Greenworks 80-Volt Battery

The battery is arguably the most important part of any rechargeable product.  It certainly is the most likely thing to fail.

Greenworks used to rate their battery's watt-hour capacity at 72 volts.  Thus, a 2.0 Ah battery was rated 144 Wh.  This was a reasonable compromise because 72 volts represents being about halfway through its useful capacity.  Now Greenworks rates a 2.0 Ah battery at 80 volts, which yields a capacity of 160 Wh.  This makes it look like a better battery, but it isn't.

I have disassembled three Greenworks 80-volt batteries.  A Torx T10H security bit is needed to open the case.  All are configured 20S1P.  All contained different cells, which are listed below: 

• Samsung INR18650-20R cells.  These have a green wrapper and are rated 2000 mAh.  Rated continuous discharge current of 20A.

• LG Chem ICR18650HE4 cells.  These have a yellow wrapper and are rated 2500 mAh.  Rated continuous discharge current of 20A.

• Unknown TIR6RXA & TIR6RX8 cells.  These have a red wrapper that is of poor quality.  I could find no information on these cells.  They are possibly from a counterfeit battery.  The BMS power board was also different (physically smaller) than the other two batteries.

The battery connections are marked:

• P-  Pack Negative (this is the return for the charger and communications as well)

• COM  Communications (a serial bitstream) 

• CRG+  Charger Positive

• P+  Pack Positive

Battery Management System

The battery management system (BMS) comprises two circuit boards.  One is located in front for cell monitoring, and one is at the rear for power switching and fusing. 

The front PCB is marked “Globetools” and a search brings up a website in China for Greenworks:  https://greenworkstools.com.cn/en/  It appears the company was started in 2007 and claims to have more than 66 patents for invention.

The BMS uses four integrated circuits in a 24-pin SSOP package.  The common marking on this IC over multiple batteries has been “8905AM05SN”.  I assume this is an analog front end (AFE) but cannot find a datasheet.  This leads me wonder to if it's a custom IC for Greenworks?

It's clear from a cursory observation that one AFE IC serves a group of 5 cells.  In this way 40, 60, and 80 volt batteries could be built using 2, 3, or 4 of the ICs, respectively. 

Transistors (Q1-Q20) for cell balancing are evident, as are 100 ohm load resistors.  This implies a maximum balancing current of ~40 mA (0.16 watts) per cell.

The microcontroller is packaged in a 20-pin plastic LSSOP marked “10268A”.   This indicates it's from the Renesas RL78/G12 family which features an ultra-low power 16-bit core.

The 2.0 Ah batteries use two IR P379Y power MOSFETs on the rear board.  The 2.5 Ah batteries have threes such MOSFETs.

Serial Communications

The two photos below are oscilloscope captures of the serial data steam observed between P- and COM from a CSB410 chainsaw.

A burst of data is sent every 200 milliseconds while the saw is ready to work.  Each bit has a high time of about 100 uS.  The line swings between 3 volts (the idle state) and 0 volts.  My Siglent oscilloscope has the capability to decode serial data, but I could not determine the content of this message.

The battery does not autonomously emit this data stream.  The motor controller must be connected via the COM wire which therefore probably initiates a query.

Battery Chargers

I am aware of two chargers for the Greenworks 80-volt battery.  Both are specified for a 120 VAC 50/60 Hz input.  Battery chargers of this type are essentially switched-mode power supplies.  This yields a high AC to DC conversion efficiency.

• GWX0800250 is rated 4 amps on the DC side.  The battery sits horizontally in this charger.

• GCH8020 is rated 2 amps on the DC side.  The battery sits vertically in this charger.

Serial communication between the charger and the battery occur via the COM connection.  Without this connection, charging will not commence.

Capacity Testing a 2.0 Ah Battery

My active electronic loads can't handle an 80-volt battery, so I used a simple resistive load and a Chinese PZEM-051 power meter (rated 100 V and 100 A) for this capacity test.  

The load was made from two Dale 20 ohm, 100 watt power resistors connected in series.  This yields 40 ohms with a 200 W rating.  With an 80-volt battery, the current draw is 2.0 A and the worst-case power dissipation is 160 watts.  As the battery discharges, the power dissipation decreases.  The test is concluded at 60 volts (3.0 volts per cell).  At that point the power dissipation is only 90 watts.  This is a less demanding test of the battery than one conducted at a constant current of 2.0 A (the 1C rate). 

I used a small fan to cool the resistors.

Hot off the charger, the battery measured 83.3 volts unloaded.  As soon as the 40-ohm load was connected, the voltage dropped to 81.6 V.

After 27 minutes, the voltage was 72.0 V.  At that point the current draw was 1.8 A.

At the 60-minute mark, the voltage was 62.9 V, drawing a current of 1.31 A.  So far, the battery had provided 131 Wh of energy.

A few minutes later the pack voltage slipped below 60, and 133 Wh had been provided.  Note the small change in energy from the 60-minute mark.  Towards the end, voltage drops rapidly and not much useful energy remains to be withdrawn.  The battery is rated for 144 Wh.  I am satisfied with this outcome.  The PZEM-051 is not a laboratory-quality instrument and I have never attempted to validate its accuracy.  Furthermore, I've rarely seen a battery provide its rated capacity.  It's possible the rating is made down to a cell voltage of only 2.5 volts, but I feel that's not good for the battery.  There's very little energy remaining when the pack voltage drops below 3.0 volts per cell anyway.

Immediately after removing the load, the voltage recovered to 61.6 volts.

Recharging with the 2-amp charger took less than 90 minutes and consumed 168 Wh of AC power.  I have not measured the input/output efficiency of a Greenworks charger, but 90% would be a reasonable guess.  This means that battery accepted approximately 151 Wh of energy. 

Heavy Load Testing

The following test shows the importance of the battery in high-load applications.  A perfectly healthy 2.0 Ah battery was tested at roughly halfway through its useful state of charge (starting at 73.9 volts, unloaded).  

The load was a series string of six 2-ohm Ohmite L175J power resistors.  These resistors are rated for 175 watts each and can sustain an overload of 10 times their rated wattage for 5 seconds.  Each wire-wound resistor exhibits an inductance of 36 uH.  A Peacefair PZEM-051 power meter was used for instrumentation.  The battery holder was repurposed from a broken charger, and the power wires are fine-strand #8 AWG with Amass AS150 connectors.

This is a challenging test to perform with the simple equipment shown.  Each test was conducted only as long as was needed to record the data (well under 10 seconds).  The negative terminal clamp was moved by hand to the required resistor tap.  This created an arc during the connection and disconnection process.  It was literally a bit like welding.  One resistor fractured during the test.  I had considered testing all the way down to 2 ohms, but decided against it as the trend illustrates my point, and it would be hard on the battery.

 The table below shows the results.  A 2.0 Ah battery would provide very short run-time in a saw rated 3.4 kW.