FAQ
TL;DR: For a 7.4 V pack, a 7.5 Ω resistor draws ~0.99 A and must dissipate ≥7.4 W; “The resistor will get VERY hot.” [Elektroda, Frank Bushnell, post #21664835]
Why it matters: This FAQ shows how to do safe, repeatable 1 A load checks with parts you can buy today, using one or two DMMs; it’s for hobbyists validating 2‑cell (7.4 V) Li‑ion battery health during quick, momentary tests.
Quick Facts
- Simple load: 7.4 V ÷ 7.5 Ω ≈ 0.99 A; choose ≥10–20 W wire‑wound to handle heat safely. [Elektroda, Frank Bushnell, post #21664835]
- Constant‑current option: AP432 (≈1.24 V), AP431/TL431/TL432 (≈2.5 V) references set sense‑resistor value. [Elektroda, Steve Lawson, post #21664842]
- Example sense resistor: 2.5 V ref → 2.5 Ω at 1 A; rate it ~5 W for margin. [Elektroda, Steve Lawson, post #21664832]
- Pass‑device heat: ~6.15 W at 1 A; use ~8 °C/W heatsink or fan. [Elektroda, Earl Albin, post #21664826]
- Metering: Put ammeter in series on 10/20 A jack; voltmeter across battery. [Elektroda, Frank Bushnell, post #21664835]
What resistor do I need for a simple 1 A load at 7.4 V?
Use a 7.5 Ω wire‑wound resistor. At 7.4 V it draws about 0.99 A (I = V/R). This meets the 1 A target closely enough for hobby checks. It’s inexpensive and easy to wire with a momentary pushbutton. Expect significant heat, so mount it on a fire‑safe surface. For visibility, clamp the ceramic body lightly in a small vise. [Elektroda, Frank Bushnell, post #21664835]
What wattage should the resistor be for that load?
Minimum dissipation is P = V × I = 7.4 W. Choose a resistor rated well above this, such as 10–20 W, to stay cool enough and survive repeated tests. Wire‑wound cement resistors are robust and widely available. “The resistor will get VERY hot,” so avoid plastic breadboards and keep flammables away. [Elektroda, Frank Bushnell, post #21664835]
How do I connect two multimeters to measure voltage and current correctly?
Connect the voltmeter directly across the battery. Put the ammeter in series with the 7.5 Ω resistor and the battery. Ensure the red lead is in the 10 A (or 20 A) jack on the meter before testing. This lets you watch the voltage sag under an ~1 A load as the pack discharges. [Elektroda, Frank Bushnell, post #21664835]
Can I test with only one meter if I only care about voltage under load?
Yes. Wire the 7.5 Ω load with a momentary pushbutton, then measure the battery voltage during the brief press. Stop using the battery once it reaches your 7.0 V cutoff. This quick check verifies usable capacity for the application without continuous instrumentation. [Elektroda, Knico, post #21664837]
Why is a constant‑current sink better than just a resistor?
Battery voltage drops during discharge, so a fixed resistor’s current falls too. A constant‑current sink keeps current near 1 A despite voltage and temperature changes, improving test repeatability. “With the OpAmp, the current will not deviate appreciably.” Use it when precision matters beyond a quick go/no‑go check. [Elektroda, Steve Lawson, post #21664828]
Which reference IC should I use (TL431/TL432/AP431/AP432), and what are their reference voltages?
AP432 is about 1.24 V. AP431, TL431, and TL432 are about 2.5 V (TL431 vs. TL432 mainly differ by pinout). Choose lower reference for lower dropout in the current sink, or 2.5 V types for simplicity and availability. This choice sets the sense‑resistor value. [Elektroda, Steve Lawson, post #21664842]
What value and wattage for the current‑sense resistor at 1 A with a 2.5 V reference?
Use R = Vref / I = 2.5 V / 1 A = 2.5 Ω. Dissipation is I²R = 1² × 2.5 = 2.5 W, so pick a 5 W resistor for margin. This keeps it within safe temperature rise during a sustained 1 A test. [Elektroda, Steve Lawson, post #21664832]
How low will a TL431/TL432 current sink regulate as the battery discharges?
It maintains constant current until the pass transistor’s VCE nears saturation. Expect regulation to fade when VCE is roughly 0.7 V. Below that, current falls because the control loop loses gain. Plan your cutoff above this region to keep results consistent. [Elektroda, Earl Albin, post #21664830]
How much heat does the pass transistor dissipate at 1 A, and what heatsink do I need?
Plan for about 6.15 W in the PNP pass device at 1 A. Use a heatsink near 8 °C/W to keep junction temperature reasonable. A smaller heatsink with a small fan also works but needs a power source. This prevents thermal runaway and protects the device. [Elektroda, Earl Albin, post #21664826]
Is there a simple 7.0 V indicator I can add so I stop in time?
Yes. A reference‑based LED indicator can watch pack voltage continuously. One implementation draws about 5 mA above 7 V and ~35 µA when below. Place it on the load’s battery side. It turns the LED off when the pack dips under your 7.0 V threshold. [Elektroda, Steve Lawson, post #21664844]
Will that indicator flicker near 7.0 V? How do I avoid it?
Without hysteresis, small noise or ripple can cause LED flicker exactly at the threshold. Add a little positive feedback (a resistor network) to introduce hysteresis and create a clean on/off transition. It simplifies interpretation during slow voltage changes. [Elektroda, Earl Albin, post #21664846]
I can’t solder tiny packages—what parts are easier to handle?
Use through‑hole TO‑92 parts. Options include a TL431 in TO‑92 and an LM385‑1.2 V in TO‑92. They are easy to wire, and the LM385 solution can draw ~16 µA when the LED is off. That minimizes idle drain on your pack. [Elektroda, Steve Lawson, post #21664848]
Any gotchas with the ammeter connections after testing?
Yes. Move the DMM’s red lead back from the 10/20 A socket to the volts socket after the test. Forgetting this is common and can short or blow the meter fuse the next time you try to measure voltage. Make it a habit. [Elektroda, Frank Bushnell, post #21664838]
Will a fixed resistor method still be accurate enough?
It’s adequate for quick health checks. Current will sag as voltage drops, but it’s far better regulated than no load and needs no calibration. For precise discharge profiling, use a constant‑current sink instead. Choose based on your accuracy needs. [Elektroda, Steve Lawson, post #21664829]
Quick how‑to: How do I perform a momentary 1 A load test safely?
- Wire a 7.5 Ω, 10–20 W resistor and a momentary pushbutton in series with the battery.
- Press the button briefly while reading pack voltage; stop if it reaches 7.0 V.
- Release, then let the resistor cool on a fire‑safe surface. [Elektroda, Frank Bushnell, post #21664835]
