|Product Performed to Expectations:||10|
|Specifications were sufficient to design with:||10|
|Demo Software was of good quality:||8|
|Product was easy to use:||9|
|Support materials were available:||10|
|The price to performance ratio was good:||10|
|TotalScore:||57 / 60|
B&K Precision Model 8600 Programmable DC Electronic Load
It's as if Christmas has come early - thanks to element14 and B&K Precision for choosing me to deliver this RoadTest review. As usual, I'll be providing a thorough impartial review of the Model 8600 based on my experiences in using it for my own experiments within the past two months. Unfortunately, as the unit arrived a month later than I had planned, I lost some review time to various other commitments (including graduating with my doctorate, reviewing a Raspberry Pi 3, presenting at a conference interstate and doing a few other things) as well as getting a few parts and thus I wasn't able to achieve everything I had initially set out to do. That being said, I did lose some sleep to try and compensate for it, and I have done a rather major study with this unit (181 cells, 4.26kg of batteries, 556 hours or ~24 days of test time) separately from the review which will be unveiled in a few days time when the data analysis and visualization and the write-up is completed. As a result, this review might feel a bit shorter than my previous efforts, but I've actually made much better use of the test gear and obtained much more operational experience in spite of it.
B&K Precision were founded in 1951, and are another one of those "familiar names" when it comes to test and measurement equipment. The Model 8600 is B&K Precision's most inexpensive standalone programmable DC Electronic Load with a 120V/30A/150W rating and constant current (CC), constant voltage (CV), constant resistance (CR) and constant power (CW) operating modes. The unit is provided with USB-TMC, GPIB and RS-232 connectivity as standard, with analog current control and monitoring and flexible triggering capabilities. The unit is capable of up to 25kHz transient operation speed, with 16-bit measurement resolution. Other features include front panel short-circuit, adjustable slew rate, list-mode operation, battery test, CR-LED mode, remote voltage sensing, power-on self testing and OVP/OCP/OPP/OTP/LRV/RRV protections. Even though it is in a "standalone" form factor, there is also a rack-mount option (IT-E151) to hold two units side by side. The unit presently retails for AU$1680 excl. GST and comes with a three year warranty.
However, the Model 8600 is merely the "smallest" member of a family of loads, which include the Model 8601 (120V/60A/250W) and Model 8602 (500V/15A/200W) standalone units, and the Model 8610 (120V/120A/750W), Model 8612 (500V/30A/750W), Model 8614 (120V/240A/1500W) and Model 8616 (500V/60A/1200W) rack-mount units which are based around a very similar feature set. For a hobbyist, the smaller capacity of the entry Model 8600 is actually quite adequate for most purposes, and the cost is more economical.
Key applications for such electronic loads include testing power supplies themselves, or power converters. This can be to gauge their behaviour under fixed loads (e.g. can they deliver the rated current, will they overheat?) and changing loads (load regulation). It's also extremely useful to test batteries for their capacity under discharge with different parameters (e.g. CC/CR/CW at different rates) to see if your batteries are still performing adequately. Another application would be in photovoltaics (solar cells) where performing I-V sweeps for characterization of the behaviour of a cell under different current/voltage combinations is desired.
To perform some of these tasks, it is possible to use a DMM and a suitably sized resistor to crudely make a test rig. I know, I've done it many times before. But such a test rig is not particularly flexible - it tests at an inherent resistance (or sets of resistances), it has a tolerance that can't easily be trimmed out, and it makes assumptions about the incoming voltage if a "fixed" current is desired. These all make fair testing of batteries difficult.
Another option is to use a current-limited power supply in "reverse", however, this has its own dangers namely depending on the design of the supply, it could be possible to cause damage to the supply and its capacitors. It is also likely to (if not terminated properly) cause battery cells or power converters to be reverse-polarized and thus damage them or cause explosions.
It's pretty clear that to do these sorts of characterizations most conveniently and accurately, an electronic load is required. There are a few "build it yourself" loads, which have various caveats, but generally they consist of some feedback mechanism driving a MOSFET attached to a fairly large heatsink. By default, such DIY units have limitations such as only operating in constant current or constant resistance modes, and with less protection and intelligence to protect the source or battery. They also lack any calibrated accuracy, which means that results can vary significantly due to temperature or operating condition.
