HW-586 Battery Discharger Module

The HW-586 is a breakout board designed to discharge batteries down to a user-defined stop voltage. It originally appeared under the name ZB2L3 and supports single cells as well as battery packs with a total voltage of up to 15V.

The board includes two 5W 7.5Ω load resistors, which can be used in parallel or in series, or replaced by different load resistors if desired—just ensure the discharge current always remains below 3A.

Overview

The HW-586 is a microcontroller-based battery discharge tester, allowing batteries to be discharged to a defined stop voltage. It measures and records the discharged energy (in mAh) for capacities up to 9.999 Ah (9,999,000 mAh).

Versions and Evolutions

When originally introduced as ZB2L3, the board used an STM8S003F3P6 microcontroller (hidden beneath the seven-segment display) and a TM1650 LED controller.

Since 2019, the board is sold as HW-586, and even though the appearance remains the same, it has undergone several silent revisions since then: to reduce costs, both the microcontroller and LED controller were replaced with cheaper, pin-compatible no-name clones (e.g., Nuvoton microcontrollers and ET6226M LED controllers).

Custom Calibration Removed

The most significant change is the reduced memory in the cheaper Nuvoton MCU, which led to a trimmed-down firmware and the removal of the calibration features.

In the original version, pressing all three push buttons while powering on the module would activate a special calibration mode to fine-tune voltage and current measurements. This functionality is no longer available.

Reliability

In-depth analysis of this module shows that it may or may not be reliable, depending on the variant.

While many users report accurate capacity measurements, others have seen errors up to 30%. This variability likely stems from the numerous lookalike clones available on the market. Some versions use counterfeit op-amps, which can lead to large measurement errors.

Since the original calibration feature is no longer present, it’s essential to verify the accuracy of your module. Compare its results with a known-good battery tester to ensure it’s suitable for your use case.

Recommendations

If your copy of HW-586 turns out to be accurate: good for you. You then have an extremely affordable battery tester.

Yet even if your module isn’t accurate enough for capacity measurement, you can still use it to accurately test your batteries: use it as a simple discharger to complement your existing battery charger:

1. Before you charge your batteries, use HW-586 to discharge them to a known stop voltage, i.e. 2.8V or 3.0V. If your HW-586 isn’t accurately summing up the discharge capacity, simply ignore it. All that matters is that HW-586 reliably discharges your batteries to a known stop voltage.
2. Next, charge your batteries as you would usually do. Most half-way sophisticated chargers report the total mAh that were charged into the battery. Since - thanks to HW-586 - all of your batteries now start charging at the same stop voltage, the charge that goes into your batteries is now a good estimate of battery capacity.

As an added benefit, this way of battery monitoring is faster and adds less strain to your batteries: since you start with already empty cells and just discharge them a little bit more to bring them to the same voltage, the overhead is minimal.

• Slow Classic Testing: In classic automated battery testing, your test device would first fully charge the battery, then fully discharge it, then again fully charge it. You would waste a full charge cycle, and testing a battery can take half a day.
• Fast Testing: When testing the battery during normal charge, you are not wasting any charge cycle. The only overhead is discharging the almost empty cell to a known stop voltage. Thanks to the high discharge currents supported by HW-586, this overhead sums up to just a few minutes. Classic testing adds 6-12 hours of overhead for a full additional charge/discharge cycle.

That said, fast testing may over-estimate the battery capacity by a few percent: total energy going into a battery during charging is slightly higher than energy drawn out of the battery during discharge.

Fast testing is perfect for routinely monitoring battery health as part of your regular charging routine.

Wiring

The board features four screw terminals, labeled on the back of the PCB:

Connect the battery holder with correct polarity, and make sure the battery holder is appropriately labeled/marked so that users later know how to correctly insert the battery. Reversing polarity will destroy the tester. You may want to add an ideal diode PCB to your setup for added safety.

Here’s a typical setup using a 18650 battery holder and an external load resistor:

The board requires a separate power supply via its Micro USB connector.

