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Old 02-26-2020, 07:08 AM   #1
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Draw That Battery Down To 10.5 Volts!

I attended an RV show this past weekend that included several technical seminars. While I attended several seminars, I really looked forward to the 60 minute discussion around lead-acid batteries, both flooded and AGM.

The RV battery expert instructing the seminar made reference several times to drawing lead-acid batteries down to 10.5V. When I pointed out to him that all of the DoD charts that I have seen indicate that a flooded battery should only be drawn down to 12.0 to 12.1 volts (50%) or you will damage the battery. He responded by saying that you can actually draw a flooded battery down to 10.5V with no problems. He stated this "fact" several times during the seminar and I found this to be bizarre.

So, has anyone else heard that you can draw a flooded battery down to 10.5V?
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Old 02-26-2020, 07:21 AM   #2
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I attended an RV show this past weekend that included several technical seminars. While I attended several seminars, I really looked forward to the 60 minute discussion around lead-acid batteries, both flooded and AGM.

The RV battery expert instructing the seminar made reference several times to drawing lead-acid batteries down to 10.5V. When I pointed out to him that all of the DoD charts that I have seen indicate that a flooded battery should only be drawn down to 12.0 to 12.1 volts (50%) or you will damage the battery. He responded by saying that you can actually draw a flooded battery down to 10.5V with no problems. He stated this "fact" several times during the seminar and I found this to be bizarre.

So, has anyone else heard that you can draw a flooded battery down to 10.5V?
Watching this as I agree with you about the 12 voltage.
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Old 02-26-2020, 07:31 AM   #3
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10.5 volts is a recipe for early replacement.

Every source I have says no further than 50% of bank capacity (or 12.1 volts).

The graph shows battery cycles (number of times it can be discharged and recharged) as a function of percent of discharge with 10% the highest and 100% the lowest number of discharge/recharge cycles.


As you can see, it drops off rapidly after 50% (about 12.1 volts).
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Old 02-26-2020, 07:48 AM   #4
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I abide with charts/graphs Herk attached. Makes me wonder how the tech seminar guy got the 'expert' moniker. Expert in battery sales maybe??
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Old 02-26-2020, 08:20 AM   #5
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There's a difference between the voltage when you're pulling on the battery and the voltage the battery will recover to once the load is gone.

If you pull a lead acid battery down to 10.5 VDC under load, turn off all load, and wait an hour, you might just see the magic 12.0 to 12.1 volts.
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Old 02-26-2020, 08:32 AM   #6
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I check my battery voltage under light load. IE: there is always something on in the trailer unless the battery switch is off.
I try to limit my discharge to 12v as many have mentioned.

It always seems the "first half of charge" lasts longer than the remaining.
This makes me think 12v under load is already below 50%

I've found the "experts" at some seminars such as you find at RV shows and State Fairs sometimes are blowing smoke to make themselves sound smart and to sell product.
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Old 02-26-2020, 08:40 AM   #7
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I check my battery voltage under light load. IE: there is always something on in the trailer unless the battery switch is off.
I try to limit my discharge to 12v as many have mentioned.

It always seems the "first half of charge" lasts longer than the remaining.
This makes me think 12v under load is already below 50%

I've found the "experts" at some seminars such as you find at RV shows and State Fairs sometimes are blowing smoke to make themselves sound smart and to sell product.
"To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid."

https://batteryuniversity.com/learn/...tate_of_charge
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Old 02-26-2020, 08:48 AM   #8
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I know the conventional wisdom (especially on this forum) is never draw a battery down that low but real life experiences of using lead acid batteries for 50+ years in many different applications say differently, especially in the last 20 years.

I'll leave it at that.
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Old 02-26-2020, 08:50 AM   #9
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Originally Posted by johnbryanpeters View Post
"To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid."

https://batteryuniversity.com/learn/...tate_of_charge

That's fine in a lab but I'm out in the woods and it's dark and I'm laying in bed watching a Tom Selleck movie on battery power.
It's not real convenient to turn everything off and wait 24 hours to check battery voltage.
So I take a look at my voltage under light load and decide if it's gonna be quiet in the morning or whether I'm gonna be running the genny.
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Old 02-26-2020, 09:34 AM   #10
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Here is an excellent article on lead acid batteries.

https://www.progressivedyn.com/service/battery-basics/

As stated prior, battery voltage drawdown has a direct relationship to battery longevity. That is, the number of times it can be charged and discharged before it is used up and must be replaced.

