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2014 136” HR
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Discussion Starter #1
This question will illustrate how little I know about batteries:

We have a simple electrical setup: 100ah AGM house battery with battery separator. No solar. No charger except the alternator. So far, this battery powers an Engel MT45 fridge/freezer, a few LED's, and USB charging. The fridge is rated at 1amp in fridge mode; depending on external temperature, it runs about 2-6 minutes/hour. On this trip, it ran about a minute every half hour.

I have a voltage meter that plugs into a socket. When we first stop, the meter reads 13.2 volts. The next morning, it will read about 12.3 volts, but the next morning after that, it will still read 12.2 volts. This makes no sense to me, given that the engine has not been run and electrical consumption is the same both nights. What am I missing?

I've read that voltage should be measured after the battery has "rested," but that's inconvenient "in the field." I do not measure when the fridge is running, but there are single-LED lights in the USB plugs that are always on. Is there some calculation to account for this?
 

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What exactly are you trying to solve? It sounds to me like you have a really good battery and a very energy efficient fridge... Good on ya!
 

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Discussion Starter #3
Why does it drop 0.9 volts in one day and only 0.1 volt the next day?

Is there a way to convert this to percent discharged if the battery hasn't "rested" when I took the measurement?

It's easy to believe that everything is hunky dory now. What I'm really trying to do is figure whether I can get away with adding maybe 6-10 minutes of a 700-W microwave per day for that two days.
 

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Voltage is not a good indicator of battery charge level. The 13.2 would indicate a full (or over) charge of your AGM but the battery will almost immediately drop to 12.8 or below with a tiny draw and it will still be fully charged. From there it dips very little- almost not enough to be measured until it is about 75% charged then a bit faster until 50% charge and something like 12.2 or so. Less than that and the battery should not be used.
Flooded acid batterie's state of charge can be tested by measuring the specific gravity of the electrolyte. Accurate but not practical in a campervan, good in a home solar setup. Not possible with AGM.

What to do? You might buy a Watt meter and find out how much power you have used overnight and calculate the use against the capacity of the battery. Easy really. See: [ame]http://www.amazon.com/Digital-Battery-Balance-Voltage-Analyzer/dp/B00EYZS6R6/ref=sr_1_11?ie=UTF8&qid=1449977336&sr=8-11&keywords=watt+meter[/ame]

What do I do? I have solar, I have a solar controller, it signals when it has gone into float mode and the battery is full. I make sure that happens every day. I don't have a volt meter because it is almost useless. With 200 watts of solar the controller usually is at float by noon. On cloudy days it usually does by late afternoon. On rainy days really dark it doesn't so I go for a drive and switch on my interconnect to the alternator.
 

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Well stated RD.

What I'm really trying to do is figure whether I can get away with adding maybe 6-10 minutes of a 700-W microwave per day for that two days.
Are you using an AC inverter or is the Micro 12V?
 

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I question your figures on the fridge. You say that it ran only 2 to 6 minutes per hour and another time it ran only a couple of minutes every half hour. I have experience with many compressor fridges designed for efficiency for marine use and I can't believe that it runs so little unless the surrounding temperature is very low.

You also refer to the fact that the machine uses only "one amp in fridge mode". Manufactures of these devices have a way of using deceptive language. What they really mean I believe is that the average use of power over time would be 1 amp per hour or 24 amp hours per day and that assumes that the outside temperature is probably only 70 degrees -- not very hot. 24 amp hours per day is doable for a couple of days if there are no other loads except LED lights.

Most of these fridges use a Danfoss compressor or similar and these typically draw 2 to 6 amps when running. Your "duty cycle" running time figures seem low to me.

I believe that you should double battery capacity especially if you plan to use the proposed microwave. I have a 700 watt microwave run off an inverter. 700 watts is the cooking (heating) power; the device will draw 1000 watts or more; that is approaching 100 amps of 12 volts. Even if only run for 4 minutes this will quickly harm a 105 amp hour battery. It is not so much the amp hours used as it is the rate of withdrawal from the battery.

Remember that batteries are best for long life to be discharged only 50 percent; that is, only about 50 amp hours out of a 100 and hour battery. At rest this would be a voltage of about 12.2. Yes the battery will read lower than that when under big load but after load removal it should bounce back to 12.2 or higher within a couple of minutes. Also fast, heavy discharges shorten life.
 

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Here is a calculator which should help you determine what you are looking for. Populate the fields as instructed, and then it will tell you what size of battery you should have for the application. Compare it to the size of the battery you actually have, and you should get you answer with a little extra math. If your Micro is A/C there's a different calculator for that. Hope this helps. Good luck!

http://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html"
 

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MsNomer you are getting good information from the folks on the forum. Battery charge and discharge although reasonably understood has so many variables that it is kind of a mystic art. The amp readings you quote sound more like the current draw when running on 120 vac than 12 vdc. Voltage readings are more useful in determining whether you are charging appropriately or discharging. RD`s system that keeps the batteries near full charge is best for battery longevity, if you need to go longer before charging more amp hour capacity is better. You can just keep going until battery failure and see how the time and cost work out but 120 vac heating appliances operating with an inverter from a 12 vdc supply are very hard on batteries that don't get immediately recharged. Like multiple attempts to start an engine that doesn't start.
 

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Discussion Starter #11
Sea pro, I have monitored the fridge's run time. This summer when ambient temps were mid-90's, the fridge would run 2 minutes out of 20--6 minutes per hour. This week with highs in the 70's, the fridge ran one minute every half hour--2 minutes per hour. Engel says the fridge uses 0.7 amps (fridge mode) to 2.5 amps (coldest freezer mode). I'm using fridge mode, thus estimating 1 amp. My run times are consistent with numbers given by users on other forums and I haven't seen anyone question the manufacturer's numbers. This is not a Danfoss compressor.

