How should power be designed for the 9704?
Series Building with the Iridium 9704 Part 8 of 10
In short
Keep the voltage steady during transmission; size the battery by data volume.
Current peaks sharply when the 9704 transmits. N3D tested 100 KB image round trips on a 3,000 mAh battery, measured the energy per message, and saw the 9704 restart by itself late in the test.
How much energy it uses
| Item | Rooftop (full sky) | Window (half the sky) |
|---|---|---|
| Per MT (download) | 5.5 mAh | 7.6 mAh |
| Per MO (upload) | 11.2 mAh | 15.6 mAh |
N3D test, 9 October 2026, Iridium 9704 Launch Pad on a 3,000 mAh battery alone, about 99 KB per message, medians.
Including idle time, each successful round trip used about 37–40 mAh; by energy alone, 3,000 mAh covers about 75–80 round trips. In practice the 9704 started restarting with about two thirds of the battery left (see below). With a poor view of the sky, the same message takes about 40% more energy.
How the voltage changes
| Item | 9 Oct 2026, rooftop | 25 Oct 2026, rooftop |
|---|---|---|
| Sending pattern | One 100 KB message every 6 minutes | Next message after the previous one returned; 1–100 KB in rotation |
| Average voltage drop | About 0.10 V per hour | About 0.08 V per hour |
| First 9704 self-restart | Minute 135 | Minute 191 |
| Voltage in the minute before | Average 3.95 V, minimum 3.61 V | Average 3.91 V, minimum 3.50 V |
| Data completed before it (each direction) | About 2.17 MB | About 2.26 MB |
The two tests sent very differently and the restarts began 56 minutes apart, but both began at an average of about 3.9 V, a transmit minimum of about 3.5 V, and about 2.2 MB transferred. So size the battery by how much data you send per day, not by how long it must run. Under heavy transmission, this 3,000 mAh battery delivers only about the top third of its energy (estimate). The cause is still being confirmed with Iridium.
What to design in
- A low-impedance supply: count the battery, wiring and connectors.
- Large capacitance: add storage such as a supercapacitor on the 9704 supply so the voltage holds during transmission.
- No large messages on a low battery: check the battery voltage and postpone large messages when it is low.
- Firmware that recovers: after the 9704 restarts, reinitialise and continue sending what is pending.
Energy by message size
| Size | Download (MT) | Upload (MO) |
|---|---|---|
| 1 KB | 0.1 mAh | 0.3 mAh |
| 30 KB | 2.1 mAh | 3.7 mAh |
| 50 KB | 3.1 mAh | 5.9 mAh |
| 70 KB | 4.5 mAh | 8.5 mAh |
| 100 KB | 6.1 mAh | 12.4 mAh |
N3D rooftop test, 25 October 2026. Estimated from the 9704's send and receive state each second multiplied by the module's specified currents; energy above idle.
About 0.06 mAh per KB to download and 0.12 mAh per KB to upload: transmitting costs more than receiving, so uploads cost twice as much.
Do not force transmissions at 0–1 bars
In the window test, 83 minutes were spent at 0–1 bars. Both directions were "sending" but little got through, which simply wasted energy. Waiting for a better signal saves energy and finishes sooner.
Parameters and sources
- Device: Iridium 9704 Launch Pad, module firmware SX-1.6.0; 3,000 mAh / 3.7 V lithium battery, no external power.
- 9 Oct 2026: rooftop 13:17–16:37, window 17:47–21:00 (Taiwan time); one message of about 98.7 KB every 6 minutes.
- 25 Oct 2026: rooftop 06:43–11:04; 1, 30, 50, 70 and 100 KB in rotation, next message after the previous one returned; 54 of 55 returned complete.
- Voltage: sampled every 10 ms (25 Oct) or 50 ms (9 Oct), with per-minute average and minimum; transmit pulses last about 8 ms, so the measured minimum may be higher than the true one.
- Energy: estimated from the send/receive state and signal bars each second multiplied by the 9704's specified currents; battery energy used is converted from voltage and is approximate.
Can it run on solar power?
Yes, with a battery or other storage in between; do not run the 9704 directly from the panel. Include cloudy days and nights in the energy budget.
What saves the most energy?
Turn the 9704 off when idle and wake it on a schedule to send and receive, and place the antenna where it sees the whole sky.
All parts in this series
From developer kit to certified product: what an Iridium 9704 product needs, the limits of the developer kit, why certification matters, how to choose hardware, where it fits, and measured timing and power. Ten parts, in order, each readable on its own.
- What do you need to build an Iridium 9704 product?
- What problems does IMT CC actually solve?
- What are the limits of the Iridium 9704 Launch Pad?
- Why do 9704 products need certification, and what does it cover?
- Who is the RockBLOCK 9704 for?
- Launch Pad, N-9704, RockBLOCK 9704 or your own design: which should you choose?
- Where can the IMT 9704 be used? Mountain rescue, maritime and monitoring
- How should power be designed for the 9704?
- Why can an IMT message take several minutes?
- How does the Iridium 9704 compare with Starlink and OneWeb?
Related notes
Related hardware
Related services
Updated 2026-10-27
N3D supplies satellite communication for alpine and industrial use. See services