ASK Cub Reporter, 05.09.2013, Your Questions and My Infinite Wisdom
Hey all, this week’s first ‘Ask Cubby’ letter is a LONG one, so I decided not to put it in the normal ‘hand written’ font thing because it takes up too much space, and a little difficult to read something of this length like that. Anyway, here we go… our first letter is from Dave, enjoy!
“Touching an old base regarding Cubbie’s LiPo discharge article from 5/2010 Hey guys, keeping ’em flat on all four?
Just cruising your website and thinking about the LiPo discharge article I came across written by Cubby May of 2010. Hope I can bring to light something that in the last 20+ years of electric racing has rankled by arse…battery advertising.
It occurred to me that current ratings by the overzealous marketers of batteries (in this case LiPos, but who can forget the pricey, good-‘ol days of NiCd semantics) have them shooting themselves in their own feet. Their advertising is unethical and deceptive, at best. In truth, their ratings are often times flat-out lies.
Don’t get me wrong, I am a firm believer of getting what you pay for. But when R/C advertising stoops to lying to get a person’s hard earned cash, they are no longer trustworthy.
My contention is that there currently is NO R/C LiPo battery that can sustain a 65C load or better on the market. Period.
Again, I am referring to sustained, and not peak advertised loads. Many rate their pack’s peak loads, but even adjusting it by half (to find sustained load figures) puts their current delivery capacity far beyond safe or realistic levels.
– A 5000 maH, 10C sustainable load LiPo plugged into the standard equation yields a sustainable output of 50 amps for 6 minutes. This would be a very old-tech LiPo, probably impossible to find for sale anymore.
– Same 5000 maH Lipo at 65C (attainable easily today through a variety of companies) should produce a sustainable output of 325 (!) amps for 6 minutes. This figure from the accepted “C” LiPo output equation.
Put into perspective for someone new to R/C, a fully charged, 3S, 65C LiPo should handily start a family sedan on its own, and perhaps even a full size pick up truck. It should not require any other power source besides the LiPo itself. Wire gauge and connection types not withstanding in this example either.
These LiPo “C” ratings were not around in 2010 that I can remember. Think you guys could fire up a video camera and do a “LiPo Discharge Part 2”? Maybe this time try using them to start full size vehicles?
My guess – they won’t even start an econobox commuter car. It drives home the point better than “numbers on paper”.
Just a thought guys…keep ’em coming.
BTW – please be careful if you try it….”
Cubby– Yo what’s up Dave, shoot Brian your snail mail for an uber BSRC sticker set. Long email with a lot of points involved, so I’ll start from the top.
You contend that battery “ratings are often times flat-out lies”. To you it might look that way, but here’s how it goes down behind the scenes. When Happy Flower Lipo Company wants to source a bunch of batteries they approach an Asian manufacture. This Asian manufacture typically has many different “lines” or “levels” of cells, and they typically group them by “C” rating. Happy Flower then selects the cells they want and cuts the huge check.
From what I’ve seen first hand in the industry, the “C” rating from the Asian manufactures is typically based on the design of the cell, not on its output. For instance, a 1000 mah cell that is said to be 100 “C” would be built with internals calculated to handle 100 amps, not tested to handle 100 amps.
Now when you Dave, get a pack in hand and it’s a 100 “C” pack, you may not be aware of this fact, you might think the “C” rating is based on being able to do 100 “C” in your car, but I do not know of any re-sellers that base the “C” rating on their packs this way. I might very well be wrong, if some manufacture out there does indeed base their “C” rating on actual bench/car tested output please complain to Brian that I lied and I’ll gladly post the correct info.
Next point, you say that there is no rc battery on the market that can “sustain a 65C load”. I’m assuming you stated this based on no testing and are just chucking it out there as an opinion, which is totally your right to do. My input here is that cells have come a long ways the last 5 years. I haven’t tested any cells at 65C on the bench, but based on the temps and voltage levels we’ve seen at lower discharge levels, I would not be surprised to see a pack that could live at a relatively insane current level. 3 years ago? No way in hell, but now days there are some damn solid cells/packs on the market.
We have equipment (West Mountain CBA w/amp) capable of testing a 1S 1000 mah pack at 120C continuous discharge. Back in the day I knew such testing was a waste of time as packs would routinely die on 20C loads, but I should find some time and see just what kind of continuous load modern cells can actually live under. I think the
result might very well surprise you and I.
About jump starting a car with a Lipo, there are a number of videos out there, including one we did a couple of years ago. We easily jump started a 6 cylinder Ford Taurus with a 3S Lipo. The 3S Lipo didn’t get hot and lived to power our rc cars for many cycles afterwards. Also, I don’t remember for sure what connector we used (we may have just soldered on clips), but we were using the stock wire that came with the pack.
In closing- Lipo batteries used to be the limiting factor in rc cars power system, but not so now days. In most typical applications Lipo’s stay cool, last a bunch of cycles, and do not dip to crazy low voltages. These facts tend to make the advertised “C” rating moot. Yes, I agree with you that the “C” ratings are mis-leading and don’t really convey much (if any) useful information to a potential buyer, and yes, there needs to be a better “rating” system. If I owned Happy Flower Lipo Company my packs would be labeled with their cell count, their cell configuration, their typical 1C capacity, and their typical output voltage under a 10C load. Sure, a noob can look at two packs and instantly think that a 65 “C” pack is better than a 20 “C” pack, but they have no idea what “C” is. Noobs just know “higher is better”, something we’ve proven to not always be the case with our shootouts. With my “Cubby Rating System (CRS LOL)” of listing voltage at 10C, a noob could also instantly see that one 2S pack rated at 7.45 volts is “better” than a pack rated at 7.38 volts, and he’d have at least a rough idea of what the numbers actually mean.
I have a xp1200 esc from my sc10 4×4 rtr and a 540sl 6100kv motor (not the micro) the specs on the esc say that it can handle the 6100kv motor would this be a good combo for a 2wd truck on a 2s lipo and will the esc really handle it.
Cubby– Yo what’s up Glen, thanks for the email. I have absolutely zero experience with the XP line of Associated RTR speedo’s, so I’m totally gonna wing it here. Enjoy!
In theory… ya, the XP SC1200 speedo should work fine in a 2wd 10th scale truck. It comes stock in the 4×4 SC10 RTR that is considerably more load for a speedo than a 2wd would be.
Can the XP SC1200 handle a 6100 kv motor? Its specs says it can and my pure guess is that you shouldn’t have an issue if you keep your 2wd truck geared appropriately. If you gear for 120 mph I’ll promise you that you’ll have issues (read- a fire).
Cubby’s helpful hint of the week- Using a 6100 kv motor is great for speed no doubt, your truck should be a rocket with such a high kv motor, but it is especially important to keep an eye on motor/battery/speedo temps if you want your equipment to live. Start with a small pinion and monitor temps as you gear taller. And… don’t go out and run for 12 minutes then check the temps, check every couple minutes during the run before things get too hot. The speedo should protect itself from overheating, but you don’t want to see temps over 140 F on either the motor or the battery. Also keep in mind that the taller (bigger pinions) you gear, the faster you’ll go, but the less runtime you’ll get. Have fun, go fast, and shoot us a massive wheelie pic of your truck when you get it going.
That’s it for this weeks ASK Cubby. I’ve been get’n pounded with mail the last few weeks, but keep’em coming. My email is Cubby at BigSquidRC.com, feel free to shoot me anything that’s on your mind. Letters that hit the big-time get a free sticker pack and the email I proclaim as “Letter of the Month” gets a free BigSquidRC t-shirt.
YOUR Cub Reporter