 15000mAh 6S3P battery which could be charged at 15A to give the 1C charge rate.

5) Lipo bags and boxes

I don’t like to get hyped up over danger. For every scary lipo fire you may see on the forums there are literally thousands of successful charges. Also, incidences seem to have reduced considerably over the last few years as technology and understanding has improved. However, there area number of things you can do to make sure you charge safely.

• Use lipo bags for containment in the event of a fire.
• Store your lithium polymer batteries in a metal case. A lot of people use ammo cases!
• Always charge on a non-flammable surface.
• Keep a small portable powder fire extinguisher near your charge station and out on location.
• Keep a log of battery charges. Changes in capacity and cell balance can indicate a battery that is degrading.

I hope all this helps to some degree. If you want to discuss anything, do get in touch!

Fly safe!

Elliott – HexCam

I am publishing this as I type… so more will keep appearing throughout the day.

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