Rendered from docs/power-solar.md in the repository at build time. Estimates are labelled as such in the source; nothing on this page is a measured result unless it says so.

BlindBot / ValiTilt - battery + solar + USB-C power (v0.4 draft, 2026-10-17)#

Status: paper design. Nothing here has been built or measured. Every figure has a cited source or is labelled (assumption) / (est.). Irradiance is PVGIS data (cited). Model: tools/power_budget.py -> docs/power_budget.png (re-run it after you measure anything). No prices in this file (see the BOM notes).

• Part D (new, 2026-10-18, at the top) - version D: panel and a low-temperature LiPo in ONE flat pane pack, charger stays room-side; includes a correction of the 6106 TS window (about +4.6 C, not 0 C, with a 10k NTC).
• Part C (new, v0.4, at the top) - cost-down re-run: 2x ULN2003A drive at 3 / 2 rpm and a 12 V 1.5 W panel. Part C supersedes the v0.3 load and solar numbers for the default build; Part A stays valid for the quiet TMC2209 option.
• Part A (v0.3) - Li-ion/LiPo cell + small 6 V panel + USB-C through ONE charger IC, recomputed with the real move (about 1 knob turn end to end, 12 rpm = 5 s per turn, typical move about 1/2 turn = 2.5 s).
• Part B (kept, v0.2) - the LiFePO4 + BQ25570 + CN3058E design (VT_POWER_SOLAR=1 in PR "solar-power"). It is still the better choice for very dim windows, but its energy table (§B3) used an old 20 s / 3.2 mWh move and is superseded by §A2.

Part D - version D (2026-10-18): one flat pane pack with the panel AND a low-temperature cell (ESTIMATES)#

Status: paper design + CAD. Nothing in this part was built or measured. Model: python3 tools/power_budget_d.py -> docs/power_budget_D.png. Same PVGIS hourly data (2015-2023), glass, soiling, 0.75x panel factor, sleep current and 47.6 mWh/day load (2 blinds x 4 moves, ULN2003A) as Part C; the version C case is re-run inside the same script as a reference and reproduces §C2 (S 89 % ... N half-shaded 13 %). Cell data are listing values, no cell datasheet seen; every assumption is labelled.

D1. What version D is#

No drilling, screws or glue: the pack stands in the cavity like version C's panel pack; the cell cover (0.6 mm) drops into a rebate. A lightening bay under the same cover (below the short cell) stows the lead slack.

D2. Cell choice (listings checked 2026-10-18; values from the listing pages, not from cell datasheets)#

Can the BQ25185 charge LiFePO4? Yes: VSET has 3.6 V and 3.65 V settings (TI SLUSF65B, RILIM/VSET table; on the 6106 replace R19 13k with 3.6k for 3.6 V / 500 mA ILIM). LFP is not used in D because it has the same 0 C charge limit as standard LiPo; its flat cells are also smaller in energy.

D3. Charger rework and the TS window (from TI SLUSF65B; computed, not measured)#

• TS biasing: ITS 38 uA (36.5-39.5), VCOLD 1.0075 V (0.9575-1.0575), VHOT 0.115 V (0.105-0.125).
• Correction to §C2 / §A10 / ELECTRONICS.md §0.4: with the 10k B3950 NTC alone the window is +4.6 C (2.8-6.3) to 54.6 C, not 0-45 C. The cold side is conservative; the hot side is above the usual 45 C charge limit. (Version C keeps its cell room-side, so this only matters for B/C cells near a warm window.)
• Version D network (passive, no GPIO): a 6.8k NTC, TDK/EPCOS B57164K682J (K164 disc, 6.8k +-5 %, R/T curve 2903, B25/100 4200 K +-3 %; R/T table from the TDK K164 datasheet), on the cell, in parallel with 39k + 2.2k in series (41.2k) at the 6106 TS pad, R16 removed:

A 10k B3950 NTC in parallel with about 35.4k also gives -20 C, but its hot trip moves to about 52 C. Fixing that in firmware would need two more XIAO pins, and the B/C pin map already uses all of them. The 6.8k NTC gives a passive -20 .. +42 C window. In the PVGIS run the cell is below -16 C only about 10 h/yr (at night, when there is no charge anyway) and below -20 C about 0.1 h/yr. So the TS tolerance costs almost nothing at the cold end. The K164 is a lacquered disc, not a bead: its size against the 3 mm wire channel (d_ch) is not checked.

• Charge current: cut SJ1 (R1 out) and replace R20 600R with 1.2k -> ICHG about 250 mA (ICHG ~ 300 / RISET). The panel alone cannot exceed about 0.17 A. USB-C, however, would charge a cold pack cell at 1 A (0.7C), well above the listed 0.2C at -20 C. 250 mA = 0.18C for 1400 mAh.
• Unchanged from B/C: R2 removed + 100k EN pull-down; MCP1700 from VSYS; VBAT divider on GPIO2 (it now measures at the box end of the lead; the voltage drop at sleep current is negligible).

