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Old 05-08-2013, 09:39 AM   #1
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Inverter problems

Since launching Annie about 6 years ago we have run the refrigerator solely off 110v. We shut down the generator in the evening and the Kenmoore side by side hold temps overnight pretty well except the ice cream gets a little mushy. .When running in the daytime we run the smaller of two gensets to charge batteries, run bilge blowers, and refrigerator. We are spending more days at anchor and don't want to run the generator just for the refrigerator so I am finally getting around to installing an inverter. I decided to experiment at home to see how things would work before doing the boat installation. I purchased one of the Kill-o-watt meters at Lowes and measured the power drain of a similar refrigerator we keep on the back porch just for drinks. It draws about 180 watts running. I purchased a 1500 watt/3000 watt MSW inverter using some arbitrary numbers that I had read. I read that surge load on an induction motor or locked rotor draw was 3-5 times run load. So I assumed that 3000 watt surge load should cover the start up on the compressor. Apparently I have erred somewhere in that when I plug the refrigerator into the inverter it will come on initially but when I come back 30 minutes later the inverter is beeping and has shut down. My conclusion is that the compressor is not happy with the modified sine wave or the surge load is much higher than I anticipated. The Kill-o-watt will not measure the surge peak so I really don't know what it is. Do I need a bigger inverter or a pure sign wave inverter or is there something else I am overlooking?? Forr my experimenting I am using a freshly charged automotive type battery with no charger hooked up.
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Old 05-08-2013, 10:05 AM   #2
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A few questions:
  • Run your inverter with a big load that is not the refrigerator to see if it kicks off
  • Is your inverter getting sufficient cooling
  • What brand, size and model of inverter
  • What do you mean by 1500/3000 watts
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Old 05-08-2013, 11:09 AM   #3
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I use a 1500 watt MSW inverter for the same purpose and it does fine. One thought comes to mind. you say you are testing with a single automotive battery. It might be that during startup at least, the inverter senses low input voltage and shuts down. If the inverter sees a 2400 watt surge, it is drawing drawing 2400 watts from the battery not counting for ineffecienciy. thats 200 amps, or more likely 220 to 250 amps. That is a lot for a single battery, even for a short while. I suggest to hook a digital meter to the input and observe the voltage for a time at start up to see that the voltage doesn't drop. Most inverters will shut down at 12 volts or so. It is easy or me to believe that your input may be dropping below that. 220 amps is roughly what happens when cranking a cold engine. It is not unusual for voltage to drop into the 11's when cranking.
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Old 05-08-2013, 11:37 AM   #4
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I think that is the issue...low voltage and high initial load. I hooked a charger to the battery to top it off.... and put a digital voltmeter on the battery. Also switched to a purely resistive load....coffee pot. When the coffee pot was switched on for an instant the voltage dropped and I saw 11.85 on the meter. The battery was charging at a 15 amps so the battery may be down. I have a brand new 6D in the basement in our storm room that also runs an inverter. Will try that one and see if the issues are the same or if that corrects the problem,
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Old 05-08-2013, 11:52 AM   #5
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Originally Posted by Capn Craig View Post
I use a 1500 watt MSW inverter for the same purpose and it does fine. One thought comes to mind. you say you are testing with a single automotive battery. It might be that during startup at least, the inverter senses low input voltage and shuts down. If the inverter sees a 2400 watt surge, it is drawing drawing 2400 watts from the battery not counting for ineffecienciy. thats 200 amps, or more likely 220 to 250 amps. That is a lot for a single battery, even for a short while. I suggest to hook a digital meter to the input and observe the voltage for a time at start up to see that the voltage doesn't drop. Most inverters will shut down at 12 volts or so. It is easy or me to believe that your input may be dropping below that. 220 amps is roughly what happens when cranking a cold engine. It is not unusual for voltage to drop into the 11's when cranking.
I think CaptCraig is on to something here. Even with a charger attached a single battery could very well drop below the low amperage rating for the inverter. I would parallel connect the two batteries, and then try it. I run both an ice maker and a 5 cu ft AC fridge on an 1800 watt true sine wave inverter with a 2900 watt surge capacity.
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Old 05-08-2013, 01:56 PM   #6
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I think CaptCraig is on to something here. Even with a charger attached a single battery could very well drop below the low amperage rating for the inverter. I would parallel connect the two batteries, and then try it. I run both an ice maker and a 5 cu ft AC fridge on an 1800 watt true sine wave inverter with a 2900 watt surge capacity.
Interesting!
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Old 05-08-2013, 02:47 PM   #7
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Interesting!
Walt, upon rereading my post, I miss stated it. It should read voltage drop below the rating for the inverter. I have a diesel truck with 2 group 27 starting batteries in parallel. With the alternator charging sometimes when a professional tool is plugged in the voltage will drop below the MSW inverter rating. The batteries don't recover fast enough.
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Old 05-08-2013, 03:56 PM   #8
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That has always been a big problem with flooded lead acid batteries, and that is voltage sag under a heavy load. If you want to stay with lead acid, you will need to size your bank to handle a heavy load with less voltage sag. Amp-hr is how a battery's capacity is determined, but it is based on a 20 hr rate. All that means is a battery rated at 100 a-hr will deliver 5 amps for 20 hours. Due to Peukert effect, the higher the draw, the less the capacity. If your pulling 200 amps, the capacity of your 100 a-hr battery will drop to about 25 a-hr.

