Charger Setups - One Man's Opinion

19 Apr 2019 5:34 AM | Marshall Henley (Administrator)

This was written in response to an e-mail by a new member. This is one man's opinion. Feel free to correct me (Marshall) on the theory.

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Full blown charge stations are generally built by the heli guys... they frequently need to charge huge batteries in huge amps. Here's an example...


https://www.rcgroups.com/forums/showthread.php?1854396-Portable-Charging-Station-Build-with-Duel-Powerlab-8s


However, you can purchase a pre-built one and save the headaches at progressiverc.com... here's an example: https://www.progressiverc.com/hardline-powerlab-1000w-combo-2982.html (although it doesn't have the parallel board or Bump Controller from Revolectrix). Look under their Charging menu for your various options.

Here's another company that makes 'em: http://www.rotorcraftrc.com/


Electrical Power Concepts

My apologies if you already know this stuff...


It helps to use the following equations with electric:

P = iV , which means power (P) = current (i) times potential (V)

Power is measured in Watts (W), potential/voltage is measured in Volts (V), and current is measured in Amperes, or amps (A). Battery voltage, while measured in V, is typically expressed as an "S" - meaning batteries in series. For LiPo chemistry, that gives you a voltage by multiplying by 4V per cell (3.7V nominal). That's why a 3S is often listed as an 11.1V battery. At full charge it's at 4.2V per cell, though, and at the low it's at 3.7V per cell, so 4V is a good in-between.


Voltage drops across a length of wire, and every wire has a resistance, which can be calculated for a given gauge for DC current by it's length. The voltage equation (Ohm's Law) is:

V=iR , which means potential in volts (V) = current (i) times resistance (R)


You can calculate voltage drop online at a gazillion calculators, such as https://www.rapidtables.com/calc/wire/voltage-drop-calculator.html.


Resistance is measured in Ohms or milliOhms. Using the above equation, you can literally say that 1 Volt = 1 Amp-Ohm or 1,000,000 milliAmp-milliOhms or 1,000 Amp-milliOhms.


The relationship between the power equation and Ohms law, combined with basic algebra, creates the following equivalency diagram, which makes my eyes cross. I just use algebra...

image.png


Battery capacity is an amount of charge, literally excess electron particles, which we typically measure in Amp-hours or milliAmp-hours (mAh). Charge would normally be measured in Coulombs, but that's not very helpful for us because we think in terms of hours or fractions of an hour. 1 Coulomb is is 1 Amp-second, meaning 1 amp of current into a capacitor or battery would store 1 Coulomb in 1 second. That means one Amp-hour (1000mAh) would be 3600 Coulombs. We also switch back and forth between Ah and mAh hour a lot, often for no particular reason. They're equivalent at a ratio of 1000. A 5000mAh battery capacity is the same as a 5Ah battery capacity. I personally prefer Ah.


The charge and discharge rates of a battery are measured as a multiple of the capacity, meaning that a 2C charge rating will allow you to charge in Amps at twice the Amp-hour rating, or in milliAmps at twice the milliAmp-hour rating for the battery. The discharge rate is always much higher, but its the same idea. On the low end we fly 20C discharge batteries. On the high end, many us who fly drones are flying 120C discharge- rated batteries. The former, and virtually all basic lipos can only be charged at 1C. This means a 2650mAh battery could only be charged at 2.65A safely. NEVER CHARGE ABOVE THE RATE GIVEN BY THE MANUFACTURER, typically on their website or a sticker on the battery. Some give conflicting information...a battery might be rated for 8C on the website, but list 1C on the battery for what I assume are liability reasons, or perhaps they didn't want to print different stickers.


High-C-rating batteries are not needed for all applications. The batteries that fly the Apprentice are about the cheapest batteries you can buy, and only need a 20C discharge rating. A 2.2Ah battery pumping out 10C is providing 22 amps. That's plenty for our Apprentice. On the other hand, plug a 20C battery into an Avanti EDF jet and it will be fried and worthless, possibly on fire, by the time you land. You'll need to know amp draw for the application.


The more-expensive lipos (60-120C or above) can be charged at 5C, 8C, even 10C, but a 3Ah battery charging at 8C would require 24A, so buying expensive batteries means you're gonna have a highly capable charger, or you're gonna want one soon. The 120C batteries are used mostly for drones, but I buy highly-capable batteries (60C and above), and buy less of them, because they charge so much faster, so it's more economical in the long run. This approach has proven to be a bad idea for CombatONEs... I've torn up a lot of expensive batteries in crashes. Trade-offs is the name of the game in electrics.


The "Right" Charger Setup

It all depends on what you're going to be flying. I built what I have after purchasing a 50cc aerobatic plane that had been converted to electric which flew on 12S 5Ah (2 6S in series), so I generally charged 4 of them at 10A each. I subsequently experienced (and learned from Jonathon Hendrickson) that these size planes are better flown with gas... anything at or above 20cc engine size rating is better as a gasser... only needs electric if you want to do it for fun, although there are a few small performance benefits. Incidentally, in our parlance, all planes are typically rated as the displacement of the engine for that size, e.g. 100cc plane or 50cc plane.