As a result, a proper electronic load such as the Model 8600 is the right equipment if you intend to characterize power converters, power supplies, batteries, LED drivers, photovoltaic cells/modules, and anything else which requires a constant power, constant current, constant resistance or constant voltage characteristic. Ultimately, it gives you the flexibility to dial in different modes and values, and the peace of mind that you're not having to second-guess your equipment or be concerned about the safety of the device under test.
If you were shopping for an electronic load today, what does the market look like and how does the Model 8600 compare? I've surveyed a bunch of test equipment and electronics suppliers and come up with this comparison table of similar products which sit roughly around the same price point and capabilities. The information is provided in good faith, and believed to be accurate at the time of publishing - I cannot be held responsible for any errors - please do your own research prior to purchasing any equipment.
Interestingly, there aren't that many choices when it comes to DC electronic loads on the market, at least, not from what I could see. The B&K offerings are fairly strong, and it seems that the Keithley offering is merely a rebadged B&K judging from the similarities in specifications and VFD configuration. The majority of the other units are higher powered units, with mostly greater expense. Where the 8600 leads is in transient response, inbuilt connectivity, software inclusions and price. However, some of their competitors do have some other special features, including the ability to parallel up a number of loads for expandability. Based on this assessment, if you're only interested in 150W or less within the 120V/30A envelope, the Model 8600 is attractive on a value-for-money standpoint as its price is one of the most competitive on the market without skimping on features.
The unit comes in a very sturdy white cardboard box with a prominent blue stripe across the middle. Despite this, the couriers did manage to bash it around a bit, but it didn't receive any critical damage.
The unit comes with quite a bit of clearance from each side, with thick foam inserts keeping the unit clear of the corners.
The unit itself comes in a zip-lock static dissipative bag, and comes with a mains power lead and a USB cable.
The amount of printed matter supplied is rather limited - a good move for the environment. A calibration report and certificate is supplied, along with a leaflet telling you how to get to the documentation for the product. Important documents include the Datasheet, Manual and Programming Manual. Two packages make up the included software, namely MDL and Battery Test. There are also IVI/LabView drivers provided online.
I was rather pleased to see the full calibration report, as it shows that this particular unit was well within the specification window, so much so, that it seems it is most likely to better any of the claimed error margins within the specifications. Whether every unit is like this is not certain, but at least this unit looks pretty good.
The unit has a very familiar bench-top look, consisting of rubberized ends, and a carry handle that can be locked off in a number of positions to either get out of the way, be used for carriage, or to prop the unit up for a better look. The handle itself is made of plastic which seems to flex slightly, so I wouldn't trust swinging the unit around on the handle alone.
The unit has a multitude of buttons including a full number pad, translucent mode buttons for LED indications, directional buttons, and a control knob, which should make everyone happy by providing a number of ways to enter and manipulate inputs. The input to the unit is via the front panel, using two screw-down post connectors which can accept larger ring and spade terminals. No connection cables are provided. I did find that on my unit, upon fully tightening the screws on the connectors, the connector shifted ever-so-slightly, making a little "clunk" noise as it did, so I wonder if the rear of the connectors are as tight as they should be. Nonetheless, no faults were observed in operation.
Both sides feature vent holes which need to be kept clear to ensure the unit is adequately cool. Cooling is performed by an internal fan, which is not visible through the grilles. In the case of a dusty environment, cleaning will require disassembly of the unit, but at least the unit features OTP and should shut down in case of over-temperature.
The unit features a full complement of connectivity, including GPIB, USB-TMC, RS-232 and analog remote controls as standard. The air exhaust outlet is in the rear as well, with an IEC connector supplying power.
With a change of voltage requires a change of the fuse due to the different rating. This was one reason behind the delay of the supply of the unit.
The analog remote terminals are on a terminal strip that is secured by screws to the main unit. It can be undone to install the necessary wiring for remote sensing amongst other features. The terminals feature rising clamp style terminals, which is good as it's less likely to cause wires to fracture when tightening.
When the unit is first powered up, it displays the "BOIS" version and undergoes a self-test. Then, it settles down in the idle display.
Note that the configuration of the display can be altered through the settings, and I have specifically enabled the time display on my unit as I find it quite useful. By default, each button press is acknowledged with a loud and shrill "beep" from the unit. It sounded a little distorted and a bit too loud for my liking, so it was something I disabled almost immediately.
Users of more traditional test equipment will be pleased that the Model 8600 uses a VFD display which is bright, high-contrast and fast. The display consists of a top row of fixed function indicators, and two rows of text matrix display with the top one bigger than the bottom. In my experience, I perceive a slight flicker when looking at the display, but nothing too distracting.
