Efficient way to discharge Li-ion to specific voltage?

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Feamane

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Hi,

I have a bunch of 14500, 18650, and 26650 li-ion batteries. I'd like to find an efficient way to drain them in batch to a specific voltage when I know I won't be using some of them for a while. Maybe 3.9v or something like that. I say drain because my batteries are more often above the desired voltage than below. Of course a solution that could either drain OR charge to a selected voltage would be even better.

I know some chargers can be configured to use a certain termination voltage. I guess if there is one that could be programed to do a discharge and then charge up to a specific voltage--and the programming could be saved to some kind of non-volatile memory--then that would probably work.

Playing with EDC flashlights and pocket knives is fun, taking optimal care of a bunch of batteries is not. Is such a unicorn available?

Thanks,
DJ
 
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chip100t

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I literally just bought an xtar vc4 plus For this reason, as I had quite a few spare batteries and had then all fully charged as it was easier than babysitting my other charger to remove the batteries when they got to 3.7 v.

The xtar has a store mode. Which automatically either charges or discharges which ever is required to bring the battery to the correct voltage for storage.

There are a few xtar chargers with the “store” mode, but check which ever model you choose is from there newer updated range that are capable of fitting protected 21700 cells. As there older models fit the shorter unprotected 21700 cells but not protected.
 
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Feamane

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Hi,

Yes, a charger that would do this automatically would be awesome! I don't consider sitting there manually babysitting the discharge of a dozen or score of batteries, one after another, to be efficient effort or time-wise. I'm looking for the lazy man's way of doing this--that's what gadgets are for after all. ;)

I have the Xtar VP4+, but don't see in the instructions where it can be made to do this. Do I have the wrong charger? Am I just missing something obvious?

If I have to buy another unit to do this, what would be the best one for performing this particular function?

Thanks much,
DJ
 
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chip100t

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Hi,

Yes, a charger that would do this automatically would be awesome! I don't consider sitting there manually babysitting the discharge of a dozen or score of batteries, one after another, to be efficient effort or time-wise. I'm looking for the lazy man's way of doing this--that's what gadgets are for after all. ;)

I have the Xtar VP4+, but don't see in the instructions where it can be made to do this. Do I have the wrong charger? Am I just missing something obvious?

If I have to buy another unit to do this, what would be the best one for performing this particular function?

Thanks much,
DJ
The xtar vc4S (old shorter slot version) and xtar vc4SL, the vc4SLis the newer version with longer slots which is probably what the L stands for. I have the vc4 plus. Look on Xtars website, all the information is on there.
 
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Feamane

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Cool, thanks for the info. I will check these out!

Thanks,
DJ
 
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Feamane

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Is the MC3000 $73 better than the VC4SL?

Thanks,
DJ
 
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sbj

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Anything where you are dissatisfied with your charger. If you're happy with yours, you don't need it.
 
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Feamane

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OK, so I got the MC3000. But either I'm not doing something right or it is a bit wonky.

I put in four 14500 cells that were in the 4.15-8v range and ran the Storage function on them. By default I think it was set to 3.8v, but it only discharged them down to the 4.05-11v range. Then I set up a program for all four slots set to 3.92v and tried again but it didn't drain any cells below 4v. Now I set it to 3.8v and with four different 14500 cells that were in the same 4.15-8v starting range it has taken them to 3.91v, 3.96v, 4.01v, and 4.05v. I put the cells in my other charger to check and it agrees with the voltages +-0.03.

So what gives? Why does it let you set the target voltage in 0.01v steps but says it is finished with the storage function when the cell is 0.25v away from the target.

Anyone else using the Storage function on this charger seeing the same thing?

Thanks,
DJ
 
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sbj

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I advised you to the MC3000. Now I won't leave you alone with the settings either.

If you want to hit the target voltage exactly even after switching off the discharge current, you must also set a low termination current for the discharge, named: Discharge Reduce. Quasi CC/CV when discharging. Because the higher the discharge current, the more the voltage jumps back up when the load is removed.

The conventional way is e.g. discharge current 0,5A; Discharge Reduce: - 0.05A The smaller this value, the less the voltage will rise again at the end. The target voltage is hit all the more precisely. The disadvantage is that this gradual reduction in discharge current takes a lot of time.

If you really want to bring a large number of batteries to storage voltage as quickly as possible, I would proceed differently.

Suppose you want to bring your 14500 cells to 3.9V storage voltage. With the 14500 LiIo, for example, I would only set a discharge current of 0.5A and initially set to 3.9V. You have to set Discharge Reduce to OFF!

Now that the charger is discharging, the discharging will stop as soon as the discharge voltage under 0,5A load reaches 3.9V. The no-load voltage then rises again and levels off after a few minutes, e.g. at 4.05V. So the targeted open circuit voltage is 0.15V too high. If you now set the discharge voltage 0.15V lower than you want to achieve later as open circuit voltage, i.e. 3.75V in this example, the discharge process runs much faster without the time-consuming discharge CV phase.

In order to hit the right open-circuit voltage value, you need a little experience. On the other hand, there is a wide range of voltages where storage is ok. So it doesn't have to be accurate to the tenth of a volt.

You can try both settings. I think that the one with Discharge Reduce OFF is at least twice as fast.
 
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Feamane

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Thanks for the info! I don't mind if the process takes more time as long as it is an automated process. I have already been playing with lowering the target voltage more and more, now I will try out this other method. If I understand correctly I can experiment with both and when I get them I can save them both so I can have both 'fast' and 'slow' storage options.

Best regards,
DJ
 
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sbj

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Sure, you can store both settings in separate memories.

With the faster discharging method, setting the specified storage voltage can become more complicated if the lowest adjustable value of 3.65V is not sufficient. Then you would have to use the discharging program instead of the storage.

For charging to storage voltage when the battery is empty, this setting is not suitable anyway.
 
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Feamane

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OK, I've been playing around with it and it is working great. Still doing some tweaking, since it seems that 14500 cells are harder to get optimize the settings for, while 26650 seem to be easy to get within +-0.02v. I'm very happy with the new charger so far, thanks for the recommendation!

Regards,
DJ
 

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