Cigarette Outlet/ Inverter voltage drop

Maxcamp8

I set up equipment to enable use of a 120v 180 watt heating pad off battery power.  Runs just fine off of truck cigarette outlet but have a 10.1volt low voltage problem with the Nucamp system and trying to figure out why.  My battery BMV monitor reads 13.4 volts DC and we’re fully charged up.

 1.  Installed a no-name 12v cigarette/usb set of switched outlets, run off the Nucamp 12v USB/cigarette dedicated fuse.  It reads 13.4v output on its digital readout normally.

 2.  Bestek 500w 12v to 120v inverter, Model MR15011BU.  

Low voltage cutoff 10-11v  High voltage cutoff 15-16v  also cuts off load 580w or 500-550w for more than 20 seconds.

 3.  Sunbeam heating pad model CAT95C 180 watt 120v

Inverter runs fine, green light, with no load on it the cigarette outlet readout is 13.4 volts.

Turning on the 120v heating pad, the cigarette outlet drops to 10.1 volts and the inverter cuts power, then comes back on... with no heating pad.  Trying to understand how it can drop this far.  (Inverter fan cycles, full green light, then cuts out, signals three flashing green times, then restarts without output to heating pad.  Bestek diagnostics says this is the low-voltage cutout.)

 

Grumpy_G

Voltage drop is caused by resistance. The resistance could be in the gizmo but also thin wiring or a bad connection. Some back of the napkin math shows 0.2 ohms of resistance causes 3.3V drop at 15A.

The 12V power sockets are not really designed for continuous high current applications (that's why they are fused at 20A or less). If you want to be able to use the full 500W you need to wire the inverter to the battery with 6AWG wire and a 50A fuse.

Maxcamp8

Thanks, I guess I should have anticipated this.  W = VxA so 500W at 12v might pull 40A if this were AC... but  I don't know much about DC or how the inverter behaves.  The voltage cycling is a surprise, seems to be an artifact of the heater.  Testing an electric drill, it did not cycle the inverter voltage.

I did a plug-in test during daytime when system was running at 14.4V and everything ran (at low cycle of 11.x V as per later test.)   Then did the install.  Didn't have a meter on it.  Later clouds dropped system to 13.6 and the voltage drop to 10.x kicked off the shutoff.  Surprise!

Yes will have to run a separate lead from the battery, and hardwire the inverter as the cigarette plug or outlet are unlikely rated nor will they accept a large conductor.  Since the factory and my replacement PD4135 wiring are all #10 awg I can't just pigtail off an existing circuit.  Criminy.

Maxcamp8

Update.  Direct wiring inverter to battery  (with fuse and switch) overcame this voltage drop problem.