When testing with a multimeter against the terminal on a fully charged pack, you should see: B+ to B- : ~20V (Full capacity) TH to B- : ~0V (unless active) ID to B- : ~0V (resistance-based)
[ Release Button / Top of Battery ] __________________________________ | | | (1) (2) (3) (4) | | B- T ID B+ | | Neg Temp Ident Pos | |__________________________________|
Store batteries in a cool, dry place and inspect the housing for cracks or swelling. If a battery is damaged, it should be recycled at a designated facility rather than used or charged.
: 21.0 Volts. Fully charged cells reach 4.2V each.
: 18.0 Volts. This is the operational average calculated at 3.6V per cell across 5 cells in series. Hart 20v Battery Pinout Diagram
In this article, we will dissect the HART 20V battery terminal layout, explain what each pin does, provide wiring diagrams, and show you how to safely use these batteries outside of HART tools.
By mastering the layout and function of the Hart 20V battery pinout, you can safely maintain your equipment, properly troubleshoot charging issues, and confidently expand your DIY power ecosystem. Always remember to prioritize low-voltage cutoffs and proper fusing whenever you step outside of official manufacturer uses!
| Symptom | Likely Cause | Pin to Check | | :--- | :--- | :--- | | Charger blinks "defective" | ID resistor out of spec or thermistor shorted | Pin 2 (T) & Pin 3 (ID) | | Tool runs, but charger won't start | Temperature sensor reading hot/cold | Pin 2 (T) to B- | | No power to tool | Blown fuse inside BMS on B- line | Pin 1 (B-) continuity | | Battery reads 0V | BMS in "sleep mode" (over-discharged) | Requires jump to B+ and B- |
This pin connects to an internal Negative Temperature Coefficient (NTC) thermistor resting against the lithium cells. As cell temperature increases, the resistance on this pin drops. When testing with a multimeter against the terminal
Located on the outer edge. This is the main positive power rail of the battery pack. It delivers the full voltage (18V nominal / 20V max) to the tool.
[ (+) ] [ (T) ] [ (ID/C) ] [ (-) ] Positive Thermal Tool/ Negative Terminal Sensor Charger Terminal Identify Use code with caution. Pin-by-Pin Breakdown
If you have a multimeter but no diagram:
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: This pin acts as a thermal sentry. By measuring the resistance here, the tool or charger can tell if the battery is getting too hot to handle, shutting down power to prevent a meltdown. The Identifier (ID)
Note: Many HART BMS boards have a proprietary IC that prevents voltage output if any cell is disconnected, even for a second. Proceed carefully.
: If your battery shows a permanent failure code (LEDs flashing in a specific pattern), it might be "undervolted." Some users "jumpstart" these by momentarily connecting a healthy 20V battery's B+ and B- to the dead one's B+ and B- to raise the voltage enough for the charger to recognize it.
Note: Some lower-capacity HART 20V batteries (2.5Ah) may lack the second temperature pin (T2) and have only 4 terminals: B-, T1, ID, B+.