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Old 11-28-2016, 01:06 AM   #1
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6v? 12v? Ah? Real World Test

Who wants to get their geek on for real world data and have a discussion?

I switched from dual 12v batteries to dual 6v batteries and decided to do a test and see how they preformed in the real world. I plan to do more tests for the batteries, power draw and solar charge. Any suggestions for test, thoughts/ideas please post them.

Question, if I have two 6v batteries at 235ah each wired in series I should actually have ~117.5 usable amp hours before my batteries hit 50% --- Am I correct?

Setup:
Batteries
Attempt 1: Two Everstart 122ah wired in parallel (Note: These batteries are rated 122ah at 1 amp)

Attempt 2: Two Duracell 235ah 6v wired in series.

Power Draw: 3way fridge in the a-frame T21DMHW camper.

Battery Monitor - bayite DC 6.5-100V 0-100A LCD Display Digital Current Voltage Power Energy Meter Multimeter Ammeter Voltmeter with 100A Current Shunt

Calculations:
Attempt 1 should give ~4.8 hours run time before my batteries hit 50%. Assumed 5amp draw in calculations

Attempt 2 should give me 23.4 hour run time before batteries are at 50% Assume 5amp draw.
I did not verify if my batteries were 100% charged at the beginning of the test. I believe the 12v setup was charged to 100% but the 6v batteries were not.

I was curious if the drain on the batteries would be linear. If they were linear I would be able to calculate actual battery drain over time for x amps. If you look at the graph jpg provided I logged data over 4-6 hours. It was random logging as I was working on other projects. The blue line is attempt #1 using the two 12v batteries rated at 122ah @1a. The orange line is attempt #2 using two 6v batteries rated at 235ah.

I logged Voltage, Amp Draw, Watt Draw and Watt Hours over time. The voltage increase at the end of the graph is after I shut off the fridge. I checked it a little later and the voltage increase a little more.

What I found interesting was the amp draw and watt draw. It appears the 6v batteries has less amp/watt draw compared to the 12v setup. Why?

I am thinking of attempting this over a longer period of time and charging all batteries to 100% before the tests begin. Any test ideas?
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Old 11-28-2016, 09:21 PM   #2
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I would check the specs on your 3 way fridge - or measure the actual current draw. The Dometic 4223 fridge installed in the "low wall" A-frames has a 160 watt DC coil - which is 13.3 amps nominal at 12 volts. I'd be very surprised to see less than 10 amps draw by your fridge when on DC. FWIW, the AC coil is also 160 watts on the 4223. The fridge does cool very well while being towed thanks to the powerful DC coil.

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Old 11-30-2016, 12:04 AM   #3
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I would check the specs on your 3 way fridge - or measure the actual current draw. The Dometic 4223 fridge installed in the "low wall" A-frames has a 160 watt DC coil - which is 13.3 amps nominal at 12 volts. I'd be very surprised to see less than 10 amps draw by your fridge when on DC. FWIW, the AC coil is also 160 watts on the 4223. The fridge does cool very well while being towed thanks to the powerful DC coil.

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2008 Hyundai Entourage minivan
I have an RM2354 fridge. Only specs I found claims 12.5amps. Assuming that is at max. I will buy a ammeter and find out for sure. I did some rough math and came out to ~10 amps per hour which is similar to what you are stating.

I started out at 12.36v and ended at 12.070 under a load. Took 5.75 hours to drain the batteries ~25%. 25% of 230ah = 57.5ah used over 5.75 hours = 10 amp draw.

Now I need to investigate why my meter is possibly showing 50% of actual amps.
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Old 11-30-2016, 12:50 AM   #4
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Question, if I have two 6v batteries at 235ah each wired in series I should actually have ~117.5 usable amp hours before my batteries hit 50% --- Am I correct?
YES

Suggest that you fully charge and equalize the batteries before testing.


FYI, I have found halogen headlight bulbs to be very accurate in their ratings. I use them as test standards for 12 volt work. Here is an example of a 60 watt rated H4 bulb:



11.99 volts times 5.04 amps = 60.43 watts, so certainly close enough for any practical testing. That gives me faith in my meter readings. Just a reminder that you want to take any voltage readings as close as possible to the actual load (not at the batteries) to account for voltage drop in the wiring. That may account for difference in draw that you see.