Choosing a Load Resistor

The discharge current is determined by both the load resistor(s) and the voltage of the battery under test.

The board includes two 5W 7.5Ω load resistors. You can configure them in different ways to adjust the discharge current:

• Highest current:
  Use both resistors in parallel (3.75Ω)
• Medium current:
  Use only one resistor (7.5Ω)
• Lowest current:
  Use both resistors in series (15Ω)

Calculating Discharge Current

You can calculate the discharge current using Ohm’s Law:

I = V / R

The actual current depends on the voltage of the battery:

Replacing Load Resistors

As seen above, discharging a 12V battery produces significantly higher currents than testing a single Li-ion cell. At higher voltages, the total wattage increases dramatically—often exceeding the board’s or resistor’s specifications.

For example, testing a 12V battery with both resistors in parallel results in a 3.2 A current (exceeding the board’s 3A limit) and 38.4 W load (almost 4× the resistors’ combined rating).

The included resistors are best suited for single Li-ion cells. For higher-voltage batteries:

• Use resistors rated for at least 50 W
• Ensure the discharge current does not exceed 3.0 A

To achieve the maximum 3A discharge current with a single Li-ion cell, use a 1.4 Ω resistor rated for at least 15 W.

Errors

Before starting a discharge cycle, the firmware performs a few checks. If a check fails, it shows an error code and stops:

If you encounter an error, here’s what to do:

The Dreaded Err3

Err3 can be especially annoying. It was intended to protect batteries from excessive load but often triggers falsely. Here’s why:

When any load is applied, a battery’s voltage naturally drops due to its internal resistance. If the drop is excessive, it’s usually a sign of too much load. The firmware monitors this during the first few seconds of discharge. If the voltage drop is deemed too high, Err3 is shown.

However, in many cases, it’s not the battery that’s at fault—it’s the battery holder or its wiring, which may have high contact resistance. For discharge purposes, this added resistance is often harmless—it just acts like an extra series resistor.

Battery Holder at Fault

Here is an example using a standard 18650 battery holder, and a 50W 2 Ω load resistor. At 4.2V, this should produce a discharge current of 2.1A:

However, in reality it just produces an Err3 error, and discharging is interrupted.

Using Better Battery Holder

When replacing just the battery holder with a higher-quality 4-wire test bay, Err3 goes away, and the same battery discharges just fine.

Note that it is absolutely not necessary to use a 4-wire battery holder. What matters is the low surface resistance between the battery contacts and the holder springs. The battery holder that worked best for me can be found at AliExpress when searching for TS457. That’s a simple internal resistance battery tester which offers high quality 4-wire battery bays as separate extras (around € 4.00-€6.00 per piece).

Workarounds for Err3

If you’re experiencing Err3, try the following:

• Upgrade your battery holder:
  Use a holder with lower contact resistance (like in the example above). This reduces voltage sag and avoids false triggers.

• Reduce discharge current:
  Use a higher resistance load. Lower current means lower voltage drop and less false alarms.

• Delay increased load:
  Start with a higher resistance, such as 7.5 Ω, to keep the initial current low. After ~10 seconds (once the firmware has completed its voltage-drop check), add a second resistor in parallel (e.g., 2 Ω) to lower total resistance to 1.58 Ω.
  For a 4.2 V Li-ion cell, this means starting at ~500 mA and increasing to ~2.66 A after the initial check.

Performing Test Discharge

Plug in a Micro USB connector to power up the test device. The device performs a quick self-test during which the three red LEDs at the right side of the display cycle.

The display shows 000 until you insert a battery into the battery holder. Once a battery is inserted, the display shows the current battery voltage.

Setting Stop Voltage

If you do not set a stop voltage, the tester picks a stop voltage automatically. This stop voltage is typically too low (i.e. 2.5V for a LiIon cell) and may damage your battery or trigger its built-in BMS.

