Here’s a metaphor I came up with that explains how I think about the Li-Ion vs Lipo for Rx power discussion.
Disclaimer : I’m not a battery chemistry expert. I’m an IT professional that plays with electronics in my spare time. If I’m wrong or inaccurate, please let me know.
Note: to avoid confusion, when I say ‘our RC planes’ I’m talking about RC high performance aerobatic airplanes with digital servos which are 50+cc in size. (flown aggressively)
Let’s pretend that flying our RC planes is like driving on the freeway. Most of the time you’re driving at 65mph but every once in a while you need to pass a car...sometimes you need to pass a FAST car. Sometimes you need to get to 90mph from 65mph as quickly as possible. (if you haven’t figured it out yet, the passing of the cars is representing hi deflection maneuvers like snap rolls, pop tops, etc etc) Let’s assume that passing another car that is already traveling at 80 mph is equivalent to drawing 20A of current in our airplanes’ servo busses.
Let’s take a Li-Ion 2600 that is good for a 11A (4.23C) discharge, weighs 3.3 oz and costs $38. Let’s use our imaginations and pretend this Li-ion pack is a 2014 Honda Civic LX which has 143hp, weighs 2750 lbs and costs $18,000. It also has a 5 star safety rating.
The 2014 Honda Civic is an amazing car. Rated one of the safest cars in it’s class. It’s dependable and Honda has a reputation as a respectable company.
A stock base model Honda Civic can drive at 65mph with no problems...and can pass people when it needs to...however, it takes a little bit longer to pass the real fast cars...and the engine begins to truggle when it gets to say about 85-90.
Lets take our Honda Civic / Li-ion comparison and bring in a 30C 2S 2200 Lipo that is good for 66A (30C) discharge, weighs 4.5 oz and costs $15.50.
Let’s do some math to figure out what the specs of a car would be that could represent our LiPo battery.
POWER:
(4.23C/143hp)x(30C/xhp)
In this relational equation, we’d be looking for a car with 1014 hp!!!
So, to make this a little more believable, let’s downgrade our Lipo and make it 20C.
(4.23C/143hp)x(20C/xhp)
x now equals 676 hp.
WEIGHT:
(3.3 oz/2750 lbs)x(4.5 oz/x lbs)
We’re looking for a car that weighs 3750 lbs.
Let’s find a CAR!!
Let’s find a car that has 676 hp and weighs 3750 lbs.
How about a 2013 Ford Shelby GT500. It has 662hp, weighs 3845 lbs, and costs $55,000. The GT500 has a 4 star safety rating. (The Civic is safer)
COST:
We have to approach cost in a slightly different manner. Let’s compare the cost difference between the Li-Pos and Li-Ions above and factor in the cost of the Civc. We can find what our GT500 would cost!
($38/15.5)x($18,000/$x)
$x = $7342.11.
What if a 2013 GT500 cost only $7400?
If your goal was to drive on the highway and you NEEDED to pass people sometimes. Why on earth would you pay so much more for for a Base model Honda Civic LX?
SAFETY:
The civic is in fact “safer†than the GT500, but both cars are fueled by explosive gasoline. Both cars can explode, it’s now just a question on how easy or hard either car is to ignite. Some might also think that since the GT500 is much faster, it’s inherently more dangerous and “less safeâ€.
Li-Ions and Li-Pos are VERY much the same, but not completely. I think the Crash Test Rating of cars is at least somewhat comparable to the “safety†of LiPos and Li-Ions. BOTH can explode, both can vent flames, etc etc. I would like to have more information on the EXACT differences in safety between the two battery types….however….I’m not really concerned AT ALL. Why?
Mainly because I trust Lipos. Mainly because I’ve never had a lipo catch fire. (that I didn’t intentionally set) Mainly because for the first 5 years of my Rc experience, I flew ALL electric airplanes and have owned and charged at least 50-100 different LiPo batteries. No venting. No danger. I am a FIRM believer in the saying, “LiPos aren’t dangerous. People with LiPos are dangerous.â€
So….if, when you read this, you feel that Li-Ions are “safer†then Li-Pos in a way more pronounced than simply using a Crash Test Rating metaphor, I ask that you explain to me with facts to help me understand.
