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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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