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Author Topic: AGM Deep Discharge Battery. What voltage needs recharge?  (Read 609 times)

N1AUP

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AGM Deep Discharge Battery. What voltage needs recharge?
« on: August 22, 2021, 10:27:02 AM »

I have an AGM class 31 battery running my shack.  I have a voltage monitor on the battery. 

Right now, having run for an hour, it's at 12.5 volts. 

When do I recharge or stop using it?  I normally keep it floating on an A&A Engineering battery charger / maintainer.

Thanks
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W7CXC

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #1 on: August 22, 2021, 10:36:43 AM »

12.05 is a charge state of 50%, 11.81 is 30% charge and is the point where you should stop using. You WILL get other opinions of course but the above is good info. In a real emergency you can go below the 30% but it is not good for the battery and will START to shorten life.  David
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K1VSK

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #2 on: August 22, 2021, 11:23:45 AM »

It all depends on what type AGM 31 battery. If it’s the typical start or ‘dual purpose’ use battery type, it isn’t designed nor intended to be deep cycled. Only a true deep cycle battery will survive in this use.
Further, a deep cycle battery should not be discharged beyond 50% of its capacity. Voltage isn’t a good way of determining available capacity so there is no valid answer to your question.

Having said that, in the absence of a battery monitor, don’t go below approx 12.3v to be safe. Remember - batteries don’t generally die - they are murdered.
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W7CXC

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #3 on: August 22, 2021, 12:45:41 PM »

AUP... not sure where the other person responding got his info. Mine comes from "East Penn" the battery mfg for all NAPA stores nation wide.

You have stated that you have a group 31 absorbed glass mat deep discharge battery. The major difference in a deep cycle battery and a "starting" battery is the number of and thickness of the lead plates. (hence the surface area). a starting battery has a large number of relatively thin plates to provide high peak current for starting. A deep cycle battery has fewer but thicker plates that favor deeper discharge without damage at the expense of peak amps. A dual use battery is a compromise between the two. A dual use battery will stand up to marginally deeper discharge without damage than a starting battery but not as well as a true deep cycle battery. (Golf cart type).

It is unclear why voltage is not a good indication of charge state. Perhaps something has changed. I would suggest you spend some time doing your own research on line at battery MFG sites and not one of the several dozen you tube sites that are of course all knowing.

When it comes time to replace your group 31 look into two golf cart 6v batteries in series.

Regards
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AC7CW

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #4 on: August 22, 2021, 01:55:50 PM »

https://batteryuniversity.com/ Great source of info. I didn't check to see what it says about your battery but you can probably move ahead nicely with what you learn there...
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Novice 1958, 20WPM Extra now... (and get off my lawn)

K6AER

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #5 on: August 22, 2021, 02:22:30 PM »

My station is 100% solar. I have eight group 31 AGM batteries to run the station and the air conditioner. My MPPT charger will float charge the batteries at 13.5 volts. Normal running maintenance charge is 12.9 to 13.2 volts. The batteries in my case are arranged in series and parallel for 29 volts to run the 4000 watt inverter for my AC needs.

Most battery chargers have a starter battery (AUTO) or AGM switch. Unless you are in a hurry to charge the battery a normal 8 amp AGM battery charger will take care of your charging needs. For my portable charging needs, I use a 8 AH charger from CarQuest auto parts. Model CQ-80CR that costs $59.00. It has digital read out with charge rates (current) and charge voltage. Their are dozens of similar charges out there to choose from.

Good Luck

Mike K6AER
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N1AUP

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #6 on: August 23, 2021, 05:02:18 PM »

The battery is a deep discharge battery designed for marine service.  It's not a starting battery.
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K5LXP

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #7 on: August 25, 2021, 04:54:48 PM »

The reason voltage alone is not a great indicator of state of charge is the voltage at any given state of charge changes with current.  Delivered amp hours will also vary based on current.  So state of charge is relative based on those operating conditions, as well as age and temperature.

A very simple rough estimate of state of charge can be determined by measuring the voltage when operating a load (delivering current) and using the following scale:

12.5V - 100%
12.0V - 75%
11.5V - 50%
11.0V - 25%
10.5V - 0%

The 50% rule is a myth.  The impact to service life changes little between 40% DOD and 80% DOD and when compounded with all the other factors that affect service life, impact from DOD is lost in the noise.  You paid for those amp hours, so use them.