When using the unit for most basic operations, the unit is fairly intuitive to use, with light-up mode and ON/OFF buttons. The values can easily be directly keyed in with the number pad and the direction arrows controlling the cursor, or the values can be tweaked using the dial. Values can be changed while the unit is running, which makes it nice for "fine tuning" a particular value.
More complex operations, such as configuring limits, external sensing, communication parameters, transient mode and CR_LED mode required first consulting the manual to get an understanding of the parameters and how to configure them. The manual is quite easy to follow "step-by-step", and the front panel does have some "hints" in the text printed near the buttons for "Shift-key" operation, but the text is a little small, so it can be hard to read. Once you have used it once, it becomes much easier, but the confusion might also be partly attributed to the limitations of the VFD display, with only limited amounts of text being displayed for menus.
I encountered no significant difficulties in using the unit stand-alone, and it behaved pretty much as I would have expected. In terms of read-out accuracy, comparing with my 5.5 digit meter and the Keysight E36103A power supply, it seems to beat the claimed specs and is good enough not to need external instrumentation for most tasks - it's about as good as some 4-digit meters in my opinion.
I decided to try an experiment to see just how well the load handles 150W, and also, how well my power supply of a similar rating could deal with supplying such a load.
Let's just say, I've spent almost an hour and a half, and both units seem to be alive and kicking, but the Model 8600 was impressive in the fact that it dissipated all of the heat with its fan running in a manner that was much quieter than my power supply which was supplying the power. The Model 8600 was almost inaudible above the whine of my power supply! The Model 8600's fan does stay "off" when on low loads, and steps through a number of speeds, but being encased inside means that the sound is mostly from the air movement rather than the whine of the fan itself. A big plus for ergonomics.
In the case that you did attempt to force the dissipation above the rated amount, the load backed-off automatically to protect itself.
I decided to use the Transient mode of the unit to generate a A-B cyclic load on my power supplies to see how they responded to various slew rates. Graphing was done with my Picoscope 2205A of the I Monitor output and the actual supply voltage.
This is my Manson 20A power supply trying to cope with a 0A to 20A load step - on the left at a slew rate of 5A/uS, and on the right at a slew rate of 0.1A/uS. The supply voltage is plotted as Channel A, with the I Monitor output as Channel B. We can see that in the first instance, the OCP protection was triggered and the supply voltage fell to zero due to the high slew rate. At a lower slew rate, the output dipped somewhat from the 7.2V set, and then recovered rather than cutting off entirely.
Repeating the same for my Manson 5A power supply with a 0A to 5A load step at 5A/uS, it seems this supply seems more competent or maybe even too relaxed when it comes to OCP, only registering a slight dip in output voltage but otherwise, not triggering any protection at all.
When testing the Keysight E36103A, I found that any load getting close to the OCP limit would trip the power supply reliably, or cause its output to ramp down over time. This suggests the OCP protection is rather aggressive.
Using a load of 1.8A, the supply does have some on and off transients when zoomed in, however, the Model 8600 also has its own overshoot as well it seems.
This proves the usefulness of the Model 8600 for characterizing power supply transient behaviour, and also for generating load on the supplies to ensure and verify correct behaviour of protections. It's also particularly useful in case you are not too confident about a power supply's output under load, to verify ripple and noise.
I tried a number of different CR_LED settings (Resistance, Forward Voltage) and swept across the voltages using the Keysight E36103A while recording the current using the onboard metering of the Keysight power supply.
It seems that the CR_LED forward voltage setting basically sets the point at which the Model 8600 will emulate CR from, and the resistance set determines the slope of the curve. This is a crude way of simulating an LED, which has a more "curvey" sort of characteristic, but is suitable to ensure LED drivers are happy. Note the low-voltage deviations in current are due to metering error in the Keysight E36103A power supply.
B&K provide two types of software for use with the Model 8600, one named MDL and the other named Battery Test. Owing to the continuous improvements, issues identified during testing and remedies enacted by B&K Precision, this review used MDL versions 1.1.5 through 1.1.7 and Battery Test versions 1.0.7 through to 1.1.0.
Installation of the software was simple. As I already had NI-VISA installed, the USB-TMC drivers for the Model 8600 were already available and installed automatically when the USB cable was connected.
MDL is a tool which is used for remote control of the electronic load and allows access of most features, including a demonstration mode.
The main page allows for configuration of the supply as if you were at a remote panel. The Waveform Recording panel allows for you to graph trends over time, provided the load has been configured as expected as recording locks out any configuration options.
