I thought the following might be of interest, and might save a few planes, and a few bush walks.
I was recently flying a model that is several years old, and has been stored for at least a year. The RX battery was about 3 years old and had charged up normally on the Futaba trickle charger, but the model's battery indicator behaved oddly - showing a full battery as expected after just one flight but, when I moved any control surface, the indicator dropped very quickly to low battery, and then settled back to full when I left the sticks alone.
The cause? Black wire corrosion. See the photo below. This is a common issue, usually manifests as the copper strands in the negative wire on a battery turning black over time, and the resistance in the wire building up at the same time. So, when checking voltage, everything seems normal, but when under load (wiggling servos), the resistance is so high that the battery voltage drops, massively, by the time it gets to the switch. Under even heavier load (pulling up elevator at speed, for example), the resistance might be too great to allow sufficient current through to the receiver and servo, and the plane would be in tatters moments later. I checked this one, and it delivered 5.7v at the battery and less than 3.5v at the receiver when two of the servos were moved together.
There is plenty of speculation, quite a bit of science and a lot of people pontificating on forums about this, and it has been a known issue for well over half a century, but the important things are:
- Keep an eye out for the signs - the example above is classic, but also look out for sluggish control movement and difficulty in charging
- Voltage without a load doesn't tell you everything about a battery. It's a bit like taking the filler cap off your car. You can smell fuel in the tank, but have no idea how much fuel based on smell alone
- This corrosion usually happens on the negative wire, and will affect the whole wire, not just the exposed strands (there is some thought that the wire coating has an involvement in creating the problem)
- It can also affect the connector and battery switch assuming the battery has sat in the plane, connected to the switch harness
- You can replace the wires on the battery (but I doubt it is worth it, given the battery will be a few years old as well) - don't solder direct onto a battery though, the heat will damage it. You have to solder onto spot-welded tags (I can't wait for the stream of people telling me that they have been soldering onto batteries for years without a problem)
- You can't clean the wires or do anything to put it right or reverse the problem other than replacement of wires
- It tends to occur more where a battery is kept below peak voltage
- It is very hard to see without stripping some wire, but you can check it by setting a multimeter to the resistance setting and comparing the resistance down the length of the red wire and black wire. You'll see no, or almost no, resistance down the red, and if the black shows a different result then the time has come to fit a new battery
- If a plane is going to be stored for a long time, taking the battery out will help (at least the switch wire won't degrade), and keeping the battery charged from time to time will also help
- If you have a charger that can cycle NiCd and NiMh batteries, it's a good idea to do so from time to time. A few deep discharges and recharges will help the battery generally (this is less important with Eneloop NiMh batteries, incidentally, due to their lower self-discharge behaviour, but still can be done every few months)