D4. Lead#

4 nets: PV+ (panel + to the 6106 VIN / terminal block), BAT+, GND (panel - and cell - joined in the pack), NTC (to TS). With a 6-way conductor, double BAT+ and GND. An assumed 0.035 x 0.7 mm FFC conductor is about 0.35 ohm per 50 cm. The about 0.23 A motor peak then gives about 0.16 V loop drop on single cores, 0.08 V doubled (estimate). Options: 6-way 1.0 mm-pitch FFC about 500 mm (part number not checked), or the BOM's 6-wire flat modular cable (2.4 mm, CCA cores; already bought for the heads). The pack's exit is an 8.0 x 2.6 mm stadium low on the cell-side edge; the box uses its existing third side exit (7.4 x 3.0 mm). Put the cell side toward the hinge.

D5. Thermal model (ASSUMPTIONS)#

T_cell = t_pack(T_out) + K_SUN x G_pack, with t_pack from §A10 (f = 0.10, about -16 C at -20 C outside) and G_pack the plane-of-array irradiance after glass, soiling and shade. K_SUN = absorptance / (about 15 W/m²K, two faces in still air): 0.06 K per W/m² for a dark skin (alpha about 0.9), 0.02 for white (alpha about 0.3). The summer peak adds cavity-air warming K_AIR = 0.02 K per W/m². Cold capacity: deliverable fraction falls linearly from 100 % at +20 C to G20 at -20 C (LT cell 0.90, standard LiPo 0.70, LFP 0.65: assumptions); the rest is "trapped", not lost.

Summer peak cell temperature (est.):

White keeps the LT cell inside its listed -40..60 C and the charger stops charging above about 42 C. The 2200 mAh option (listed to 50 C) fits north windows only. Long hours above 45 C still speed up calendar ageing (not quantified).

D6. Heater from the panel, gated by the NTC (checked, rejected)#

A resistor heater fed from the panel only when the cell is below the charge limit. December, standard 0 C LiPo, dark skin: it uses 21-34 mWh/day of panel output on S/E/W/N, and the lowest charge changes S 50 -> 50 %, E 7 -> 15 %, W/N still empty. The cell area loses about 40 mW/K, so a 16 K lift needs about 0.65 W continuous. At 300 W/m² the panel delivers about 0.34 W electrical, while the same sun falling on the cell area already puts about 0.9 W into it as heat. Verdict: no heater. A cold-rated cell does the job without one.

D7. Winter budget (47.6 mWh/day, PVGIS 2015-2023, ESTIMATES)#

December mean net vs load and the lowest reachable charge over 9 years (charge that can be used at the cell's temperature at that hour):

LT cells: 0 cold charge-inhibit days with the -20 C trip (white skin), and no hot-inhibit days that cost charge.

Lowest reachable charge by month (%, ESTIMATES):

Plot: docs/power_budget_D.png. As in Part C, the 0.75x panel factor is the weakest input. The cold-capacity factor of the LT cell is the next one.

D8. Versions compared#

D9. Not verified (open)#

• LT cell: no datasheet; -20 C charge rate, 60 / 50 C upper limits, cycle life, cold capacity (G20 = 0.9 assumed), PCM temperature rating and the pouch's real thickness/swelling are from the series pages or assumed. Order samples.
• The TS window is computed from TI's thresholds and TDK's R/T table, not measured: bench-test it in a freezer and an oven. The NTC disc size against the 3 mm channel is not checked.
• Pack temperatures (K_SUN, K_AIR, f): log a dummy pack with a thermometer for a cold week and a sunny week.
• FFC part number, conductor size and whether it survives the gasket; the cable choice under the gasket.
• Panel output (0.75x factor) and the panel pad position, as in Part C.
• Fit in the cavity: the gap and the slat clearance are not measured (same as version C).
• Weekly re-home assumes the open-loop position drift over a week stays small; not tested.

D10. Dark winters: panel factor, weekly re-home and a low-battery mode (ESTIMATES)#

Model: python3 tools/power_modes.py (same PVGIS data, glass, cells, cell temperatures and charge windows as §C2 and §D7; the pack sits on the outer pane in front of the slats, so closed slats do not shade it). Conservative sensitivity on the panel: F = effective panel current / P124 current = 0.75 (current model), 0.375 and 0.19, standing in for darker winters, snow and dirt on the glass and the unknown low-light behaviour of the 12 V panel.

Loads (recomputed): one 1/2-turn move 2.42 mWh, one re-home 6.99 mWh, sleep 14.27 mWh/day.

Low-battery mode (design): at midnight, if the reachable charge is below 20 %, each blind makes only one move that day and skips the re-home; the mode ends at 30 %. Button presses always run (not counted in the model).

