Understanding Lithium Polymer batteries

RC pilots need the best available batteries that are not only cost effective by means of recharging capability but the highest energy density that is possible to obtain. At this point in time Lithium Polymer batteries are the best fit-for-purpose batteries available on the market today for RC and some other businesses.

Excellent reply. Thank you.


I will never understand electricity ... so please excuse my foolishness. Electricity is the work of Satan ... and man will never control it ;)

Obviously not . but it would seem to me the typical hearing aid battery would be an option ... very light, long lasting. Could probably supply one to each motor .... of course there needs to be a speed controller between each one. Whatever ... I don' know nuttin' 'bout dem there batries. I suppose it is a matter of energy capacity.

One thing I do know ... 5-30 minutes flight time barely justifies the enjoyment.

I will never understand electricity ... so please excuse my foolishness. Electricity is the work of Satan ... and man will never control it ;)

Obviously not . but it would seem to me the typical hearing aid battery would be an option ... very light, long lasting. Could probably supply one to each motor .... of course there needs to be a speed controller between each one. Whatever ... I don' know nuttin' 'bout dem there batries. I suppose it is a matter of energy capacity.

One thing I do know ... 5-30 minutes flight time barely justifies the enjoyment.
I strongly recommend you learn some basics on electricity and RC hobby can make the learning more fun. Failing to learn could mean learning the hard way after the fact.


Some of my post was in humor. Your advice is well taken. Always room for more knowledge. I believe I have a respectable degree of the basics. Thank you for the advice.


New Member
Thanks a lot Bartman for this very informative post... Was a real pleasure to read every single line!

For further reading, I would like to share some experiences I got with Tattu 4S 16000 mAh batteries on my last built for others to be cautious about the issue.
I try to respect the 80% rule, but crashed my frame before it reached the 3.5v limit.
After further investigation, I noticed the following:

- I measure the voltage at two points: on the balance plug of the battery through the Taranis telemetry and in the Naza v2 through the iOSD
- I have an alarm set at 3.7v at the battery level
- I noticed that I have a difference of about 1.25v between the two measure points
- On my last flight, I landed when the Naza showed 13.4v on the iOSD and the battery 3.65v per cell on the Taranis
- Under noload, the battery went back to 3.8v per cell just after landing
- I charged the battery and put back 9700 mha

Here is my issue: as you can see, I'm quite far away from the 80% rule regarding the capacity (12800mhA for a 16000mhA battery). Can I go under 13.2v (3.3v per cell) under load measured within the Naza and displayed on the iOSD if I don't use more than 80% of the capacity? If i do so, will I risk a sudden voltage drop leading to a crash?

Many thanks in advance for your precious opinions...


I don't know all I'd like to know about anything. But I have 2 brand new "HI-C" batteries, 2250/65C, that must be bad. (no name dropping). They drop to 3.5 per cell, a lot faster than any of my other batteries of the same capacity, but very quickly jump back up to 40% total after you shut them down. Kinda acting like a deep cycle boat battery. Is that normal for the HI-C's ??? or do these just need a few cycles to straighten out ???


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Hard to say Cooter, if the capacity is lower but the C is higher they'll still run low very quickly.

Just thinking out loud but if the C rating is high then big swings in voltage might make sense as the battery is allowing large/fast outlays of electrons so they've got to come from somewhere and that might allow larger swings in voltage. Never really thought about larger voltage fluctuations being a consequence of higher C ratings. Maybe with larger capacity batteries you won't notice it as much but with your 2250mah packs it's more obvious?



It was pointed out before that your LiPo cells are nominally rated at 3.7 volts per cell. When they're finished charging they will actually be at 4.2 volts per cell. As the cells discharge during use the voltage being produced will quickly go down from the 4.2 value and remain fairly steady in the range of 3.75 to 3.8 volts per cell. Under a heavy load they will drop as much as .3 to .5 volts but when relieved of that load they will return to and remain at approximately 3.7 volts until almost fully discharged. It is, therefore, this voltage level that represents the rating of the battery.