Only you can decide whether discharging to a minimum of 20% capacity remaining and the resultant reduction in battery cycles is "worth it" to you.

Most everyone agrees that discharging below that is to be avoided.

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Old 02-26-2020, 09:55 AM   #11
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Totally agree with KyDan. I would bet no one camps overnight on battery power, then wakes up, disconnects the batteries to remove any load. After doing that, wait 24 hours for things to stabilize, then check the voltage to see if they need charging. Not too likely IMHO! Most people would look at the volt gauge or battery monitor, see it down to below about 12.1 with just the light load of propane sniffers, fridge boards ect and just charge up the battery bank. Jay.
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Old 02-26-2020, 10:40 AM   #12
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Drawing a battery down to levels below 50% may reduce life and cycles but how many times is really important.
If you draw your battery down even 10 times a year you may only reduce the life by a small amount. Think about it. 10 x5 years is only 50 low cycles in the whole life.
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Old 02-26-2020, 11:10 AM   #13
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I know the conventional wisdom (especially on this forum) is never draw a battery down that low but real life experiences of using lead acid batteries for 50+ years in many different applications say differently, especially in the last 20 years.

I'll leave it at that.
Something always overlooked in these conversations is how long the battery remains at it's low state of charge.

If it's run down then recharged ASAP, that's entirely different than running the battery(s) down on a weekend of boondocking, driving home and parking the RV for a week before recharging.

I wonder how many people with golf carts worry about the "50% Rule"? Especially golf courses.
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Old 02-26-2020, 12:03 PM   #14
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I know the conventional wisdom (especially on this forum) is never draw a battery down that low but real life experiences of using lead acid batteries for 50+ years in many different applications say differently, especially in the last 20 years.

I'll leave it at that.
X2.
But I do enjoy following all the experts in the forum and the detailed analysis of amps, volts, charts, graphs, and 27 8x10 color glossy pictures with circles and arrows and a paragraph on the back
Then I go out boondocking and 'abuse' my batteries horribly on a daily basis. And yes, after about 5 years, I have to replace them due to said abuse......
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Old 02-26-2020, 12:29 PM   #15
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X2.
But I do enjoy following all the experts in the forum and the detailed analysis of amps, volts, charts, graphs, and 27 8x10 color glossy pictures with circles and arrows and a paragraph on the back
Then I go out boondocking and 'abuse' my batteries horribly on a daily basis. And yes, after about 5 years, I have to replace them due to said abuse......
Ad those who "follow the rules" and baby their batteries may get 6 years out of their lead acid batteries.
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Old 02-26-2020, 01:34 PM   #16
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this is the exact reason I installed battery monitor so it tellls you exact state at any instant without having to wait or overshooting on the low side.
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Old 02-26-2020, 01:45 PM   #17
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X2.
But I do enjoy following all the experts in the forum and the detailed analysis of amps, volts, charts, graphs, and 27 8x10 color glossy pictures with circles and arrows and a paragraph on the back
Then I go out boondocking and 'abuse' my batteries horribly on a daily basis. And yes, after about 5 years, I have to replace them due to said abuse......
You can get anything you want at Alice's restaurant.
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Old 02-26-2020, 01:56 PM   #18
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You can get anything you want at Alice's restaurant.
And there it is . I guess that dates us......
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Old 02-26-2020, 03:39 PM   #19
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Bottom line is I don't really give a damn, I just go about my life everyday and not let trivial things like batteries consume my time. Diesel engine is another story, maintain the hell out of that, big price difference in cost for 4 batteries than shorter life of engine.
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Old 02-26-2020, 03:40 PM   #20
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You can get anything you want at Alice's restaurant.
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And there it is . I guess that dates us......
I always liked that Arlo Guthrie fellow!...
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