Inside View

A generous Dragonfly owner provided the following photos of an original 3.2 battery which had been damaged by water. Upon disassembly, it was found that one parallel group of cells measured zero volts.

But because the bike was purchased secondhand, it's not known how water entered the battery.

I have made some assumptions/comments based on these photos, but would need the circuit boards in my lab for a more thorough analysis.

Closeup of water-damaged area.

Overview of entire battery assembly.

Arrangement of 112 cells. There are 12 rows of 5 cells across, interspersed with 13 rows of 4 cells across.

Samsung INR21700-40T cell.

Manufactured in South Korea.

The Dragonfly 3.2 battery uses Samsung INR21700-40T cells, which have the following specifications according to 18650batterystore.com:

4000 mAh (8P yields 32 Ah, likely relating to the 3.2 designation)

35A max discharge (8P yields 280A)

67 grams (x 112 cells = 7.5 kg)

Rated 300+ charge cycles

Cost $3.85 (USD) each in 10-piece quantities.

Bottom view of circuit boards reveals components are soldered to topside only. Board on the right is the “high power” section. Board on the left is the “low power” section. Using separate boards helps isolate electrical noise.

Battery negative connection (strap) is at the far right. See photo below also.

Negative connection between battery and “high current” circuit board shown at top of photo.

Positive cell connection lead, which appears to be brass. The left end was spot-welded to the cells. The right end connects to wire cables. In order of decreasing size, the cables are: Discharge, Charge, BMS power.

The three copper-colored components near the right side of the circuit board appear to be shunt resistors to measure current. I assume they are all connected in parallel.

Unlike Electric Motion batteries that use electromechanical relays (contactors) on the positive side for the Charge and Discharge functions, Amopack uses MOSFETs on the negative side. I assume the Discharge MOSFETs are under the heatsink.

The four visible MOSFETs are Infineon IAUS300N08S5N012. These appear to be connected in parallel. Although these may be for switching the Charge function, they don't seem to be physically where I would expect them.

Closeup of “low power” circuit board. Group of repeated circuits outlined in yellow across the top is for cell balancing and cell voltage measurements. This BMS could handle up to 15 series cells.

Identifiable Components

Microchip PIC18LF26K83, Microcontroller. Features:

Program Memory Size (KB): 64

RAM: (bytes) 4096

Data EEPROM (bytes): 1024

ADC maximum resolution (bits): 12

ADC Channels: 24

Texas Instruments BQ7694003, Analog Front End (AFE) for BMS. Features:

Pure digital interface

Internal ADC measures cell voltage, die temperature, and external thermistor

A separate, internal ADC measures pack current (coulomb counter)

Directly supports up to three thermistors

Overcurrent in Discharge (OCD) detection

Short Circuit in Discharge (SCD) detection

Overvoltage (OV) detection

Undervoltage (UV) detection

Infineon IAUS300N08S5N012 is an N-channel MOSFET rated 80V, 1.2 milliohm on-state resistance, 300A
Texas Instruments ISO7521CDW is a low-power 1 Mpbs 5 kV rms dual-channel digital isolator

SMPS is a buck converter that drops the battery voltage down to around 3.3 to 5 V to run the BMS. Flexible 8-conductor flat cable seen near bottom-center of photo connects BMS to front panel diagnostic LEDs and Test button.

Credit Texas Instruments: Simplified schematic of Analog Front End (AFE) integrated circuit for BMS.

Battery external connections.

External Battery Connections

Heavy Red wire: Discharge positive.

Heavy black wire: Discharge negative.

Thinner red wire: Charge positive.

Thinner black wire: Charge negative.

Blue and White wires: likely for external communication with BMS via charging connector.

Learning from Samsung's INR21700-40T Datasheet

I found it curious that Amopack rates the entire battery for 500+ charge cycles, whereas 18650batterystore.com describes the individual cells as being good for “300+ charge cycles.”

Amopack undoubtedly knows more about this than I do. In an attempt to reconcile the difference, I came across Samsung's “Confidential Proprietary” datasheet for the INR21700-40T. As with most topics, the deeper you dig, the more complicated it gets. (Note that the datasheet was published in December 2017, and so may not contain the very latest information. It was also marked “Tenative”.)

Most importantly, there are two different methods to describe the cell's characteristics: Standard and Rated. The Standard method implies a more conservative usage than the Rated method, and yields more desirable numbers.

The Standard Charge current is 0.5C (2 amps), whereas the Rated Charge current is 1.5C (6 amps).

A quick calculation reveals the Dragonfly's cells are only being charged at about a 0.25C (1 amp) rate. Here are the particulars:

The INR21700-40T is a 4000 mAh cell.
The Dragonfly's charger is rated at 12.5A.
The constant-current portion of the Dragonfly's charge cycle takes about 1.5 hours.

Having eight 4000 mAh cells connected in parallel means a 1C charge rate would require 32A. But 12.5 / 32 = 0.39. This number is further diminished by taking more than an hour to charge (0.39 / 1.5 = 0.26).

Similarly, the cell's Standard Discharge capacity is a more conservative measure than its Rated Discharge capacity.

The Standard Discharge capacity is ≥ 4000 mAh, and defined as:

“...the initial discharge capacity of the cell, which is measured with discharge current of 800 mA (0.2C) with 2.5V cut-off at 23 ℃ within 1 hour after the standard charge.”

Whereas the Rated Discharge capacity is ≥ 3900 mAh, and defined as:

“...discharge capacity of the cell, which is measured with discharge current of 10 A with 2.5V cut-off at 23 ℃ within 1 hour after the rated charge.”

The test for Cycle Life is a worst-case condition described as being:

With Rated charge (6A, 4.2V, 100 mA cutoff) and

Maximum Continuous Discharge (35A, 2.5V cut off)

the Capacity after 250 cycles is ≥ 2400 mAh (60% of the standard capacity at 25 ℃).

Some Takeaways

Unsurprisingly, the lifespan of a cell (or battery) depends on the severity of the duty it must endure.
The major factor in determining lifespan is depth of discharge (DoD). A lesser DoD yields a longer life.
End-of-life is generally considered to be a reduction to 80% of new capacity.
Charging current may inherently be limited by the cost to manufacture (and power needed to operate) a high C-rate charger.
For further reading, see: https://www.electricmotiontech.com/home/ev-tech-101/battery-care-and-feeding

Bonus Photos

Series interconnect, zinc strip

Parallel interconnect (brass?)

Page updated

Google Sites

Report abuse