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Thread: Selbuilt Review of the Xtar WP2-II Li-ion Charger

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    *Flashaholic* selfbuilt's Avatar
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    May 2006

    Wink2 Selbuilt Review of the Xtar WP2-II Li-ion Charger

    I will keep this review brief, as I see there are already two excellent reviews and discussion threads for this charger:
    HKJ: Review of / Measurement on Xtar WP2 II charger
    gopajti: New XTAR WP2 II Li-ion charger

    My goal is not to repeat everything that has already been covered, but to give my testing and handling results.

    To start, please see my earlier review of the original Xtar WP2 for more background on my testing method, and comparison to other chargers. If you are looking for more information on how to perform measurement/testing on chargers, please see HKJ's excellent Measurement on flashlight page.

    WP2-II Specifications
    • XTAR WP2 II 14500/ 14650/ 17670/ 18500/ 18650/ 18700/ 10440/16340 3.7V Li-ion battery charger
    • Charging qualifications and structures
    • Operation Temperature: 0~40℃
    • Input AC: 12V DC / 1000mA
    • 500mA CC Current: 500m A±5%
    • 1000 A CC Current: 1000m A±5%
    • Cut-Off Voltage: 4.2±0.05V
    • CV Cut-off Current: <100mA
    • Standby Current: <20.0m A
    • TC Current (Battery Volt 2.0~3.0V): 80±20m A
    • Auto Recharging Threshold Value Voltage: 3.9±0.15V
    • USB Discharging qualifications and structures
    • Operation Temperature: 0~40℃
    • USB Interface Output Floating Voltage: 5.0V
    • Output Floating Voltage Ripple: <120 mV
    • Reactive Current: <10m A
    • Maximum Output Current: 500mA
    • Battery Output Cut-off Voltage: 2.80±0.15V
    • Car charger adapter is included with some dealers (not necessarily all)
    • MSRP: can be found online for < $20

    My WP2-II came with both a standard AC power adapter and DC car adapter. The cabling and overall build seem good (good length on the cables, solid unit, etc.).

    The AC adapter with my unit supports 100 – 240 VAC, 50/60Hz. Standard North American plug on mine, but you could use a plug adapter for other jurisdictions.

    The battery trays are spring-mounted, and can easily accommodate any size from 14500-18650. With spacers, you should be able to easily use RCRs.

    Note the charger has two channels, so you can charge two cells independently of each other.

    Here's the interesting feature – the WP2-II features two charging currents (1A or 500mA), and an output to USB (for charging USB-based devices off a Li-ion cell).

    Unfortunately, none of this is properly labelled – you set the switch to "2" for 1A charge, "1" of 0.5A charge, or "0" for USB output.

    Just plug in your USB charging cable for phone or other device, and the unit gets to draining a Li-ion cell to charge it. Note this won't work if you have the AC power connected to the charger.

    There is a USB light light that lights up on the top of the unit when it is charging (note a Li-ion battery has to be in the left-most channel for USB charging).

    The other two LEDs on the top of the unit are for the charge status of the two channels (red when charging, green when no cell in place or when it is fully charged).

    Voltage/Current Measurements

    To examine the performance of the charger, I have directly monitored charging current and voltage with a data-logging DMM (on separate runs).

    For these tests, I have used one of my standard AW protected 18650 (2200mAh) cells. To deplete the cell, I used my regular test bed of a fully-regulated JetBeam Jet-III IBS on Max output – and wait until the cell’s protection circuit gets tripped. The cell is then immediately loaded into the charger for testing and recording (depleted resting voltage typically ~3V).

    Here is the WP2-II charger (1A charge rate):

    And the original WP2 charger:

    Overall, both charges do a reasonably good approximation of a CC/CV charger. The initial charging current was accurately measured at 1A on the WP2-II.

    The "dips" in the graphs above have to do with how the charger operates – it actually stops charging once every second or so, in order to accurately measure the cell's voltage. This causes an intermittent drop in current and voltage readings. While this drop is only a fraction of a second in length, it is enough that my data-logger will pick it up on every third or fourth reading, on average (i.e. gets caught as the DMM drops down and back up). My sampling rate is only once every 30 secs for the tests above, hence the why you don't see a continuous "wall of noise", but just sporadic dips.

    At the end of the day, the effect of this intermittent drop is negligible on the charger performance. It just seems to be a method for the charger to gauge the most accurate voltage level of the cell (without the effect of the charging current/noise getting in the way). While not a classic CC/CV algorithm, it shouldn't cause a problem.

    Note that the 18650 cell used in these tests was a "retired" cell from my runtime testing, with a lot of cycles on it - and thus suffers from higher internal resistance. This presumably explains in part why the 1A charging current start to drop after half an hour, and the final resting voltage was only 4.14V.

    Without the measurement leads in place, I have recently tested a couple of newer 18650 cells, and get under 3 hour 15 mins to a full charge (4.16V resting voltage). That suggests to me two things:

    • On newer, good quality cells, I would expect the "regulated" 1A charging current would run for longer before dropping down (e.g. likely well over an hour).
    • The charger tends to terminate a little early, before 4.2V is reached. While this means your cells may not be quite as fully charged as some other chargers, it should lengthen their lifespan (i.e. it is much better to slightly under-charge than over-charge - over charging is very hard on the cells).

    Note also that the WP2-II does not actually completely terminate when the light goes green. Although my DMM dropped to zero on my 10A port, when I switched to my DMM’s mA/uA port, I could still detect a low uA current. But this was low enough to be irrelevant.

    Preliminary Discussion

    I like this new WP2-II charger – it is a definite upgrade from the original WP2.

    One obvious new feature is the dual 1A/0.5A charging switch. You can now supply the most appropriate current for different capacity cells.

    But an even more important distinction is the much lower charging rate at the time of termination (i.e. <80 mA in my tests above). This is much safer for lower-capacity cells, which suggests to me you should be good to use just about anything in this charger.

    At the end of the day, the 1A current is best for 18650, 17670, and 18500. I would recommend 0.5A for 14670, 14500, and 16340 (RCR). Not sure if I wouldn't want to run 10440 in the charger, but it may be ok at the 0.5A level.

    The USB output charge is an interesting option. As a general rule, I am always a little wary of draining one Li-ion cell to charge another, but it seemed to work well in my testing. A nice option to have, it you run out of juice on your cell phone without AC power handy (but a supply of Li-ion cells on hand).

    While the intermittent current drops to check voltage may seem unusual, all the other characteristics of this charger seem very good. For the price, I don't think you could do much better out there at the moment.


    WP2-II charger supplied by Xtar for review.
    Last edited by selfbuilt; 09-28-2011 at 06:43 PM.
    Full list of all my reviews: Latest hobby: New: Selfbuilt's Spring 2018 Sale
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