You may be asking (as a true LiPo Ninja would), how do you determine when a battery is depleted and it's time to land the helicopter? There are a few rules of thumb to determine how much of the battery's capacity can be used safely. One rule of thumb says to not use more than 80% of the battery's capacity. To know if you've gone beyond 80%, it helps to have a charger than can keep track of how much capacity (Total mah) was put back into the pack during the charge cycle. Divide the mah put back into the pack by the rated capacity of the pack (and then multiple by 100) to determine the % that you used and try not to exceed 80%. If you are keeping track of how much time you're flying and how much mah is being returned to the pack with each charge it isn't hard to come up with time guidelines that will keep you from exceeding the 80% discharge rule of thumb.

Another rule of thumb (the one I prefer) is to not let your cells go below 3.3 volts. There are inexpensive and very lightweight battery monitors/alarms that plug into the battery balancing tap/plug and that can be velcroed to the side of the battery. They are generally set to alarm when any one cell in a pack goes below 3.3 volts. The alarms may activate if you quickly increase throttle causing the voltage to sag but if they stop when your reduce throttle then you're generally safe to continue. A continuous beeping alarm during normal hovering flight indicates it's time to end your flight (preferably with a nice landing). Again referring to the information provided by a good computer controlled balancing charger, it's possible to see exactly what the individual cell voltages are in the pack when it's hooked up for charging. This is a good opportunity to decide if it's safe to extend your flight times a little or if they actually need to be shortened.

It's important to know that the cells in your LiPo packs must be kept between a range of 3.3 volts and 4.2 volts. The upper limit is managed by your battery charger in that it should stop the charge cycle when 4.2 volts per cell is reached. It is up to you though to keep from discharging your batteries below the lower 3.3 volt limit. Multi-rotor helicopters are unique in that we don't generally use any voltage protection features within our ESC's to protect the batteries from low voltage damage. If we did, we'd have helicopters falling out of the sky as the ESC's tried to protect the batteries by shutting down the motors. Some flight control systems have methods to alert you of low voltage condition. Consult your user manual or online wiki for more info.

There's another detail on the battery label that is important to understand and it too is listed with a capital C. It indicates the limit at which you can charge the battery. Modern LiPo balance chargers will usually auto-detect the voltage of the pack and will choose an appropriate voltage setting for the charge cycle. It's up to you though to select the amount of current that will be applied to the pack during the charge cycle and this will determine how quickly the pack will finish charging. The capital C again stands for capacity (mah of the pack). To understand this let's look at a 4S pack with a capacity of 5000mah and a max charge rating of 2C. THe 2C rating indicates that the pack can be charged at a rate of two times the capacity. Divide the capacity of 5000 mah by 1000 to get 5 and then multiple it by 2C. The maximum current (aka amperage, aka amps) setting you can use on the charger is 10 amps. Some batteries may be rated as high as 15C but keep in mind that charging at maximum rates will likely reduce the life of the battery. A ten minute charge may sound nice but it will likley shorten your battery's life which isn't so nice.

Also regarding chargers and charging, there are a lot of chargers that advertise the ability to charge multiple batteries simultaneously using what is called "parallel charging". This means that groups of packs are connected to each other and to the charger in order to be charged (as far as the charger can tell) as one large battery pack. It should go without saying that if you don't fully understand how to charge one pack at a time then please don't attempt to parallel charge multiple battery packs. A significant danger involved with parallel charging occurs when packs are connected together where one pack has significantly more charge remaining than the other packs. What happens is that the greater voltage in the less-discharged pack will cause current to flow into the more discharged pack with the lower voltage. To put it another way, the higher voltage pack will act like a battery charger on the lower voltage pack! The problem is that this happens very quickly and without any control so it has the potential to start a fire. The rule of thumb for parallel charging is that the individual cells within the packs should all be within .1 volts of each other prior to being connected to the charger. This means all of the batteries being connected need to be at a similar state of discharge.