Each cell: lowest reachable charge 2015-2023 for daily re-home / weekly re-home / weekly + low-battery mode, or empty N d = days per year with no reachable charge; "(low-battery mode N d)" = days per year spent in it.

2 blinds x 4 moves/day (daily re-home / weekly re-home / weekly + low-battery mode):

2 blinds x 8 moves/day (daily re-home / weekly re-home / weekly + low-battery mode):

¹ The 2200 mAh cell's listed upper limit is 50 C; the estimated summer peak is about 53 / 50 / 55 C on south / east / west windows (§D5). Use it on north windows only. Its south/east/west rows are for comparison.

Does the low-battery mode keep every window off empty? No. With weekly re-home it closes the gap wherever the deficit is short:

• At F 0.75 every case stays above zero at 4 moves. At 8 moves only D 1400 mAh on a half-shaded north window is left (under a day a year).
• At F 0.375 it fixes all of C and D 2200 mAh at 4 moves, and all of C at 8 moves except half-shaded north (4 d/yr). D 1400 mAh still runs empty on half-shaded north windows (6 d/yr at 4 moves).
• At F 0.19 north windows need C or D 2200 mAh plus the mode. Half-shaded north windows run empty 133-175 days a year in every version: even the 19 mWh/day low-battery load (three quarters of it is sleep) exceeds what the panel makes there for most of the year.

Recommendation (estimates):

Default settings for B/C/D builds: weekly re-home (7 days), plus a re-home after any power loss, missed step or manual override. Low-battery mode on (20 % enter / 30 % exit), reporting Matter BatChargeLevel = Warning so the user is told to top up over USB-C. Daily re-home only where position accuracy matters more than winter reserve. Firmware switches VT_REHOME_DAYS, VT_SOC_LOWBATT_ENTER / EXIT and VT_POWER_MODES match these numbers. Weekly re-home still assumes the open-loop drift over a week stays small; that is not bench-tested.

Part C - v0.4 cost-down re-run (round 5, 2026-10-17): ULN2003A drive + 12 V 1.5 W panel (ESTIMATES)#

All numbers in this part are model estimates (python3 tools/power_budget.py; v0.3 comparison with --v03). Nothing was measured. Same PVGIS data, glass, shading, sleep and cell assumptions as Part A.

What changed against v0.3:

C1. Move energy and daily load (2000 mAh LiPo, Thread sleepy device 150 uA)#

The ULN build roughly halves battery-only life (version B). The cause is the 4x longer move at 3 rpm at full coil current, and the daily re-home at 2 rpm (about 30 % of the daily load is sleep, the rest is motion). If the coils really are 200 ohm (the motor listing's value) the motor power is about 0.16 W and the 2 x 4 load is about 26.0 mWh/day -> about 209 days. Measure one coil before trusting either figure. Levers if B is too short: re-home weekly instead of daily (saves about 12 mWh/day for 2 blinds), fewer moves, or the quiet TMC2209 option.

C2. Solar balance, version C (panel inter-pane on the outer pane = placement B, 2 x 4 moves, 47.6 mWh/day)#

December mean harvest into the cell minus load, and the lowest charge over 2015-2023 (PVGIS hourly):

• v0.3 (P124 + TMC2209) for comparison: South +248 mWh/day / 97 %, North 86 % unshaded.
• At 2 x 8 moves (66.9 mWh/day) half-shaded North runs empty about 28 days a year; East/West stay above 12 %.
• Room-side panel placement (behind the slats) is much weaker: South 77 %, East 29 %, West 23 %, North empty about 25 days/yr at 2 x 4 moves. Keep the panel between the panes.
• June surplus is large everywhere (about +0.8 to +1.7 Wh/day), so the cell refills over the summer; the risk is the December-January deficit on East/West/North windows.
• Cold: the charger only charges above about +4.6 C (NTC on TS after the 6106 rework, ELECTRONICS.md §0.4; "0 C" in older text - see §D3). The cell stays in the room-side box, so the 0 C gate does not block winter charging.

Plot: docs/power_budget.png (v0.4). The 0.75x panel factor is the weakest number here: log one week of charge current on a south window before relying on the December figures.

C3. Fit (from the CAD echoes)#

Heights inside the box use two ASSUMED component heights (terminal block 8.5 mm, mated JST-PH + wire bend 10 mm): tb_h and jst_h in ebox.scad.

