--- title: "ModBee Battery Pack 18650" space: "Battery Pack 18650" url: "https://docs.modbee.org/battery-pack-18650" updated: "2026-10-25" --- # ModBee Battery Pack 18650 ![ModBee Battery Pack 18650](/files/gitimg-d18b5bfd0d2a9052597750aafb093e364b18eb30.webp) A 3S (three cells in series) Li-ion battery pack for standard 18650 cells, with on-board protection from a **TI BQ7791501** protector IC. It gives 11.1 V nominal (12.6 V fully charged). It runs standalone, with no microcontroller or configuration: the protection thresholds are fixed in the IC at the factory. This repository holds the hardware documentation: the schematic (PDF), 3D renders and this README. > [!WARNING] > **Lithium-ion cells can catch fire or vent if they are damaged, short-circuited, or charged or discharged outside their limits.** The protection on this board is a last line of defence. It does not replace a correct charger, a fuse and matched cells. Read [Cells](#cells) and [Safety](#safety) before fitting cells. ## Features - **Protector IC:** TI BQ7791501PWR, configured for 3 cells (CCFG tied to VSS, VC5/VC4/VC3 tied to the top cell). - **Protection:** cell overvoltage, cell undervoltage, overcurrent in charge, two overcurrent-in-discharge levels, short circuit, open wire (cell disconnected from the board), and over- and under-temperature in charge and discharge. - **Low-side MOSFETs:** separate charge (CHG) and discharge (DSG) FETs in the PACK− path. A 10 mΩ resistor (R7) senses the current. - **Passive cell balancing:** about 10 mA per cell, while charging. - **Temperature sensing:** an on-board 10 kΩ NTC thermistor (RT2) for the protector, with a solder jumper (JP1) to bypass it. - **Charger temperature sensor:** a second, independent 10 kΩ NTC (RT1) brought out on its own connectors for an external charger. - **Outputs:** one 4-pin pack terminal (J3, 5 A max) and two 2-pin JST XH connectors (J4, J5, 3 A max each), all wired in parallel to PACK+ / PACK−. - **Mounting:** four mounting holes (H1–H4). The assembly renders show the board in a 35 mm DIN rail enclosure. ## Specifications Thresholds and delays are the factory settings of the BQ7791501 variant (TI datasheet SLUSCU0L, Tables 6-1 and 6-2). The current trip points follow from those voltage thresholds and the 10 mΩ sense resistor fitted on this board (I = V / 0.010 Ω). | Parameter | Value | Notes | | --- | --- | --- | | Configuration | 3S, 18650 Li-ion | One cell per position, no parallel cells | | Nominal voltage | 11.1 V | 3 × 3.7 V | | Full-charge voltage | 12.6 V | 3 × 4.2 V; set the charger to this | | Cell overvoltage (OV) | 4.25 V, 1 s delay | Clears below 4.05 V (200 mV hysteresis). Stops charging only | | Cell undervoltage (UV) | 2.8 V, 1 s delay | Clears after the load is removed and the cell is above 3.2 V (400 mV hysteresis). Stops discharging only | | OV / UV accuracy | ±10 mV / ±18 mV | At 25 °C. ±40 mV over −40 to 85 °C | | Overcurrent in charge (OCC) | 2.0 A (20 mV), 10 ms delay | | | Overcurrent in discharge 1 (OCD1) | 3.5 A (35 mV), 180 ms delay | Delay set by R17 (100 kΩ) on the OCDP pin | | Overcurrent in discharge 2 (OCD2) | 6.0 A (60 mV), 180 ms delay | Same delay as OCD1, so OCD1 always trips first | | Short circuit in discharge (SCD) | 12 A (120 mV), about 1 ms delay | | | Current trip accuracy | ±20 % | Plus the tolerance of R7 | | Overtemperature, charge / discharge | 50 °C / 70 °C | 4.5 s delay, about 10 °C hysteresis | | Undertemperature, charge / discharge | 0 °C / −20 °C | 4.5 s delay, about 10 °C hysteresis | | Open-wire detection | 4.5 s delay | Turns off both FETs if a cell connection is lost | | Cell balancing | About 10 mA per cell | Only while charging, cells above 3.8 V. Set by the 200 Ω cell input resistors | | IC supply current | 8 µA typical | Datasheet NORMAL mode, not balancing | | Connector ratings | J3: 5 A; J4, J5: 3 A each | As printed on the board | | IC operating temperature | −40 to 85 °C | Cell limits are usually narrower. Check your cell datasheet | | Size (L × W × H) | 118 × 78 × 45 mm | | The current protection sets the practical limits. With the ±20 % tolerance, OCC can trip from 1.6 A and OCD1 from 2.8 A: - **Charge current:** keep it below