diff --git a/docs/BOM.md b/docs/BOM.md
index fd73007..ab803ae 100644
--- a/docs/BOM.md
+++ b/docs/BOM.md
@@ -53,3 +53,21 @@ type: reference
 - M.2 B KEY is **not** mechanically compatible with Mini-PCIe.
 - Supply voltage should be ~4.0 V for reliable operation.
 - DO NOT hot-plug while powered.
+
+## RTC backup battery
+
+**Status: research pending.** Part choice and footprint not yet verified against NVIDIA's schematic.
+
+| Field | Value |
+|-------|-------|
+| **Purpose** | Keep the RTC (`rtc0`, `nvvrs-pseq-rtc`) running while the board is unpowered, so the clock is correct at boot without network or GPS |
+| **Carrier board** | P3768 (Orin Nano Developer Kit): pads present, **connector not populated** (needs soldering) |
+| **Connector (per NVIDIA docs, as quoted on the forum)** | Wieson AC2651-0011-003-HH, 2-pin, 1.25 mm pitch. A community-used substitute is Molex 53398-0271 header + 51021-0200 housing |
+| **Battery** | TBD: coin cell with leads; confirm chemistry/voltage against the carrier board spec (SP-11324-001) |
+| **Extra parts** | Forum reports also populating R560 (0.56 Ω, e.g. Panasonic ERJ2BQFR56X); confirm in the P3768 schematic |
+
+### Notes
+
+- Without it the clock boots at a stale date (seen 2026-10-08: June 4) until NTP syncs. See the [time sync report](impl/reports/TIME-SYNC.md).
+- Forum sources: [RTC battery on Orin Nano DevKit](https://forums.developer.nvidia.com/t/rtc-battery-on-jetson-orin-nano-developer-kit/296732), [How to add RTC battery](https://forums.developer.nvidia.com/t/how-to-add-rtc-battery-in-jetson-orin-nano/255792).
+- A reported side effect: the battery lead and socket make the case cover sit unevenly.