My adventure into LiPo power!|
June 04 2010
Until recently I've avoided LiPoís due to how complex they soundedÖ C Ratings, Balancing, Voltage Cut Offs, Human sacrifice, Dogs and cats living together... Mass hysteria!Ö you get the idea (Internet high-five if you got the Ghostbusters reference). Well, after doing some research and finding a great deal online for LiPos I decided to take the plunge and let me say, Iím NEVER going back to NiMHís again!
Iím writing this article as an introduction to LiPoís and not as any sort of engineering guide on the subject. Who cares that Lithium is #3 on the periodic table and LiPos differ from Li-Ion batteries by not containing an organic solvent? Although it makes me look
totally amazing and uber smart like I can use Google, it's of zero use to you when youíre at the store looking to buy oneÖ
So lets skip all the stuff you donít need to know, and get right down to the stuff thatís actually useful shall we?
Pros, and Cons of Using LiPos
|They are aprox 30% lighter then an equivalent NiMH
||They require balancing periodically
|They provide longer runtimes
||You need a LiPo specific charger if you don't have one already
|Discharge rate is consistent (You get full power for the whole pack)
||Using them till they are dead is bad mmmmkay
|Donít heat up as much while in use or while charging
||Require more maintenance/attention then NiMHs
|Zero battery memory (NiMHs claim this as well, but they still do)
||See "Safety Warning" section
|They retain a charge longer when not in use then NiMHs
You probably heard that LiPos can be dangerous, and yes they can be but no, they aren't going to spontaneously go nuclear leaving a small crater where your house was. There is no reason to be over-paranoid about this warning as LiPos arenít any more dangerous then the Propane tank under the BBQ, fire extinguisher in the cupboard, or mom's hairspray you
use to blowtorch your sisters toys never touch.
However, since there are people out there that need to be told the small pack of silicon they get with their electronics is in fact NOT free candy, here is a list of pretty common sense rules when using LiPo batteries:
That's it! If you want you can even get an item called a "LiPo Bag" to keep/charge them in. These bags are fire resistant and can be picked up pretty cheap from your local hobby store. Of course, it wont stop your house filling with toxic smoke, but hey, its better then nothing right? :smile:
- Make sure you use a charger designed for LiPos and ensure settings are correct for the pack you are charging
- Don't leave them unattended while on the charger
- Donít exceed the recommended charging rate (more on this later) for the pack
- Keep them stored at room temp, no hiding them in the furnace or deep freeze
- If the outer casing ďpuffs upĒ, stop using the battery
- When your done using them, don't leave them in your RC
- Dont drain/use them below thier low voltage rating (Aprox 2.7v)
On to the basics:
There are few numbers to know when shopping for a LiPo: Capacity, Voltage, Cell Count (which is directly tied to voltage) and Discharge Rate.
Capacity (Known as mAah)
This number indicates how much energy the battery can store. Its like a gas tank for the battery. The larger the number, the more it can hold; meaning the longer it will run. There is some math behind this number, but again... who cares? The only math you need here is: The higher the mAh = the longer you have fun.
Voltage (Known as v - I'm not even making this up people!)
NiCad and NiMH batteries use cells that produce 1.2v per cell whereas a single LiPo cell produces 3.7v. The more cells in the pack connected in series (more on this in a minute), the more voltage the LiPo can provide.
1 cell x 3.7 volts = 3.7v
2 cells x 3.7 volts = 7.4v
3 cells x 3.7 volts = 11.1v
... and so on
Voltage also affect how fast your RC goes. If you have a motor/esc that uses between 7.4v and 14.8v, the 7.4v will be slower because it cant provide as much power. But don't
be an r-tard and throw a 22.2v LiPo into a 7.4v RC, every ESC/Motor has a Min/Max voltage rating and if you exceed it you'll just end up frying the whole thing.
Cell Count ("S"):
This is a good time to bring up cell count because it ties directly to voltage. You already know each cell is 3.7 volts so unless your talking to a real nerd you generally wont hear people say "I am looking for a 18.5v LiPo". Instead most people simply refer to the number of cells in the pack.
1s = 1 cell (3.7v)
2s = 2 cells (7.4v)
3s = 3 cells (11.2v)
... and so on
So if your RC takes 22.2v, you'd be looking for a 6s battery. That's all there is to that.
