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Old 02-22-2020, 11:42 AM   #1
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Difference in Charging Between Generator and Shore Power

I spent two weeks boon docking in January in Quartszite and had trouble keeping my batteries charged with my generator. Here are the specs.

2 Trojan T-105 batteries (less than 1 year old and attached to shore power when
not camping.
New Honda 2200EUi generator. (less than 10 hours of use)

WFCO 55A Power Converter (Model 9855)

I have a Dometic CFX65 compressor refrigerator that is supposed to draw 8.2 amps. I choose this fridge because of its supposedly low power draw. This was the main 12v use as well as sometimes charging my phone and tablet.

My only previous boondocking was for no more than 3 days and I usually had to run the generator on Day 2 for 2 hours to bring up the charge.

In Quartszite after about Day 3 I couldn't seem to keep my batteries charged to a sufficient level. I decided to record the battery readings.

After 3 hours on the generator 13.62
Unplugged from generator: 12.85
3 hours after charge 12.65
7 hours after charge 12.29
overnight/next morning 12.32

With this discharge rate I had to run the generator everyday, sometimes once in the morning and then again in the evening if I wanted to turn on any LED lights as well as charge my phone or tablet.

My guess was that the fridge was drawing way more power than the stated specs. I could see it drop down to 12.36 when the compressor kicked on 3 hours after the generator charge. The compressor usually only ran for about 90 seconds every 3 hours during the day. The temperature dropped pretty quick after sunset and I rarely heard it kick on in the evening.

When I got home I decided to run an experiment. I cooled the empty fridge on shore power down to the same temp I had set in Quartszite. In Quartszite nights were in the high 30s low 40s, days between 58 and 70. I couldn't exactly replicate the day time temperature (highs in the mid 50s) but the nights were the same. Also the fridge was empty so it would have to run more to cool. I unplugged from shore power. Here are the numbers:

Fridge cooled, shore power. 13.08
24 hours later 12.72
48 hours later 12.71
72 hours later 12.45

So, after 72 hours following a shore power charge the number was higher than after 24 hours from a generator charge. Also, I was not using EcoThrottle on the generator.

Any ideas?
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Old 02-22-2020, 12:22 PM   #2
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I have lots of ideas and experience in running a DC refrigerator while dry camping. I have two 6V batteries equivalent to yours. I have a DC refrigerator that draws 14A while running and runs nearly continuously. I have 600W of solar that can produce 40A or more of charging directly at the batteries. I have a 3400W generator to boost charge my batteries. I upgraded my WFCO unit with a PD4655. And even with all my charging ability I cannot run the DC refrigerator continuously.

Here’s the problem. First the weak WFCO 55A charger can only produce 10A of charging at the batteries. My WFCO charger never charged above 13.8V. Therefore it’s almost useless to charge 6V batteries with the WFCO charger as it would need to run for 24 hours. Your refrigerator uses 7A. Times 12 hours, that’s 100Ah if the refrigerator ran 50 percent of the time. That’s 10 hours of generator charging per day just for the refrigerator. Then more generator time for other use such as 24Ah for just having the Airstream in use mode. Another 20 to 40Ah for pumps, lights, fans and miscellaneous use. 7A per hour of furnace use. Like I said before, the generator would need to run continuously.

The WFCO converter was designed for continuous use while plugged into shore power. Not for quickly charging with a generator. Upgrading the charging unit with a PD4655 will help because mine at least charges at 14.2V. It still can’t put many Amps to my the batteries. It takes at least five hours of generator charging to get out of bulk charging at 80% charged.

Your problems are the weak WFCO charger, high voltage charging parameters of 6V batteries and a fairly power hungry refrigerator. The problem is not the generator. It’s that the generator would need to run continuously like when on shore power.

With 400W to 600W of solar you might be able to boondock with your refrigerator while being frugal with other power usage.
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Old 02-22-2020, 01:02 PM   #3
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I agree, it is not the generator...red herring...it can supply the same voltage and current as shore power to the charger for battery charging. One of the things that always puzzles me when folks ask these kinds of questions is how they are measuring battery voltage. Voltage is a poor way to measure battery state; the battery needs to be at rest for some time before the voltage has any correlation to state of charge and when comparing, the temp has to be the same also. If you really want a valid experiment you need to use specific gravities, but Air Miles nailed it with his conclusion that you were just not getting enough charging time on the generator.
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Old 02-22-2020, 03:00 PM   #4
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Maybe this a dumb question, (but consider the source).
Does the 9855 have the correct charge profile?
You may need a bit more punch, especially for equalization.

