25-07-2016, 09:11 PM
(25-07-2016, 11:49 AM)BOB Wrote: HI, Barry and Andre. The question, someone told me quite a while ago that you can discharge batteries using a light bulb plus low voltage alarm?
Hi Bob,
We could spend hours talking about that topic. This might become a long post
For starters, let's define two useful equations:
1) V = I x R, voltage equals current times resistance, known as "Ohm's Law"
2) P = I x V, power equals current times voltage, sometimes called the "Power Law"
Most classic incandescent bulbs are a simple coil of wire. The house variety are rated for 240 volts, at say 60 watts of power. Plug those numbers into the second equation, and it's easy to calculate the current flowing through a 60W bulb:
P = I x V, 60 = I X 240, 60 / 240 = I
In other words, current through a household 60W bulb is 60/240 or 0.25 amps. In turn, we can use that result in the first equation to find the resistance of the bulb's filament:
V = I x R, 240 = 0.25 x R, 240 / 0.25 = R
The filament resistance of a 240V 60W bulb is 240/0.25 or 960 ohms.
Assume for a moment that the resistance is a constant at all voltages and temperatures (in reality it's not), and imagine that a 3S 2200mAh lipo (12.6V when fully charged) is connected straight to a 240V 60W household lightbulb. We can calculate the current by using Ohm's Law:
V = I x R, 12.6 = I x 960, 12.6 / 960 = I
Current flowing through that bulb would be a measly 12.6/960 or 0.0131 amps, and that probably wouldn't be enough to make it light up at all. At that rate, the lipo would take about a month to discharge, so clearly household bulbs aren't the thing for discharging lipos!
A more suitable alternative is in the next post...
The same two equations ALWAYS hold. Let's now use a 12V 21W car bulb, of the sort that cars used to have in their tail and indicator lamps (before this new LED fandanglery). Assuming the same full 3S 2200mAh lipo:
P = I x V, 21 = I x 12.6, 21 / 12.6 = I
Current through the 12V 21W automotive bulb is 21/12.6 or 1.66 amps. We can calculate the filament resistance by plugging those numbers into Ohm's Law:
12.6 = 1.66 x R, 12.6 / 1.66 = R
Filament resistance of a 12V 21W automotive bulb is 12.6/1.66, or rather 7.6 ohms.
Discharging a 3S 2200mAh lipo at 1.66A is pretty much exactly what an average hobby-grade lipo charger will do, except it has to dissipate that ~20 watt power into the air as heat, with its tiny 40mm fan whirring itself to bits in the process.
Rigging up two of the 12V 21W bulbs in parallel would double the current, thereby halving the time-to-discharge. Four bulbs in parallel quadruples the discharge current, and now we're discharging at 80W - which is a discharge rate of [(1.66 x 4)/2.2=] 3C, which means around 20 minutes to totally empty, or about 15 minutes of gentle flying with an Easystar.
The trouble with automotive bulbs is that they themselves don't have voltage sensing or a low voltage cutoff (LVC). That quad-bank of 12V 21W bulbs in parallel will happily run the lipo all the way down to 0%, if left alone to do it.
A lipo low-voltage detector can deal with that issue, kind of. If hooked up across the lipo's main wires, it will beep/scream when the voltage reaches a predetermined level. Trouble is, most are designed to sound the alarm at say 3.0V/cell or thereabouts, which means they won't be much use if attempting to discharge from full to storage voltage (3.85 V/cell) under load.
The other problem is that you cannot discharge 6S batteries with the same setup, because at 25.2V (full) they far exceed the voltage rating of a 12V automotive bulb. That problem can be rectified by hooking up the four bulbs in a 2-by-2 configuration, two parallel lines of two bulbs in series, but when it's all said and done that's all a lot of fiddling when you could instead...
... buy one of these for $28
http://www.ebay.com.au/itm/AOK-3in1-Batt...OSw5VFWJK5~
That thing will discharge at up to 150W, far more safely and accurately than the bulb method.
For 28 bucks there's really no point fiddling with bulbs and bulb sockets, and wiring, and lipo voltage detectors, and having to listen out for the alarm... The automated 150W lipo discharger is a simpler and better solution. <-- That's the short summary of the entire post.