No matter how much or little you use a lead acid battery you want to restore the charge as soon as practical.  Don't worry too much about using it as much as you want, that's what it's for but between uses you should keep it fully charged. A battery tender/maintainer takes care of that very well.

Mark K5LXP
Albuquerque, NM
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AC7CW

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #8 on: August 28, 2021, 09:12:17 AM »

The different battery types discharge differently. I worked with some pacemaker batteries that would have an unchanged no-load voltage when they were at end of life. They "discharged" only by a steady increase in resistance. I used to test them by measuring them unloaded and with a known load and from that I could calculate the output resistance. I'd say that with any battery the unloaded voltage and the loaded voltage will tell us all we can know about them via testing. I'm assuming that resistance is the same whether charging or discharging, am unsure if that is true for all batteries...
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Novice 1958, 20WPM Extra now... (and get off my lawn)

K5LXP

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #9 on: August 29, 2021, 08:25:17 AM »

Right, measuring battery impedance under load is one way to estimate state of charge.  Because impedance is driven by age and condition as well as type there's no hard and fast reference to use by default but one way is to measure the battery you have at known states of discharge and come up with a baseline to use.  I've done it this way for years.

Mark K5LXP
Albuquerque, NM
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KM3F

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #10 on: February 15, 2022, 10:56:08 PM »

State of Charge is the resting  >>open circuit voltage<< after the battery has been at rest for the cells to settle down to inactivity and the surface charge is low.
As already stated, 12.5 is about 50% SOC.
The battery can go to 12.9 SOC depending on system design and applicaion.
Station use is far different than motor vehichle use and how the mangemewnt is desiged to handle the AGM battery. The requirements and intent are far greater in vehchle aplications..
How is the battery resistance figured?
Open circuit voltage minus the loaded voltage divided by the loaded circuit Current.
It's just Ohms law  E/I= R.  Esentiually a battery tester does this but presents it in a rough Red-Green, Good, Fair, Bad  scale display.
As the battery R goes up, the ability to supply power to the intended load goes down due to the voltage division between the total circuit resistances.
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KD8IIC

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #11 on: February 16, 2022, 07:15:04 AM »

I was an auto electric tech, now retired. I worked on fire trucks, over the road trucks tractors, you name it.
THE RULE OF THUMB FOR CHARGING LEAD ACID/SLA IS THE CHARGE @ 110% VOLTAGE.
It MUST be at least 110% of the rated voltage. 13.9 is a good place to start. I prefer 14.3.
Leave it on til there's no more current draw I use a regulted pwr supply with amp meter.
I do not ever use any automotive chargers.
 
« Last Edit: February 16, 2022, 07:18:56 AM by KD8IIC »
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K7AAT

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #12 on: February 16, 2022, 09:04:02 AM »

The battery is a deep discharge battery designed for marine service.  It's not a starting battery.

For the record, if your battery has automotive connection posts in addition the bolt terminals, it is a dual purpose battery, NOT a true deep cycle battery.   You have not made clear what you have.....
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WA3SKN

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #13 on: February 16, 2022, 12:23:53 PM »

2021 post.
However... look at the load.
Most radios are rated at 13.8 volts plus/minus 15 percent.  That's 13.8 volts minus 2.07 volts (15%) or 11.73 volts. Below this the radio will "motorboat".  So if you are powering a radio you will want to recharge, or keep your voltage above 11.73 volts. With a lead acid battery you will want to keep the voltage above 10.4 volts or the plates will warp and damage the battery.  You will have to look up other chemistries yourself, but what did the battery maker recommend?
73s.

-Mike.
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ROYSBOY

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Re: AGM Deep Discharge Battery. What voltage needs recharge?
« Reply #14 on: February 16, 2022, 04:04:52 PM »

I discovered a charger that is quite unique. The Noco, 10 amp Genius charges 6 and 12, fla, agm, lithium, has a desulph mode, a force mode and a optimization at the end of charge, most likely an equalization function. Can be used as a power supply as well. So far, it is the only tender that I have found that wont boil a battery dry. $100, no affiliation, I just really like it, have 2 and would like to get the 25 amp Pro model. $600.00 is quite high, it just went up $100 this month, prob due to supply chain issues.

https://no.co/genius10

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