How is your LiPo holding up?
by Red Scholefield ([email protected])
Do your LiPos seem to have lost some of their punch? LiPos deteriorate over time. To track how much, I found a helpful technique demonstrated on YouTube.
You need a wattmeter (the kind used to check power-system performance) and a load. The video uses a pair of 12‑volt halogen bulbs wired in parallel to give an approximate 100‑watt load with a 3S LiPo pack.
- Plug your battery into the wattmeter and note the voltage after approximately 10 seconds. Write this down.
- Connect the load for 10 seconds and record the voltage and current while the load is applied.
- Subtract the loaded voltage from the unloaded (initial) voltage.
- Divide that voltage difference by the current (in amps).
- Multiply the result by 1,000 to get the internal resistance in milliohms. Record this value.
Periodically check each pack the same way (using the same load) so you can see if, and how much, the pack’s internal resistance changes. Perform tests at roughly the same ambient temperature for best results.
Notes:
- The first voltage measurement should be taken with a small load (for example, about 1C) rather than an open-circuit reading. This gives a more consistent starting point. Open-circuit voltage can be misleading.
- Internal resistance of a LiPo pack is not strongly rate-dependent in most cases, but open-circuit voltage can give a false reading.
Give each pack a unique identifier so you can build a consistent history. Using a spreadsheet (for example, Excel) record:
- date
- ambient temperature
- battery identification
- starting (pre-load) voltage
- loaded voltage
- current under load
- calculated internal resistance (milliohms)
You can try tests at different temperatures later if you wish. The YouTube video gives a clear, straightforward description of the process and other useful tips for understanding your packs.
2.4 GHz Antenna Guide
An antenna guide is a convenient way to position 2.4 GHz antennas for maximum coverage. The installation can be built into your model so that when you change receivers you can maintain the correct antenna separation. Slip the antenna wires into the tubes you form by heating them.
Parallel Charging of LiPos
Questions about parallel charging come up often. My thoughts:
- NiCd and NiMH packs should never be charged in parallel. These are recombinant systems: gases generated in overcharge are recombined in the cell, and the heat of recombination changes the cell voltage at full charge. If you try to charge multiple NiCd/NiMH packs in parallel, one pack may reach full charge and its voltage will change, causing the charger to continue supplying current to that pack (potentially damaging it) while leaving other packs undercharged.
- Lithium packs can be charged in parallel provided they have the same cell count. They do not need to be the same capacity. Each pack will reach the charger’s set voltage and the charge will terminate. The main issue with parallel charging LiPos is that few chargers provide parallel charge balancing, so take care when using this method.
It’s Seldom One Thing
Many people blame “the battery” based solely on voltage readings. A fully charged small-capacity pack (for example, 100 mAh) can show the same voltage as a large pack (for example, 2,000 mAh), yet deliver far less capacity. Relying only on voltage can mask a nearly dead pack that will fail in flight.
If you’ve been crash-free so far this season, check the actual capacity of your packs. A charger that can discharge a pack and report delivered capacity is inexpensive and good insurance.
Don’t depend on what the charger reports as the pack input; it often has little bearing on real usable capacity. You can input 1,000 mAh into a nearly dead battery and still end up with a pack that won’t perform despite “good” voltage readings.
Transcribed from original scans by AI. Minor OCR errors may remain.
