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Lead acid battery minimum discharge level

HANKMARS

Jul 28, 2019
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I have read many recommendations that a 12 V lead acid battery should not be discharged to a voltage lower than 10.5 V. Why is this. Does it have to do with the acidity of the electrolyte which may determine the temperature at which the electrolyte freezes? Or at lower voltage levels does the power demand increase current flow to a harmful level? My personal emergency lighting system is driven directly from the battery bank, bypassing the inverters. Although noticeably dimmer, the emergency lights will operate at least down to the minimum LED forward breakdown voltage of 6 V. It is my desire to keep my storage batteries healthy and happy.
 

crutschow

May 7, 2021
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It is my desire to keep my storage batteries healthy and happy.
Discharging them to 10.5V (or below) won't.

Discharging standard lead-acid batteries to a low level can damage the plates due to shedding of lead sulfate from the plates.
Thus, for best life, it it recommended that standard Pb-acid batteries be discharged to no more than 50% of it's capacity, which is about 12V for a nominal 12.6V battery.

Deep cycle and/or glass-mat batteries are designed to reduce the damage from deep cycling and can be discharged to about 20% of their capacity (about 11.6V) with minimal damage.

Discharging to 10.5V is very near 0% capacity, and will likely damage either type of battery.
 
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HANKMARS

Jul 28, 2019
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Discharging them to 10.5V (or below) won't.

Discharging standard lead-acid batteries to a low level can damage the plates due to shedding of lead sulfate from the plates.
Thus, for best life, it it recommended that standard Pb-acid batteries be discharged to no more than 50% of it's capacity, which is about 12V for a nominal 12.6V battery.

Deep cycle and/or glass-mat batteries are designed to reduce the damage from deep cycling and can be discharged to about 20% of their capacity (about 11.6V) with minimal damage.

Discharging to 10.5V is very near 0% capacity and can damage either type of battery.
Wow. Good to know. Thanks. I'm going from memory here on a WAG but I'm thinking that the conversion on the face of the lead plates is a result of the charging process and during discharge, that lead sulfate is converted back into solution with the electrolyte. So I can see how shedding could strip away some of the plate material therefore shortening battery life. Also I suspect shed material would collect on bottom of battery case, perhaps to a thickness that would contact the bottom of the plates resulting in a shorting effect.
 

HANKMARS

Jul 28, 2019
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"As the story goes" if a typical 12 volt car battery is set down on bare concrete, it will discharge in a short time, a matter of a day or two. If the battery is set down upon plain dirt, it will remain unaffected. I can not recall a specific instance of these events but am sure I have heard these explanations. Sound familiar or have you even heard these notions? Perhaps BU.com may mention this in the dos and don'ts article. I will be readings those articles a little later, Thanks.
 

crutschow

May 7, 2021
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"As the story goes" if a typical 12 volt car battery is set down on bare concrete, it will discharge in a short time, a matter of a day or two.
See no reason for that.
There has to be a current path between the two battery terminals to discharge it, and just setting it on concrete wouldn't do that.

Sounds like an "old wives tale".
 

Kiwi

Jan 28, 2013
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I never put batteries straight on to concrete. This has nothing to do with the "old wives tale" about them discharging, but because any acid on the bottom of the battery will damage the concrete.
 

Bluejets

Oct 5, 2014
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I never put batteries straight on to concrete. This has nothing to do with the "old wives tale" about them discharging, but because any acid on the bottom of the battery will damage the concrete.

Saw this on a string of 40 series 12v batteries used on one of the hospital ups systems spilled out of the provided racks in a specially ventilated containment room and into the generator hallway for more capacity.
Reminding of the dangers of the release of hydrogen gas in and around possible spark source didn"t seem to phase anyone, concrete all white around the batteries and all sitting there humming, waiting............one day for sure.
 

HANKMARS

Jul 28, 2019
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Discharging them to 10.5V (or below) won't.

Discharging standard lead-acid batteries to a low level can damage the plates due to shedding of lead sulfate from the plates.
Thus, for best life, it it recommended that standard Pb-acid batteries be discharged to no more than 50% of it's capacity, which is about 12V for a nominal 12.6V battery.