Transient operation allows you to set up A-B style transients, whereas buffer operation allows you to configure the load to read a number of readings at high speed, store it in its internal buffer, and then have the software poll the supply for the results.
Finally, list program allows you to use the internal memory of the load and access/program/run the lists within. External program allows for you to run a program under the definition of the software with direct PC control. It doesn't seem there are any ways to make "flow" decision-making loops in these programs, although basic repetition is allowed for.
The battery test application is broken into two sub-sections, the first being Battery Discharge, and the second being a more fully-featured Sequence Test Program. The Battery Discharge program is most simple and screenshots are shown below in the following experiments - it offers CC/C-rate and pulse modes but not any other (CR/CW) modes. Sequence Test program is a more sophisticated interface which allows for more fine grained conditions and automated cycling (in conjunction with a B&K Power Supply).
In early versions of the software, it was not possible to use Sequence Test without a connected B&K Power Supply, however, it has changed with the latest few updates. Functionality improvements have also been found, as a number of small bugs in regards to how capacity was calculated in the Sequence Test program were uncovered. I contacted B&K directly via e-mail, and even spoke to them in person over their toll-free US number and managed to get swift responses and resolutions to the problems, which is a testament to the dedication of B&K's support staff. Even those the programs are intended to be "start-up" in nature, and users are expected to write their own programs to meet their needs, the quality of the supplied programs and their functionality is very close to what I needed bar the small bugs and its existence really saved me a lot of time and effort.
Sealed lead-acid batteries are a bit of a "curse" upon most hobbyists, because they always have some lying around salvaged from gear or purchased for a project, but they never really know what state they're in. As a result, I tried to test my whole batch of SLA batteries to see just how they fared using the Discharge Test program.
I had three cells of various ages, so I cycled them at two rates.
The results were as follows:
Battery 1 at C/20 and C/3
Battery 2 at C/20 and C/3
Battery 3 at C/20 and C/3
The interesting thing is that it did identify some batteries with cell imbalance - a weak cell contributes to a voltage knee in the curve.
I had two larger batteries, so I tested one at a low rate and the other at a high rate. I was being a little more ambitious, so this time, I had to set up remote sensing cables to ensure correct voltage read-out on the higher current test.
The results looked as follows:
In both cases, both batteries exceeded their specifications. That's unusual, but a testament to the quality of the batteries. In case you were wondering, yes ... I did get these from element14 a year ago!
This one was a bit of a "throw in" because it was salvaged from the university I worked at. I looked at it on the "recycling" shelf, and it didn't look dead to me, so I bought it home and gave it a few cycles with equalization on the charge, and at 0.6A (C/20) discharge rate for testing.
Interestingly, giving it a bit of exercise seems to have increased the capacity slightly, to about 68% of its rated capacity. It's probably not good enough for its intended application, but it's far from worthless. Slight variances are likely due to differences in charging strategy.
Since the Discharge Test program had a pulse option, I decided to play with it. The pulse feature is only "experimental" and does expose an issue - namely the measurement speed is not fast enough to accurately capture fast pulses, thus resulting in strange measurements and termination when using the unit alone.
I decided to proceed anyway just to see what would happen, but it's not exactly useful information. The capacity graph is also particularly alarming, as it should be continuously heading towards the right over time rather than jumping back and forth.
I tried a continuous low-load discharge as well, although it seems affected quite severely by temperature swings. It seems that I should pay more attention to the specs, as programming a 30mA test current is not likely to be quite that accurate due to it being at the low range of the scale (0.05%+0.05% FS = up to 1.515mA error).
Due to the current indication error potential, I decided to do some spot-checks in standalone mode:
|Set||Metered by Model 8600||As measured by Keysight U1241B|
Given that the unit came from element14, what would be more appropriate than to use it to test their "own brand" of alkaline cells under CC (400mA), CR (3.75Ω) and CW (0.6W) modes to a cut-off of 0.8V which are considered very heavy loads.
Owing to limitations in earlier versions of Sequence Test Program which required the connection of a B&K power supply to function, the CR and CW tests were done using an external switch and MDL's Waveform Recording feature without any discharge termination ability, zooming out and exporting data for post-processing analysis.
A summary of the results are as follows:
There is some cell to cell variance, but we can see that these three "naively" equivalent loads (400mA at 1.5V = 0.6W = 3.75R), at the nominal rating are actually quite different on the cell and allow for different amount of energy to be delivered. This is purely a consequence of how current varies throughout the discharge.