about **1.5 A**. - **Continuous discharge:** keep it below about **2.5 A**. Higher loads can trip the discharge protection, even though the connectors are rated higher. ## Board overview | Part | Function | | --- | --- | | BT1 | Three 18650 cell holders, in series | | IC1 | BQ7791501PWR protector | | Q1 | Discharge FET (DSG) | | Q2 | Charge FET (CHG), with a 12 V Zener (D2) gate clamp | | R7 | 10 mΩ current-sense resistor, between BATT− and the FETs | | R18 | 470 kΩ load-detect resistor from the LD pin to PACK− | | RT2 / R19 / JP1 | Protector temperature sensor. JP1 selects the RT2 thermistor or a fixed 10 kΩ (R19) | | RT1 / J1 / J2 | Charger temperature sensor (not connected to the protector) | | J3, J4, J5 | PACK+ / PACK− outputs, in parallel | ## Connectors **J3 – pack terminal (5 A max)** | Pin | Signal | | --- | --- | | 1, 3 | PACK+ | | 2, 4 | PACK− | **J4, J5 – JST XH, BAT+ / BAT− (3 A max each)** | Pin | Signal | | --- | --- | | 1 | PACK+ | | 2 | PACK− | Charge and discharge use the same terminals. Connect the charger and the load to any of J3, J4 or J5. The 5 A and 3 A figures are connector ratings. The current protection trips well below them (see [Specifications](#specifications)). **J1, J2 – charger temperature sensor ("NTC CHARGER TEMP SENSOR")** J1 and J2 are wired in parallel across RT1, a 10 kΩ NTC thermistor on the board. They carry the thermistor to a charger's battery-temperature input, so the charger can see the pack temperature. RT1 is separate from the protector's own thermistor (RT2). Connect it to the charger's TS input and its ground or return pin, for example the TS / 10K NTC terminal on the [ModBee MPPT Charger](https://github.com/mod-bee/modbee-mppt-ti-c3). ## Temperature sensing and JP1 The protector reads its temperature from the TS pin, biased by a 10 kΩ resistor (R16) from the VTB pin. Solder jumper **JP1 ("NTC BYPASS")** selects what is on TS: | JP1 | TS connected to | Temperature protection | | --- | --- | --- | | Pads 1–2 bridged | RT2, 10 kΩ NTC thermistor | Active, at the thresholds in [Specifications](#specifications) | | Pads 2–3 bridged | R19, fixed 10 kΩ | Off. The IC always reads about 25 °C | The factory temperature thresholds assume a 10 kΩ, 103AT-type NTC with a 10 kΩ pull-up (datasheet §8.3). A thermistor with a different B value shifts the trip temperatures. RT2 measures cell temperature only if it is in thermal contact with the cells. ## Fault recovery The board has no reset button. Each fault clears on its own once its condition has gone, as below (datasheet Table 9-5). The board silkscreen gives the short version: *"Reset BMS UV fault: disconnect load. Reset BMS OC faults: connect charger."* | Fault | Turns off | How it clears | | --- | --- | --- | | Overvoltage | Charging | The cell falls below 4.05 V | | Undervoltage | Discharging | Remove the load and charge the cells. Discharge resumes once every cell is above 3.2 V | | Overcurrent or short circuit in discharge | Charging and discharging | Remove the load, or connect the charger | | Overcurrent in charge | Charging and discharging | Disconnect the charger. The fault clears when the IC detects a load on the pack | | Over- or undertemperature in charge | Charging | The temperature returns about 10 °C inside the limit | | Over- or undertemperature in discharge | Charging and discharging | The temperature returns about 10 °C inside the limit | | Open wire | Charging and discharging | Restore the cell connection | ## Cells The silkscreen reads **"ONLY USE 3.7V Li-Ion 18650"**. The thresholds suit standard 3.6–3.7 V nominal Li-ion (4.2 V full charge). Do not use LiFePO4 or other chemistries: their voltage window does not match these thresholds. Use three cells that match: - the same make and model, from a reputable supplier - the same capacity, ideally from the same batch - the same state of charge. Charge or discharge them to within a few tens of millivolts of each other before fitting them. The balancing current is small (about 10 mA). It can trim small differences during charging, but it cannot correct badly mismatched cells. Mismatched cells reach the overvoltage and undervoltage limits early, which wastes capacity and stresses the weakest cell. ## Getting started 1. Check JP1. Pads 1–2 bridged uses the thermistor (RT2). Bridge pads 2–3 only if you deliberately want temperature protection off. 2. Check the three cells as described in [Cells](#cells). 