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Old 06-16-2019, 06:52 AM   #1
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How long to charge batteries from genny?

We are in Deep Creek Lake state park in Maryland at the moment. Got here yesterday (sat) at 9:30am. No hookups.

So right now itís about 8am (Sunday). So almost 24hrs. Been parked here using the house batteries to keep the fridge running. Havenít really used anything else electrical except the occasional LED lights on ceiling. (But did use coffee maker for 2 cups yesterday!)

The fridge is still running strong. Everything still works. Batteries havenít died yet. Iím impressed. Damn near 24hrs. Iím tempted to just continue to let it run without charging up off the generator just to see how long I can actually go with these house batteries. My gauge shows Iím still in the Ďyellowí!! Woohoo

My question is .... how long should the generator run to get full charge on the house batteries? Didnít know if it was a quick 1hr thing, or if it will take several.

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Old 06-16-2019, 07:03 AM   #2
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One hour each day will extend my batteries (2 golf cart) for several days. A full charge will take hours as the charge drops off a lot when I am around 90% of a full charge.


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Old 06-16-2019, 07:12 AM   #3
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there's no exact computation of 'how long'...as there are too many variables - your situation will impact the 'time'...

such as:
- what are the battery levels WHEN the charging starts
- how MUCH amperage(output) is the charger providing during the charging period
- what OTHER items are also being used while the charger is active
- what is the temperature around the battery bank during charging
- what is the condition of the batteries(age, usage, etc)


sometimes we want a specific number, but in the RVing world, that's a rarity because there are just too many differences in situations, weather, usage, etc...

What you can do is a test: Try running the gen for only 30 minutes. When the generator/battery charging is then turned off, let the battery bank sit unused for several minutes, then check their Voltage level. If you see a high 12.8 as a consistent reading, then you are good. If you see a lower number, and it starts dropping over the next few minutes, then you are not, yet.

Some will say that you must charge your battery bank for at least several hours before they could be fully charged, but that can't be necessarily true because every situation is different. It all depends on the levels of the batteries WHEN the charging starts, and the AMOUNT of amperage charging that is occurring during the charging time.

If you have a Magnum ME-RC remote panel for your Magnum Inverter, or a similar setup, you may find that you have a SHORE MAX button, or setting. This tells the BATTERY CHARGER how much Amperage it can make use of while you are charging, while on Shore Power, or running the Generator. The higher the setting, such as 50, means that you are letting the Battery Charger use the MOST of the amps it has access to, while if you set it down to, say 10, you are really pairing back on the amount of amps, giving you more of a 'trickle charge' since the Charger will not be able to provide near as much charging, even with the generator running full bore. You are in control.
(this setting is designed to allow you to pair back on the battery charger while using roof air conditioners and other 120v items without tripping a breaker on the generator, especially during hot conditions, etc, but it's quite useful when you want to make sure that the Battery Charger is getting the MOST out of the generator run time, when little else is needed during that time)
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Old 06-16-2019, 07:31 AM   #4
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I believe you're about to kill your batteries. You should not let your battery get below about 12.0 volts. Looks like they are already down to 11.5 right now.
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Old 06-16-2019, 07:34 AM   #5
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See this.
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Old 06-16-2019, 07:36 AM   #6
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Quote:
Originally Posted by dan-nickie View Post
I believe you're about to kill your batteries. You should not let your battery get below about 12.0 volts. Looks like they are already down to 11.5 right now.


thatís kind of what I wanted to see. If Iím boondocking and Only using the fridge and the occasional led overhead light, how long will the batteries go? Thatís the question Iíd like to figure out. Just so I know roughly how long I have.

Iím assuming itís not bad/damaging to run the batteries all the way down correct? Then just kick on the genny on to charge up ...
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Old 06-16-2019, 07:38 AM   #7
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Turn your generator on now.
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Old 06-16-2019, 07:44 AM   #8
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In my test when the rig and batteries were new, it took about 8 hours to run the batteries down to 12 volts with mostly nothing but the refrigerator running. That's when my AGS starts the generator.
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Old 06-16-2019, 07:46 AM   #9
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What Dan said. 11.5v is darn near dead, something like 20% of charge. But that's based on voltage NOT under load which your battery is now. Still not good to discharge them this low though.

There is no battery used in our vehicles or trailers that can be charged in an hour. Battery voltage is only a valid indicator of state of charge when the battery has been at rest (neither charging nor discharging) for at least 12 hours. It's all about amp hours. Battery charging is dependent on voltage differential. Consequently as battery voltage increases the rate of charge decreases. Smart chargers compensate for this to a degree but you can't put 100 amp hours into a battery with a 10 amp charger in 10 hours. Will take closer to 20.

A true battery junkie will want an amp hour meter. These measure the current flow out/in the battery and compare it to the known capacity of the battery. These will show the true state of the battery allowing you to recharge soon enough to prevent damaging the battery. Not even deep-cycle batteries like to be deep cycled. Here's one that will track true state of charge on two batteries like a house and starting battery.



