Batt Protect
Monitors a single Li-ion cell for over-charge (>4.30 V), over-discharge (<2.5 V), over-current (a few amps), and short-circuit (tens of amps) faults, and disables either or both pack MOSFETs to interrupt the offending current path.
Three precision comparators continuously compare the cell voltage to OV/UV thresholds set by laser-trimmed band-gap references. A current-sense comparator looks across the source-of-the-low-side-MOSFET to detect over-current after a programmed delay. A separate fast comparator handles short-circuit detection in under 10 µs. When any comparator trips, gate-drive logic pulls the corresponding MOSFET gate low through a small charge pump.
In plain terms
Like a vigilant electrician at a residential meter — the moment voltage or current crosses the safe line, the breaker flips before anything downstream catches fire.
Why designers use it
- Monitoring and disconnecting a battery or load under the fault conditions supported by the selected protection circuit.
- Keeping charging, power selection, monitoring and protection requirements distinct.
- Designing protection thresholds and current paths for the actual cell chemistry and system requirements.
Best for
- Single-cell pouches
- 1S2P packs
- Power tools
Key specifications
- Cell count: 1S – 16S
- Over-voltage threshold: +50 mV above float
- Under-voltage threshold: −500 mV below float
- Over-current trip: 1 ms – 100 ms response
- Quiescent current: 0.5 – 5 µA (Sleep)
When not to use it
- Where a full BMS with cell balancing is required (>2S packs above ~5 Ah) — the protection IC alone won't equalise SoC across cells.
- When the load can spike a few amps for a few seconds (e.g., motor inrush) — the over-current threshold may need to be set high enough to bypass useful protection; pair with a more configurable IC.
Common mistakes
- Choosing back-to-back FETs whose total RDSon × pack current exceeds 100 mV at peak load — the chip mistakes this drop for over-current and shuts the pack down mid-discharge.
- Skipping the 100 nF cap across the cell pins — switching transients couple into the OV/UV comparators and cause spurious trips during charging.
Where you will find it
- Analog Devices' battery-system article separates power selection, charging, monitoring and protection into different functions.
- Its LTC4231 example illustrates controlled connection and protection in a specified system. It is not a universal certification recipe for every lithium battery.
A short history
Battery-protection circuits monitor conditions such as cell voltage and current. A common design controls a MOSFET to disconnect a cell when a monitored condition leaves its permitted range. Hysteresis can prevent repeated switching as voltage recovers. The protection functions and thresholds must match the particular battery and application.
Good to know
- A protection IC is the silent guardian inside every Li-ion pack — it disconnects the cells from load on over-voltage, under-voltage, or over-current. Forgetting one is how phones catch fire.
- Two N-MOSFETs (one for charge, one for discharge) sit in series with the negative terminal; a protection IC's gate signals open them in failure conditions.
- Multi-cell BMS chips also balance cells: small bypass currents around fully-charged cells let weaker cells catch up — extending pack life by years.