With that in mind, now my setup (PowerLab8 with Bump Controller, Revolectrix paraboard, 24V 1500W PSU and 24V dual wheelchair battery power for when no AC is available) is probably more "nice to have" than "need to have." However, when I want to charge six 4S drone batteries or six 2S indoor batteries, I'm a happy camper. Not to mention wanting to fast-charge two 6S 5Ah batteries at 20 amps each. The Bump Controller let's me parallel charge any batteries with the same chemistry and number of cells without thinking about anything. I can bump a 3S 2.7Ah Turnigy Nano battery and then two 3S 2.2Ah ThunderPower batteries and one 3S 3.2Ah Revolectrix batteries and the Bump Controller will charge at the lowest safest rate for that mix. The Turnigy can charge at 3C = 3x2.7 = 8.1A, the ThunderPowers can charge at 5C = 5 x 2.2 = 11A, and the Revolectrix can take 3C which is 9.6A, so the BumpController won't charge at higher than 8.1A x 4 batteries = 36.4 Amps, but it WILL charge at 36.4 amps, so that's nice to have.


For fixed-wing applications, if you follow the previous prescription to fly gas engines in 20cc size aircraft or larger (and I mean gasoline, NOT messy, expensive glow), then the only time you'll use the bigger batteries would be to fly EDF jets like the Avanti S we all love. Then you'll be into expensive 6S 5Ah batteries. If you want to charge them relatively, fast, then a setup like mine starts to make some sense. I have a 1344W, 40A charge ability at 8S or 32V. For example, I find myself often charging 2 6S 5Ah batteries at 20amps or 4 of them at 10 amps, which is 40 amps X ~25V = ~1000W). I can charge at 8S (8 x 4.2V) at 40 amps, which is where the 1344W rating comes from for the PowerLab8. With rotor applications, all bets are off. Buy the best charging setup you can, typically. The dual PowerLab 8 build above was for a 600-700 size heli pilot flying 3D.


I don't know much about Jim Beerman's charge station, but if you can find out the power rating (watts) and the S-rating (which turns into volts when you multiply by 4V per cell) and the current rating,  you can easily figure out what level of charging you can do with it. . I know he flies EDFs jets using it to charge their big batts. You could go that route and then buy a paraboard. I always recommend Revolectrix paraboards because they're so well-made and safe. It may not even be necessary to buy a paraboard if you can quickly charge two batteries and you purchase fast-charging batteries with a 3C or 5C or above charge rating.


As for brands, so much of RC is almost unnecessarily complicated. Things often are that way in a lot of hobby industries. If you bow hunt, the number of bow manufacturers is ridiculous. I'm not sure how they all stay in business. Same thing could be said for chargers. Having said that, I obviously like the Revolectrix stuff. I also really like the iCharger stuff that I've seen. I believe the pre-built brands sold by ProgressiveRC are all good.


You're gonna need storage, too. Most of us use ammo cans for safety, but there's a lot of stuff you do for care of your batteries. Jonathon wrote a good battery storage article on our website: https://www.slrcfa.com/Electrics/3868530. I keep my batteries inside ammo cans in a minifridge. I suppose a catastrophic fire could burn the mini-fridge, but the oxygen would be in short supply. My favorite non-refrigerated way to store lipos is in ammo cans on a concrete floor at least 3 feet from anything else. There are some great articles out there on Flitetest.com about building cinder-block-based charging "stations" - love the idea, but have never implemented because I charge in a section of my basement that is all concrete. You would also have to rig up some long charge cables, and doing so would require 10 gauge or 12 gauge wire so you don't lose too much voltage across the wire (see above equation). Finally, if you're gonna get into electrics beyond the basics, here's a bare minimum you'll need to consider (purchase list highlighted):

  • What am I going to fly and what LiPo batteries will I use? Bear in mind that what you're flying will likely change drastically over the next few years.
  • Where am I going to charge safely? Lipo bags needed. Possibly backer-board charge station like on Flitetest.com
  • Where am I going to store my batteries? Ammo cans needed.
  • What am I going to use for a charger based on my needs?
  • What am I going to use for a power supply based on my charger needs? Do I have adequate grounded AC outlet for that (might need a 20A wall plug circuit, or two 15A wall circuits on separate breakers depending on your PSU)?
  • Am I going to use a marine battery at times instead of a power supply? If so, how will I charge that (I use the Battery Minder).
  • Do I want to increase my parallel charging capability with a paraboard?

  • How will I measure voltage safely and quickly at field? PowerLab can show you individual cell voltage but much easier to pick up a quick LiPo voltage checker. The kind I have is far more accurate than the ones we use for QuickSTART.
  • If you go with a Bump Controller, you'll need to purchase batteries from Revolectrix with bumptags installed, or buy bumptags from Revolectrix and install/program them on your batteries (more likely the latter).

Based on the above, if I were you I would start with something less expensive like Jim's and then sell it when you're ready to go bigger and know better what you're going to be flying, unless you just like the ease of something like the Bump controller. That's what most of us did, often for years before upgrading. Of course, "less expensive" could still be in the $200-300 range.


Most of all, spend time online researching stuff before you pull the trigger on a purchase, the $50 GT1000 charger offer notwithstanding - that one was so cheap and good it was low risk.


Best wishes!

-Marshall


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