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Old 06-16-2019, 07:48 AM   #10
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Is it a big deal to run them down? Other than the fact that the fridge will eventually go out?
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Old 06-16-2019, 07:49 AM   #11
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Is it a big deal to run them down? Other than the fact that the fridge will eventually go out?
Yes it's a big deal. You will shorten the life of the batteries If you consistently run down below 50%.
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Old 06-16-2019, 08:16 AM   #12
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Cool thanks for all the quick responses guys. Genny been running since 8 at this point.
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Old 06-16-2019, 08:26 AM   #13
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that’s the problem with some saying that you are ‘killing’ your batteries - Nonsense... batteries are made to be used, and having them fall below the mysterious ‘50%’ level is not going to suddenly and automatically somehow ‘kill’ them... baloney.

It’ s true that manufacturer’s rule of thimb is that discharged CONSISTENTLY below a 50% State Of Charge, on a very frequent basis, could shorten the long life of a battery, but to say that just because it happens you suddenly have problems are over-the-top scare tactics... a Voltage reading, while under load(things are currenty using the battery) is NOT the same as State Of Charge, which is the ‘at rest’ levels of the battery.

If you use a 11.9 to 12.0 general ‘under load’ level rule, then start the battery charging, you’ll be just fine. Now, if you don’t have this type of ‘digital’ readout on your panel - check the battery itself, and see how that correlates with the panel reading - then you’ll know more of how to process what Red, Yellow, and Green really means.

If you happen to let the levels fall below those numbers, it’s not the ‘end of the world’...

our original stock 6v battery bank lasted 5 good years before I recently replaced them - after 90,000 miles and many, many off-grid overnights, and using 11.9 as a general setting for my AGS to start my generator(which triggers it to run after two continuous minutes at that level), and using 11.5 as the LBCO(low battery Inverter cut off), which has happened on several occassions thru the years.
The batteries were still ‘alive’ and useful after all that time.
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Old 06-16-2019, 08:27 AM   #14
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Big deal absolutely! Batteries abused by deep discharges get damaged. You only get so many deep discharges before the batteries are damaged beyond repair.

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Old 06-16-2019, 08:42 AM   #15
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It's funny how some posts go on and on about the correct way to measure batteries etc. But in the middle of that Post it agrees that Using 12.0 as the point to start charging Is a good method.

Why make it so complicated when a guy out camping just wants to know if he should let his battery run down to 0.
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Old 06-16-2019, 08:47 AM   #16
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thatís kind of what I wanted to see. If Iím boondocking and Only using the fridge and the occasional led overhead light, how long will the batteries go? Thatís the question Iíd like to figure out. Just so I know roughly how long I have.

Iím assuming itís not bad/damaging to run the batteries all the way down correct? Then just kick on the genny on to charge up ...
If you let your batteries run all the way down, how are you going to start your generator.
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Old 06-16-2019, 08:54 AM   #17
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All good guys. Again, much appreciate the responses. Iím not a super noob but I was just wondering about he Berk specifically.

I was asking also because Iíve talked to other Berk owners that have told me the house batteries will only last 4-8hrs max just running the fridge by itself. And once I had them last 12 straight hrs. So it got me wondering how long MINE could last.

On THIS trip, theyíve lasted 24hrs. So again, it got me thinking about how long they could theoretically go.

I donít do it very often. And Iím of the mindset that every now and then, running them down and charging them back up isnít bad for them at all. Of course ďabusingĒ them would mean Iím doing it constantly which just doesnít happen. I actually WISH I was running them down constantly and abusing them ..... because that would mean Iím using my RV all the time and boondocking regularly!! Some day
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Old 06-16-2019, 08:56 AM   #18
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How long to charge batteries from genny?

Quote:
Originally Posted by dan-nickie View Post
If you let your batteries run all the way down, how are you going to start your generator.

Good point too. Obviously starting the engine could solve that problem, but yea, you wouldnít want to get to that point if you could help it.

I have my answers.

Iím happy as long as I can boondock from 8p-8am roughly. But knowing I can get 24hrs is even better. I love this RV.
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Old 06-16-2019, 09:01 AM   #19
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Don't let the voltage drop below 12v. I found the flooded set I had needed 2 hours at night and the same in the morning, especially making coffee. I now have AGM and they last longer and charge 2x faster.
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Old 06-16-2019, 09:07 AM   #20
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Don't let the voltage drop below 12v. I found the flooded set I had needed 2 hours at night and the same in the morning, especially making coffee. I now have AGM and they last longer and charge 2x faster.


Thatís what I was thinking too. These wet batteries are old tech. Iím from the motorcycle world its AGM or nuthin!

So maybe I should Ďabuseí these batteries a bit more so I can get my AGM set
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