Why balance charge anyway you might ask? In the early days of LiPo batteries, it was common to charge the packs like any other battery pack by connecting the charger to the power plug and charging it as if it were one big battery cell. It wasn't that this didn't work, it's just that when one cell within a pack became out of balance with the other cells in the pack, it created the potential for the pack to overheat and catch fire. There are many stories on the internet of people that connected their chargers to their car battery, started a charge cycle and walked away only to find a few minutes later that their battery had caught fire along with their car! Houses, workshops, cars.....they've all been lost to LiPo fires that started during battery charging.

It didn't take too long for the LiPo manufacturing and using community to realize it was important to monitor the individual cells during the charge cycle and to control/charge them as individual cells. When you charge a four cell pack with a balancing charger it is actually running four charge circuits (one for each cell) and speeding up or slowing down the charging cycle for each cell as they lead or lag each other. If one cell takes a charge more quickly and its voltage begins to exceed the voltages of the other cells in the pack, your balancing charger will slow it down so as to keep it at a similar voltage as the other cells. It is "balancing" the voltages of the individual cells as the pack is being charged! Get it? The potential for batteries to catch fire during charging is always there so please only charge your batteries on a non-flammable surface like cement, outdoors if you can, and try not to leave your charger unattended while charging.

If you've come to LiPo batteries after having used NiCad or NiMh packs you may have wondered what that extra set of wires coming off of the battery was. The extra wires are connected to the balancing plug and the wires/plug together are commonly referred to as the balancing "tap". Some chargers will charge only through the balancing tap, others will charge through the main power wires but also through the tap for control of the process. There are charging guides that say it is safe to balance charge every ten or twenty charges but, in my humble opinion, if you have a balance charger, use it every time you charge your batteries.

Internal resistance is yet another concept that relates to the overall health of your LiPo battery packs. Even a simple length of copper wire has a certain amount of resistance in it. Resistance (measured in Ohms) is the characteristic of an electrical circuit that prevents electricity from flowing. It won't necessarily stop the flow of electrons but it will just make it harder for them to flow. Make it too hard for the electrons to flow and the current passing through the wire will produce heat. Make too much heat and we're back talking about, you guessed it, fire. High resistance in your batteries will make them run hot for a little while and there will be some signs things aren't good before they think about catching fire but if you ignore the warning signs completely a fire may be just around the corner.

Chances are if your battery packs aren't delivering the same power they used to, resistance is building inside the packs and it may be time to considering disposing of them. Other warning signs would include packs that are hotter than usual after use and puffing which is what it's called when your normally flat and tightly packed LiPo battery begins to take on a more inflated and rotund appearance. Slight puffing isn't the end of the world but a LiPo Ninja would take it as a sign to watch things more closely and to monitor/measure the cells' internal resistance on a more regular basis.

Here's a video on how to measure internal resistance

The video demonstrates a procedure for a 3S pack so you'll have to modify the procedure if you're using packs with different cell counts. If you're checking your packs on a regular basis, logging the values on the cells will help you to see when the values begin to increase indicating the pack is aging and isn't as happy as it once was.

Some advanced chargers will be able to display the internal resistance of the cells. Refer to your user manual to find out of your charger has this feature.
I found this to be extremely helpfull.



That's a HORSE???:p I was kinda wondering and was going to ask him but I didn't want to hurt his feelings:D Thanks for taking care of that for me Kratos;)


Iargan Ambassador
You insult my entire race you Monkey Boy! It's not a horse we were once Sea Otters. Search Iarga the beautiful high-G water world only 12 light years north. We are your next door neighbor & many of us have incarnated here to provide Doctors for this High Security Insane Asylum. :D If you want creepy that would be the Suclars
I forget where they are from but it's a swamp. They are basically a big duck billed toad who carries a big stick & has real Jedi powers. George Lucas based Yoda & Howard The Duck on them. Did you think he makes all this up? He is well informed.
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