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Old 02-05-2011, 07:08 AM   #3 (permalink)
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2. The circuit:

Let’s have a look at the electrical diagram of the first prototype:

How does it work ?

In short:

The lipo pack is simply connected to the circuit by the deans connector and the balance plug, and all the rest starts automatically, the circuit will stop discharging and switch off the pack once it is discharged to the programmed alarm level in the Cell-Log monitor.

In detail:

The connected Cell-Log monitors all voltages during the discharge, we program it for a “normally closed” (NC) setting on the alarm output port, which means this output will be closed as long the Cell-Log judges that the actual voltages are above the alarm level, like on a fully charged pack. This makes the (normally open) relays contacts close, and by that the pack will be connected in series with the high-power discharging resistors. So now the pack is discharging, at a rate determined by the resistors value. Once the voltages have dropped enough, the Cell-Log will activate the alarm port, which will open, and the relay contacts will subsequently open as well, so no more discharging, the pack is not connected to the resistors anymore. Note that the pack voltage will rise a bit again without the discharge resistors attached, and the circuit will restart discharging again for a short time. This will happen a lot of times before the pack reaches a low enough voltage to make this stop. It’s not really a big problem, as you could stop the discharging yourself at that moment, but it is hard on the relay contacts. That’s the price to pay for such a simple setup. The blinking of the connected light and the sound of the relay contacts opening and closing gives an extra indication that the discharging process is (almost) finished. BTW, we’re already thinking about a different version where this won’t happen anymore, but you’ll need to give me a few more weeks before we’re there. There are several possible solutions, but I want the most simple and reliable one.

Note: if the relay coil would fail and refuse to close, there’s no harm done with this kind of setup, the lipo will simply not start discharging.

The manual of the Cell-Log clearly indicates how the alarm port works. It is a transistor driven port, capable of switching up to 50V/500mA, which is largely enough for a relay coil. FYI, the coil of my very ordinary relay needs 21 mA at 24 Volt. I attached a copy of the Cell-Log manual, it explains all about the alarm features and settings. On page 9 you’ll find this typical application, and that’s also how we’re going to use it:

The diode connected over the relay coil suppresses the back EMF, don’t worry too much about it. Note though that it has to be mounted the “wrong” way: the negative pole is connected to the positive pole of the pack !

The small 24V light bulb is just an extra optical indication that the pack is discharging, but it’s not really necessary, the Cell-Log’s screen will also tell you that.

The relay and the light bulb will take (small amounts of) power from your pack while discharging, and will work on slightly lowering voltages during the discharge, but that causes no problems.

The high power resistor(s) now: I picked a value of 7,8 ohm in this case, because that will give me a (for a 6S pack): 25V/7.8ohm = 3.2A current, which will make for 3.2A x 25V = 80 Watts of dissipation power. I found two resistors of 3.9 ohm in series to be convenient, as these are cheap in a 50 Watt version, and that way they will each only need to cope with 80W/2 = 40 W. If you find a single 7.8 ohm/100Watt power resistor, that’s fine also, and even easier. I also found some high power resistors up to 250 Watts each, but I considered these too expensive.
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Attached Files
File Type: pdf CellLog_8S.pdf (851.4 KB, 585 views)
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