lipo, voltage, mahs oh my! Lipo understanding needed

[totalgeek]

I saw the awesome FAQ in the sticky but nothing seems to touch on lipos. Is there a lipo FAQ someone can point me too? I'm using a battery my LHS lent me (thanks Mark @ HC&M Reno) as I went budget busting getting my heli up - thankfully I'm not married anymore :)

Here are some questions:

1. I'm using a 3S 11.1v 2100mah pack. My current setup is off-brand 3400Kv brushless motor and an Align 25A ESC. Once I'm out of hovering phase what is my continous flight time on that pack? Since I'm learning I constantly go up and down and now have the copter spun up continously so my flight times look like 40 minutes.

2. With above pack what happens if I only went with a 2-cell? Am I looking at less flight time, less amperage, or would I know the difference with going with a 2 cell versus 3 cell.

3. Are you able to quick charge lipo yet? It seems to take me almost an hour and half to charge using a simple charger. From my RC car days I was able to quick charge quickly with Nimh.

4. Whats the difference with number of cells and voltage coming off the pack isn't only voltage important when using a BEC? (you want to make sure you're above your BEC cutoff mines is 5v so anything higher would be fine - I'm guessing) Or does it increase flight time.

5. What is this discharge rate I keep hearing about? I hear getting a battery with a 15C or higher rating is good. But what does that mean?

6. Is lipo dangerous? I hear about fires all the time

7. My lipo charger is warm when charging but the battery I'm charging is only room temperature. Is this right? I remember Nimh getting warm.

Sorry to bother everyone again with an easy question. I guess I don't know how to use the search function. I searched for 'lipo' and too many non-relevant items were returned so now I bow down to the masters in order to get some help with understanding lipo before buying my own packs. TIA.

[spork]

There's a brief section on batteries on my web page (link below this post). Just click on the "just starting out" link on the main page and scroll down to the batteries section.

1. I'm using a 3S 11.1v 2100mah pack. My current setup is off-brand 3400Kv brushless motor and an Align 25A ESC. Once I'm out of hovering phase what is my continous flight time on that pack?

That will depend mostly on your head speed and how you fly. High head-speed, and pushing it hard, figure on something on the order of 6 minutes. Lower head-speed just flying around will be more like 10 minutes. You need to use a timer, a LiPo alarm, or both to make sure you don't drain more than 80% of the rated capacity (that will destroy a LiPo in a big hurry).

2. With above pack what happens if I only went with a 2-cell? Am I looking at less flight time, less amperage, or would I know the difference with going with a 2 cell versus 3 cell.

LiPo's are rated at 3.7 volts/cell. A 2-cell pack will therefore give you only 7.4 volts. Normal flight and basic maneuvers will still require the same amount of power, so you'll be trying to draw more current (amps) from the pack than it can deliver. This will yield poor performance and damage the batteries quickly. Never let your LiPo's get above 140 deg-F. This will destroy them quickly too.

3. Are you able to quick charge lipo yet? It seems to take me almost an hour and half to charge using a simple charger. From my RC car days I was able to quick charge quickly with Nimh.

The common wisdom is to charge LiPo's at no more than 1C (charge in one hour). I think some are charging at 2C (1/2 hour) or even 3C (20 minutes), but do so at your own risk. Be sure not to charge them while they're still warm, and be sure not to let them get too warm while charging. NEVER charge a LiPo with a non-LiPo specific charger. Not only can this destroy the pack quickly, it can burn your house down. This isn't like a lot of warnings - this one is real.

4. Whats the difference with number of cells and voltage coming off the pack isn't only voltage important when using a BEC? (you want to make sure you're above your BEC cutoff mines is 5v so anything higher would be fine - I'm guessing) Or does it increase flight time.

Look at the "battery configuration primer" on my webpage to get a basic understanding of this. If you put your cells in series you add the voltage of each cell. If you put them in parallel you get a pack with the voltage of a single cell, but add the capacities. The BEC supplies 5V to the RX, gyro, and servos, but it supplies full pack voltage to the motor (switched rapidly on and off to throttle it). Generally speaking, more capacity increases flight time; more voltage increases flight performance.