Deep cycle and/or glass-mat batteries are designed to reduce the damage from deep cycling and can be discharged to about 20% of their capacity (about 11.6V) with minimal damage.

Discharging to 10.5V is very near 0% capacity, and will likely damage either type of battery.
I will be heading to Battery University searching for battery capacity info, of which I need a better understanding. Let me know if you have any recommended reading on the subject. Thanks.
 

crutschow

May 7, 2021
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This has nothing to do with the "old wives tale" about them discharging, but because any acid on the bottom of the battery will damage the concrete.
That shouldn't be a problem with most modern sealed (zero-maintenance) batteries.
 

HANKMARS

Jul 28, 2019
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Saw this on a string of 40 series 12v batteries used on one of the hospital ups systems spilled out of the provided racks in a specially ventilated containment room and into the generator hallway for more capacity.
Reminding of the dangers of the release of hydrogen gas in and around possible spark source didn"t seem to phase anyone, concrete all white around the batteries and all sitting there humming, waiting............one day for sure.
If you had a mind to, you most likely could get their safety officer dismissed, possible reward?, doubt it. But civil men don't first do that sort of thing. No! We bang our heads on the wall trying to get the attention of somebody that donates a fecal nug, or better yet, someone in authority to make a work order and a competent crew to whom to give the order. I've often knocked myself unconscious with this method and have resolved to not dabble in error correction unless matter of life and death, and then only upon further consideration. Nope! If I can't get the Captain's attention because he is too busy drilling holes in the bottom of the boat, so as to drain it more rapidly, my time will be better spent searching for a viable flotation device. In earnest, I would probably be to a point of annoying trying to rectify the evidential presence of an explosive gas underneath a couple hundred living beings. Contrary to crutschow's opinion, I contend there is significant amounts of sulfuric acid and or hydrogen fumes released from zero-maintenance batteries to corrode the face of the concrete. Zero-maintenance considerably slow then flooded lead acid cells, which I remember being used in the lab's UPS. Nontheless, It is my understanding that lead acid cells demand venting. Consider the temp at which electrolyte vaporizes and at what temp those fumes condense to liquid, and you are virtually sprinkling droplets of sulfuric acid on all adjacent surfaces. Bluejets, if you can peruse that area again, look for a forced air vent system. Wouldn't take much to rinse away such a slow drip of fumes and screw the concrete. That's why we use crete. It's durable. I suggest making a holder to hold litmus test strip. Strip should be 3/4" to !" above floor. If no acidic indication on strip in 24 hours, rest easy, until vent system shuts off. If litmus strip turns high acid content before you finish setting it down, run away like hell in a hysterical manner screaming madly, or tell someone with a vested interest about the possible consequences of the situation.
 

HANKMARS

Jul 28, 2019
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That shouldn't be a problem with most modern sealed (zero-maintenance) batteries.
Sealed, yet vented. I'll guess that there is obtainable evidence to indicate acidic fumes surrounding a zero-maintenace battery. Test would be performed in non-ventilated enclosure for a yet undetermined time, but WTHM, (What The Hey, Mo?) Is testing worth the time to further scientific knowledge? It's a possibility.
 

HANKMARS

Jul 28, 2019
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Discharging them to 10.5V (or below) won't.

Discharging standard lead-acid batteries to a low level can damage the plates due to shedding of lead sulfate from the plates.
Thus, for best life, it it recommended that standard Pb-acid batteries be discharged to no more than 50% of it's capacity, which is about 12V for a nominal 12.6V battery.

Deep cycle and/or glass-mat batteries are designed to reduce the damage from deep cycling and can be discharged to about 20% of their capacity (about 11.6V) with minimal damage.

Discharging to 10.5V is very near 0% capacity, and will likely damage either type of battery.
In reading at BattU, I was quite surprised to find the proper charging procedure for the various lead acid battery types. Quite honestly, my major relationships with lead acid cells is strictly automotive. Somebody left car lights on? Jumper battery or recharge and start car. Couldn't be simpler. Right? Wellllll. I have only recently become a solarelectric dependent being and am hoping to avoid costly errors.
 
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