How To Look After LiPos Part 2
Hopefully you had a chance to read the overview of LiPos given in the first part of these articles. Not an exhaustive list of the technology, but hopefully a few pointers to those new to the technology and a few tips for those who have been using them for a while. In this second part Jamie Cole aims to go into a little more detail on certain areas and cover a few essential points.
An interesting point that a lot of people overlook with LiPo cells is running them in, but it is commonly accepted that running in packs should be considered to get the very best from them. The best thing way to approach this is to think of it as taking it easy for the first handful of charges, like you would take it easy with an IC engine, not loading up and not pushing too hard. Just for at least the first six to 12 charges. So don’t over discharge them.
My advice would be to charge the cells at each cycle as normal, and then run them for 25 to 50% of the duration you would expect to run them for, take the first few cycles easy and then crack on as normal.
There is no real official advice on this, but I would recommend it. I have done it from day one and have had little issues with my packs other than through my own stupidity.
We explained previously the massive importance of balance charging, it’s the only safe way to ensure you are charging the batteries correctly. But with so many different cells out there what are the chances of them keeping the balance leads the same? Well surprisingly there are two types that have stayed over the last few years, and it’s not as terminal as selecting Betamax or VHS or HD or Blu-ray. They are both current and will work with most of the chargers and balancers out there.
There are a number of cell choices available for us to choose from in the RC model marketplace. You have FlightPower, Thunder Power, Align, Overtech, E-flite, Outrage, Esky and EnErG to name just a few. All of the above packs have one of two types of balancing lead, this is a JST or the Thunder Power and FlightPower type (TP/FP) as they are generally referred to. There are four options for balancing:
- A straight forward balancer which plugs in to the balance leads from the pack.
- An inline balancer, which plugs into the balance leads and has the power leads plugged into it, which then in turn connect to the charger.
- A balance port charger (charges through the balance leads).
- A charger with a balance port (charges through the main output leads).
All of the above options share the fact that they use one of two types of balance ports, the JST or the TP/FP type (also shared with the later Align packs). Pretty much all the balance solutions offer connectivity to one of each of the types of packs. If they don’t as standard there will be an adapter available to fit most packs.
Regardless of which packs you use the balancer or charger will require you to get the corresponding correctly matched balance board or leads. There are some pictured for reference.
My preferred method of charging in the above scenarios is to opt for a charger with a balance board, see the pictures for examples. You will see that some chargers require you to plug in a board, then to plug in balance leads from the cells to the board, be this JSP or FP/TP type connectors. The main power lead of the pack then plugs in to the main power leads of the charger and off you go. This will support high charge levels and in many cases allow you to monitor the voltage of each cell so you can see exactly what’s going on and how well the cells are in or out of balance. Most chargers of this sort or type will cut the charge cycle if there is a problem with a cell voltage, or even the over all voltage.
Using the in-line balancers, such as the FlightPower V-Balancer offers in-line balancing and charging. Basically you plug the balancer in to the cells and the balancer then plugs in to the charger. It will give you a rough indication of charge status for each cell, up to 6S. the balancer will then flash on each cell that is high to indicate that it is putting a load on that cell to bring the voltage in-line with the others. At any point in time you can hold down the connect button, a yellow light flashes up and you are ready to charge. If at any point in time a cell goes above its 4.2V per cell figure the balancer will cut out to stop the charge cycle. A great fail safe, and a proven way to charge cells, but it doesn’t give you the same visibility as the balance charger with individual voltages per cell.
There are chargers around that allow you to charge through the balance leads itself rather then through the main leads. These are great and work very well, especially in ensuring each and every cell has the right voltage, but if you are going out to spend some money on these then buy one with a display so you know how much you are putting in to the packs each time. Why? Well quite simply so that you can monitor how much you are using per flight and to figure out your flight times. If you are using larger capacity and voltage packs these may not be the best idea as the tiny cells may not be able to handle the amps you are pumping through them. It is however arguably the best way to charge cells as they each have their individually carer, charger and discharger.
Straight forward balancing would allow you to simply balance the cells before, during and after charging. Probably my least favourite method but better then nothing, just! Obviously having no feed back to the charger, or any fail safe type cut off does mean that the cells could be at risk of going over voltage, especially if the balancer cant keep up or if the cells did come how go way out of balance.
Getting the power down, or up, or more to the point how are you going to connect the cells to your speed controller?
Options really depend on the application and the numbers of cells as well as the loading. A receiver LiPo pack (detailed later on) can use a conventional top quality standard receiver connector. A T-Rex 250 can be the small red two-pin BEC connector type, but a 10S setup pulling 50A, peaking at 100A will clearly need something a little beefier. Options for power systems connectors:
- 2.5mm gold – smaller applications, mini helicopters etc.