5. What is this discharge rate I keep hearing about? I hear getting a battery with a 15C or higher rating is good. But what does that mean?

Any battery is capable of putting out only so much current. We describe that in terms of xC (1C, 2C... 15C, etc.). This means 15 * capacity (per hour). In other words a 1C battery could discharge itself completely in one hour. A 15C battery could discharge itself completely in 4 minutes (because it can put out 15 times the current as a 1C). Trying to draw more current than a battery is rated for will heat it up. Doing this too much, or for too long, will drive the battery temp above 140 deg-F, which will damage/destroy it quickly.

6. Is lipo dangerous? I hear about fires all the time

LiPo's can be relatively safe if you handle them properly. They can be extremely dangerous if you mishandle them. Only charge with a proper LiPo charger set to the proper number of cells. Use a balancer when charging, or use a balancing charger. ALWAYS charge them outdoors away from anything flamable, or in a fire-proof safe. Never walk away from a LiPo that's charging.

7. My lipo charger is warm when charging but the battery I'm charging is only room temperature. Is this right? I remember Nimh getting warm.

That sounds normal.

[totalgeek]

spork thanks for the answers and the link to your web page. You actually have a lot of good 'starting' out device. A shame I didn't see your link come up in google on of the first million pages or so when searching for 'rc heli beginner'.

Your post and comments are very informative. So from the sounds of it, even though your web page only recommends 2 packs, we should start out with 4 packs. Because once one is used, and wow 6-10 minute flight times I thought it would be much higher, and we wait to cool it down to charge again it can be several minutes before the copter is up and flying again.

I noticed my ESC comes in a 35A flavor as well. How do I know that I will need a 35A ESC?

Spork thanks for your response and an informative website. I'm going back to reading it some more as there are tidbits of information on there that I find helpful.
You mention a balancer what is the purpose of that device? Do we need one? My charger doesn't seem to have a balancer on the charger or that I can find in the manual. So I need to find out what the maximum voltage is that goes to my engine? Can putting more voltage to the engine then its rated for damage it? Or are we safe in going to higher rated voltage packs?

[spork]

There's certainly nothing wrong with getting 4 LiPo packs. But a lot of beginners find their knees are shaking after a couple of packs anyway. So taking a break to let the packs charge isn't the worst thing.

As far as needing a 35A ESC that will depend again on the motor and pinion you're running, and how hard you push it. It's unlikely that you'd need the 35A ESC for a while. The best way to tell if you need one is by logging the onboard current with a data logger, or simply asking on the forums about the particular combo you're using.

The balancer is used to make sure all the cells in your pack stay at the same voltage. Unlike NiMH's and NiCAD's, LiPo's do not tend to self-balance. This can lead to overcharging, and damaging (or even exploding) one cell because the charger is trying to bring the pack up to voltage while another cell is low. Balancer's or balancing chargers are a must.

Putting too much voltage in the motor can damage it. But you can't put too much current in it. The motor will only draw the current it needs. I assume (for some reason) you're flying a T-Rex. If so, you should stick with a 3S pack in the 2000 mAH range for now. You may later decide to go 4S and make the necessary changes to ESC and maybe BEC.

[totalgeek]

Then does that mean we can only have so much voltage or so many cells in a series? So if we have 5 cells and the voltage is around 15+ is that too much for a copter engine to handle or does the ESC only draw what it needs voltage wise? I need a primer on electricity - off to google.

I fly an HDX450 similiar to a TREX, it will be a TREX soon with all the Align parts I put on it.

[spork]

Given the appropriate voltage, the motor and ESC will only draw what's needed. There's no such thing as having too much capacity (mAH) or too much current capability (e.g. 30C). However, if you use a pack that has too high a voltage, it's like drinking from a firehose. The motor WILL end up swallowing more current than it should - and then the smoke gets out. :shock:

For our motors and ESC's, I can't say what value that is. But 4S LiPo is the highest voltage I've seen used on a T-Rex 450 (for what that's worth).