ryan
10 kW
I'm evaluating some old Headway 38140S cells. What should my benchmarks be for a good or bad cell? I'd like to see how many can be used in a new pack.
Headway LiFePO4 38140S cell benchmarks for testing old cells
- Thread starter ryan
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999zip999
100 TW
Like cells like like cells
ryan
10 kW
For example, if a cell charges to 3.5v and is at 3.4v after 3 hours, and 3.2v 24 hours later, is that a bad cell? Are there any best practices here?
scoot
1 kW
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A Headway cell that does that is probably just fine. Pretty typical actually.
Headways often do not effectively retain a surface charge (for lack of a better description) when they are new or unused... at least that has been my experience. It often takes many charge/discharge cycles to have their legs stretched enough to have them retain their higher voltage for longer periods and achieve their maximum capacity potential (pun intended). After a while, they may retain the higher volatges for longer periods, but it is really not particlaurly useful information other than saying 3.45 volts and higher indicates a full cell at the time of measurment for all pracitcal purposes. Some people use internal restance measurments to get a better idea, but the only real way to know IMO is to perform a discharge test of some type and measure their actual capacity somehow. Voltage measurements alone will tell you very little unless of ourse there is something catastropically wrong with the cell.
Headways often do not effectively retain a surface charge (for lack of a better description) when they are new or unused... at least that has been my experience. It often takes many charge/discharge cycles to have their legs stretched enough to have them retain their higher voltage for longer periods and achieve their maximum capacity potential (pun intended). After a while, they may retain the higher volatges for longer periods, but it is really not particlaurly useful information other than saying 3.45 volts and higher indicates a full cell at the time of measurment for all pracitcal purposes. Some people use internal restance measurments to get a better idea, but the only real way to know IMO is to perform a discharge test of some type and measure their actual capacity somehow. Voltage measurements alone will tell you very little unless of ourse there is something catastropically wrong with the cell.
ryan
10 kW
Thanks. How do you recommend setting up a simple discharge test?
scoot
1 kW
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- Apr 30, 2009
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Well, I'm quite lazy so I use a CBA II battery analyzer for discharging (google Powerwerx). I usually do 8 cells at a time connected in series with a CellLog8s connected to balance leads attached to the 8s pack. Nice to see the graphic discharge curves for comparing cells that way. I have seen many DIY setups for one cell at a time (never tried mself) using DC light bulbs and amp & volt meters and a calculater. The draw back I see with that sort of setup is you have to watch it like a hawk to avoid over discharging a cell. maybe the other members here who are expereinced in this sort of thing can offer their wisdom on this approach.
ryan
10 kW
Finally got my battery pack back up and running strong. Here's what I learned... Even though I was able to get many "bad" cells up to a decent voltage, they dropped very quickly. When rebuilding my pack I even had a few cells surprise me and after getting the pack all together there were still three cells that didn't hold up to load or even a few days of sitting.
