In collaboration with Electronic Labs OpenNEXEN builds the units and delivers them to homes. Electronic Labs draws the schematics, analyses the design and writes a plain-language explanation of how it works, printed and left in each home for anyone curious about electronics.

OpenNEXEN · BCKUP//ENRGY

Lights that stay on
when the grid doesn't.

Outages are routine in Kampong Thom. BCKUP//ENRGY is a low-cost, battery-backed lighting unit: it charges from the wall while the power is on and switches to its battery the moment it drops.

Prototype · Grid-charged · Solar planned
Safety design ↓ Build log ↓
FUSE INPUTGRIDAC mains 12V · 10APSU120W switching CC / CVBUCK9A · 12.0–12.3V cap 40WUPS BOARD12V in · 4 out 2 × 10WLIGHTS12V LED PLANNEDSOLARcharge ctrl · Rev E 3S · 40ABMSprotect · balance 18650PACK3S1P LOW-VOLTAGE CUTOFF · BMS OPEN

Pack voltage—
State of charge—
Lights—
From grid / solar—

SIMULATED VALUES · measurements in progress

Battery pack 3S1P · 18650 Panasonic 3400 mAh · ≈ 37 Wh
Protection 3S 40A BMS over/under-voltage, overcurrent, short
Power board 40W UPS · 12V in 2 × 12V (1.5A) · 9V · 5V barrel outputs
Load 2 × 10W LED 12V, ≈ 1.7A combined
Charging 12V 10A PSU → 9A CC buck pack capped at 85–90% for cell life
Modeled runtime ≈ 1.5 h both lights, 3S1P · 3S2P ≈ 3 h
Why

Light is the first thing an outage takes

In rural Kampong Thom the grid drops often, and evenings are when it matters most: homework, cooking, getting around safely. The goal is not to run a household. It is to keep two lights on for a few hours, at a cost that makes it realistic to build for many homes.

LINE DOWN GRID HOME
Fig. 1One cut line, and the lights go with it.

The unit has one job in each state of the power-flow display at the top of the page. While the grid is on, a 12V supply feeds a current-limited buck converter, and the UPS board splits that power between the lights and the battery. When the grid drops, the UPS board switches to the pack with no interruption. When the pack runs low, the BMS opens the circuit before the cells can be over-discharged.

The same unit already powers the routers in my own home on a 3S2P pack. Early testing found that a drained pack pulled over 60W through the UPS board's input diodes. That finding shaped the current-limited charging design below.

Safety

Built to fail safely

Lithium cells in a family's home need layers of protection, so no single part is the only thing preventing a fault. Each layer is marked built or planned.

3S BMS BALANCING BALANCED
Built in

Battery management

A 3S 40A BMS cuts off over-charge, over-discharge, overcurrent and short circuits, and balances the cells near full charge.

INPUT POWER TIME → LIMIT NO LIMIT WITH BUCK
Built in

Current-limited charging

A constant-current buck converter caps both the charge current and the pack voltage at 85–90%. A drained pack can no longer pull 60W+ through the board.

HEATSINK UPS BOARD
Built in

Heat control

Heatsinks sit on the UPS board's input diodes, MOSFETs and inductor. The 120W supply runs below half its rating.

PACK FUSE FAULT
Planned

Pack fuse

A DC-rated fuse on the pack's positive lead, sized above the load and below a single cell's continuous rating: 5A for 3S1P, 7.5A for 3S2P.

− + PANASONIC PANASONIC PANASONIC 18650 18650 18650
Built in

Cell quality and matching

Genuine Panasonic cells, capacity-tested and matched in voltage before assembly, with a full balancing charge every month or two.

MAINS 12V SEALED SUPPLY LOW-VOLTAGE DC
Built in

Low-voltage downstream

Only the sealed 12V supply touches mains. Everything after it, including the battery and lights, runs on low-voltage DC.

Status Items marked planned are not yet installed on the prototype. Safety claims on this page will be updated as each layer is added and tested.
Mainstream

Frugal in mind

The cost for each home is $0.

SPONSOR BOREY LY RANY housing company SPONSORSHIP PRODUCT BCKUP//ENRGY battery-backed lights LIGHT HOME $0 COST TO THE FAMILY
Fig. 2Sponsorship pays for the unit. Families pay nothing.

By securing generous sponsorships from Borey Ly Rany, a complex and housing company, we were able to fund the project without charging families in the province.

Build log

Revision history

ABCDE A B C Router testDONE Current limitingDONE Lighting unitCURRENT InstrumentationNEXT Solar chargingPLANNED
Fig. 3Where the project is today: Revision C of five.
RevStatusChange
ADone
Concept and router test Built a 3S2P pack with BMS and UPS board to keep home routers online. Ran it for 3+ months and tracked cell voltages, which stayed within 20 mV.
BDone
Current-limited charging Found the UPS board drawing over 60W from a drained pack, with its input diodes running very hot. Added a 9A constant-current buck converter with a voltage ceiling of 85–90%, plus heatsinks.
CCurrent
Household lighting unit Reworking the design for two 12V 10W LED lights, with a pack fuse and a target cost of about $25 per home before delivery. Planning a pilot across 5+ homes.
DNext
Instrumentation Measure real runtime, charge current and heatsink temperatures to replace the simulated values in the display above.
EPlanned
Solar charging Replace the wall supply with a solar charge controller on the DC input, so the unit recharges even when the grid is out for days.