Customer Question: Preventing Inverter shut-down when engine starts?

Posted on 2/25/2016 by Jack C. (Professor Jack)

Customer: Recently I was camping in my RV watching TV and noticed every time the water pump came on, the 200 watt inverter momentarily shut off. When the inverter shut off, the TV also went off. It took several seconds for the inverter to restart and longer for the TV to come back on. What could be wrong?

Wagan RV Install

Wagan Tech: This was happening because either the “house” battery was weak or the RV wiring was not adequate. Rewiring the RV circuit can be difficult, here is an easy solution to prevent momentary low voltage shutdown.

 

A 200 Watt 12 volt inverter only draws a maximum of 20 amps when it delivers 200 watts. If the inverter was powered by a nearby small 12 Volt lead acid battery, it could continue to power the inverter if the RV power dropped low for a little while.

Add a small 12V battery to your system. The battery has to be a 12 volt sealed lead acid type from 4 to 7 amp hours (Ah). You will also need to obtain and install a small diode. Please refer to the chart below.

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While the RV is in motion the house battery is likely being charged. Keep the plug inserted and the small battery will be charged. The diode has a small voltage drop of ¼ to ½ of a volt so the small battery will never be as high as the RV voltage so it is unlikely it will be overcharged.

Just make sure you get polarities right. The diode will charge small battery as long as the outlet voltage is higher than that of the small battery. When the pump comes on, the outlet voltage drops and the small battery still powers the inverter because the diode acts like a one way switch.

So, how long will the inverter operate when the house battery voltage is low or off? That depends on how much AC power the inverter delivers and the small battery capacity. For example, A 200 watt AC appliance draws 20 amps from a 4 amp hour 12 volt battery. A good estimate would be five minutes of inverter operating time because of battery derating factors.

About Diodes
A diode is an electronic device that allows current to flow in one direction only. Diodes act like check valves. All diodes have two elements, an anode and a cathode. The anode is the triangle part of the symbol, the cathode is the line. Conventional current defines current as plus to minus flow in direction (electrons actually flow in the other direction). Conventional current through the diode symbol is in the direction of the arrow.

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