3. Fit the cells, following the + and − markings on the holders. A reversed cell can damage the board and the cell. 4. Measure between PACK+ and PACK− on J3, J4 or J5. It should equal the sum of the three cell voltages (12.6 V at most). 5. Fit a fuse in the PACK+ lead, sized for your load and wiring. The board has no fuse. 6. Charge with a CC/CV Li-ion charger set to **12.6 V**, at a current inside the limits in [Specifications](#specifications). If the charger has a battery-temperature input, connect it to J1 or J2. 7. Connect the load to PACK+ / PACK−. ## Connecting packs together The BQ77915 can be stacked to protect more cells in series, but this board does not support that. Its stacking signals (CTRC, CTRD, CBI, CBO) are not brought out, and each board is a standalone 3S protector. - **Do not connect boards in series.** If one board turns off its FETs, the voltage of the rest of the string appears across its FETs and protector pins. The board is not designed for that. - **In parallel,** bring every pack to the same voltage before connecting them, so that no large current flows between packs. Each board protects only its own three cells. ## Safety - Use cells with no visible damage. Retire any cell that is dented, swollen, leaking, hot, or has a torn wrap. - Never bypass the protection, and do not leave JP1 on pads 2–3 unless you have other temperature protection. - Charge on a non-flammable surface and keep the pack away from flammable material. Do not leave it unattended while charging. - Lithium cells and packs are dangerous goods for transport. Follow the carrier's rules when shipping them. - Recycle used cells at a battery collection point. Do not put them in household waste. ## Schematic [`schematics/modbee-battery-pack-18650.pdf`](https://github.com/mod-bee/modbee-battery-pack-18650/blob/main/schematics/modbee-battery-pack-18650.pdf) (rev 0.01, 2025-05-16) The schematic is the reference for the circuit. Where the README and the schematic's own notes differ, the README uses values derived from the fitted parts and the TI datasheet. ## Renders | | | | --- | --- | | ![Assembly 2](/files/gitimg-8102cd02dcf820ad8621febe6eb995d515879b8b.webp) | ![Assembly 3](/files/gitimg-24e979922043f8d75cb16ff1b58a2985a291cf68.webp) | | ![Assembly 4](/files/gitimg-9d7b9108cdc5758a8a50e4e2ece892f1f1730bf2.webp) | ![Assembly 5](/files/gitimg-d20843821b1ac0dfa8bfffe60ad35ade139e9696.webp) | | ![PCB 1](/files/gitimg-d5cc5d1878461bbc740de0409fb61554aa9bb3f9.webp) | ![PCB 2](/files/gitimg-8173363da211453f7842043dc39221c9070af214.webp) | | ![PCB 3](/files/gitimg-bd7596c4c21086c0eaf3c39c1018faf71b5bc246.webp) | ![PCB 4](/files/gitimg-9f1e556cd65bf8b5ffe1d3a6369fd83558544600.webp) | The images are 3D renders. Placeholder 3D models can carry markings that differ from the parts fitted to the board. The schematic is the reference. ## References - [TI BQ77915 datasheet](https://www.ti.com/lit/gpn/bq77915) (SLUSCU0L, revised August 2023): thresholds, recovery methods, balancing and thermistor requirements - [TI BQ77915 product page](https://www.ti.com/product/BQ77915) ## Related repositories - [modbee-mppt-ti-c3](https://github.com/mod-bee/modbee-mppt-ti-c3): solar MPPT charger for 2S to 4S lithium packs, with a 10 kΩ NTC input ## Licence The schematics, images and documentation in this repository are licensed under [Creative Commons Attribution-ShareAlike 4.0 International](https://github.com/mod-bee/modbee-battery-pack-18650/blob/main/LICENSE) (CC BY-SA 4.0). You may share and adapt them, including commercially, if you credit ModBee and release adaptations under the same licence. Texas Instruments and other product names belong to their respective owners. Manufacturer datasheets are not redistributed here; the references above link to the official source. ## Disclaimer This design is provided as is, without warranty of any kind. You are responsible for the safe build, use and installation of any battery pack made from it.