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Discussion Starter · #1 ·
I’m bench testing a six module Tesla pack, about 150V, 230Ahr.
How can I safely discharge it, say 20%, so I can test the charging system?
Thank you.
 

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Might be a job for incandescent light bulbs wired in parallel. Best way to switch them on or off is with a contactor. Length of time to discharge would depend on number and size of bulbs.

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Yup, two 120v 100W incandescent light bulbs would work well. I have two wired in series, one in each wire of a lamp cord, for this very purpose. This works well for voltages <250v, above that, it would be better to have lamps designed for 240v.

Batt+}-------lamp-------(
Batt-)--------lamp-------(
 

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Or depending on how fast you want to discharge the battery one or two 120v 1500watt water heating elements 8-10$ at lowes/home depot in large tank 7kWh,46Ah is about 20% if you meant down to 20% you need to use up 27.6KwH or 184Ah. There are 120v water heater element with a higher wattage I have seen them up to 3500w.
David Poz has a couple videos where he uses water heater elements.
Later floyd
 

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Just throw a wrench at it. Err... okay, you want to do it safely. So, stand far far away, and throw a wrench at it. Everyone likes fireworks, right?

Or connect them to a cheap thrift store electric kettle (or two, in series). Fill with water, and sort of keep an eye on them as they gradually boil themselves dry and need to be topped up.

Or if you have an electric stove, it should be 240v, pull out a burner, set it on top of a cookie sheet or something, and put a big soup pot on top full of water, you'll have a lot more time that way.

Or an electric clothes dryer element (it'll have ceramic tombstones to hold it away from a metal pan), and a box fan pointed at it.

Or an electric stove element. These will be fine in open air indefinitely (they stay on for 3 hours when your oven self-cleans). Two in parallel would hog a whole hell of a lot of power.

Your local landfill might let you nab these from them, or any local appliance repair or sales place that does delivery (when they deliver the new one, they take away your old one, and just take it to the dump, they could probably set one aside for you and have you one within a day or two with only a minute or two's effort from their end.
 

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Why is everyone determined to build something which works as poorly as possible? Use an electric heater with multiple settings to control the discharge rate. It has a fan to get rid of the heat, because heating air is what it is designed to do.
 

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Why is everyone determined to build something which works as poorly as possible? Use an electric heater with multiple settings to control the discharge rate. It has a fan to get rid of the heat, because heating air is what it is designed to do.
That will work also but don't plan on turning it on and off using the OEM switch.

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True, the switches used for AC don't last well in DC service.
They likely will not work at all. I.E. Once an arc is struck you won't be able to shut it off.

And the fan will almost certainly not tolerate DC.

Which is why I suggested various heating elements and either a large amount of water as a themal sink or a fan just plugged in pointed at them.
 

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They likely will not work at all. I.E. Once an arc is struck you won't be able to shut it off.
Same as any of the other schemes - shut it off by whatever moderately dangerous hack was planned for some other random heating element. People use angle grinders intended for 120 V AC on DC supplies from welding trucks all the time... what I was told decades ago was that the switch dies after a while and they usually just replace it with a jumper and turn it on and off by plugging it in an unplugging it, although I'm not suggesting that...

And the fan will almost certainly not tolerate DC.
If it's an induction motor, it won't work on DC; it will be immediately apparent if that's the case.
If it has more than one speed it's not going to be an induction motor - it's likely a universal motor like every other small home appliance made for decades. Universal motors are brushed series motors, and don't care if they get AC or DC (which is why they are called that).

You can also avoid the fan issue by using a convection-style heater, which is designed to dissipate the heat without a fan.
 

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If it has more than one speed it's not going to be an induction motor - it's likely a universal motor like every other small home appliance made for decades.
Mmm nope... most household fans, including multi-speed ones, are still AC fans. Usually with multiple windings they're switched between, or a single winding that's toggled to be partially active.

I'd venture to say I've never once seen a universal motor inside an appliance to drive a fan. They're wholly unsuited to fans.

You give DC to one of those fans, they'll catch on fire.

 

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This is what the big people use(beaucoup bucks):Battery Load Unit/Capacity Tester/Discharge Tester | 240 A | DV Power
This would take days:DL24M 600W Car Battery Tester Electronic Load Charge Voltage Indicator Power Bank 18650 Pack Capacity Monitor USB DC 12V Checker Intelligent Discharge Resistance Power Tester with Bluetooth: Amazon.com: Electronics

If you had PWM series motor controller, you could control the power going to a large resistive load through a main contactor. Better yet, with a good BMS (which you should have anyway) you could monitor the individual cell groups in each module. A good BMS would turn off the main contactor when a set or minimum cell group voltage is reached. The good BMS would most likely have a cell group health record(internal resistance and voltage) and the total amount of power discharged, battery temp(s), etc.
 

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Or depending on how fast you want to discharge the battery one or two 120v 1500watt water heating elements 8-10$ at lowes/home depot in large tank 7kWh,46Ah is about 20% if you meant down to 20% you need to use up 27.6KwH or 184Ah. There are 120v water heater element with a higher wattage I have seen them up to 3500w.
David Poz has a couple videos where he uses water heater elements.
Later floyd
Can you leave a link of where he uses the water heater? I'm interested in that video too.
 

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Discussion Starter · #20 ·
From my DC mentor,
Discharging a HV pack is Dangerous. Plan your work, and work Carefully.
There are two components to this problem:
1. The switch.
2. The load.
The switch must be able to handle the voltage And the sustained current that will flow through it, and it must be able to reliably turn off the current.
The load must be able to handle the current in a controlled manner without self-destructing.

When you’re talking 12V, there are many kinds of switches and loads that are able to be used safely. But when you’re talking 144V or 360V, it’s a whole different, and dangerous, situation.
  • The Safest route is to break down the pack to lower voltages.
  • Always use a contactor or HV rated switch only.

fwiw, I didn’t want to break down my pack as I was simply trying to cycle the charging system. I‘m also not smart enough to experiment with HV. So I connected my pack via contactors to a motor controller and DC motor. Problem solved safely.
4E6ED634-F16F-4666-8329-DA071A9D1EFA.jpeg
 
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