The current (amps) can be taken anywhere in the circuit, since it is the same at any point.
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Old 11-30-2016, 12:56 AM   #5
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After posting the above I contemplated if the wiring is correct on the shunt. I just checked it and the camper negative was on the battery terminal and not the shunt. Quickly re-wired and turned the fridge on....13.5amps initial draw

I will repeat my attempts and post up. This weekend I will be off grid camping and plan to log what I use and how long. Shall see... I do know I will add two more 6v batteries before I go lol.
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Old 11-30-2016, 01:03 AM   #6
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FYI, I have found halogen headlight bulbs to be very accurate in their ratings. I use them as test standards for 12 volt work. Here is an example of a 60 watt rated H4 bulb:



11.99 volts times 5.04 amps = 60.43 watts, so certainly close enough for any practical testing. Just a reminder that you want to take any voltage readings as close as possible to the actual load to account for voltage drop in the wiring. The current (amps) can be taken anywhere in the circuit, since it is the same at any point.
Thanks for the H4 suggestion. I would be curious if the voltage drops between my meter and the shunt. Also, is that meter actually accurate. Now I have a way to get close enough... thank you.

I probably should start a post of my entire build once I figure out these minor details
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Old 11-30-2016, 06:12 AM   #7
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I have an RM2354 fridge. Only specs I found claims 12.5amps. Assuming that is at max.
I believe you will find that the DC cooling of the fridge is unregulated, and it will draw 12.5 amps (nominal) continuously. There is no easy (cheap) reliable method to switch 12.5 amps DC, so it just stays on. The actual current is going to vary with your DC voltage - it's just a fixed resistance coil in your fridge.

AC is easy to regulate with a triac (what your light dimmers use in your house). And you are only looking at 1.5 amps or less on 120V AC.

That said, 12.5 amps is a good load check for a deep cycle battery.

Fred W
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Old 11-30-2016, 06:40 AM   #8
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The reason many folks go to 6V golf cart batts is due to their deep cycle construction.

The plates are usually thicker as the spacing is wider than the thinner 12v plates in the same case size..

This should translate to a longer operating life , IF the weight is the same.

Its the lead weight that counts , as much as the construction style.

For house service start batts are totally unsuitable , and the multi style is also not a great choice in LA, although might work with AGM.
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Old 11-30-2016, 07:30 AM   #9
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For house service start batts are totally unsuitable , and the multi style is also not a great choice in LA, although might work with AGM.
Not necessarily. If one only utilizes full hook up campgrounds and never camp self contained "start" batteries are just fine. Used them for many years in a motorhome without any issues. No special maintenance is required and they're also a money saver. Just a thought.
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Old 11-30-2016, 08:42 AM   #10
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Think you have a math error. Two 235AHr batteries in series should provide approx 117AHrs to about 50% capacity while 2 122AHr batteries in parallel should provide approx 122AHr to about 50%. So the tow battery configurations should be fairly close to equal. The advantage in the two 6V gold cart batteries is you can theoretically draw them down a little lower without damaging them.
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Old 11-30-2016, 10:21 AM   #11
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The 122ah batteries states on the battery 122ah @ 1amp. I guess they are not tested at the 20 hours like the new batteries are.
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Old 11-30-2016, 10:25 AM   #12
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Originally Posted by pgandw View Post
I believe you will find that the DC cooling of the fridge is unregulated, and it will draw 12.5 amps (nominal) continuously. There is no easy (cheap) reliable method to switch 12.5 amps DC, so it just stays on. The actual current is going to vary with your DC voltage - it's just a fixed resistance coil in your fridge.

AC is easy to regulate with a triac (what your light dimmers use in your house). And you are only looking at 1.5 amps or less on 120V AC.


Fred W
Your saying with DC it just draws the power as it never switches off vs AC is able to regulate?
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Old 11-30-2016, 02:25 PM   #13
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Your saying with DC it just draws the power as it never switches off vs AC is able to regulate?
The DC mode of the fridge is only intended for use while towing - the alternator in the tow vehicle is supposed to be providing the power. When camping, the fridge has thermostatic control in AC or propane mode.

