Design Files

Open source design files for the 12V 7AH SLA Low Voltage Disconnect can be obtained from Circuit Maker. Circuit Maker is a free PCB design tool from Altium. Designs can be imported into Altium Designer.

A PDF version of the schematic can be obtained here.

Description

A basic, no-frills low voltage cut-out for 12V, 7Ahr Sealed Lead Acid Batteries.

Using two Keystone 3571 female PCB terminals, this PCB assembly attaches directly to the terminals of a 12V 7AH SLA Battery with 4.75mm Spade Terminals.

The Maxim Integrated MAX8212 Micropower Voltage Comparator with 1.15V reference switches a high side P-Channel MOSFET to disconnect the load. The MOSFET has a low on resistance of 4.1mOhms and a maximum current capability of 90 Amps well exceeding that of the battery.

The MAX8212 can operate with a programmable threshold and hysteresis (Populate R3/R6/R7) ensuring the load doesn't oscillate when the low voltage trip point is reached and the battery terminal voltage rises. However, if this behaviour is not desirable, this circuit also allows the low voltage event to be latched by populating R4/S1/R7 instead, as per Maxim Application Note 926.

When the load is switched off, the MAX8212's 5uA quiescent current ensures the battery is not discharged.

The 102k/953k/86.6k voltage divider sets a low voltage cutout at 11.9V and a reset high threshold of 12.87V.

Three printed circuit boards fabricated by OSH Park costs $11.15 and the BOM cost is approximately $12.15 from Digikey.