Finally the last number on the battery that actually means anything to you. The C rating is simply how fast the battery can "give up" or discharge its power. The larger the number, the faster you can drain it. There's a ton of math involving big words and a formula, and if your into that sort of thing
you need more hobbies that's cool, ill leave you to Google it :)
The math that you do need to know is how many amps your ESC can draw. Bad things can happen when your ESC asks for more power then the LiPo can produce so its important to figure this out, luckily its fairly simple.
(mAh x c ÷ 1000 = amps)
Hang on its really not that hard. Say you have a 100amp ESC. and you wanted to pick up a 5000mAh LiPo for it... well you would need at least a 20c rate because 5000mAh x 20c ÷ 1000 = 100amps.
However, if your ESC was 120amps, a 20c battery wouldn't be enough and you would have to go to a 25c (5000mAh x 25c ÷ 1000 = 125amp) or higher.
Other slightly less important stuff you should know:
Series vs Parallel
The concept of this is simple, but explaining it
is a bitch not so much. Cells (or batteries) connected in series will multiply the voltage, and cells (or batteries) connected in parallel will multiply the mAh. Are you following me camera guy? (Another high five for those who got the Shamwow reference).
So... say you have two exactly the same 2s 5000mAh batteries.
Connected in series and you get the equivalent of a 4s (14.8v) 5000mAh
Connected in parallel and you get the equivalent of a 2s (7.4v) 10,000mAh battery
Here's where your head may explode: Some batteries will come as a combination; For example you can buy a 3S2P 5000mAh LiPo. This means inside the battery there are two separate sets of 3s (11.1v) packs hooked up in parallel to double the capacity. So even though it says its a 5000mAh battery, its ACTUALY two 3S 2500mAh batteries connected together, but all wrapped up as one neat little package.
Standard or Hard Case?
Despite what they look like online, standard LiPo batteries are not soft gel filled bags of power (keep the breast jokes to a minimum people!). They are actually fairly ridged and durable, and hold up just fine to ďevery dayĒ type of abuse. Hard Case LiPos have a thin, plastic "shell" and are more or less used in situations when there is a risk of the LiPo being damaged by debris or in a crash. You might want to check with your local track before you buy, some tracks require you to have a specific type either for safety, or just to keep things fair among racers.
As you now know, your batteries are made up of multiple cells. These cells need to be charged, and discharged at the same rates. Thatís all that balancing is. Most decent LiPo chargers will monitor each cell in your battery and ensure they charge at the same. If you get a charger that doesnít balance while charging, you can pick up a balancer. Itís a small device you plug into the LiPo and it will discharge each cell independently until they are at the same voltage. Its then you can pop it on your charger and you should be good to go. Personally, I own a balance charger and would highly recommend them as it makes the whole charging process as easy as charging NiHMs.
Low Voltage Protection
When a LiPo is almost out of juice its voltage drops off very quickly. If any cell goes under aprox 2.7v (this number can vary between manufacturers) they can be ruined and may not properly hold a charge again. To protect the LiPo there are 3 methods available:
- Low Voltage Cut-off (LVC) Ė Best Method
Higher quality ESCís usually have a LVC built right in. When it detects the battery getting too low it will simply shut down the RC or lower its power usage so you can drive/fly it back to you. If you donít have one built in, there are external LVCís available.
- Low Voltage Monitor (LVM) Ė Meh, It works if you pay attention
A LVM is like your carís fuel light. It doesnít really do anything other try to get your attention by sounding a buzzer, beep, or flash some LEDs to warn you when your running out of power. If you happen not to see/hear this warning you run the risk of ruining your battery.
- Noticable drop in power - If your reading this article, probably not the best way to go
When a LiPo reaches it's low voltage threshold it is pretty noticeable. It looses a good 20% - 30% power and even then you still have time to get the RC back to you safely. This drop in power is your "last warning" before you damage the battery. LVCs and LVMs can fail occasionally, so even if you have one and you notice this drop of power, bring your RC in right away.
There's quite a bit to learn about LiPos at the start, but overall they arnt much more complex then NiMHs. I've had mine for +8 months (at the time of writing this) with zero issues at all and the only regret going LiPo is that I didnít do it sooner!!!
Hopefully I have been able to clarify some of the basic terms and concerns for anyone new to the world of LiPos. See you at the track!
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