Best way to discharge a cell prior to top balancing?

D

Deleted member 23531

Guest
Wanna try IRL and tell us what really happens?
Haha, if you can afford it I want to see the results, but yeah that's a ballpark estimate. I bet there is some nonlinear effect which we have not considered which kicks in after a short amount of time, like maybe electrolyte flow to the anode/cathode, or some heating effect due to all the current. Some brave or unlucky soul who has accidentally shorted their battery terminals could tell us what actually happens.
 

Hedges

I See Electromagnetic Fields!
Some brave or unlucky soul who has accidentally shorted their battery terminals could tell us what actually happens.

That would be 3.5V not 160 mV, and my calculation is 20kA.
I have more confidence in that figure than the charging of one cell from another; the I/V curve of charging may not be same as discharging.

One guy did short his 48V pack, but didn't tell us measurements, just aftermath. A (poorly connected?) busbar became a fuse.
 

Supervstech

Administrator
Staff member
Moderator
That would be 3.5V not 160 mV, and my calculation is 20kA.
I have more confidence in that figure than the charging of one cell from another; the I/V curve of charging may not be same as discharging.

One guy did short his 48V pack, but didn't tell us measurements, just aftermath. A (poorly connected?) busbar became a fuse.
I have had my bussbars swing over and hit studs... the studs disappear...
 

BarkingSpider

Carbon Lifeform
IMG_20210127_100132.jpg

While balancing or just in general use, I sometimes want to lower the voltage on a particular cell in my battery bank quickly and safely.

After trying numerous methods, I found this approach to be quick and easy and it does not require any special equipment or tools.

I used the household extension lead in the picture below with a brand new 280ah LifePO4 cell.

I used an old 14 gauge 50ft cord with a simple loopback (yellow wire in the picture) effectively making the wire 100ft long.

You could use any long extension lead, of smaller or larger gauge. Just check the resistance is between 1-10 ohms. Any cheap multimeter can be used to check resistance.

I connected to the lead Live/Neg prongs with alligator clips using some existing cables from my garage.

You could connect the wires many different ways, just ensure the connections are solid.

The total wire resistance when tested, measured 2 ohms, 1 - 10 ohms is the sweetspot.

There were no sparks, nothing eventful connecting the wires. No wires got hot, not even slightly warm. You do not need to uncoil the lead as the EMF effect is cancelled out by utilizing the reverse wire.

Remember - You are not using 120v or 240v AC, but 3.2v DC. Very different! Do the math. It's not the same as putting a screwdriver across the battery terminals. If you don't understand the physics, please don't post untested contraversial shock theories.

I checked the voltage every minute with a multimeter. The voltage dropped from 3.6v to 3.3v in 15 minutes

All in all, a success. I would recommend this method over any other, as it's safe, fast and practical.
 
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Just John

Photon Sorcerer
The math says so.
Wanna try IRL and tell us what really happens?
I've come pretty close to that voltage differential and didn't even get a spark. Likely because at that end of the knee, it's less than 5 amp hours, thus the current drops nearly instantly.
 
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