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XTAR Over 4 Slim Review by KingPin!


In my defence, I was left unsupervised ^^
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Hello Folks
Xtar sent me through their latest charger just short of a month ago now, claiming it as “the fastest charger ever”. There are more details here: (


Manufacturer Features:
  • Max 2x 4.1A, the fastest charger ever
  • Impressive low temperature rise
  • Battery internal resistance testing protection
  • Battery temperature testing with overheat protection
  • Optional current for each slot (1A / 2A / 4.1A)
  • Dual 5V / 2.4A USB ports
  • Informative LCD screen display
  • Reverse Polarity protections
  • Low voltage activation
  • 3 stage charge cycle (TC,CC,CV)
  • Flame retardant material
Manufacturer Specifications
  • Input voltage: DC 12V 4A
  • Output current: 1A / 2A / 4.1A
  • Cut-off voltage: 4.2V
  • USB Output: 5V 2.4A
  • Unit Operation temperature: 0-40C
  • Size: 75 x 149 x 33mm
  • Weight: 173g
  • Supports Li-ion (IMR/INR/ICR) 18650, 18700,20700, 21700, 22650, 26650

First thing to address is the name being confusing “Over 4 Slim” this bears no relationship to the number of bays as you would expect rather its related to the amount of amps it’s maximum possible charge rate is. That’s right a whopping 4.1A charge rate! They claim you’ll be able to charge a cell from the low cut off point (approx. 3V) to 4.2v in just 30 minutes! This is a bold claim, to do it safely at least, so rather than bang on about likes and dislikes first let’s get out my test results first and go from there as a discussion point


4.1A results 18650’s

As you can see from the table the charger does keep inside the 30ish range for the 2500mAh cells (HE4 & VTC5A) but doesn’t do it for the 3000mAh 30Q’s. This is bloody quick but it comes at a price the cells get pretty warm. I noted 49.C as the maximum temp reached across all my tests although the charger is set to limit charge rate if it hits 51.C. Secondly the charger never seems to terminate at the 4.2V mark, for one of the tests it cut off well short at 4.13V even though the screen said 4.2V & 100% complete.

2A results 18650’s

Temperatures were 9.C cooler across the board at 40.C although again the termination seemed to fall slightly short even though it said 4.2V & 100% complete on the screen (4.17 was the lowest recorded). The charge time was consistent with what I tested on the Xtar Dragon VP4 Plus maybe a little quicker in fact.

1A results 18650’s

Whilst it took a lot longer to charge the temperature was significantly reduced as you’d expect. But notably it terminated almost all the time at the 4.2V.


So with the results done …I know that XTAR employs a 3 stage charge cycle ….1) Trickle Current to introduce charge without shocking the cells 2) Constant Current hitting the peak of the charge cycle and remaining there through roughly two thirds of the cycle 3) Constant Voltage reduces current to maintain the charge state of the cell…..and I suspect it’s the final stage of charging that’s the issue here. Instead of a smooth transition and ramp down into the final CV stage its choppy at the end because the cells are still too warm and are trying to come around from the rampant charge they just received. Ill have to wait for some bench testers to validate this theory though and if im wrong ill come back to update.

I just want to finish this portion of the review by saying these are the tests i recorded specifically however continued to use the charger extensively for the rest of the month before this review.

My other main observations:

I tested this with two devices attached via USB capable of 2A charging (the Aspire Skystar and Yihi SX Q mini). At the same time loaded in two cells and selected 4.1A for the stress test (a total of 6 18650’s charging at the same time). The charger handled it well taking 2hrs 51minutes to complete the cycles. I recorded a maximum temperature of 41.C. in this time. The slim is dropping the charge rate significantly to handle that lot and does seem to favour the USB connected devices first over the onboard cells, but at least proves it can be done although the on screen charge rate is maximum possible over actual, something i repeatedly pick out of the XTAR charger reviews as a con.

The charger is designed not to house anything less than an 18650 li-ion. This is a positive step forward from the VCT2 which was also designed for Li-Ion cells only, but you could still load it with NI-MH / NI-CD cells and it would start charging!.