- Tamiya connector: 25A – stick to cars with these.
- EC3 (3.5mm Gold with connector) – 60A – good for all 450-sized stuff up to 600-sized models such as the T-Rex 500 and 600.
- 3.5mm Gold – 50A – As above
- Deans connectors – 50A – widely used.
- 4mm gold – 75A – good for the larger stuff, perhaps a more suited solution to a T-Rex 600.
- 5mm gold – 150A – best suited to anything over 6S.
- 6.5mm gold – 200A – the beefiest.
To be honest looking at the above as a rough guide has made me have a rethink as to what is the best solution.
I love the idea and safety of the Deans, but the rating is only up to 50A. I currently use 4mm gold connectors for most of my batteries, however I think I may replace the bigger stuff (10S configuration) with the 5mm or 6mm golds as the 4mm golds are currently near the recommended limit of the current rating and I know I am pulling over 75A in my MA Razor when I’m giving it some abuse!
Looking at the above I would really use bullet connectors for more if not all of my applications above a 500-size model!
One thing to note is to be very careful with all of the connectors, especially the bullet connectors. The last thing you want is for these terminals to touch or short out. If they do it’s not pretty. Hopefully you can learn from my mistakes. Trust me shorting out a 10S, 42V, 4350mAh pack when the positive and negative terminals touch is quite a fright. Not to mention the flash being very scary. Luckily for me the flash resulted in the 4mm gold connector just disintegrating before my eyes before any serious damage was done. For the record the cells were fine and are still in use.
LiPo Dos And Don’ts
- If you can use a permanent sheath/insulating sleeve over the connectors
- If you can t so the above, place some fuel tubing over it when in storage
- Treat these batteries with the respect they deserve
- Treat them as you would petrol or fuel
- Use an apparently sized connector/lead
- Read up on how to solder correctly
- Get a dry solder joint
- Store the cells without guarding the terminals
Using LiPos in a model as the power source for the receiver is something I have done for years. In fact we have done a couple of articles on LiPo receiver packs and voltage regulators. I guess this opens up a number of areas, from dos and don’ts down to common misconceptions.
First off and foremost LiPo packs as a receiver and servo power supplies are designed to be used in conjunction with a voltage regulator. Normally you would use a 2S or 7.4V LiPo Pack in conjunction with a voltage regulator. Normally the voltage regulators come in a variety of voltages, and load ratings in amps as well as variable voltage options.
Normal voltages are 5.1V to 6V with varying voltages in between. What should you use? Well this really depends on the ratings of the servos and receiver that you are using. If you are using a conventional 4.8V rated system then I would opt for a 5.5V or lower option. This is dangerous territory for us to really quote on, and we risk getting a slap from the radio manufacturers, you will struggle to get an official answer as to what should be used from a manufacturer, to be honest many of them quote 4.8V, but here is the reason why I would recommend 5.5V as a safe bet.
Take a four-cell NiCad, fully charge it and take a note of the peak voltage. Now depending on what rate you charged the pack at and how much you slammed in you should find the peak voltage is around 6.4V. Now take this pack off charge, leave it for 30 minutes and stick it on a loaded volt meter with a 1A load. You will find the voltage is around 5.3V to 5.5V, may be even higher! In fact if you put a loaded voltmeter on a pack and it kicked out 4.8V you really should not fly, these cells are flat and need charging. So with this in mind and the fact that a fully charged NiCad will kick out 5.5V I would opt for this as a safe bet as it’s the equivalent of a charged NiCad pack. To be safe use an inline voltage stepper or regulator for those servos that claim they need to be 5.1V.
So why are manufactures of radios reluctant to quote usages of LiPos? Well to be honest I don’t blame them. There have been rumours of people plugging a LiPo pack straight in to a receiver with no regulator. Why is this bad? Well take a fully charged 2S LiPo and stick a 1A load on it and what do you get? About 8V! That’s a lot to go to the servos, nearly a 50% voltage increase. This is going to make a number of servos go pop, not to mention the receiver. These guys then send the equipment back as faulty!
So what do you so? Opt for a decent RX LiPo pack and a decent regulator, with a suitable rating. I use a 5A one in most of my models and a 3A one of the smaller ones. From experience I have used 5.5V Regulators and higher, where approved by the manufacturer, for the last four years or more. I did it cautiously, but have never looked back. Using the right regulator, balancer, and charger you know exactly where you stand, how much you are using per flight and the best bit of all is you get the following:
- Maintain a constant voltage under load
- Higher capacity
- Better suited to larger high power servos
- Noticeable difference in consistence
- Noticeable improvement in-flight when the servos are under load
- Not many, but if you want to be picky then more components – i.e. voltage regulator.
