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Old May 22, 2015, 09:30 AM
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Thanks, hoppy. I think I'll get one of those. But it appears the minimum discharge is 2v. For disposal of a bad pack doesn't one need to get to 0v or as close to 0 as is possible? Or is that completely unnecessary?
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Old May 22, 2015, 09:51 AM
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2 volts is fine. You strip and twist the leads together after discharging to prevent voltage rebound anyway.

mw
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Old May 22, 2015, 10:04 AM
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Originally Posted by Mark Wood View Post
2 volts is fine. You strip and twist the leads together after discharging to prevent voltage rebound anyway.

mw
Wow... I didn't know that would be a safe thing to do. I just keep learning new things from the fine folks here on RCG!!
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Old May 22, 2015, 10:45 AM
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Yeah well, you've been getting a whole lot less "ignernt" lately too.

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Old May 22, 2015, 12:33 PM
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Quote:
Originally Posted by ignernt_dweeb View Post
Wow... I didn't know that would be a safe thing to do. I just keep learning new things from the fine folks here on RCG!!
Look at what the capacity (energy) left in the battery is at 2V.
Even at 3V/cell, there is very little energy left in the cell.
SOC = State Of Charge
OCV = cell voltage
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Old May 22, 2015, 01:06 PM
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Quote:
Originally Posted by ignernt_dweeb View Post
Wouldn't too rapid discharge be a fire hazard? And wouldn't decreasing resistance increase discharge rate? I'm sorry if I'm misunderstanding.
Yes, but using a 2 Ohm resistor to discharge a 3S battery results in a drain of 6.3A peak. LiPos have a drain limit of 20C minimally and can be as high as 90C. For a 1000mAh battery, that's a limit of 20A at the least. Discharging at 6.3A is well within tolerance. Even if you were to use a 2 Ohm resistor on a 6S pack, that would be a lot of power, but a 12.6A drain is usually within tolerance. You'll run into the power limitation of the resistor before you run into the drain limit of the battery.

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I used to parallel charge until my house almost burnt to the ground that one time.
Parallel charging is hazardous due to the potential for large equalization currents when cells are initially connected. One has to be vigilant in checking voltages are similar prior to connecting the cells. A big charge mismatch is going to cause a problem without exception. Once charging there is also the potential of a bad cell causing a problem, but that can happen charging a single battery as well.

An option is to use a parallel charging board that provides protection against large equalization currents. It's kind of a necessity really.
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Old May 22, 2015, 01:10 PM
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Parallel charging is hazardous due to the potential for large equalization currents when cells are initially connected. One has to be vigilant in checking voltages are similar prior to connecting the cells. A big charge mismatch is going to cause a problem without exception. Once charging there is also the potential of a bad cell causing a problem, but that can happen charging a single battery as well.

The other option is to use a parallel charging board that provides protection against large equalization currents. It's kind of a necessity really.
I use the BuddyRC ParaBoard V3 with fuses, that should be good right? When I plugged in two 3S lipo's the other day, one with each cell ~3.6V and the other had each cell ~3.9V, they equalized within seconds. Only the power (XT60) connector was plugged into the board, I waited a minute before plugging in their balance connectors.

I think this should minimize any potential for issues?
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Old May 22, 2015, 01:17 PM
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They typically have fuses to protected against large currents so yes that should cover you. Of course it's always better to avoid those large currents. It's not so much the battery supplying the current, but the battery receiving a large current suffers unusual wear. Though the fuses will protect against an occurrence of fire, they won't protect against premature cell wear.
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Old May 22, 2015, 02:02 PM
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, one with each cell ~3.6V and the other had each cell ~3.9V, they equalized within seconds.?
I thought about that for a minute and decided to calculate what the current would need to be for that to occur.

If it was a 1000mah pack with one pack is at ~5% capacity and the other pack at ~63% capacity. That would be about a 300mah change in both packs in 3 sec.

Think about how much current would I need to pour into a pack to add 300mah in 3 sec.
A 300mah pack would reqiure 0.3A for 1 hour.
Or 3A for 6 min.
Or 30A for 360 sec.
Or 360A for 3 sec.

That's assuming 0 resistance which is not the case. But it's theoretically a large surge current.

I think that if you had 2 similar packs and connected-disconnected them in 3 sec, you would not find them equalized.

But since the packs didn't go thermal or puff or heat up, what's the diff.
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Old May 22, 2015, 02:16 PM
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Old May 22, 2015, 03:01 PM
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Yeah I just glossed over that, but there's no way two cells will equalize that quickly. DC resistance for a 1000mAh cell would be 25mΩ typically. With two cells in parallel total is 50mΩ. If there's a difference of a half volt the resulting current flow is 10A. Doesn't sound like much, but that's 10C for a 1000mA cell and Li-Ions are much less tolerant to the level of current going in than going out. Also, bigger cells have lower DC resistance. It's about 6mΩ for a 2200mAh 20C cell. With a difference of half a volt equalization current between two cells would be over 40A. Still that's nowhere near the 360A required for 3 seconds. You would not want to subject a cell to that anyway.
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