Part A - v0.3 Li-ion + 6 V panel + TMC2209 (quiet option)#

A0. Summary (v0.3)#

A1. Architecture (v0.3)#

 6 V panel (inter-pane) --VIN--+                     +--SYS (3.0-4.5 V)--> TPS7A02 3.3 V LDO --> XIAO ESP32-C6 3V3
                               |   BQ25185           |                 \-> TPS61023 boost 5 V (EN) --> 2x TMC2209 VM (gated)
 USB-C 5 V (CC 5.1k) ---VIN----+   (power path)  ----+
                                       |BAT
                                 1S LiPo 2000 mAh (+PCM), NTC on TS (0-45 C charge window)

• One input, two sources. The BQ25185 has a single VIN. Panel and USB-C 5 V are joined through two Schottky (or ideal) diodes so neither back-feeds the other (assumption: diode drop is negligible against the VBAT + 330 mV VINDPM point; with USB-C the panel diode simply blocks). The Adafruit 6092 board already has the USB-C (5.1 k CC) and separate VIN pads.
• Why BQ25185. From the TI datasheet (SLUSF65B, https://www.ti.com/lit/ds/symlink/bq25185.pdf): input 3.0-18 V (25 V tolerant), up to 1 A charge, power path with SYS regulated at 4.5 V, battery-tracking VINDPM = VBAT + 330 mV (lets a weak panel sit near its maximum-power point without collapsing), IQ_IN 0.75 mA typ / 1 mA max while the input is valid, IQ_BAT 4 µA typ / 5 µA max in battery-only mode, 30 µA sleep input current, TS thermistor input, 6 h safety timer.
• Why not the boost-converter harvester here. The BQ25570 (Part B) needs about 0.5 µA and real MPPT, so it harvests far better in dim light (see §A5 table: north/room-side cases), but it can't take USB-C 5 V directly as a fast charger. Use Part B when the panel can only go room-side on a north window.
• Sleep rail. TPS7A02 (TI, https://www.ti.com/lit/ds/symlink/tps7a02.pdf): 25 nA Iq, 200 mA, dropout <= 270 mV at 200 mA (85 C). Below about 3.5 V cell it runs in dropout and the ESP32-C6 (3.0-3.6 V) follows the cell. Do not use the Adafruit 6092's on-board 3.3 V buck for the sleep rail: Adafruit names it TPS62569; the similar TI TLV62569 has 35 µA operating Iq (TI datasheet), which would add about a quarter to the sleep load. Hold its EN pad low, and remove or cut the always-on green 3.3 V LED.
• Motor rail as in Part B: TPS61023 boost (0.1 µA shutdown, TI) enabled only while moving; TMC2209 VIO also gated, because an idle TMC2209 draws mA-level supply current.

A2. Move energy, from the real move (ESTIMATES with cited inputs)#

Result (tools/power_budget.py): 1/4 turn 0.34 mWh, 1/2 turn 0.58 mWh, 1 turn 1.04 mWh; daily re-home (1/2 turn to the CCW stop at 6 rpm + 1 s slip, then back at 12 rpm) 1.31 mWh per blind. The old v0.2 figure (3.2 mWh per 20 s move) assumed about 4 turns of travel; the measured 1-turn span cuts the move energy by about 5x.

A3. Sleep current and daily load#

Energy is counted at the 3.7 V nominal cell voltage (the LDO passes current 1:1).

Usable capacity = 85 % of nominal (cell to about 3.5 V under load) (assumption); days include self-discharge. Moves/day assumption: 4 (open in the morning, two adjustments, close at night) and 8 (automations).

A4. Light at the panel - sources and through-glass losses#

• Irradiance: PVGIS v5.3 (EU JRC), PVGIS-SARAH3 + ERA5, hourly 2014-2023, generic grid point in southern Finland, about 60 N (not a specific address; the exact query point is LAT, LON in tools/power_budget.py), vertical plane, DEM horizon: https://re.jrc.ec.europa.eu/api/v5_3/seriescalc?lat=<LAT>&lon=<LON>&angle=90&aspect=<0|-90|90|180>&startyear=2014&endyear=2023&outputformat=json (aspect 0 = S, -90 = E, 90 = W, 180 = N). Monthly means of G(i) on the outside of the glass, kWh/m²/day:

For contrast, northern Finland (PVGIS PVcalc 65.0 N 25.5 E, vertical, 2005-2023 monthly H(i)_d): south December 0.03, June 3.67 kWh/m²/day - in the north the battery alone has to carry about Nov-Jan.