As I said, regulating 13 amps of DC is not particularly easy or reliable with cheap components. A heavy duty relay is one option - but even then avoiding burned contacts over the long term is an issue. A transistor or other solid state device would need a very substantial heat sink (and waste a lot of energy) for partially "on". Or you add in an oscillator of some kind to generate AC from DC, and then regulate the power with cheap AC devices.

It's much easier, cheaper, and reasonably effective to put in a smaller (wattage) DC coil and let it run continuously. That's why DC mode is NOT part of the "Auto" mode switching, and has it's own on/off switch. And no, the fridge thermostat does not do anything in DC mode.

Larger RV fridges do not even have a DC mode.

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Old 12-01-2016, 07:30 AM   #14
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"The 122ah batteries states on the battery 122ah @ 1amp. I guess they are not tested at the 20 hours like the new batteries are.

Its a con by the batt mfg, every batt will have a higher rating with a 1 amp drawdown.

ONLY the 20 hour rating is used to rate deep cycle batts for the past 40 -50 years

Another batt mfg con is the MCA (marine cold start ) instead of CCA rating which comes out higher as it is done at a higher temperature.
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Old 12-01-2016, 10:20 AM   #15
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I'd suspect that your 12V batteries actually have a 20 hour rating of less than 100AH. As pointed out in previous replies, rating a battery with a 1A draw nicely inflates the AH rating. If you multiply the 12V battery's "reserve capacity in minutes" by .4167, you'll get the actual AH capacity at a 25A draw. If you're drawing two batteries in parallel down to 50% of charge, this is also a bit smaller than the number of amps you'll have available to run your fridge. There's more available for the fridge because as the rate of drain on a battery decreases, the usable capacity of the battery increases. It's also why the AH rating test that's used for the battery's advertised capacity is done at 1A.

Reserve capacity for a golf cart battery is determined using a 75A load. (There's also a different multiplier to convert to AH capacity.) The significantly lower drain rate for normal operation of an RV, with the exception of an inverter, should contribute to a higher effective capacity in RV service. Golf cart battery life in cycles is also usually provided for two different percent of capacity discharge cycles, 50% and 80%. The number of 80% discharge cycles for a golf cart abttery is usually close to the 50% cycle rating of 12V hybrid (marine) deep cycle batteries.

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Old 12-01-2016, 02:05 PM   #16
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Not to hijack the thread... Are you changing out the power supply/charger as well? The WF-9800 series that came with my rig will not charge 2-6v batteries to their full potential. You may need to verify the 2-6v battery voltage requirements during Absorption/Bulk/Float modes.
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Old 12-02-2016, 01:07 AM   #17
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Correct on the battery advertising 122ah when its really much lower.
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Old 12-02-2016, 01:12 AM   #18
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Not to hijack the thread... Are you changing out the power supply/charger as well? The WF-9800 series that came with my rig will not charge 2-6v batteries to their full potential. You may need to verify the 2-6v battery voltage requirements during Absorption/Bulk/Float modes.

Do you mean converter?

I added two more 6v batteires yesterday and upgraded battery chargers. Im now using a ctek 25000 and four 6v batters wired in series and parallel. My other battery chargers didnt like large batteries. The ctek 25000 can do up to 500ah. The batteries were at 12.4 and now 13.22.
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Old 12-02-2016, 09:02 PM   #19
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13.22V is the converter float voltage. Batteries need to be disconnected for a few hours for the voltage to be an accurate measure of battery charge. Fully charged batteries will read 12.7V.

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Old 12-03-2016, 08:22 AM   #20
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Do you mean converter?

I added two more 6v batteires yesterday and upgraded battery chargers. Im now using a ctek 25000 and four 6v batters wired in series and parallel. My other battery chargers didnt like large batteries. The ctek 25000 can do up to 500ah. The batteries were at 12.4 and now 13.22.
Sorry, Yes I did mean the converter/charger.
The CTEK has some nice specs.

One other question? Are you performing your tests with similar ambient temperatures?

re: "I am thinking of attempting this over a longer period of time and charging all batteries to 100% before the tests begin. Any test ideas? "
Additional testing ideas - Although your batteries are different, Lifeline Batteries has some real world testing on their batteries. You may wish to skim thru the document for ideas. Graphs start at page 30.
http://lifelinebatteries.com/wp-cont.../12/manual.pdf
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