In CC, the current remains constant throughout, so when the voltage falls, the delivered power reduces over time. In CR, the resistance remains constant throughout, thus the current falls as the voltage falls, thus the delivered power falls even more as the cell discharges. The CW mode represents a fixed power consumption, thus, the current rises as the voltage falls to compensate, and is thus much harder on the cells, forcing its voltage down faster.
A later, updated version of Sequence Test Program allowed for users to operate the load without the need for a connected power supply, remedied issues with the cell capacity calculations for different discharge modes, and provided a Wh read-out as well. I could have also used the Battery Discharge program, but the Sequence Test Program is more versatile and allows for CW/CR discharges as well, although the Battery Discharge program allows for pulsed transient operation which the Sequence Test Program does not.
I decided to conduct a basic capacity test of two Panasonic CGR18650CG cells at the prescribed 430mA rate. The capacities came out to 2.0212Ah and 2.0486Ah, which is less than the claimed 2.25Ah. The achieved capacities were 89.8% and 91.0% which is fairly good considering these cells were pulled from an old laptop battery. Termination occurred pretty much as expected, which is important to avoid damage to the cells under testing.
While the unit features GPIB, RS-232 and USB-TMC, one of the new features a lot of instruments are coming with is LXI-LAN. After having a taste of the simplicity of SCPI-Direct Port 5025 control of my Keysight E36103A, I wanted to do some direct control of the Model 8600 through its serial interface as it would be very driver and platform independent.
I thought I would accomplish this through direct serial connection to a hardware COM port, or via my Moxa 5110 RS-232 to LAN interface configured as a TCP Server on Port 5025 with appropriate baud rate, flow-control and terminating character (0x0A) settings.
The manual states that the Model 8600 uses RTS/CTS flow control, but despite selecting Hardware or RTS/CTS flow control, I was unable to establish communications with the unit. Even a basic *IDN? was not obtaining a response. However, the hardware was operating correctly, and using the Battery Test tool with the direct COM connection was successful.
The only way I could get it to work was actually to disable flow control on my RS-232-LAN interface, and the same with my desktop RS-232 port. It appears this workaround does work but ignores the flow control requests from the unit. The reason, it seems, is because the Model 8600 only has RXD, TXD, GND, RTS, CTS pins connected, and does not use DTR or DSR. I think that the serial ports I have, when set to RTS/CTS flow control, also assert DTR and expect DSR from the equipment before they send any bytes down the line and as a result, "hang" waiting for a signal that never comes. Once flow control was disabled, communication could be established, but the reliability is uncertain.
The B&K Precision Model 8600 DC Electronic Load is the smallest in the family, but is good value for money compared to its peers, with a full complement of features, software and connectivity, backed by a three year warranty. The unit's range of 120V/30A/150W is quite suitable for many bench-scale investigations, and its transient load abilities and CR_LED abilities make it more useful. On standalone operation, the unit performed solidly. Basic features were easy to use, with all manner of input methods catered for. More sophisticated features did require consultation with the manual partly due to the limited display functionality, however, the manual was easy to understand and follow. The display is a fast, bright, contrasty VFD and although slightly flickery, is a delight.
When PC-connected, the included software distinguished it from its competitors by being included, and fairly featureful, covering most application requirements. This means that users don't have to spend their own time building their own test software using third party tools or "direct" communications. While the software is intended as "start-up" software, it is quite comprehensive. However, a number of basic bugs were identified and B&K contacted. To their credit, they followed through with my feature suggestions and bug reports and attended to them quickly, proving that they do have a competent and responsive support department.
The Model 8600 proved to be my reliable partner in an investigation (soon to be published online for all), and managed to survive various forms of torture without adverse effect.
Amongst the things I didn't have a chance to test include the IVI/LabView drivers, as I don't actually have LabView at the moment. I also didn't have a chance to test how discharge rates affect Li-Ion battery capacity nor produce I-V curves of photovoltaic cells. I didn't characterise any power converters using the unit yet, however, as I've won another RoadTest of an evaluation board - it's something I will do fairly soon.
The only flaws with the unit are a carry handle feels a bit weak, and the front panel connectors that creak slightly when tightened fully. Aside from that, the unit has been a solid performer, with all the features, connectivity, support and value we have come to expect from the B&K Precision name.
Thanks to B&K Precision and element14 for providing this unit for review, and supporting the community. Please keep a look-out on my personal blog at http://goughlui.com for other reviews, experiments, and more.