• PVGIS knows terrain only, not buildings or trees: a "shaded" case multiplies irradiance by 0.5 (assumption).
• Glass (EN 410 direct solar transmittance ST, Pilkington Glasfakta 2024, https://www.pilkington.com/-/media/2ace97721b82435f8a0f32d606639783.pdf): 4 mm clear float 0.86 (6 mm: 0.83); double unit 6-16Ar-4 with one Optitherm S3 low-E 0.55 (clear double unit 0.76). ST is the conservative metric for c-Si (light transmittance is 0.91 / 0.81). v0.3 first used 0.88, which is the g-value (includes re-radiated heat) - corrected. Typical Finnish MSE window: single outer pane, double unit in the inner sash, blind between (Solar Kaihdin MSE guide https://solar.fi/wp-content/uploads/2022/04/mitoitus-mse.pdf; Safetyset https://www.safetyset.fi/artikkelit/ikkunarakenteet) - so an inter-pane panel looks out through one pane. Dust and incidence angle: x0.95 (assumption; PVGIS quotes 2-4 % AOI loss for a bare module).
• Placement B, inter-pane (on the inner face of the outer pane, outside the slats): T = 0.86 x 0.95 = 0.82.
• Placement A, room side (on the inner pane, light passes the slats): T = 0.86 x 0.55 x 0.5 (slats mostly horizontal by day, assumption) x 0.95 = 0.22.
• Panel: Voltaic P124, 6 V, 66 x 113 x 2.6 mm, Voc 7.28 V, Vmp 6.23 V, Imp 220 mA, 1.35 W peak (Voltaic data, https://voltaicsystems.com/1-watt-6-volt-solar-panel-etfe/). Model: panel current = 220 mA x G/1000 W/m² (assumption: linear, Isc-like, valid while the VINDPM point is below Vmp); charge into the cell = (I_panel - IQ_IN) x 3.7 V x 0.98 coulombic (assumption), hour by hour. That subtraction is why a linear charger's input quiescent matters in December: at G = 20 W/m² behind glass the panel gives about 4 mA, of which 0.75 mA (BQ25185) or 1.1 mA (bq24074) is lost.
• BQ25570 comparison (Part B path, low-voltage panel of the same area): module efficiency 18 % (P124 class: 1.35 W / 74.6 cm²), x0.90 low light, x0.75 boost charger (assumptions), no threshold.

A5. Solar balance - December vs June (mean mWh/day into the cell minus load), 1 panel, LiPo 2000 mAh#

Recommended charger BQ25185, load 2 blinds x 4 moves = 21.4 mWh/day (x 8 moves = 26.0: subtract 4.6 more).

(cell room-side, so it can always charge; for a cell inside the cold inter-pane pack see §A10.)

Other chargers, same panel (December harvest, mWh/day): bq24074 about 1-8 % lower in the brighter cases and much lower in the dimmest (room side N open 0.0 vs 0.4; inter-pane N shaded 1.7 vs 3.4) because of its 1.1 mA input quiescent. BQ25570 (low-voltage panel, same area): inter-pane N open 26 (net +5), room side N open 7 vs 0.4 - many times better in the dimmest cases, about 20 % better in bright ones.

Reading the table. December is negative only on north / room-side / shaded windows, and the deficit is at most about 21-26 mWh/day - i.e. the whole load, which the 2000 mAh cell carries for about 7 months. Spring refills it in days. So with this panel and cell year-round operation without USB-C is expected in every modelled case; USB-C is for the first charge, a long dark spell in the north, or a Wi-Fi build.

A6. Recommended parts (availability checked 2026-10-17; prices only in the private report, not here)#

A7. Fit in the sleek shell#

• Head (housing_slim) - unchanged inside; no battery in the head (a 2000 mAh pouch would double its depth).
• Controller box (cad/ebox.scad, power_variant = "liion_solar"): outer 65.6 x 47.6 x 34.6 mm, fully rounded (R6). Cell bay under the board: 54 x 44 mm, 11 mm high, for the 50 x 40 x 8 mm pouch (0.8 mm feet that keep it clear of the rounded inner floor edge, +1 mm swelling allowance (assumption), 1.2 mm under the board), perfboard on two end ledges above it. Room on the deck for XIAO + 2 x TMC2209 (about 51 x 21 mm) and the 30.2 x 19 mm Adafruit 6092 beside them (23 mm free). The box's USB-C opening is the charger's USB-C (usbc_z / usbc_y - align to your board), plus a 2.6 mm exit for the panel lead. USB-only box (no cell): power_variant = "usb", 61.6 x 37.6 x 23.6 mm.
• Panel (66 x 113 x 2.6 mm) goes inter-pane in the flat pack cad/pane_pack.scad (3.3 mm panel-only, or 4.2 mm with the cell beside it - §A10). No glue on the glass; the cavity mounting is not designed yet (gap between outer pane and slat tips NOT measured). Lead through the sash gap (thin 2- or 4-core flat cable) to the box.
• Heat / cold: keep the cell in the room-side box; never charge below 0 C or above 45 C - wire a 10 k NTC to the BQ25185 TS pin (datasheet §TS). The inter-pane space can freeze in winter and pass 50 C in summer sun: only the panel lives there.

A8. Firmware follow-ups (NOT changed in this round)#

1. A VT_CELL_CHEM switch (LFP / LIION): Li-ion constants VT_CELL_MAH 2000, VT_CELL_NOM_MV 3700, VT_VBAT_MIN_LOADED_MV about 3300 (LDO dropout + C6 minimum), and a Li-ion OCV->SOC table instead of the LFP plateau.
2. VT_E_MOVE_MWH is "per 20 s move" (3.2) in power_source.cpp / power_policy.cpp: re-express it per second of motor time (about 0.20 mWh/s from §A2) so short 1/2-turn moves are not over-counted 5x.
3. VT_E_SLEEP_MWH_DAY 14.1 for Li-ion (12.0 is the LFP value).
4. Optional: read the BQ25185 STAT pin (charging / fault) into the Power Source cluster (BatChargeState).
5. Optional: skip the daily re-home when the day already included a move to the CCW (closed-up) stop - saves 1.3 mWh/blind/day.

