LELTx5, a 5-channel LiPo-Cell Electronic Load Tester kit

GUSTECH

LELTx5 is a 5-channel LiPo-Cell Electronic Load Tester

Configured as five independent (up to) 100.0mA constant current loads (each), the LELTx5 is a versatile and valuable piece of test equipment for any experimenter working with the quickly expanding Lithium-Ion and Lithium-Polymer (LiPo) battery technologies and their many varied applications and circuits. Using on-board switches, it can present constant current loads of: 100.0mA, 200.0mA, 300.0mA, 400.0mA, and 500mA, per LELTx5 board. Two LELTx5 kits will get the loading up to 1.0Ampere; four will get 2.0Amperes, etc.

PICTURE with Overview:
TOP RIGHT CORNER: Block Diagram of 1 Channel of the LELTx5
TOP LEFT CORNER: Schematic of 1 Channel of the LELTx5
BOTTOM RIGHT CORNER: Printed Circuit Board Assembly Drawing (2.85” x 2.85”) depicting Components' locations.
BOTTOM LEFT CORNER: Board Assembly view with Components Installed.

WHY:
Characterizing the discharge behavior of batteries is an important task for optimally selecting the best solution for many battery-operated applications, including (but, by no means limited to): medical and healthcare devices including a huge assortment of “Wearable’s,” small to medium size robots, a wide-spectrum of wireless sensors with ISM radio-based communications (think: RF-digital, Wi-Fi, Bluetooth, XBee, Active-RFID, ZigBee, Radio Modems, et.al), even the exploding field of UAV’s, to mention just a few.

A simple resistor-load will NOT do. Unlike active circuits (microcontrollers, radio transceivers, sensing system and some low-power actuators), resistors do NOT ‘look like’ constant (on average) loads to batteries and/or cells since the current drawn by them drops as the voltage drops over time. The purpose of Electronic Loads in battery testing systems is to present an unchanging current-or-power drain regardless of the battery or cell’s voltage level, emulating the power drain of the (future) application circuitry.

HOW:
The useful voltage range in most LiPo-cell-powered circuits is +4.2volts maximum (for a 3.7volt rated cell just after it has been completely recharged) down to +3.2volts minimum (just starting into the sharp knee drop-off towards total discharge). So, as a minimum design goal, it would be ideal for an Electronic Load to maintain a constant loading on the cell down to at least 3.2volts if it is to perform as a practical Electronic Load for LiPo cell testing and performance analysis purposes. This is a perfect task for a low-dropout linear voltage regulator.

The LELTx5, per channel, consists of a low-dropout regulator that maintains a constant voltage on a fixed resistor (and LED) load regardless of the voltage input applied to it, at least down to 3.2volts. Non-precision resistors can be used because the actual voltage output is adjusted so that exactly 100.0mA is flowing into the LELT-channel circuitry, compensating for variations in component values due to wide-tolerances. This enables precision operations from extremely inexpensive components. As shown in both the block diagram and the schematic in the figure with this brief, the Green (jumbo, 10mm) LED, whose actual forward voltage drop varies significantly from unit to unit, is in parallel with a series resistor string composed of 5% tolerance resistors.

When the Green LED(s) turn(s) off, the LiPo-cell being discharged by the one or more LELTx5 devices has dropped its output voltage to the point where its integrated Protection Circuit Board [PCB] has electronically disconnected the cell from its wiring to prevent over-discharging the cell(s). More details about the PCB’s can be found in the Owner’s Manual for the LiBaC kit.

BREADBOARD VERSION:
The “ELOAD#” voltage levels for the breadboard version of the LELTx5, after per channel calibration to exactly 100.0mA loading each, measured from a low of +2.66volts to a high of +2.69volts. For the LP2951, the typical ‘headroom’ when controlling 100mA is nominally 400mV (at 25C) and goes up to about 430mV (at 50C). This means that 100mA loading can be maintained when the LiPo-cell voltage drains down to a low of +3.09volts (at 25C), which is well into the drop-off knee for a typical discharge curve.

The LELTx5 channels can be adjusted for lower constant-current loadings, with a low of 50mA already tested during the LiBaC development. Additional data regarding non-100mA-per-channel operations are included in the kit instructions.

When running the LELTx5 per-channel load levels at the maximum of 100.0mA (each), the maximum “V+” input voltage (in case the kit-builder wants to use it for other battery and cell testing purposes) is: +14volts (at 25C), or 12volts (at 50C) or a low of about 8volts (at 85C).

There are more details about how the LELTx5 works, including a couple of classic-form discharge curves, in the LiBaC Instructions, steps 38, 41, and 49. More curves are included in the instruction set for this kit as Performance Data.

THRU-HOLE ONLY = BEGINNER LEVEL:
All of the JameCo-only components for the LELTx5 are thru-hole components, making this kit easy to build, even as a very first kit, for the beginner kit-builder. Assembly time is about an hour, not including calibration. Any voltage source greater than 3.2volts (LiPo-cell, 9volt battery, power supply, car battery, etc.) with a Multimeter that has a 200mA ammeter scale is all that is needed to calibrate each channel (one at a time) of the LELTx5 after assembly.

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