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Old 02-22-2020, 05:25 PM   #5
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Thanks so much for that run-down AirMiles. I'll admit I'm an absolute rank newbie when it comes to electrical stuff. The one thing I think you might have overlooked in my post was that, in my experiment at home, I unplugged from shore power. What confused me was that starting out with 13.62 after shutting of the generator, and 13.08 after disconnecting from shore power, why the refrigerator could barely run a day after being charged with the generator and ran for nearly 3 days after being charged with shore power.

You confirmed what I suspected, that the converter might be part of the the problem. Since solar is not in my immediate future, looks like I need to run the generator longer when boon docking.
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Old 02-22-2020, 05:30 PM   #6
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To measure voltage using your AS monitor, you are supposed to wait at least an hour after disconnecting from shore power, or after shutting down the generator. The voltages you just posted are giving you a false starting point IMO. Most regular batteries [not sure about your 6v pair in series] will read 12.8 volts fully charged at rest IMO.

When a compressornor other alliance kicks on, it is normal for the voltage to drop quite a bit, until the device shuts off.

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Old 02-22-2020, 05:34 PM   #7
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I agree. With your setup you need to run the generator for a lot longer. I find that even with a propane rifigerator we need longer generator runs than I thought at first. Getting to full charge takes a lot of time. That is not so noticed when on shore power. 8 amps sounds like a pretty strong draw. What percentage of the day is ithe fridge actually running?
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Old 02-22-2020, 06:17 PM   #8
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Our Boondocker charges the same on SP as it does when plugged into the Honda 2000i, I believe that's the way it should be.

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Old 02-22-2020, 06:33 PM   #9
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It runs about 45 seconds, sometimes up to 90. It kicks on about every 3 hours during the day. And, because it was cold at night (lower 40s), and the very well-insulated compartment has ventilation from the outside, it doesn't run at night. So, all told the compressor probably only runs about 10 minutes total during 24 hours.
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Old 02-22-2020, 07:07 PM   #10
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For boondocking with a generator you need a converter that can be forced into boost or bulk mode such as the Progressive Dynamics. The WFFCO won’t do this and will require too much generator time to supply sufficient amp hours.
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Old 02-23-2020, 09:13 AM   #11
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Hi

If you are measuring the voltage with any of the "stock" Airstream monitors, they all are only guessing at the battery voltage. Depending on what sort of loads are running, they can easily be off by a couple tenths of a volt in either direction. The only accurate number is from a multimeter on the battery posts. Even that (as noted above) only counts on a battery "at rest". If it's actively charging or discharging, that impacts the voltages you see.

Something like a BMV-712 is a much better way to monitor the state of your batteries.

====

To get full capacity out of your T-105's you have to take them to pretty high voltages *and* do it for quite a few hours. Three hours at 13.x volts simply isn't going to fill them up. If you want to fast charge, you will need to go to Lithiums.

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Old 02-23-2020, 09:19 AM   #12
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So if you are boondocking with a generator are you better off with the standard 12 volt flooded cell or the conversion to the 6 volt batteries? Would the proper converter help the OP’s issue?

I boondock a couple weeks a year and not quite ready to drop 3 grand on lithium and a converter and a monitor.
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Old 02-23-2020, 09:39 AM   #13
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Wire Size

For many years I had problems trying to operate the fridge on our boat on 12V. Long story short, it turns out that the boat manufacturer had not used heavy enough wire from the panel to the fridge. The fridge would quickly drain the batteries down to the low-voltage shut-off. I upgraded that wiring by several sizes and finally the fridge would run reliably on 12V. Yes, I still had to run the onboard genny for an hour morning & night with the fridge running on 110V during the charging (auto switchover). Last season, I augmented the charging with wonderfully quiet solar panels. Have no idea what the wiring size was on my Airstream ('97 34FK), but with the issues you're having, I'd recommend at least taking that into consideration when diagnosing the problem.
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Old 02-23-2020, 09:45 AM   #14
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Quote:
Originally Posted by Bill M. View Post
So if you are boondocking with a generator are you better off with the standard 12 volt flooded cell or the conversion to the 6 volt batteries? Would the proper converter help the OP’s issue?

I boondock a couple weeks a year and not quite ready to drop 3 grand on lithium and a converter and a monitor.
Yes...the 6v batteries being used need a bulk charge rate of at least 14.82v, not available with the OEM unit.

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Old 02-23-2020, 10:01 AM   #15
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Quote:
Originally Posted by Bill M. View Post
So if you are boondocking with a generator are you better off with the standard 12 volt flooded cell or the conversion to the 6 volt batteries? Would the proper converter help the OP’s issue?