A9. Tests before trusting any of this#

1. Measure Thread SED average current on the real build (Nordic PPK2 or similar), Thread and Wi-Fi.
2. Measure one 1/2-turn move with the real knob load: mWh from the cell.
3. Panel short-circuit current at the planned spot on an overcast December day (both placements), slats open/closed.
4. Confirm the BQ25185 input current at very low light (does it hold VINDPM near VBAT + 0.33 V, what does IQ_IN do).

A10. Flat inter-pane pack (thickness first) and cold (-20 C design minimum)#

Stale (v0.3 numbers): the result table below is from the v0.3 load. Version D (Part D) re-runs the in-pack cell case at the v0.4 load with sun warming, and uses a low-temperature cell instead.

Requirement (2026-10-17): the inter-pane pack must be as flat as possible; panel and cell side by side if that is thinner; thin walls; design minimum -20 C outdoors.

Cell options for a flat pack (listing data, not verified on a sample):

Thickness (from cad/pane_pack.scad, echo):

Walls: 0.6 mm back / cover (3 layers at 0.2 mm), 0.4 mm front skin over the cell, 1.2 mm rims, all outer edges rounded (R1.2, corners R4). The pocket allows +0.3 mm pouch swelling (assumption). Panel retention: 4 snap hooks (or thin foam tape inside the pack - nothing touches the glass). Mounting in the cavity is not designed: the gap between the outer pane and the slat tips and the sill/bead shape aren't measured.

Inter-pane temperature (labelled assumption): series thermal resistances, T = T_out + f x (T_in - T_out), with ISO 6946 surface resistances Rse 0.04 / Rsi 0.13 m²K/W, an unventilated cavity about 0.18, and a low-E inner unit (about 0.74 without films). Mid-cavity f = 0.12; on the outer pane f = 0.04; clear inner unit mid-cavity f = 0.25. Design value for the pack f = 0.10 -> at -20 C outside / +21 C inside: about -16 C (range -18 to -10 C). Ventilated outer sashes (normal in MSE windows, against condensation) push it toward the low end. Sunshine warms the pack in the day, which the model ignores (conservative). PVGIS/ERA5 hourly T2m at the generic point, 2014-2023: minimum -24.6 C, about 1 h/yr below -20 C.

Consequences:

• Li-ion/LiPo cells are charged 0-45 C (all the listings above): the BQ25185 TS pin with a 10 k NTC on the cell blocks charging below 0 C. In the model a pack cell charges only when the pack is >= 0 C.
• Result, 2 blinds, BQ25185, inter-pane (placement B), lowest charge 2015-2023, 4 / 8 moves:

Battery-only: 1100 mAh about 146 / 123 days, 1000 mAh about 134 / 112 days (4 / 8 moves).

• LiPo deliverable capacity drops at -16 C (often to 60-80 % - assumption, not from these listings), so the north / 8-move pack cases (28-34 % lowest charge) have little margin. Summer: a cell behind the outer pane in sun can pass the 45-60 C limits (not modelled). Both points favour the panel-only 3.3 mm pack + cell in the box.

A11. Cross-check with the team research report (2026-10-17, internal)#

team/research/report.md (internal, not part of this pack) agrees on the method and flags these points:

Part B - v0.2 LiFePO4 + BQ25570 design (kept for dim windows; §B3 energy figures superseded by §A2)#

B1. What the SwitchBot Blind Tilt does (public sources only)#

We don't own the device. Everything comes from public pages, the FCC filing and one published teardown. V = verified in the cited source. I = my inference from verified facts. ? = unknown or unverified.

B1.1 Verified facts#

B1.2 Inferences and guesses (not verified)#

Discarded: one SEO page (electronics.alibaba.com "What Is SwitchBot Blind Tilt…") claims a "500 mAh battery, 1.2 W 5.5 V panel" and a Reddit "survey". That contradicts V1/V2 and is not used. Reddit threads about "panel not reaching level 6" in winter exist (r/HomeKit 157nbp8, r/smarthome 10bqqg5) but could not be fetched, so they are not quoted.

