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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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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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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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Old 03-24-2018, 02:07 PM   #11
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or switch to a pair of paralleled 12V batteries (with the same total amp hours), which will decrease the internal battery resistance losses.

have not had the inverter alarm,
Yes, works for me although I seem to be the only one with a paralleled 12V set-up. Both for microwaving, 1 or the other for normal house use. I think that my total amp hours work out to be a little less than two 6's but I've never had an inverter alarm.

My own self imposed guideline is no microwaving if the batteries aren't pretty close to fully charged.

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Old 03-24-2018, 04:04 PM   #12
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note to you guys reading 13.x V before running the load, thats charger float voltage. batteries will stay at that upper voltage for awhile after being taken off a charger (either solar or the AC converter), but generally an hour or so later, you'd see the 'real' voltage of a fully charged battery, which is 12.6-12.7V typical. 12.1V at rest is 50% discharged, and is my normal lower threshold for wanting to recharge the batts ASAP... under any sort of heavy current load, you'll always read less, but a few minutes after switching off the load, it should return to the rest voltage.
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Old 03-24-2018, 06:40 PM   #13
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As a footnote as I didn't mention it, I was going into these test with the new batteries at 12.6V.

I found an additional issue after one of the load test when I noticed the cable joining the two 6V batteries together was rather warm! but none of the other cables were. The nuts on the cable were very tight but one of the crimps had very slight movement when I pulled on it. After correcting it. the cable no longer got warm under load and with a full three minute microwave run, voltage only dropped to 11.6 . I believe the lose crimp was costing me some amps and thus causing my other problems.

Dang, why are the simple solutions so hard to find.
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Old 03-24-2018, 08:26 PM   #14
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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.
or get a pair of 6 volt AGM's. Also expensive, but lower internal resistance, which also speeds up charging, all else being equal. Once our trailer is wired up with our AGM's later this summer, I will be able to report performance with our microwave, but it should be better than with flooded batteries.
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Old 03-25-2018, 02:34 AM   #15
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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).
Yes, the wattage used in the description of a microwave oven is the power output (of microwave energy into the food), so the input electrical power to the appliance is always significantly higher. In addition to imperfect efficiency of the microwave generator, there is a fan and light and controls to operate. The input electrical power is listed on the usual appliance information label, which may be on the back panel but is often in front, visible when the door is opened. In the traditional type which cycles on and off, the power draw when on is always that listed maximum.
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Old 03-25-2018, 02:39 AM   #16
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I found an additional issue after one of the load test when I noticed the cable joining the two 6V batteries together was rather warm! but none of the other cables were. The nuts on the cable were very tight but one of the crimps had very slight movement when I pulled on it. After correcting it. the cable no longer got warm under load and with a full three minute microwave run, voltage only dropped to 11.6 .
Good troubleshooting.

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I believe the lose crimp was costing me some amps and thus causing my other problems.
It was adding resistance, which would reduce current through a fixed-resistance load, but wouldn't reduce the current taken by the inverter. It would certainly reduce the voltage across the battery bank for any given voltage... which would actually slightly increase the current the inverter would need to take to meet a particular output power requirement.
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