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Discharger Update 02

By murankar, Sep 6, 2018 | |
  1. murankar
    Last night I ordered the power resistors for the discharge bank. Now lets get into what I got and why i think its going to work?

    Up front I want to say is sorry for the delay in this part. Things are still not ideal but are getting better. Its going to take some time for this project. Lots of reasons why, one money, times are getting better but still not ideal as stated earlier, two: Voltage + Amps + Watts = FIRE if your not careful along with $160 USD of bad batteries; so no reason to rush the project. Three; I am not an electrical egineer and this is all new teritory for me. There is a lot to learn and I have not even began to sctratch the surface. I have a passion to make stuff and this is just another one of those things.

    For the write side I will document this project as I make forward progress on it. As of right now I have the load resistors on order from amazon and I still need a few more parts before the bank is ready for assembly. I'll save the resistors for a little later. To cover over what else I would like to get here is the list in no particular order.

    • A pre made voltage/amp meter: I want to be able to track the voltage and amp draw as the pack discarges. I could have the arduino track that part but at the moment its on the back burner. I want to get a functioning load bank before ading any kind of "brains" to it.
    • Solid state relay that can handle 50 amps and 50+ volts: I want the circuit to open and stop discharge at a particular voltage limit. Its feature that I want so the packs dont over discharge. The idea is to keep the packs healthy not stress test them.
    • A low voltage alarm: These things are really neat in this project. We use them in the hobby to protect our packs when we fly. If the pack drops below a certain level an alrm sounds. With that functionality I can redo the circuit to send that alarm signal to the solid state relay instead and shut down the load bank.
    • Cooling: This is going to be the most critical part of the system. The resistors have to stay cool to warm. They can not over heat at all. I will be pushing about 150 watts more than the rated capcity of the bank. I dont think it will make that big of an issue, then again I am not an electrical engineer. I fugure if I have the correct amount of cooling the bank should be just fine. My solutions for cooling is either going to be passive on the resistors (no additional heat sinks) with high aor flow fans in a case or some kind of giant heat sink with fan (think cpu cooler). Still trying to work out this problem. As for the fans I found a set of 2 case fans (120mm) that work on 110V which means I would need an additional power source. I would like to use the battery pack as the power source for the fans. Again I will need to develop a way to step down the voltage with a voltage divider or by a capacitor bank. Not sure yet. I have 2 voltage regulators and their max voltage is 32 volts if I remember correctly. I have yet to find one to handle 50v. Then again I have not done any extensive searching yet. Any ideas are welcome. One last thing here: Liquid cooling? Not sure if I can get this to work. plus I think the cost would be higher but one would get bettter heat removal.
    • Case/chasie/box: This is going to have to be housed in something. I am thinking a wood box that is assembled to be air tight at the joints. I don't want the fans pulling air from the corners, just the in/out lets. I am think air tunnel style. Heat needs to be move out quickly to keep the resistors from failing.

    FInally the resistors that I got. I found on amazon a 2 pack of 100w 4 ohm load resistors for just over $6 USD. I got 4 packs at $25 USD with shipping. These resistors will allow me to discharge at around 44 amps at .5 ohms. I will be needing to dissipate about 986 watts of heat. I only have 800 watts of resistors. so 186 watts is going to have to be dissipated through cooling. I am hoping that the resistors can handle more than the ratings on the resistor or the heat management allows the resistors to handle the extra wattage. I do not know how this is going to play out. I have fiddled with all kinds of numbers and 8 100 watt 4 ohm resistors gave me a decent amp draw that will work for me. I push the packs to about 50 amps during my flying so I know they will handle 44 amps. Another thing to consider with this is that a solid state relay gets expensive above 50 amps. trying to keep the price of this respectable while it not taking all day to discharge is tough. the idea is to speed up discharge not take longer.

    That is where I stand at the moment with this project. As time and money permit I will be adding the components listed above. I hope to have it functional soon, at leat the load bank part which will consist of the meter, box, cooling. the relay is going to be needed once the arduino is ready to be put into the mix.

    Well the resistors came in today 09/07/18

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