In the end, I ask you to think to yourself and answer honestly the following two questions:
1. Would driving a GT500 65 miles per hour for 95% of the time make you feel less safe than doing the same in a Honda Civic?
2. In the year 2014, are you actually AFRAID of 2S 2200 LiPos? So much that you’d rather pay so much more for a Honda Civic?
CONVENIENCE:
Let’s talk about the storage charge requirement of LiPos. Let’s equate this to having to check the oil level of your car. On the Civic, it’s a small motor and you REALLY don’t need to check your oil OUTSIDE of every 3,000 - 5,000 miles when you have to change it no matter what. Let’s assume that with our Mustang, we are required to just CHECK the oil at the end of the day and add oil if it’s too low. (We can equate adding oil to storage charging batteries because discharging and charging is the exact same action on our part...just pressing different buttons on a charger.
Is that convenience reason enough to pay so much more for a Civic?
CONCLUSION:
In this metaphor, keep in mind that the Power, Cost, and Weight of the battery to car comparisons are factual and based on real numbers. This is a very accurate comparison.
The highway driving scenario, safety rating, and convenience are more conceptual comparisons and are admittingly open to debate.
ENOUGH OF THIS METAPHOR!
Here are my final and honest non-metaphorical thoughts and opinions:
Safety revisited:
Lipos are safe. Learn to handle them correctly and take care of them and they’re safe. Li-ions are safer. That doesn’t automatically make Li-pos UNsafe. Li-Ions being “safer†is not a big enough reason (to me) to use Li-Ions in our planes. If it is to YOU, that’s your decision and you will pay for it in certain ways. (more voltage drop, more money, longer charging times, and slightly lighter battery packs)
Convenience revisited:
I baby my Lipos. I have no problem taking them out of a plane to charge. (I do it in my electric planes all of the time...you probably do too.) I have no problem keeping them at a storage charge if I’m not going to use them for more than a week. The storage charge requirement is thus not a reason for me to use Li-Ions. If it is for YOU, that’s totally fine and you will pay for that convenience in certain ways (more voltage drop, more money, longer charging times, and slightly lighter battery packs)
Longevity:
People claim that Li-Ions (particularly Fromeco Relions) last a very long time with little to no storage loss. Fromeco claims the life of their Relions is two years. We all know that Fromeco under rates alot (this is a good thing) and that figure is probably just to be safe.
Many of those same people also have the notion the Li-Pos don’t last as long in comparison.
Let’s be careful about that assumption and look at some more numbers.
A Relion 2600 running 7A is actually (based on 4.23C) running at 63.6% (7 divided by 11) of its capacity. Double up 2600s and you’re running both packs at 31.8% of their capacity. (7 divided by 22) Those numbers look just fine.
Let’s do the same with Lipos:
A Glacier 30C 2S 2200 running at 7A is running at only 10.6% of it’s capacity. (7 divided by 66) Double up the LiPos and they’re churning at only 5.3% of their capacity!
Ask ANY electrician and I bet they’ll tell you that an electrical component running at 5.3% has a MUCH better chance of lasting a long time compared to something running at 31.8% capacity.
In my personal experience….I have a pair of 4 year old Thunder Power 2S 1350s that have only been used for rx power in various 50cc planes that still act as new. They’ve never been pushed in the least bit...and they still work wonderfully.
Load Checking:
What about load testing Li-Ions? For everyone who claims this is one aspect that makes Li-Ions a better choice and more reliable….please read the following very carefully:
drumroll.....
You can load test a LiPo. There’s nothing stopping you. The Fromeco 8-ball doesn’t have a Li-Ion-only connector on it. The 8-ball won’t blow up if you hook it up to a LiPo pack.
Load testing is not a special characteristic of Li-Ion batteries.
It’s a good idea, I'll give you that!! In fact, I think I’m going to order an 8-ball myself. (however...load testing is in no way shape or form something that makes Li-Ions better or worse. It’s not a feature of the chemistry...it’s a procedure!)
In the end, there is no right or wrong. There is only a better choice based on your own situation.
I hope/wish that this write-up makes you at least THINK about what you want in a battery and the actual differences. I ask that you weigh the options and make the decision for yourself based on some real numbers.
