I’ve discovered that adding the low voltage battery disconnect (LVD) is key for maintaining a happy and healthy accessory battery in my truck. Previously, the accessory battery was electrically isolated from the starting battery, which allowed me to run down the battery without worrying about getting started, but nothing protected the accessory battery from overdischarge. Dropping a rechargeable battery’s levels down too low means the chemistry is irreversibly changed and the plates have sulfur collecting on them, which limits the ability to recharge. The alternator also has work much harder and longer to recharge a totally dead battery compared to one that is discharged to a safe level of only 50% charge. Battery life is much longer when you don’t overdischarge the battery, so that 50% consumed, actually can be longer then 80-90% consumed after a few deep discharges. A good deep cycle battery should be able to go into the 75% range safely, but why risk damage when one can disconnect it at a higher voltage? Moreover, you have to leave a bit of battery voltage above the danger range, to keep the low voltage disconnect powered, even though it only uses about 70 milliamp (which can add up over time).

A low voltage disconnect is a combination of a electronic device and electro-mechanical relay that monitors system voltage, and if voltage drops below a certain level for a period of time, the relay opens to disconnect the load from the battery. This keeps the battery from overdischarging. On my truck, I use a BlueSeas 60-amp LVD ($50 on Amazon), which was a nice integrated unit that has both the voltage monitoring circuitry and a 60-amp relay built into the unit, and come with a wired remote for overriding the system or changing the targeted voltage. Read the instructions carefully on wiring the remote – they are a bit confusing. I have the remote hooked up, but I have yet to use it. There are other models out there, but most of them have a separate disconnect controller and relay, and aren’t as elegant.

The low voltage disconnect on my truck is always monitoring the voltage of the accessory battery. As long as the voltage is at 12.1 volts or higher, the relay (switch) inside it remains closed and supplies power to my truck cap lights, inverter, CB radio, dash camera, and all other accessory loads I have connected to it. When the voltage drops to 12.0 volts for more then 30 seconds (a delay to avoid disconnects on temporary high start-up loads of inductive loads like electric motors or the starting of the inverter), the relay opens, disconnecting the load. This means that the lights in the truck cap, inverter and all accessory loads are disconnected. That can plunge you in the darkness, but it also spares the accessory battery. You can always hit the door unlock button, or hop in the truck, and have interior light to get the truck started again.

The low voltage disconnect is constantly polling the battery voltage. When you start the truck up, after 30 seconds of voltage over 13.5 volts, the load is automatically reconnected to the battery, automatically reseting the inverter, turning back on the lights, and other load you have connected. The delay in reconnecting the load is good for the alternator, because it gives it a chance to start recharging the low voltage – a high amperage drain – before you put even more load on it from the accessories.

Prior to the installation of the low voltage disconnect, I relied on both the inverter faulting out for low voltage, and keeping an eye on the battery voltage to know when I should recharge things. But that was a hardly a perfect solution. The inverter wouldn’t raise a fault until 11.8 volts or so, which is closer to 75-80% discharge, and would beep but allow the battery voltage to fall well into the danger range for low voltage, before finally disconnecting. Other loads wouldn’t be disconnected at all. The inverter would chirp and beep when the battery voltage was low – a rather annoying alarm – and require a full reset by reaching down and turning the power switch on an off. Left parked for an extended period of time, sometimes predatory loads from the various USB ports and other minor electronics would discharge the accessory battery below a safe level. It was far from an ideal situation.

The low voltage disconnect does it’s job, silently. It disconnects the load when the battery voltage is low, without any further prompting or annoying beeps or alarms. It reconnects the load when the engine is powered back up and the battery charge is at a safe level once again. It works without fiddling with it or monitoring the voltage yourself. It’s a good safety measure that I would recommend for any rechargable battery-powered application.