Size your bank by your calculated needs for 24 hours taking into account the Peukert effect and the fact that a lead acid battery should not be discharged past 50% DOD (depth of discharge). When sizing, build your bank as a single series string, not a bunch of 12 volt batteries in parallel. To make life easy on your back, keep each battery or cell below 90 lbs. Example, if your 24 hour drain is 4000 w-hr, then a good battery choice (if you want to stay with lead acid) would be 700 a-hr AGM single 2 volt cells, (6) in series for 12 volts, 700 a-hr.

I used to design systems for off grid remote homesites. Today we have a much better choice for batteries, and that is large single cell LiFePO4. They come in sizes from 40 a-hr to 1000 a-hr, have nil Peukert effect, don't sag under heavy loads and will cycle 3000 times at 80% DOD vs 500 cycles at 50% DOD for lead acid.

Here is a picture of (4) 100 a-hr 3.2 volt LiFePO4 cells that weighs 32 lbs. This small bank would start and run your fridge at 25% compressor run time for over a day. The reason it can and a same rating lead acid can't is because of how the cells are rated and Peukert. I mentioned how the lead acid gets its rating at an easy 20 hour draw down, well the LiFePO4 cells are rated at a high current 1 hour draw down, and only to 80% DOD whereas the lead acid test for capacity pulls them all the way down 100% DOD. A-hr ratings are completely different, so is performance.
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Old 05-08-2013, 05:33 PM   #9
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I run a 3 cu ft fridge like this...



...on a 1000W MSW cheapo Xantrex inverter (2000W surge) and it runs fine. I can also power either a small 700W microwave or a 750W coffee pot with the fridge without issues.

My unit has a low voltage alarm at 11.0V and an auto shut down feature at 10.5V to protect the battery from depletion. I'll be interested in your followup with the larger, healthier battery bank powering the inverter.
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Old 05-08-2013, 05:40 PM   #10
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deckofficer,

How many of those 4-battery modules would I need for comparable performance in place of my 660AH 6V golf cart lead acid bank (6 batteries)? (They are wired for 12V)

My GC house bank cost $450 and I expect to get 6-8 years of use out of it. How much would this comparable bank cost and what's its life expectancy?
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Old 05-08-2013, 06:03 PM   #11
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Al,

Current price for 220 a-hr 6 volt golf cart batteries are $100 apiece or $600 for your bank. If you cycle only to 50% DOD you should expect 500 cycles which is very good life for a lead acid battery. My picture shows (4) 100 a-hr cells, so would be a bit small for a replacement of a 660 a-hr lead acid pack. A 300 a-hr LiFePO4 would give close to the usable capacity of the 660 a-hr lead acid bank. Each cell weighs 21 lbs and costs $375. (4) gives you 12 volts (13 volts nominal) for $1500. You can cycle to 80% DOD instead of 50% DOD for lead, and get 2000 cycles instead of 500. The weight savings if that is important is 378 lbs.
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Old 05-08-2013, 06:17 PM   #12
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Thanks, Bob. And one more thing...

Can they be charged with existing smart chargers with Lead-acid/AGM/Gel charging profiles or do they need a custom charger?
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Old 05-08-2013, 06:31 PM   #13
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If you don't need fast charging then a bulk charge at 14.2 volts would be fine. I need a fast charge so bought a special charger for LiFePO4 that outputs constant voltage at 14.8 volts and current stays constant until the knee and then quickly tapers off till 14.8 and the charger stops. The main issue with fast charging at this higher voltage is the cells have to be balanced or if not you stand a chance of one of the cells going above 4 volts which shortens its life. A lot of folks that don't want to take the time to do a bottom to top balance and also want a plug and play install will also include a BMS (battery management system).
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Old 05-09-2013, 06:12 AM   #14
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The problem is the TYPE of battery you chose for your experiment.

Auto start batts make great high amperage (lots of surface area from many thin plates) so the batt starts the fridge with no problems.

A start batt will loose its ability to supply sufficient amperage after a very short time , as you noticed.

Install the setup in the boat with genuine deep cycle batts and it should work.

About 20A will be required from the set for each our of operation.
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