Xtar VP2 Indicator Question

[exsion]

Currently I'm using a Xtar VP2 to charge my 18650 cells. I noticed that the voltage indicator shows a quick increase during charging and even reaches 4.2V before the charging led turns green. If I take out the cell and put it back in, the voltage reads something under 4.2V which is more accurate as it corresponds to a DMM reading that I used to make sure.

I was wondering if the misreading is not an issue I should worry about? Thank you for any input.

 

[ChrisGarrett]

Currently I'm using a Xtar VP2 to charge my 18650 cells. I noticed that the voltage indicator shows a quick increase during charging and even reaches 4.2V before the charging led turns green. If I take out the cell and put it back in, the voltage reads something under 4.2V which is more accurate as it corresponds to a DMM reading that I used to make sure.

I was wondering if the misreading is not an issue I should worry about? Thank you for any input.

I was the first on this site to have a VP2, but then it was recalled due to a faulty formulation of plastic.

It's a great charger, even though it's somewhat dated, has only two bays and does no analyzing.

There is a termination point where cells reach the intended 4.20v and then the charger terminates the charge. This is within spec (+/- .05v). After the charge terminates, the cells 'settle down' voltage wise and they might measure 4.19v, or less, depending on the condition of the cells, themselves.

I have an Xtar WP2 II and it mostly would undercharge (to me) cells and I'd just take them out and reinsert them, thinking that 4.20v was where they needed to be, but that's not an elegant solution.

Slightly undercharging cells is better for them.

Don't worry about it. The VP2, while somewhat limited, is a very safe charger for our li-ions, even if they don't always read 4.20v 30 minutes after we remove them from the charger.

Chris

 

[HKJ]

Check some charge curves for LiIon cells, the reach 4.20V long before they are full. If you take them out of the charger the voltage will drop, but it will raise fast again when you put them back into the charger.

You can find a lot of charge curves on my website and a lot of other stuff about batteries and chargers, a lot of it is also posted here. This is practical if you want to ask questions about it, I am subscribed to just about anything I have posted and will see questions posted.

 

[fmc1]

That's the way a good charger should operate first constant current or CC then constant voltage or CV. If the charger just stopped as soon as the battery reached 4.2V it would not be fully charged. Good chargers will charge a LiIon battery at a selected current let's say 1000mA until the battery reaches 4.2V or very close to it. Then the charger will reduce current down to certain point some were around 100-150mA then terminate. A good charger like the Opus BT-C3100/3400 adjusts the termination current according to the charge current you select, higher charge current gives you a higher termination current. On a very good charger like the Skyrc MC3000 the charge current the target voltage and the termination current are all fully adjustable.

Frank

 

[Gauss163]

Probably the VP2 is displaying the voltage measured while the charge current is on. This voltage will be higher than the resting voltage of the cell. By Ohm's Law the difference is roughly the product of the current I and the internal resistance R of the cell, i.e. I * R. In order to minimize this difference the standard CC/CV charge algorithm includes a second CV (Constant Voltage) phase which gradually reduces the current (necessary in order to keep the voltage from increasing beyond the target value).

Typically at charge termination the current will be reduced by a factor or 10 or 20, which reduces the difference I * R by the same factor. This smaller difference causes the resting voltage to be much closer to the programmed termination voltage (vs. terminating at the full programmed current). However, if the internal resistance R is very large (e.g. old cells) then the voltage drop at termination current may still be significant (which can be remedied a bit by using a lower termination current if your charger supports such).

Symmetrically, under load (discharge) there will be an I * R voltage drop, and the voltage will increase once the load is removed.

 

[tatasal]

Currently I'm using a Xtar VP2 to charge my 18650 cells. I noticed that the voltage indicator shows a quick increase during charging and even reaches 4.2V before the charging led turns green. If I take out the cell and put it back in, the voltage reads something under 4.2V which is more accurate as it corresponds to a DMM reading that I used to make sure.

Does it happen to all your batteries? I have instances similar to yours, and in all my chargers with voltage read-out, but only to my "unhealthy" cells, on their way to trash.

 

[exsion]

Thank you so much for the relevant information. By chance, what would you recommend as a more modern charger these days?