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Old 03-23-2018, 08:36 PM   #1
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how much voltage drop with inverter

I am trying to determine if I have a battery problem or a inverter problem. So, I am curious what voltage drop others experience when running the microwave from the inverter. I have new batteries and if I turn the microwave on for say 1 minute, the battery voltage drops to about 11.2 volts within 15-20 seconds and then holds there. That seems far too low to me for new batteries. What are others experiencing?
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Old 03-23-2018, 09:17 PM   #2
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What is the reading after the microwave stops? I thought I was damaging our dual 6V batteries just by making a 4 cup pot of coffee. It would go from around 13.5 down to 11.9, but then after turning off coffee pot it would jump back up.
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Old 03-23-2018, 09:25 PM   #3
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That seems pretty normal. As long as the battery recovers then everything's OK.

On mine, given something like a 3 minute burst, the battery recovers back to the original reading. A couple of minutes more use and it might be .1 volt down.

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Old 03-24-2018, 01:10 AM   #4
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a typical microwave is 600 or 700 watts when its actually nuking. lower power settings typically turn them on/off/on/off with varying duty cycles. 600 watts is 50 amps at 12V, thats a LOT of current, so yes, it draws the battery voltage down considerably.

question, is the inverter directly attached to the battery via short heavy gauge wires? are you measuring the voltage at the battery, or somewhere else in the DC system ?
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Old 03-24-2018, 07:03 AM   #5
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Originally Posted by John in Santa Cruz View Post
a typical microwave is 600 or 700 watts when its actually nuking. lower power settings typically turn them on/off/on/off with varying duty cycles. 600 watts is 50 amps at 12V, thats a LOT of current, so yes, it draws the battery voltage down considerably.

question, is the inverter directly attached to the battery via short heavy gauge wires? are you measuring the voltage at the battery, or somewhere else in the DC system ?
The wiring is standard Escape inverter wiring. I checked the voltage at the 12v outlet and at the battery with a meter, they both read the same. The engineer at Samlex says that is too much voltage drop and I tend to agree. At around 10.6-10.7v, the inverter will sht down and give a low voltage imput error code.
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Old 03-24-2018, 07:05 AM   #6
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What is the reading after the microwave stops? I thought I was damaging our dual 6V batteries just by making a 4 cup pot of coffee. It would go from around 13.5 down to 11.9, but then after turning off coffee pot it would jump back up.
The voltage returns to near normal within about 1 minute.
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Old 03-24-2018, 07:10 AM   #7
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What I failed to mention is that when voltage reaches 11.2, the inverter starts giving off a continuous shrill beeeeeep. On the old batteries, when the voltage reached 10.8, the beep would then become a rapid series of beeps just before the inverter shut down from low voltage imput.
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Old 03-24-2018, 09:23 AM   #8
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Originally Posted by thumper-tx View Post
What I failed to mention is that when voltage reaches 11.2, the inverter starts giving off a continuous shrill beeeeeep. On the old batteries, when the voltage reached 10.8, the beep would then become a rapid series of beeps just before the inverter shut down from low voltage imput.
What was your no load voltage? I reviewed your posts and missed any mention of it. Based on what I remember as the volt drop on our new 6*2 volt battery pack, when loaded with the ~276 watts required by our 3 way fridge (a short experiment), I think the ~1.6 volt drop referenced earlier from the coffee pot (probably kind of close to your load) in the thread is from a healthy system. I.e., you probably can't expect less without more battery power.
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Old 03-24-2018, 12:48 PM   #9
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What are others experiencing?
I ran my 950w microwave full power (1200w to the inverter) and the voltage dropped to 11.7 volts at the inverter. The voltage prior to the test was 13.3 volts. The inverter didn't beep or seem unhappy and the battery voltage recovered shortly after I turned off the microwave.
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Old 03-24-2018, 12:55 PM   #10
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Originally Posted by John in Santa Cruz View Post
a typical microwave is 600 or 700 watts when its actually nuking. lower power settings typically turn them on/off/on/off with varying duty cycles. 600 watts is 50 amps at 12V, thats a LOT of current, so yes, it draws the battery voltage down considerably.

question, is the inverter directly attached to the battery via short heavy gauge wires? are you measuring the voltage at the battery, or somewhere else in the DC system ?
The actual wattage of most microwaves is higher than the labeled wattage, i.e. my 700 watt RCA microwave supplied by Escape actually draws 95 amps at full power (1140 watts at 12V).

As John said, this will definitely pull your batteries down due to their internal resistance. Too long or too small wiring will cause voltage drop, which will show at the input terminals of the inverter, but not at the batteries. If the voltage drop is caused by the internal resistance of the batteries, (i.e. the same voltage measurement at both the batteries & the inverter) larger wiring will not help. One expensive & heavy solution is to add another pair of 6V batteries, or switch to a pair of paralleled 12V batteries (with the same total amp hours), which will decrease the internal battery resistance losses.

I have the 2 6V batteries & the RCA 700 watt microwave, and the Escape installed wiring, and have not had the inverter alarm, although I generally have fully charged batteries - 320 watts of solar.
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