I boondock a couple weeks a year and not quite ready to drop 3 grand on lithium and a converter and a monitor.
The 6 volt Trojans will have a total rated capacity of 225 A-h, which is almost triple that of a typical group 27 battery. Even if you have a pair of 12 volt batteries in parallel, the Trojans will outlast them. As noted by others, what is needed for generator charging is a converter that can maintain a solid 20 amp or higher output so that the batteries get charged in a reasonable amount of time.

What we have found is that even adding a couple 100 watt panels and an inexpensive PWM solar controller makes a huge difference to the daily power balance. On a moderately sunny day, it consistently puts several amps back into the battery on top of the steady (propane) fridge load. That setup can be had for as little as $350 these days.

In terms of monitoring the battery, get a shunt type current sensor like the Victron or the Tri-metric so that you can see the total amount of charge gain and loss over time, as well as the instantaneous current draw. It can help you figure out parasitic draws also.
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Old 02-23-2020, 10:19 AM   #16
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Quote:
Originally Posted by Bill M. View Post
So if you are boondocking with a generator are you better off with the standard 12 volt flooded cell or the conversion to the 6 volt batteries? Would the proper converter help the OP’s issue?

I boondock a couple weeks a year and not quite ready to drop 3 grand on lithium and a converter and a monitor.
You are better off with true-deep-cycle batteries. These could be 6V or 12V, but not 12V RV/Marine batteries. The cheapest true-deep-cycle batteries are 6V golf cart batteries such as the Duracell EGC2 battery from Sam's Club or Costco, which also have both automobile posts and stud posts to work with the stock Airstream battery cables. https://www.samsclub.com/p/duracell-...c2/prod3590220 Read this article: https://marinehowto.com/what-is-a-deep-cycle-battery/

The reason true-deep-cycle batteries are the best "inexpensive" battery is that they are designed for 80% depth of discharge because of their solid lead antimony plates. https://www.solar-electric.com/learn...le%20Batteries A pair of Duracell EGC2 or Trojan T105 batteries will provide as many usable amphours as a pair of Battleborn Lithiums when used with stock Airstream loads under 10A.

Sure, the Lithiums have many advantages over these 6V golf cart batteries but at a significantly higher cost. Some advantages of Lithiums include: no maintenance, faster charging, provide full amphours even at significantly higher discharge rates, longer lifecycle. But the ability to get 200AH of usable storage with a fairly long life for $250 makes 6V golf cart batteries very attractive. Even Battleborn uses the Duracell EGC2 battery for the comparison of their battery to lead-acid batteries: https://battlebornbatteries.com/comp...teries-series/

My pair of Duracell EGC2 6V golf cart batteries provide all the power I need each day to boondock with the addition of at least 400W of solar.

P.S. A proper charge profile is needed to charge 6V golf cart batteries with shore power or a generator. The stock WFCO charger is very inadequate. A Progressive Dynamics or Boondocker true multistage charger upgrade is needed or, better yet, a solar array with a programmable solar controller such as a Victron 100/30 (400W) or 100/50 (600W).
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Old 02-23-2020, 10:52 AM   #17
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Two more possibilities

I love a good detective story and this one has all the right ingredients!

Two points that came up as I read the thread...

1. There are multiple ways to hook a generator to a trailer for battery charging. I didn’t see anywhere that clearly stated that the Honda is connected through the shore power connection (and therefore through the converter) and not through the 12 volt output of the generator using alligator clips to the battery bank. The standard EU2200i doesn’t have a 30 amp receptacle (the companion model does) but it does have a 12 volt outlet that Honda says is there for “battery charging”. I’m assuming the shore power connection was used since the original post mentions the converter but just to be sure...

2. The original post said it was in the 30s at night at Quartzite and maybe 70 in the day. How were you keeping the trailer warm? While the furnace gets its heat from propane, it moves the warm air with a 12 volt powered blower. When we’ve dry camped with temps in the 30s, that blower gobbles battery juice.
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Old 02-23-2020, 11:38 AM   #18
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^
I thought the same...then this popped out on my second read.
SNIP>>>"After 3 hours on the generator 13.62
Unplugged from generator: 12.85
3 hours after charge 12.65
7 hours after charge 12.29
overnight/next morning 12.32"

I read unplugged as shore power cord.....rong?

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Old 02-23-2020, 12:57 PM   #19
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. What confused me was that starting out with 13.62 after shutting of the generator, and 13.08 after disconnecting from shore power,
Another thing that SHOULD have been mentioned is how long it was charged on shore power. This part that caused the OP confusion could be that the batteries were only actually charged enough to get out of bulk mode when they were charged on shore power not on gen.
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Old 02-23-2020, 02:10 PM   #20
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External Charger

What about charging the batteries with an external 15 amp or more charger off of the A/C plug on the generator, not thru the trailer system?
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