Auto Switching 12 Volt Battery Under Load

Voltage sensing: Make a voltage divider to bring the charged battery voltage within range that can be measured by your micro-controller (5V, 3.3V or whatever your adc pin allows). Make sure your adc pin never sees a value which is out of range. For ex - If battery has a max voltage of 15V and my adc can accept max voltage of 5V, I can use this circuit:simulate this circuit Schematic created using CircuitLabThis will allow you to sense the battery voltages. You will know which batteries are ready for getting fed into the inverter.For switching, you need a high current mosfet - one for each battery. Parametric search on digikey will give you some results. If your max current can be 200A, better to select a value higher than that - probably 2-3 times.Another solution (cheaper) is electromechanical relay but at that current, arcing might kill it very often.If you are using mosfets, you will have heating issues so you will definitely need to attach massive heatsinks to your mosfets. Having a common heatsink for your mosfets might help because only one mosfet will remain ON at any particular time. Due to the big size, cooling will be more effective as compared to a small detached one. Assume a power dissipation of 150-200 watts in your mosfet while carrying 200A. If that's a concern, you probably shouldn't go this way

I have a 120 V / 60 Hz 2000 watt inverter which can draw up to 200 amps from a 12 V DC battery source. An AC oil heater drawing 6 amps max 15 amps is running from the inverter constantly. I want to automatically change batteries without interrupting the current to the inverter.

The scenario is: several batteries are supplying the inverter. When a battery becomes depleted it is replaced with a charged battery. The invertor cannot go off line so a battery must be on line at all times. The depleted battery is switched to a charging circuit until it is fully charged then it is available to be switched to the inverter load. The battery being charged must be isolated from the inverter during charging.

Having many batteries is not an issue. Having many battery chargers is not an issue.

Currently this is a manual process and I'd like it to be automatic. I'm looking for: the best way to detect the depleted battery, swap the battery from the inveter to the charge circuit, detect the fully charged state then return the battery to the load circuit.

This application is an off grid. Any advice will be helpful.

·OTHER ANSWER:

I have a 120 V / 60 Hz 2000 watt inverter which can draw up to 200 amps from a 12 V DC battery source. An AC oil heater drawing 6 amps max 15 amps is running from the inverter constantly. I want to automatically change batteries without interrupting the current to the inverter.

The scenario is: several batteries are supplying the inverter. When a battery becomes depleted it is replaced with a charged battery. The invertor cannot go off line so a battery must be on line at all times. The depleted battery is switched to a charging circuit until it is fully charged then it is available to be switched to the inverter load. The battery being charged must be isolated from the inverter during charging.

Having many batteries is not an issue. Having many battery chargers is not an issue.

Currently this is a manual process and I'd like it to be automatic. I'm looking for: the best way to detect the depleted battery, swap the battery from the inveter to the charge circuit, detect the fully charged state then return the battery to the load circuit.

This application is an off grid. Any advice will be helpful.