Would running the engine while microwaving make any difference?
 

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I bought THIS and for the price, it works VERY WELL. Add THIS to the mix you'll have a good and very inexpensive system. Could be a good option to keep your battery conditioned and topped off when fuel consumption and storage is a concern. You could do some sort of permanent install or just keep it portable like me...

P.S. The solar panel inverter will charge your battery directly when you don't have access to AC power. When you are at home, the onboard tender/charger will keep your battery conditioned.

Solar Panels
Onboard battery charger
 

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Tracy, I'm not seeing any of your attachments.
Ms,

Click on the words THIS for a link. (not my THIS but Tracy's THISes)

As far as the microwave, I'm at the same place. 700w but uses about 1000w. 100AH battery with separator.

Harbor Freight 2000W inverter that works well, but the microwave pulls just a bit too much power. The solution is going to be a second battery in the spring probably.

No solar.... I'd love to but even if I could squeeze it in my carport's 3" clearance, it wouldn't do much generating under the carport!

Ed
 

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I question your figures on the fridge. You say that it ran only 2 to 6 minutes per hour and another time it ran only a couple of minutes every half hour. I have experience with many compressor fridges designed for efficiency for marine use and I can't believe that it runs so little unless the surrounding temperature is very low.
I have the Engel MT45 too and, in fact, it is extremely efficient and its insulation is great. I use it in the summer in The Gorge where temperature averages between 80 and 95F, and I don't think the compressor is on more than 10 min/hour. So, I think that MsNomer's 2 to 6 min/hour is about right.

By the way, I noticed that my MaxxAir fan running only at 10% all night draws way more electricity than my Engel.

I just love my Engel. If I was rich, I would buy the company ;)
 

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proeddie and MsNomer,
Why not flex solar panels glued or double sided VHS taped to the roof? You don't need to charge the solar under your carport if you have 110V nearby, just get a charger! Solar is the BOMB and solves so many problems! If I had a microwave (never) or other 110V appliances I would go 300-400 watts of solar and 200+ AH battery set, easy peasy! Why are we struggling so hard to avoid the obvious solution? I know some folks have carports, others like to camp in the shade, others want every electrical appliance known to humanity even resistance heat and hot water! Living in your van is like a marriage- compromises will be made- accept it, move on.
 

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12.3V after 1 night seems pretty low.
Right after a charge there is a 'surface charge' that dissipates after awhile. That is why your voltage drops quite a bit the 1st nite. Measuring voltage is not the best way to measure state of charge, but it still works reasonably well after it's rested with no load, or a very small load on it. I've measure voltage after a proper rest, and with a small load on it such as you have. The difference is about 0.1 V. I've called lifeline a couple of times with questions about my battery and every time, one of the 1st questions is what is the resting battery voltage.

If your voltage is 12.3, perhaps we can call it 12.4 resting voltage, Referencing this table7 here:
http://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
, that would be about 60 or 65% charge.

The PM regulator holds the voltage at around 14.1 V, which is a little low to charge up a deep cycle AGM battery at warm temperatures. At colder temperatures (<70F) it is too low. AGM's are more finicky than FLA (flooded lead acid).
It looks to me like your battery is not getting fully charged or has gotten sulfated. You should put a charger on it when you are home, charge it completely at the voltage recommended by Trojan, let it rest overnite, then measure the voltage.

Another solar option is this renogy portable:
http://www.renogy-store.com/Renogy-100W-12V-Mono-Portable-Solar-Suitcase-p/kit-stcs100d.htm
If you want microwave, that won't be enough tho.
 

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Discussion Starter #19 (Edited)
Some more fridge info--apparently I had interpreted the numbers wrong and the usage is less than I thought. Someone had an Engel rep explain to him that the draw on this fridge is a constant 2.5 amp when it is running. The 0.7-2.5 amp-hour range references how long it runs per hour. By that criterion, my fridge at my setting of 37° interior temp is using:

At 90° ambient: 2.5 x (6/60) x 24 = 6 amps per 24 hours
At 70° ambient: 2.5 x (2/60) x 24 = 2 amps per 24 hours

Except night temps are lower than those daytime highs.

This is consistent with my latest reading: yesterday when we got home after 6 hours' driving, I immediately turned the fridge off and emptied it. Today, the voltage is 12.3.

This suggests to me that my usage has been effectively noise and that the battery is not getting a full charge as Papab suggests.
 

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Just to confuse everyone a bit more - I have a similar solar set up to RD's other than I have 300w of PV panels to his 200w and I have 2 - 12V AGM batteries vs his 2 - 6V flooded golf carts.

I have always kept my house batteries isolated from the starting battery with both an isolator and a mechanical on/off switch just because I don't want my alternator charging my AGM batteries and my PV panels charging my starting battery. The only time I do combine them is while camping and/or driving at nighttime or early in morning when the house batteries are usually down to 12.3v or 12.4v after running the fridge all night. Then I may start the engine up in order to charge up the house batteries from the alternator so as to speed up recharging them but even then I only keep them combined for a short time 15 minutes or so is usually long enough it seems.

My question to you all is - Am I being paranoid about leaving both systems connected together while the engine is running and the sun is shinning? It seems to me the steady 14+v from the alternator is not a particularly good way to charge AGM batteries for any extended length of time and it sort of defeats the idea of the solar charge controller as when they are combined the solar charge controller starts to link indicating the AGM's are fully charged when, in fact, they may not be. The last thing I want to do is have my alternator and solar charge controller fighting with each other while trying to provide the correct charge to each system!
 
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