Black wire corrosion

I'm a keen builder and pilot of radio control planes. They are a great way to push battery systems to their limits, so I get to play whilst learning more about batteries...probably.

I was recently flying a model that is several years old, and has been stored for at least a year. The receiver battery in the aircraft (the one that makes the radio equipment work and powers the servos that move the flying surfaces) was about 3 years old and had charged up normally on the overnight charger, so all looked well...or so I thought..., but on each model I have a battery indicator, which tells me whether the battery is good enough for another flight, and this one started behaving 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 control sticks alone. 

The cause? Black wire corrosion. See the photo. This is a common issue for photographers, radio equipment users, power system designers and the like, and usually manifests as the copper strands in the negative wire on a battery circuit 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 (such as wiggling the servos on my plane), the resistance is so high that the battery voltage drops, massively, by the time it gets to the switch, which leads to the radio receiver and servo control centre. Under even heavier load (pulling up elevator at high speed on my plane, for example), the resistance might be too great to allow sufficient current through to the receiver and servo, and the plane would be on the ground, in tatters, moments later. I checked this one, and it delivered 5.7v at the battery (correct) and less than 3.5v (hopeless) at the receiver when two of the servos were moved together. With RC planes, you don't fly if you aren't sure, so I grounded this one in one piece and replaced the battery. Lucky!

There is plenty of speculation, quite a bit of science and a lot of people pontificating on forums about black wire syndrome, black wire corrosion etc, 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 for a radio control plane, but it would show up differently in a camera, or a radio, or a car or boat (yes it happens there too, and can seriously affect motor starting) - check the "earth" wire from the battery to see if it feels hard and rigid (bad) or soft and flexible (good)
• 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. Voltage under load is more telling. If a car battery drops below 10.6v when starting the car, reach for the phone and the credit card 
• 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 cable coating has an involvement in creating the problem)
• It can also affect the connectors and battery switches in the circuit
• You can replace the wires - it's seldom worth it on nickel cadmium and nickel metal hydride batteries, but is certainly worth it in a car or boat (and also worth using nice thick cables, and please use lovely braided cables on classic cars, if appropriate)
• You can't clean the affected wires or do anything to put it right or reverse the problem, once it is there, other than replacement of wires
• It tends to occur more where a battery is kept below peak voltage, and no current has been flowing for a while
• 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 solve the problem
• and, if it is of any comfort, it is happening just about all the time. Old cabling is generally not going to perform as well as new cabling, once it has been connected ot a battery for a while

Bottom line - if your battery is doing something strange, if your remote control plane or car doesn't seem to be working quite right, if your classic car isn't starting as well as it should, or if your negative wires are feeling a bit stiff - there's work to be done, preferably before the black wire causes a bigger problem!