How to discharge spare battery?

  • Hi team,

    So I bought myself a low mileage spare pack for my XR. The guy had upgraded to a CBXR pack so didn't need his original.

    Only issue for me is it's fully charged, holding 62.8v. I'd like to get it down to a safe storage voltage cause it's a spare and I hope I won't need it for a long time.

    My current battery box is virgin and I don't want to open it until I need to. Any good ideas on a safe simple discharging method?

    I have a voltmeter to monitor it and can make up an XT60 lead to something that could take the voltage? Don't have a 15S balance lead plug.


  • @stinkyface Probably going to have to be a DIY solution. We can ignore the balance leads.
    You're best bet is to wire it up to a power resistor and drain it down to something more suitable like 57v (3.8v per cell)

    Here's an article going over discharging a Lipo but of a smaller voltage and for the purpose of disposal.

    You'll want to spec your resistor for this battery. To keep the pack healthy a "slow" drain will be ideal so I'd suggest keeping the current no more than a few amps. Bear in mind you'll probably see sparks connecting this up, just a heads up ;)

    If it were me I'd be using a single 50 ohm 100w resistor.
    Using Ohm's law at 63volt max with a resistance of 50 ohms the current draw would be 1.26 amps while max power would be 80w, well within the resistors specs so it will probably only get hot but not "mega owwie" hot. Best to place it away from the battery just in case still and off anything that might burn/melt.

    Once connected just keep an eye on it and eventually it should get near 57v at which point disconnect it and keep it stored away safely. This solution won't stop it going past 57v so don't forget about it ;)

  • @Lia thanks, somehow I knew it would be you who replied. 😂

  • @stinkyface Aha no problem, I’ll admit I did wait a little just in case anyone else wanted to reply 😅

  • Dodgy setup creating some heat... About 180 C from the twin resistors. Glad I took the time to put it all on this spare ceramic tile.

    Found a couple of 10W resistors to get the job done. Should take about 11 hrs to get down near a good voltage.


  • this is a good trick to know; I have a 800 mi battery that I'll swap for my og 2400mi battery soon, can't justify the price of the cbxr right now after investing in carevpower 40v ryobi, will eventually get a BYOB for the spare xr batt.

  • @stinkyface Wow that’s hot!

    If my 6am maths is correct you’re pulling near double the power rating for them so definitely don’t leave them unattended. Maybe wire in a fuse just to be safe incase one of them fail shorted.

  • Hard to get a good read on temp, I'm just using an infrared.

    Is there a standard for XT60 polarity? Either Future motion or Hobby King hasn't read it...🤔

  • @Lia the 2 x 470 ohm parallel resistance is 235 ohm, which the meter agreed with, so I'm burning about 16W with 8W thru each. They are rated for 10W each.

    I have an inline power meter (HobbyKing) but because the XT60 polarity is opposite, I can't see the actual amps or power. Sadly I've already blown the fuse in the fluke meter and haven't gotten around to replacing it yet.

  • @stinkyface Knew 6am maths was a bad idea, yep I was doing wattage as if wasn't shared. All good then :)

    In the same boat with my metre, blew that fuse testing some mains years ago and haven't bothered replacing it.

    How low have you gotten the voltage by now?

  • @Lia all done, nice and controlled down to 56v in the end.

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