I really like the inclusion of the temperature sensor and would like to see this as a regular feature going forward on other chargers as well. I would propose to have them located in 3 places rather than just the middle though because some cells might have irregularities causing them to heat faster away from just the centre.

I didn’t test how it would handle loading a cell with a damage wrap and contacting the metal temp sensor whilst charging, sorry I’m not prepared to take one for the team on that aspect without knowing more. I would personally like XTAR to put information on their sites product page regarding this important safety issue, as i expect as some point this is very likely to happen.

I’m not a big fan of the positive terminal on the Slim. Both of them have an extra nipple which causes cells to slide about easily, leading me to my next point – internal resistant checks. The Slim only allows you to charge at 4.1A (automatically) if it detects the internal resistance is low enough. It’s often off because you are still fiddling about trying to get the cells to settle in the bays since both are extra wide to house up to 26650. You literally have just a few seconds to get it in and settled before “computer says no” and switches to 2A.


If you switch the slim off at the mains then place the cells in and it’s still not happy about the internal resistance…there’s an override feature “The Fuck It” option as I like to call it. The manual simply says doing so could cause irreparable damage….erm then why allow it!?

Both of these above two points could have been easily rectified by not commencing the charge until the user clicks on a button to physically start. If the internal resistance is still over then it sets charge at a rate that’s suitable for it, or just take frequent readings throughout and adjust accordingly at that point although this is far more tricky to do without disrupting the cycle itself.

The interface is very easy to use. The buttons simply allow you to select charge rate and that’s it. You can change the charge rate in the middle of a charge cycle. In terms of the displayed information I found the combination of charge percentage and voltage confusing, it would be at 4.2V even below 100% for a while. I also really miss the capacity restored which is the hallmark of a lot of XTAR chargers.
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In my defence, I was left unsupervised ^^
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Member For 4 Years
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Smaller observations:
  • When I tried to remove that sticky label on the screen out of the box it started to pull the screen cover off with it! Not a great start.
  • Reverse polarity worked ok.
  • Bay 1 seems to be favoured until the end of the charge cycle where it balances both out.
  • Pressing buttons once the cycle is at 100% complete does not reset anything or start charging again.
  • If you charge at a low rate the Slim always defaults back to 4.1A when removing batteries.
  • The coating is rubberized and white which marks easily and dust just sticks to its surface. It wouldn’t surprise me if it discolours over time as well.
  • Does not work as a power bank if any devices are connected via USB and power is disconnected.
  • There is a really audible high frequency noise when charging, those like me sensitive to high frequency it’s really irritating.
  • Introducing cells that are partly charged (around 4.08V was fine charger picked up and carried on from 82%).
  • The charger doesn’t go into low power mode when not in use. You can hold one of the buttons to dim it but that’s it
  • Plug and play - there are no other features outside of straight charging available
  • Nice bright display with good viewing angles available.
  • Although the batteries heat up but the rest of the charger remains cooler throughout the cycle.
  • I did note differences of the internal resistance checks on occasion albeit they tended to show higher on the slim than testing off the charger, this does bother me.
  • The Negative terminals are spring loaded with smooth travel. Due to its size this charger will not support smaller batteries by default even the smaller Li-Ion cells.
  • The charger itself comes with an additional brick (power adaptor to use it) making it less portable.
  • Doesnt require batteries to be loaded to utilise the USB ports.

Final Thoughts

A charger utilising 4.1A has to be spot on with its checks and there really isn’t room for too much error. I think the slim introduces some really unique features I’ve not seen before and would like to see these ideas developed further in terms of temperature sensing, cooling and resistance checks, but accuracy is the key to employing these things, so allowing users time to get everything settled in before doing its thing is something I feel the Slim lacks.

Personally I don’t want to see chargers go the way of big sub ohms using stupid high wattages (or charge rates in this case) it’s just ends up in a dick swinging contest between the big names in the charge scene. At the end of the day you can’t perform miracles with these things, cells still need time to accept a charge without shortening its lifespan or just heating them over prolonged periods of time, so if these higher rates are emplyed the sensors and terminations need to be spot on over repeated cycles for my buy in.

I ended up using the slim at 1A far more than the other rates mainly because it terminated accurately for me there, although I cannot deny the slim did smash through the higher rate charge cycles really impressively quickly without overcharge.

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