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Old 09-11-2019, 05:52 PM   #1
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New member with a battery bank issue

Not the greatest intro, I'll admit. I haven't even set up my profile.

Our HR 2002 Vacationer is fairly new to us. We've had it about two years. We're running a four Interstate GC2's in a series/parallel setup with 400 watts of solar (Renogy) and a Zantrex 2000W inverter/charger. Batteries charge to full capacity on either system. They'll hold a charge for months with the house battery disconnect switch off. There's not even a problem leaving the disconnect switch on with no shore power and the solar switched off for a few weeks even with the usual parasitic losses.

But, when on battery only, if I run the microwave to heat a cup of coffee for one minute the indicated battery voltage drops to 10.5V on the monitor panel and the green/yellow/red state of charge indicator goes red. The monitor panel says the microwave is drawing 150A during the 60 seconds that the microwave is running. 150A for one minute seems like a load that the four batteries should take in stride. The batteries do charge back up to green in a short amount of time. The batteries have been equalized about every other month.

I pulled the four fully charged batteries out and tested them individually after letting them sit for four days.. All four read 6.4V on the multimeter and 1.30 when tested for specific gravity. All cells read this except one that reads 0.02 lower that the others, which my chart says is acceptable.

I don't have enough experience or knowledge of deep cycle batteries to decide if these (unknown age, came with the motorhome) are just at the end of their service life or they are acting like deep cycle batteries should (which I highly doubt.

Any help much appreciated.

Bud
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Old 09-11-2019, 06:01 PM   #2
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Do a capacity test.

You have about a 400 AH bank.

The 20 hour rate is 5 amps per 100 AH

Put a 20 amp load on them, without solar or charger, and they should last 20 hours before the voltage drops to 11.6 volts or 10% state of charge.

If they seem to be dropping much faster, they are nearing the end of life.
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Old 09-11-2019, 06:16 PM   #3
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Thanks for the quick reply.

I'll put it all back together as soon as the new paint in the battery bay drys solid and give that a try.
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Old 09-12-2019, 12:11 AM   #4
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https://itstillruns.com/manufacture-...s-8192048.html

That site might help you figure out how old your batteries are.
Microwaves are energy hogs. You can count on them hitting a battery bank hard when using 12 volt
Personally, I fire up the gennie to use the microwave (RV) and on my Lobster Boat (no genset) we wait until we get shore power to use the microwave. The kids don't like that..... Best of luck figuring out your electrical setup.
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Old 09-12-2019, 04:06 AM   #5
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Ditto on using the geny while running the microwave! Consider this: When you convert 12 volts DC to 120 VAC you are drawing 10 times as much current from your batteries as the microwave is drawing from the inverter. That's a lot of current! And that is only if your inverter is 100% efficient (which none of them are). So 15 amps to the oven is going to be more like 160 - 165 amps from the batteries. And for 60 seconds. Imagine if you cranked your starter over for 60 full seconds without giving your battery a rest. The geny is your friend!
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Old 09-12-2019, 04:07 AM   #6
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What about getting a much smaller microwave? If you were pulling 150 amps through your inverter, then I assume your microwave is well over 100 watts. You could pick up a 750 watt microwave that would be much friendlier to your batteries like the ones that are sold to truckers.

Or, what if you ran the microwave at a lower power setting? Not sure, but I suspect that would reduce the draw on your batteries.

For us, I carry a small propane single burner stove which I use for times when we're not plugged in. I hate running the generator, especially in the morning when I'm waking up. All I want is a quite cup of coffee and this lets me do that: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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Old 09-12-2019, 05:30 AM   #7
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The 150 amps is split between both series sets of batteries, so its only 75 amps per 12 volt pair. That's not out of the realm of discharge of 200+ AH batteries.

The Trojan specs for a comparable battery ( T105 ) say you can draw a steady 75 amps for 125 minutes before full discharge.

A minute or 2 at that rate is a drop in the bucket.

Besides who wants to start a generator for a minute of run time ?
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Old 09-12-2019, 07:31 AM   #8
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Id look for a bad or loose /corroded connection in the large battery cables. If u have an infrared it will be hot under load. According to OHMs law power = current x voltage so a bad connection will have a voltage drop across it under heavy current and that lowers the voltage under load and releases power as heat. Under little or no load everything will seem fine.
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Old 09-12-2019, 07:38 AM   #9
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Or theres a possibility your 12v cables are undersized, if they get hot under load you need larger cables. Longer Length of cables also means larger cables for the same load.

As example, My roof A/C draws 150 DC amps and my 4/0 cables get just slightly warm but their about 3/4” diameter and maybe 7ft long max.
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Old 09-12-2019, 08:48 AM   #10
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To get back to the original question, I've got my 1500-watt water heater set up to run from the inverter if needed. It draws about as much as a microwave. We have 780 Ah in our battery bank. The microwave will pull the voltage down to about 11.5v while it runs, pulling about 150 amps. Posting for comparison.
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Old 09-12-2019, 08:57 AM   #11
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Same battery set up in our '02 DSDP. Ran the Advantium to heat food or water for years without problems.
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Old 09-12-2019, 09:10 AM   #12
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Bud,

While the microwave is running off of the batteries compare the voltage at the batteries themselves to the indicated voltage on your panel. You may find a significant difference between the two with the actual battery voltage being higher.
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Old 09-12-2019, 10:11 AM   #13
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Wow! Thank you, all of your input is most helpful.

The battery bank connections and the cables running to the inverter/charger are all 2/0 wire as specified by Zantrex for their unit. The longest runs are the inverter input wiring and those are just under six feet long. From the wire gauge charts, 2/0 should handle anything the system throws at it except a direct short and in that case, it's fused. Everything looks clean with no visible signs of heating up or corrosion at the terminal lugs.

After heating a cup of water in the microwave (before pulling the batteries last weekend) I checked the cables to see if anything was heating up. All were cool to the touch except the cable to the disconnect switch, a run of about two feet. It was warm at the switch end only. I'm not sure why that didn't register at the time. I'll pull that today and check for a loose connection or evidence of the contacts being overheated or dirty.

Like some of you, I think this set up should be able handle 150 amps for more that a minute without depleting the battery bank. ...and I don't like the noise of the gen set in the morning either. What's the point of 400+ amp hours of battery capacity and solar to match, if my wife and I have to flip a coin to see who gets the one hot cup of coffee in the AM. So it's find my problem and fix it, buy new batteries, or rethink my 400A system.

More troubleshooting today. And, again thanks for the help.
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Old 09-12-2019, 10:34 AM   #14
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Quote:
Originally Posted by Jon_C View Post
Id look for a bad or loose /corroded connection in the large battery cables. If u have an infrared it will be hot under load. According to OHMs law power = current x voltage so a bad connection will have a voltage drop across it under heavy current and that lowers the voltage under load and releases power as heat. Under little or no load everything will seem fine.
Start by checking the voltage at the batteries (2people) under heavy load as the above is your problem. I had the same problem plus the generator would not spin fast enough to start in the morning...was going to add a large diode and another battery for the generator. Fortunately I tackled the bad connections first tightening most (including those in the rear run box) and cleaning the ones on the negative battery cables and the shunt to the frame. I still have 0.4V difference under heavy loads (test only as you run the generator when using a heavy load for more than a minute) and now my generator will start in the morning.

PS: I have 880AH of AGMs and 900W of solar and still crank the generator to make coffee in the morning.
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