Lipo batts put your ordinary NiMH or NiCD to shame, and I am not going to spend a lot of time going over the reasons why, but I will mention a few key things about lipo:
- Lipo holds Voltage a LOT better. Whereas your NiMH battery will be going downhill lap after lap in it's performance, with Lipo, you would almost not know the difference until the battery is near empty
- Lipo Offers higher power density, allowing you to run high powered 1/8 and 1/5 rigs that draw a LOT of amps.
- Modern Lipo chemistry allows you to safely charge and discharge at much higher rate than NiMH.
Trying to keep the same general format of my previous post, I'll start with a little bit of Lipo specification explanation and then go into some formulas and examples.
Any lipo on the market is going to have several parameters that determine it's application:
- Cell count. Every lipo cell is roughly 3.7 Volts. Total amount of cells determines batteries total voltage. Ex: 2S = 7.4 Volts; 3S = 11.1 Volts etc.
- Milli Amp rating. Thing of it as lipos fuel tank. You'll see anywhere from 50Mah to 12,000Mah batteries out there. Mah amount directly affects battries form factor, so make sure to measure your battery tray before you go out and buy a 12,000Mah batt hoping to run it for 2 hrs straight.
- Discharge Rating: It's indicated with a small "c", preceded by a number. This number, along with the batteries capacity in Mah is going to determine the max amp output of your lipo. I will elaborate on this later.
- Charge Rating. Indicated with a capital "C", preceded by a number. This number combined with batteries capacity is going to determine it's maximum safe Amp/hr intake when charging.
Example: 2S 5,000Mah 40c 5C
^ ^ ^ ^
Cell Count Capacity Disch. Rate Charge Rate
*Any battery is going to have a min and max discharge rate. Example above is 40c-50c
So now that we've covered basics, lets go over actually figuring out what each of those numbers does and how to figure out what battery you can get for your particular application.
Here are some factors that are going to determine which battery to get:
- Battery tray size
- Cell count permitted by race regulations or Motor/ESC limits
- Motor Amp draw at peak power
List above is in no particular order, and any of the first 2 factors can be more important to you. Only thing to keep in mind, is that both Mah rating and Cell count affect final battery size, so what I would do, is decide on cell count first, and then try to find the largest Mah amount battery that will fit your battery tray.
Most manufacturers clearly specify their battery dimensions, so it's definitely something to pay attention to.
Once you picked you cell count and maximum Mah capacity your battery box allows you, you should look at the motor's manual and figure out it's maximum amp draw. Reason it's important, is because there are lipos rated for slower discharge rate and will cause your system to overheat and cog.
So at this point, you know the size and voltage limitations of your battery, and you just found out what kind of Amp draw your battery is going to have to pull, so lets dive into some formulas on how to determine that:
To calculate max continuous amps your battery could provide:
Multiply packs capacity in Mah, by it's discharge rating in c.
so with the example I used above, we get 5,000Mah x 40C = 200,000 Milli Amps, which is 200 Amps.
Because most batteries list two "c" ratings, continuous and "burst" discharge that lasts around 10 seconds, you can calculate the second value using the same formula.
If you found that your battery rating does not reach your max motor amp draw, you need to look at similar batteries with higher discharge rate or you will create unhappy components. Generally, you want to over-match you motor's max draw by as much as you wallet allows you to. And you'd be surprised, there is almost no $ difference between a 20c and 40c battery on HobbyKing.com. I've yet to find a person who suffered from higher discharge rate. Key thing to understand is, even though your battery can potentially provide 200Amps, your motor will only draw as much as it possibly could, so there's no danger in running a high c rating.
Few things should be said about handling a lipo. It's VERY important to treat your lipo nice, or it will burn down your RC or your house all together, so lets go over few things to do and not do with a lipo.
- Never short the leads, if you are soldering connectors, undo shrink wrap or unsolder from old connector ONE WIRE AT A TIME. These things are no joke. Some are capable of turning over your car engine, so don't piss your battery off.
- Never charge or discharge your lipo above manufacturer's recommended ratings. Both ways, you are going to piss it off, and pissed off lipo puffs, bursts and smokes a bunch or goes up in flames.
I'll note, some lipos are rated to be safely charged at 5C, which is way more than conventional 1C charge rating, so make sure to read the label and figure out what your battery is capable of. Generally speaking, higher density ( discharge rate) yields higher charge rates as well.
- Never puncture or mess with lipo wrapping unless you know what you are doing and plan on taking a cell out or smth like that. Chances are, if you are reading this, you don't yet know what you re doing. so just don't do it. Lipos get very pissed off when you poke em with sharp objects.
- Never leave your lipo unattended while charging. Yes its safe if you treat it right, and nothing should happen, but why chance it? In fact, I'd recommend getting a "lipo sack" for charging, which protects you from any fire accidents.
- Don't leave your Lipos fully charged/discharged. Lipos like to be stored at 3.7Volts per cell, so try to charge your packs before you go out, and discharge/charge em back to 3.7 per cell when you are done. It makes lipos happy, and happy lipos will make you happy too.
All of the above is equally important, so if you re planing to keep it safe and get a lot of use out of ur packs, follow the advice above.
If you didn't read any of what's written above, or didn't get any of it, this last section is going to provide you with a few examples which will hopefully clear things up!
Lets pretend you are picking a battery for your 4x4 short course truck that is running a motor that is capable of drawing 130Amps. You need to run 2S and your tray wont fit anything larger than 5,000Mah.
Here are some batt's you are choosing from:
1) 2S 5,000Mah 20c-30c 1c
2) 2S 5,000Mah 30c-40c 2c
3) 2S 5,000Mah 40c-50c 5c
For battery #1, using the formula to find out it's continuous amp draw capacity, we get 5,000Mah x 20 = 100Amps
100Amps is not enough for out 130Amp motor! It will create heat, cogging, resistance, some more heat and so forth in a vicious circle.
20c discharge rate is no good for our truck, and we'll let some other fools buy that battery.
For battery #2, we get 5,000 x 30 = 150Amps. Is it enough to run the motor we got? Yes! In fact, we got 30 amps to spare, so we should be OK for a while. Only one problem... Lipo batteries loose their "punch" or c rating over time if not handles properly and or get used a lot. So is this a good choice for that truck? Yes, but... if there's any more buffer room we can create by getting a higher discharge rating, I would totally do it. This is my personal opinion from the little experience that I had, so take it or leave it.
For battery #3, we get 5,000 x 40 = 200Amps of continuous discharge, which is plenty more than out motor requires, so our battery would be able to supply it with full power time and time again, even under high heat or high load conditions, even after the battery deteriorates some over time. This would be MY battery of choice for the application.
Note on charging:
Notice how my 40c battery is rated at 5C for charge rate? It can safely take up to 25Amp charges, which reduced my charge time to under 15 minutes, which lets be honest... kicks (bleep)(bleep)(bleep)! Next thing is finding a charger and a power supply that can provide a 25Amp charge Luckily, they re out there, you just gotta be willing to spend some cash.
I think I am just about out of ideas and suggestions for you guys. I'll proof read a little later, so if anything doesn't make sense, or something is incorrect, please correct me or ask me to rewrite. Otherwise, Enjoy! Hope this helps!
Took me almost an hour! Daaaaamn
Can I have a cookie? Or at least a URC decal to make me "Cooool"?