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



## selfbuilt (Sep 28, 2011)

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. :thumbsup:

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. :thumbsup:

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.

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WP2-II charger supplied by Xtar for review.


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## CamoNinja (Sep 28, 2011)

*Re: Selbuilt Review of the Xtar PW2 II Li-ion Charger*

Awesome review, thank you.


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## samgab (Sep 28, 2011)

*Re: Selbuilt Review of the Xtar PW2 II Li-ion Charger*

Thanks for the review. 

Here is the reason why it isn't necessary for a true CC/CV charger to pause charging periodically to take an OCV (Open Circuit Voltage) reading:
As the charger charges at full current (1A), there will be some voltage hysteresis, so yes it's true, 4.2V under full charge current would be a little less under OCV. But when using CC/CV method, the current reduces whilst retaining 4.2V as the cell's capacity is topped up. As the current gets lower and lower, the VUL (Voltage Under Load) and the OCV come closer and closer together, until at the cut-off point of 100mA, they are near enough to each other as makes no difference. The ending voltage will be very close to 4.2V.
For that reason, stopping the charge periodically to measure OCV on a true CC/CV charger is completely pointless. Of course, it's also completely harmless, so who cares. It will just increase the charge time a very slight amount.

Now on the other hand, charging NiMH/NiCad cells using constant current and a combination of termination tests, such as overall max voltage, negative delta of voltage, etc; that is where pausing charging periodically to test the OCV becomes important.

Is it possible that the charger designer wasn't aware of these basic facts? I don't know. I am unsure as to why the OCV test pauses have been integrated into this charger. Maybe it has an internal circuit design - or uses an IC - that allowed for the option of NiMh charging as well, if required, or if activated or set up that way in another model charger... Not sure.


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## selfbuilt (Sep 29, 2011)

*Re: Selbuilt Review of the Xtar PW2 II Li-ion Charger*



samgab said:


> As the current gets lower and lower, the VUL (Voltage Under Load) and the OCV come closer and closer together, until at the cut-off point of 100mA, they are near enough to each other as makes no difference. The ending voltage will be very close to 4.2V.
> For that reason, stopping the charge periodically to measure OCV on a true CC/CV charger is completely pointless. Of course, it's also completely harmless, so who cares. It will just increase the charge time a very slight amount.


Thanks for the comments. What you suggests makes perfect intuitive sense to me for Li-ion - with a slow drop in charging current, there really shouldn't be a need to stop and check the voltage under no load repeatedly. But at the same time, it shouldn't cause a problem, except for a slightly slower charging cycle. :shrug:



ITPython said:


> Earlier today I wouldn't have considered this charger due to its early termination, but after some wise words from a few CPF members in a thread I made yesterday, I am starting to come around.
> EDIT: Oh I wanted to ask, did yours com with spacers to make charging RCR123's possible? I've read from some people they didn't get any with the WP2 II and then there's some who did. Cause I know the WP6 II comes with 6 spacers by default.


Yes, the concesus definitely seems to be that it is better to routinely slightly under-charge than over-charge a cell.

As for spacers, my WP2-2 didn't come with any. I routinely use aluminum spacer blocks purchased from battery dealers.


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## HKJ (Sep 29, 2011)

*Re: Selbuilt Review of the Xtar PW2 II Li-ion Charger*



selfbuilt said:


> Thanks for the comments. What you suggests makes perfect intuitive sense to me for Li-ion - with a slow drop in charging current, there really shouldn't be a need to stop and check the voltage under no load repeatedly. But at the same time, it shouldn't cause a problem, except for a slightly slower charging cycle. :shrug:



I have a suspicion that the reason for stopping is noise. The charging circuit makes a lot of electric noise (not audible) and that may make it difficult to get a exact measurement of the voltage.


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## aimxplode (Feb 13, 2012)

*Re: Selbuilt Review of the Xtar PW2 II Li-ion Charger*

Great review, thanks for explaining what the (2, 1, 0) switch selectors.


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## stickydoorknob (Oct 14, 2012)

Awesome review. Definitely considering picking one of these up for the USB charging function.


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## roadkill1109 (Oct 15, 2012)

Check out Also the Miller ML102 USB Charger. More compact.


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## Shadowww (Oct 15, 2012)

roadkill1109 said:


> Check out Also the Miller ML102 USB Charger. More compact.


It has it's drawbacks too - not perfect CC/CV, needs USB input (and won't charge at 1A from PC's USB input), and just single-cell. So for people for whom that matters, WP2 II is a better choice.


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