B2. Same principle, original design#

SwitchBot: panel → USB-C 5 V → linear Li-ion charger → 3.7 V cell → (regulator) BLE SoC + DC motor, bridged to Matter by a hub. BlindBot keeps the principle (a self-charging window device with USB-C backup and a sleeping radio), but every block is different:

flowchart LR
  PV["PV panel ~60 cm2<br/>4-5 cells, Voc ≤ 3.3 V"] --> BQ["BQ25570<br/>MPPT boost charger<br/>OV 3.50 V, VBAT_OK 3.0/3.2 V"]
  USB["USB-C 5 V (backup)"] --> CN["CN3058E<br/>LiFePO4 linear 3.6 V"]
  BQ --> BAT[("LiFePO4 18650<br/>1500 mAh + 1S prot.")]
  CN --> BAT
  BAT --> LS1["load switch<br/>ON = VBAT_OK"] --> MCU["XIAO ESP32C6 3V3 pin<br/>Matter/Thread SED"]
  BAT --> BOOST["TPS61023 → 5.2 V<br/>EN = MOT_PWR_EN"] --> TMC["2x TMC2209 VM"]
  MCU --> LS2["load switch → VIO_MOT"] --> TMC
  MCU -- "ADC D2" --> BAT
  MCU -- I2C --- VEML["VEML7700 at the panel"]
  MCU -. "Matter over Thread<br/>Power Source cluster" .-> TBR["Thread border router"]

B3. Energy budget (ESTIMATES)#

Cell: 1500 mAh x 3.2 V x 0.9 usable ≈ 4.3 Wh → about 110 days at 39 mWh/day with no sun at all (est.; LFP self-discharge of a few %/month not included).

B4. Cell choice#

The 18650 (65 x 18.3 mm) doesn't fit today's ebox.scad (61 x 37 x 25 mm). The box grows to about 80 x 40 x 28 mm (est.) - still hidden low. A 26650 LFP (about 3-4 Ah) works if a bigger box is acceptable.

B5. Harvester / charger IC trade study#

B6. Irradiation in southern Finland behind glass (PVGIS) and harvest (v0.2 - superseded by Part A §A5)#

Source: PVGIS v5.3, EU JRC, PVGIS-SARAH3 2005-2023, DEM horizon, ~60 N southern-Finland grid point (v0.2; Part A uses a generic southern-Finland point, about 60 N), vertical plane, fetched 2026-10-17: (v0.2 query at a ~60 N point; superseded - re-run with the Part A URL) and .../MRcalc?...&horirrad=1&d2g=1 (horizontal + diffuse fraction).

Mean daily irradiation on a vertical window, kWh/m²/day (PVGIS H(i)_d; S_diffuse = my isotropic diffuse + ground-reflection estimate for a south window whose low winter sun is blocked by buildings/trees):

Notes: horizontal December is only 0.124 kWh/m²/day with a 70 % diffuse fraction (PVGIS). The south December figure is 4x that because PVGIS assumes the very low sun (≈ 6° at noon at the solstice) reaches the glass - in a built-up or forested street it usually won't. The worst December in 2005-2023 for south was 9.39 kWh/m²/month (0.30/day).

Harvest model (est.): E = H x A x T_optical x η_sys, η_sys = 0.12 (small mono panel at low light) x 0.75 (BQ25570 at mW level) x 0.95 (MPPT 80 % Voc) x 0.95 (charge) ≈ 0.081. T_optical: placement A = 0.65 (three panes, some low-E) x 0.50 (slats mostly horizontal by day) ≈ 0.33; placement B ≈ 0.70 (outer glazing only); placement C (frame face) ≈ 0.02.

December harvest, 60 cm² panel, mWh/day (EST.) vs load 39 (2 blinds) / 25 (1 blind):

Panel area for December break-even (2 blinds), cm²: S 29 / 13 (A / B), S-shaded 215 / 100, E 133 / 62, W 162 / 75, N 365 / 169 - i.e. a self-sufficient December is only realistic for south with an open horizon.

Seasonal buffer simulation (1500 mAh LFP, 60 cm², start full on 1 Oct, daily steps, lowest charge in year 2):

Verdict for southern Finland, v0.2 LiFePO4 design (ESTIMATES)#

• December alone does not pay for itself except on a south window with a clear view of the low sun. December harvest is ~6-17 mWh/day on a typical E/W/N or shaded window vs ~39 mWh/day of load.
• Year-round is viable through the battery buffer: the deficit from about mid-November to early February is ≈ 1-3 Wh, against ≈ 4.3 Wh usable. From late February to October the panel refills the cell in days.
• Marginal cases: north windows, heavily shaded windows, or slats kept closed all day in winter. Expect one USB-C top-up in January/February, or use placement B, a 100 cm² panel, or one blind per cell. The firmware's low-battery policy (section 8) stretches the reserve further.
• Measure before trusting this: put the VEML7700 where the panel will go, log lux for a winter week, and calibrate the model (section 11).

