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A Reproducible Way to Size Ultra-Thin Solid-State Film Batteries for GPT48-X / GPT50

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Turn reports/day into mAh/day, choose a thin‑film capacity band, and apply firmware/assembly levers to hit real‑world runtime targets.

Why this post exists

If you can’t explain runtime in mAh/day, you can’t control it. This guide shows a reproducible path from reporting policy to capacity selection for thin‑film solid‑state cells in Eelink GPT48‑X / GPT50 trackers. It includes a vendor‑agnostic selection matrix and a free workbook so you can plug in your own GNSS/TX numbers.

The minimal model (copy‑ready)

Define a “report” as the sum of energy events:

  • GNSS: I_gnss (mA) × t_gnss (s)
  • TX: I_tx (mA) × t_tx (s)
  • Overhead: I_over (mA) × t_over (s)
  • Sleep baseline: I_sleep (µA) across the day

Compute:

mAh_event = (I_gnss * t_gnss + I_tx * t_tx + I_over * t_over) / 3600
mAh_day   = mAh_event * reports_per_day + (I_sleep / 1e6) * 24 * 1000
days      = Capacity_mAh / mAh_day
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Start with measured currents if you have them. Otherwise, use conservative defaults from our workbook (GNSS ≈25 mA × 10 s; TX ≈180 mA × 3 s; overhead 5 mA × 2 s; sleep ≈5 µA) and tighten the numbers as you collect field data.

Profiles you can reuse

There is no single “right” report frequency. Use a profile that matches your application:

Profile Reports/day Notes
Low traffic 1 Batching recommended
Medium 6 Good default for many apps
High 24 Strongly batch transmissions

The workbook converts any profile into predicted days for TF‑5/10/20 mAh bands so you can spot if you’re under‑budget before ordering hardware.

Choosing a thin‑film cell: the four‑axis view

When selecting an ultra‑thin solid‑state battery, look at more than just nameplate capacity:

  • Capacity: 5, 10, and 20 mAh bands solve most GPT48‑X/GPT50 covert installs.
  • Thickness: Aim for < 0.6 mm stacks to keep mechanical freedom (laminates, PCB‑embedded).
  • Current capability: The cell must tolerate short TX/GNSS draws without large voltage sag; verify with your PMIC and protection stack.
  • Cost/Wh vs. volume: Use USD/Wh at 1k/10k/100k volumes to talk total cost of ownership with operations.

A good rule of thumb: if you can batch and cap GNSS, 5–10 mAh is often enough for weeks to months. For always‑on telemetry or poor networks, step up to 20 mAh or add energy harvesting.

Firmware levers (practical, not theoretical)

  • Batch uplinks: Amortize attach and protocol overhead by grouping data.
  • PSM + eDRX: Keep paging infrequent and predictable.
  • Attach caching: Reuse results where allowed; apply exponential back‑off on failure.
  • GNSS policy: Prefer hot‑start; cap at a fixed time; accept coarse position when necessary.

Assembly and reliability checklists

  • Volume: Confirm stack height after adding adhesives and covers.
  • Flex: Check min bend radius; avoid repeated flexing near the cell.
  • Protection: Validate PMIC over/under‑voltage, over‑current, and short protection under load.
  • Temperature: Observe min/max operating temperature for both the battery and device; account for hot‑table shipping.

Putting it all together for GPT48‑X / GPT50

  1. Pick a report profile (1/day, 6/day, 24/day or custom).
  2. Use the minimal model to compute expected mAh/day.
  3. Select a thin‑film cell with enough capacity for your runtime target.
  4. Apply firmware levers to minimize consumption.
  5. Assemble with proper mechanical and electrical protections.

Validating your math in the field

Lab numbers rarely match the real world perfectly. Validate with a production‑like device:

  1. Run your reporting profile in the target network environment.
  2. Measure actual average current draw over 24 hours.
  3. Compare measured vs. predicted mAh/day.
  4. Adjust GNSS/TX timings, duty cycle, or capacity as needed.

The workbook

Download the companion workbook to experiment with your own numbers:

  • Adjust current draws, timings, and report rates.
  • See predicted days for various capacity bands.
  • Test trade‑offs between capacity, thickness, and cost.

Closing

Sizing ultra‑thin solid‑state cells doesn’t have to be guesswork. With a simple energy model, vendor‑agnostic selection, firmware/assembly levers, and a free workbook, you can confidently match cell capacity to reporting policy and runtime requirements.

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