Lithium Polymer batteries for radio control planes need special care and attention that Nicads and NiMh batteries just don't need. These special considerations are briefly summarized on this page. I then go on to describe my LiPo charger, and a power supply I made for it from a discarded PC computer power supply.
A lipo battery pack is made up of a number of cells, which are usually rectangular and a few millimetres thick, sandwiched together. Each cell gives about 3.7 volts, so a 2 cell pack gives 7.4v and a three cell pack gives 11.1v. Each cell is in its own individual plastic sack, and you should always be careful to examine the cells for signs of swelling, which indicate that the pack has been abused, and is now damaged. The chief type of abuse is to discharge it too much. Nicad cells are happy to be discharged until they are almost completely empty, but LiPo cells must be kept above 3v. If you accidentally leave your plane switched on all night, so that the receiver is draining the batteries, the cells will get discharged too much, swell up, and be no good.
Deciding whether you need a 2 or 3 cell pack is really down to how many volts your motor can handle. Many motors will be ok on either 2 or 3 cells, so you could run your plane on a 2 cell pack for easy flying, or on 3 cells if you want more power. My Smoothe goes well on 2 cells, and is quite capable, but on 3 cells it is a beast, capable of vertical flight from a hand-launch.
The capacity of a lipo pack is measured in mAh, meaning milliamp hours, for example 1000mAh, and really just indicates the amount of charge that the battery can store. So, the more mAh, the longer your plane will fly for. You can work out how long it will fly for (approximately) by dividing the mAh by the current drawn by your motor. So, if you have a 1500mAh pack, and a motor that draws 10A, then 1500mAh (which is 1.5 Amp-hours), divided by 10 gives you 0.15 hours of flight (or 9 minutes if you prefer).
Just to complicate matters, some batteries have their cells arranged in series, and some have them in parallel. So, there could be four cells arranged as 4 in parallel, 4 in series, or 2 series sets of 2 parallel cells! What it really means this: Putting cells in series means you add their voltages together, and putting them in parallel means you add their capacities together. So if we have four 1000mAh cells in parallel we'll have a 4000mAh battery pack that gives 3.7v. If we put them in series we have a 1000mAh battery pack that gives 14.8v. And if we put 2 in series 2 in parallel, we get a 2000mAh battery pack that gives 7.4v. These four combinations are termed 1S4P, 4S1P and 2S2P respectively.
Maybe I should draw a diagram here!
Some packs just have a simple two-wire charging lead, but others have a complicated multi-wire charging plug as well as the simple lead. The complicated wire is so that the cells in the pack can all be charged individually on a special charger that seeks to balance the cell charging process, giving each cell the same amount of charge. This has been found to prolong the life of the battery packs.
If you charge your pack on the wrong voltage or current rating, you could make the pack explode. This is serious. Also, if you store the pack in a hot location such as in your car during hot weather, the pack could explode. As already mentioned, you must never allow the pack to be discharged to below 3v per cell, as this will ruin the cells.
I store my packs in the attic, which is stupid of me. Really it would be safer if I put them in a fireproof container, in the garden shed. People have said that you should treat LiPo battery packs in the same way that you treat live ammunition.
I have an Apache LiPo charger, that only charges LiPo batteries. I think this is a good idea, as it eliminates the chance that I'll charge a LiPo battery on Nicad settings.
I use this Smart Charger 2500. It draws its current from an adapted PC power supply (see below). It can charge 1 to 4 cells from 250mAh to quite a lot.
(Also I will soon provide details on a low capacity charger I built for sub 100mAh cells)
Details of how to build a 12v power supply from an old PC power supply, coming soon too.
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