That’s why you should always set your own stop voltage. To set your own stop voltage, press and hold + until the desired stop voltage is shown in the display. You can set or change the stop voltage when no discharge test is running. Once you start a test run, the stop voltage can no longer be adjusted. You would have to abort the test run by pulling the Micro USB plug, then start over again, if you needed to change the stop voltage.

Caveat: Dangerous Stop Voltage

Always make sure you check the stop voltage by pressing + before you start any discharge test.

The test device has an awkward and potentially dangerous way of setting stop voltages automatically:

• it does not remember your stop voltage from previous runs
• it sets the stop voltage to half of the batteries’ start voltage

If you accidentally insert an almost empty LiIon cell into the test device, it will set the stop voltage to 50% of the current voltage. So if the cell is at 3.1V, the stop voltage would be 1.5V.

If you perform the test now without checking (and correcting) the stop voltage, the tester would go ahead and discharge your LiIon cell to 1.5V. Discharging a LiIon cell to below 2.8V permamently damages the cell.

So always manually set the stop voltage prior to every discharge run!

Starting Test

To start the discharge test, press OK. The display briefly shows the set stop voltage, then the discharge test begins.

The display automatically switches between Ah (total discharged capacity), A (discharge current), and V (current battery voltage) in short intervals.

Press OK to temporarily lock the display to one of these units. After 15 seconds, the tester starts cycling the units again.

Ending Test

There is no way to prematurely stop (or pause) the test. If you must interrupt the test, pull the Micro USB power plug.

Once the battery has reached the stop voltage, the display shows the total discharged capacity (Ah), and the display blinks permanently.

Press OK to confirm the test result. Press OK again to start a new test. The display now shows the battery voltage again, and you can start over with a new battery.

Conclusion

I routinely test all of my batteries using a commercial battery tester, then attach a sticker to each battery with the measured capacity and the date of test. This provides a great way of evaluating battery health at any time, and also identifies fake batteries immediately so they can be returned in time.

Charging Turned Into Battery Test

Thanks to HW-586, I can now turn each regular charging cycle into a full battery test:

Before I charge an “empty” battery, I use HW-586 to fully discharge the battery to a known level, i.e. 3.0V.

This way, when I then charge the battery using my regular charger, I can directly compare the total charge that the charger reports at the end of charging to the original battery capacity that I initially measured.

The great thing about this setup is that it comes at almost no extra cost or effort: since HW-586 can discharge with up to 3A, it takes almost no time to bring “empty” batteries to the common 3.0V stop voltage. The subsequent charging process is the same charging process I would do anyway.

Load Resistor

The batteries I am using are all good for at least 3A discharge which is why I picked a 50W 2 Ohm load resistor.

It discharges “empty” batteries (around 3.4V) at 1.7A: 3.4 V / 2 Ohm = 1.7 A.

Precision and Experience

The HW-586 works very well for me. Even when I place a fully charged battery into the battery holder and discharge the battery all the way, HW-586 reports roughly the same capacity that my commercial battery tester found initially, too.

HW-586 reliably cuts off discharging at the defined stop voltage.

Let there be no misunderstanding: the kind of battery testing I perform aims at getting a good battery health estimate. It does not have to be accurate to the milli-Ampere. I can live with a 5% error. What’s more important is repeatability.

Challenges

HW-586 is dirt cheap. The most recent batch I ordered was around €0.70 per piece (including load resistors).

However, you should invest in a high quality battery holder. I am using the battery test stand for the TS457 internal resistance tester.

The contacts can be unscrewed from the test stand holder and then screwed in your own test stands.

The larger contact has an internal spring mechanism that can be pressed approximately 1cm in. This makes it very convenient to insert or remove batteries. In the center of each contact, there is an additional pogo-like pin. Overall, the surface resistance is very low. The contacts work well with discharge currents of up to 3A. I did not test any higher currents.

Here is a 3D-printable test stand that works with most LiIon round cells:

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