More must-know LiPo Info

Nov 22, 2012 3 Comments by

Last week, we discussed the basics of LiPo batteries in this post. Now, let’s get into what the numbers mean and how to choose a battery that’s right for your helicopter.

Voltage
All Lithium Polymer cells have a nominal voltage of 3.7v per cell, and when fully charged the cell should read 4.2v. Another important voltage to remember is that, when discharged, each cell should never fall below 3v. If a cell should fall below 3v, then it can sustain irreversible damage. But the real question is, how much of the battery capacity can I safely use without hurting it, and to get the most life out of the battery. The quick answer is to always keep the battery voltage between 4.2v and 3.7v. But this leaves a gray area between 3.7v and 3v, and to examine this area we have to be concerned with battery life cycles. If the battery is never discharged below 3.7v you can expect to get between 100 and 150 cycles out of the battery. This is where a cycle is defined as a charge and discharge. But should the battery be discharged below 3.7v, but not down to 3v, the number of cycles of the battery will be reduced. As an example, suppose you fly until the battery is discharged down to 3.4v. That may reduce the useful cycles to something below 100, let’s say down to 75. There’s no way to really tell how much damage is done to the battery, other than to say it has been hurt and will not give the normal amount of cycles it was originally capable of producing. Note also during this discussion I am referring to individual cell voltage, so you must multiply these numbers by the total number of cells to get the overall battery voltage.

S – Number of Cells – LiPoly RC packs are made up of layers of multiple cells. If the battery’s rating is 3S this means it has 3 layers of 3.7v each, for a total rated voltage of 11.1v. In other words, it’s a 3 cell pack.

Weight/Size
For a battery to be right for your model it must fit within the models battery compartment and also balance the plane correctly.  It’s temping to choose the biggest and most powerful battery your model can handle, but this will sacrifice flight performance, and if the voltage is too high it will destroy the ESC or motor. Check the ESC and motor specifications to ensure you have the right pack and then check the models CG (Center of Gravity) to decide on the right battery weight.

 

Paul Tradelius

About the author

A regular contributor to Model Airplane News, he is also the columnist for our “Rotor Speed” helicopter column. Paul has been flying RC helicopters since the early ‘80s and now enjoys all types of rotary machines, including scale and aerobatics, and he continues to experiment with modifications to improve performance.

3 Responses to “More must-know LiPo Info”

  1. Bennett Gottlieb says:

    So how do you measure the voltage? Does any one sell a voltage meter for these types of batteries?

  2. Paul Tradelius says:

    Yes – there are several meters on the market to make these measurements. I would recommend you bring your battery to your local hobby shop so he can see the type of connectors you are using. He should have a volt meter that will measure the overall voltage of the pack as well as each individual cell voltage. It would also be a good idea to get a balancer while you are there, which is used to vary the voltage of each cell to ensure they are all the same. This will prolong the life of the battery.

  3. Norm says:

    There are many options out there. I recommend something like this that does everything- balances your packs, measures amp and watt draw on your setup and checks voltage on packs up to 6s. http://www.hobbypartz.com/26p-twometer.html There are also voltage monitors with low voltage alarms small enough to connect to the balance plug and leave in your plane while you fly. When any cell reaches the preset alarm voltage , it emits a very loud alarm that is audible from the airborne plane. I’ve been using these for years.

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