RC Battery Monitors and Maintenance

Battery Monitors and Maintenance

The Questions
How do I know the true condition of my airborne NiCd battery packs? Is my pack good enough for one more flight? How close am I getting to the 'danger' point of a low receiver pack?

Common Misconceptions
1) I had my batteries on charge overnight - they have to be OK. (wrong!)

2) I check my flight pack with an expanded scale volt meter. That tells me that the pack is safe to fly. (wrong!)

3) I always fly 'X' number of flights, that's how I know it's safe to fly that many today. (wrong!)

Get To Know Your Batteries!
There is no single method of knowing the condition of your NiCds. You must get to know your batteries and act accordingly. There are several things you can to to almost guarantee you won't have battery related problems when at the field. These suggestions come from over 30 years of experience in R/C - watching countless unnecessary crashes due to battery neglect and/or misunderstanding.

Install an On-board Battery Monitor

This is very important! I'm not speaking of a monitor that just displays the voltage when the system is idling (no servos being moved). The monitor must display the LOWEST voltage the battery saw during the last flight - under full load of the servos in actual flight conditions. The BC6 'glitch counter/battery monitor' is such a device. I am not affiliated with them in any way, I just used the monitor and am very impressed with it's functions.

Upon landing, the BC6 will show you the lowest voltage of your pack during the flight. It has a row of LEDs (lights) that indicate this voltage. As the pack drops in voltage during your flights, the LEDs indicate this drop. So, when you you stop flying (or recharge your pack)? It takes some testing because we are all flying different battery and servo configurations. For example, using the same servos in a plane with a 600 mAh pack will not yield the same results as using an 1800 mAh pack in that same plane. The BC6 will indicate, for example, that the pack is at 1/2 capacity, but 1/2 capacity of a 600 MAh pack may only be good for one more flight, whereas 1/2 capacity of an 1800 MAh pack in the same plane may be good for 4 more flights.

Run the Tests
So, to determine when to stop flying, it is important for you to perform the following test. Fly your plane/helicopter , keeping track of your total flight time, until the BC6 indicates a half charged battery, then stop flying and put your pack on a battery cycler (an AccuCycle works fine for this - more on that later). The cycler will display the current left in your pack and, with a simple calculation, you can determine the remaining flights you could have had. The calculation is:

First, subtract the remaining capacity from the total capacity of your pack to get capacity used.

Then, multiply the total capacity of your pack by the total flight time in hours and divide that number by the capacity used from above.

The result is the total flight time you could have safely used your flight pack.

600 - 200 = 400
600 * 1.2 / 400 = 1.8

You can see that you could have safely flown 1.8 hours instead of the 1.2 that you flew. Next time out, you know you can fly until you get a lower LED. Once you know which LED to call your 'stop flying' indicator - you're done. Just fly until that LED is lit and go home or recharge!

Battery Cycling
The best way to know your battery packs are healthy is to cycle them. There are quite a few battery cyclers on the market today. I use the Hobbico AccuCycle just because it is relatively inexpensive and does everything I need it to do. It's about $100. You should cycle your batteries once every couple of months. Here's how. First, give your pack a full charge. Then, let it set for a couple of days and cycle it. If you are getting less than 80% of it's rated capacity, it's time for a new pack. How else do you know? I've seen packs that last anywhere from a month, all the way up to 5 years. And don't worry about the cycling process taking life from your packs, as NiCds are good for about 1,000 cycles!

As we go to more and more expensive plane and/or helicopters, some thought should be given to redundancy. In my more expensive birds, I run dual batteries and switch harnesses. Check out this link if you're interested in how it's done.


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