B7. USB-C backup charge path#

• The USB-C on the box (J1, 5k1 CC pull-downs, PTC) goes to CN3058E (3.6 V LFP, ISET ≈ 500 mA → about 3-4 h from empty, est.) and to BAT+. The BQ25570 keeps running in parallel; both regulate to ≤ 3.6 V.
• Motors and MCU always run from the cell, so USB only charges. A USB-C power bank for an hour is enough in a dark winter (about 1.5 Wh per hour of charging, est.).
• The XIAO's own USB-C is for programming only. Don't plug both in the solar variant unless the XIAO is isolated from 3V3_SYS (it would back-drive the rail).
• CN3058E TEMP pin: an NTC on the cell (or GND per the datasheet if unused). The box is on the room side, so no cold charging.

B8. Matter: Power Source cluster + low-battery behaviour#

Cluster (stub in firmware/main/power_source.cpp, API names checked against esp-matter release/v1.4 headers): Power Source (0x002F) on endpoint 0, features Battery + Rechargeable, EndpointList = the window-covering endpoints.

ICD/SED: sdkconfig.defaults.battery (MTD + ICD server, slow poll 5 s). A 5 s slow poll ≈ 5 s command latency - fine for blinds; longer polls save energy. Limit Thread TX power to about 0-9 dBm (TX peak 119 mA at 0 dBm vs 305 mA at 20 dBm, C6 datasheet).

SOC on LiFePO4: the voltage plateau (~3.2-3.3 V) says almost nothing, so SOC = energy ledger (sleep µA x time + motor-seconds x energy per move, plus harvest credit from VEML7700 lux x a learned factor (TODO)), re-anchored at the plateau ends: ≥ 3.40 V after charging = 100 %, ≤ 3.18 V knee ≈ 12 %, ≤ 3.10 V ≈ 5 %, 3.00 V = 0 % (generic LFP curve, est.).

Adaptive move scheduling (firmware/main/power_policy.cpp):

Deferred moves aren't errors in Matter terms. The firmware should re-report Target = Current so controllers don't show a phantom move (TODO, verify the esp-matter behaviour).

B9. Mechanical placement + looks#

A välikaihdin sits between the inner and outer panes, so the panel can be:

Keep the panel ≥ 10 mm from the glass edge seal. Don't cover the sash lock. Cable: the same white flat cable + clips as the motor cables, joining the box via a JST-PH 2-pin. The VEML7700 goes next to the panel (same light), on a 4-wire tail, or inside the box behind a light pipe if you only want room light.

B10. Electronics changes (summary - netlist in electronics/ELECTRONICS.md §10)#

• New: PV1, U5 BQ25570, U6 CN3058E, BT1 LFP 18650 + 1S protection + F2, U7 system load switch (VIN = BAT+, ON = VBAT_OK), U8 TPS61023 5.2 V boost (EN = MOT_PWR_EN), U9 VIO load switch, U10 VEML7700, 1M/1M VBAT divider.
• Pins: STEP_B → D3/GPIO21, DIR_B = DIR_A (D1, shared; one motor at a time), D2/GPIO2 = VBAT_SENSE (ADC1 ch2), D10/GPIO18 = MOT_PWR_EN (100k pull-down), LED → on-board GPIO15. Firmware: -D VT_POWER_SOLAR=1.
• The TMC2209 is re-configured over UART after every motor-rail power-up (its registers are volatile). The UART TX goes hi-Z and STEP/DIR/EN_N go low while the rail is off, so nothing back-powers the driver. The EN_N 10k pull-up goes to VIO_MOT (gated).

B11. Bench / field tests (add to DESIGN §16)#

1. Sleep current of the whole node (PPK2 or µA meter at the cell) after commissioning: target ≤ 150 µA avg.
2. Motor move energy: integrate cell current over a 0→100 % move (expect ~3 mWh).
3. Panel I-V at the planned spot (placement A, slats horizontal / closed), a sunny and an overcast winter day.
4. A week of VEML7700 lux logging at the panel spot in Nov-Dec → calibrate ETA_SYS/T in tools/power_budget.py.
5. BQ25570 dividers: verify OV 3.50 V, VBAT_OK 3.00/3.20 V with a bench supply in place of the cell.
6. Brown-out: run a move at 3.0 V cell; the loaded-sag abort must trigger before the rail drops.

B12. Risks#

B13. About REA (https://github.com/morluto/rea)#

Cloned and README read (2026-10-17). REA is an MCP/CLI toolkit for reverse-engineering software artefacts: native binaries (Hopper/Ghidra/IDA), JS/Electron, .NET, Android APKs, firmware images (binwalk/unblob), packet captures, logic-signal recordings, websites. It has no feature for FCC PDFs or teardown photos. The public material here (PDF exhibits, photos, manuals, web pages) was handled with poppler (pdfimages) + PIL crops instead. REA was not installed or used: no public Blind Tilt firmware image exists, and analysing the SwitchBot app wouldn't reveal the power hardware. It would help later for BlindBot itself: analyze our own ESP-IDF ELF (symbols/sections), decode Thread/BLE pcaps of the ICD poll pattern, or decode a sigrok capture of the TMC2209 UART. All of that is our own material.

Figures