- Maybe cost, but a branded NiCad would be about £30, and a voltage regulator and receiver pack need not cost you more then £40 to £50 with a wealth of benefits.
Using a LiPo receiver pack was probably the single biggest improvement to my radio system for a long time. Noticeable to most who saw or flew my machine, including myself.
On a side note to regulators there are now a number of receivers with built-in regulators, such as those available from Spektrum and JR. So just plug the receiver into the LiPo and the regulation is already taken care of. Be careful to make sure you have the right receiver before you plug something in!
Higher Cell Count Configuration
Okay, so you have your new electric model, with its LiPo receiver pack, regulator, 80A high voltage compatible speed controller and 10S motor set-up with appropriate gear ratios. You need more than a 6S configuration of LiPo cells, what do you do?
Well in this situation I would opt to go for two separate packs, in the example here we have a 10S pack made up of 2 x 5S packs. You can join these packs in series by quite simply connecting the positive of one pack in to the negative of the other. This is wiring the packs in series to give you two x the original S rating of the pack, i.e., 2 x 5S packs become a 10S configuration. The packs will need to be of identical type and of exact or similar charge cycles.
Why do it this way? There are two reasons really the first and foremost is so that if you do spank the model, as we all occasionally do, it is unlikely you will damage both cells, and if you do then these things happen, but if you do only damage one set of cells you still have 1 x 5S pack left over, which can be used as a spare or for another application at a later date.
The second reason is it can make charging easier, if you only have an 8S charger and a 10S pack you can charge the cells individually without investing in a new charger. This method will also give you a wider choice of cells.
The above is common place with the T-Rex 500 using two 3S packs. If you are using bullet connectors this is simple, if you are using the Deans connectors or EC3 connectors you will need to make or purchase an adapter, these are readily available from model shops.
This has taken me a while to get my head around, but after much deliberating and getting it wrong I am finally there.
Working out the gear ratio on an IC machine is easy, very easy in fact, Quite simply if you have an engine that has a peak power of 15,000rpm and a gear ration of 8.18:1 the you divide the rpm by the ratio, this will give the head speed, in this case it’s 1833rpm. Or to work it backwards if you want 1900rpm on the head from an engine giving 15,000rpm then you need a ratio of 7.89:1. Not rocket science, but how do you figure out the electric motor RPM and hence head speed if you know the ratio or alternatively how can you tweak the ratio to get the desired head speed. You will need the following information: motor KV; cell count; gear ratio. The motor KV is quite simply the RPM rating per volt.
Each cell in a pack is normally quoted as 3.9V under load as a fully charged cell. This is the general consensus of opinion and some views will differ depending on who you speak to and the cells used. In practice I have found this to be the best guide.
So a 10S battery is kicking out around 39V loaded. If we take the Hacker A50-12 ET motor it has a KV rating of 640. 640KV X 39V = 24,960rpm on a 10S pack. To get a head speed of 2000rpm for a 50-sized model we need a gear ration of 24,960 /2000 = 12.48:1
You can use this same methods to work out the head speed of your machine although use it as a guide and not a definitive measure as you will need to factor in drag from the drivetrain etc. There are other variables to this including motor timing that will need to be taken in to account.
Charging And Watts
So you have your 6S cells, lets say hypothetically for your T-Rex 500 and it’s a 3200 pack. To charge this pack at 1C (3.2A in this case) you will need a charger that will output watts at the relevant level. In this case peak voltage is 4.2V per cell x 6 cells = 25.2V. To work out the watts it’s volts x amps = Watts So in this case 25.2 x 3.2 = 80.64W. Hence we need a charger that can cope with this output, or if we wanted to charge at 2C (check with the packs, but this is possible) then you would need a 161.28W charger.
Make sure you buy a good charger with as high a Watt rating as possible for your money whilst charging as many cells as possible.
I have just invested in a new Bantam BC8DX which allows the charging of up to two 8S packs whilst balancing and up to 150W per side. This a monster charger with a 300W combined output, which is massive. There are others available from Graupner which I own and use that have 120W output.
Battery technology is advancing in leaps and bounds. The latest cells have a higher capacity than the previous versions with higher C ratings than ever before. The best bit of all is higher charge cycles, which if you combine with the best advice, best chargers you can put your money on, run in the packs, store them properly and treat them with the respect they deserve. If you do you will have a great investment with fantastic performance of the packs, be this from powering your receiver or to powering your machine. It’s a cost effective, even a relatively cheap solution to obtaining consistent and reliable electric power.