Feedback on Battery Experiment

[Luciano151]

About a week ago, I accidentally unplugged the charge cord to my van, which took the batteries to way below 50% I don't know how long, but my battery monitor showed  all kinds of lights and warnings that the power was sucked out of them.

To check for damage, I plugged the batteries in and let them charge for almost a week. At this point the float charge was 13.1. I unplugged the cord at 8:30 a.m. to start my experiment.

From Wednesday 9:00 a.m. the charge was 12.5 all the way to 10:30 p.m. where it showed 12.5. On 8:45 a.m. Thursday. It was still 12.5. At 5:30 it was 12.4, which I would estimate is about half of the batteries. At this point I plugged it back in again because I didn't want to take the chance that it would go to critical low state again.

Based on this, do the batteries seem pretty solid? They are 4 AGM 6 volts in parallel. The only thing running was an Engle fridge which eats about 1 to 2 amps per hour. I would rate my batteries at 400 to 450 ah.

Based on the fact that it took so long for the batteries to go from 12.5 to 12.4, do you think I could have gone another day in an emergency?

 

[John61CT]

First if all, if you have (what I would call) a real battery monitor, it should be telling you what your state of charge is as a percentage.

But of course for that to be accurate you would need to input your actual 20-hour AH capacity, not guesstimating to the nearest 50AH.

Voltage has a very loose relationship to SoC.

A good properly sized charger (in your case 80-120A) would get them right back to 100% full overnight, no need for longer.

As for your question, you can always do what you like, run the bank down all the way, but you are drastically shortening their lifespan, to the extent you're doing so regularly.

But it is also just as important for longevity to get them back to 100% Full as quickly and often as possible.

The way you've written so far makes it seem likely to me that may not be happening, it does not just happen, most charge sources at factory settings fall short.

It takes certain equipment, a little knowledge and most importantly a strong desire to take such proper care of your bank.

 

[highdesertranger]

I think you are over estimating the AH draw of your Engel. at 20% duty cycle mine draws .5AH. highdesertranger

 

[jonyjoe303]

50 percent for your batteries is 12.1 volts, you can take it down to that voltage every day without any problems. if you go below 11.5 volts thats when you start to damage the battery. They can handle it a couple of times but do it 3 or 4 times and the battery will start to lose capacity. 

Go below 10 volts and the battery will have a hard time to recover. It will still charge up and act like its full but as soon as you put a load on it, voltage drops quickly.

You voltage of 12.4,12.5 volts looks good, if you were using them as you normally do. If you put a big load on them and the voltage drops quickly below 12 volts, then they might have lost capacity. How long it can handle a big load before it drops below 12 volts will give you an idea of their condition. 

 

[Ticklebellly]

......the AH draw of your Engel.  at 20% duty cycle mine draws .5AH.  

That's pretty much what I measure for my Engel.   Worse duty cycle I have had was 80% in 43 Degree C heat.

 

[SternWake]

I plugged the batteries in and let them charge for almost a week. At this point the float charge was 13.1. 

What is your plug in charging source?
In general if batteries midl life are inadvertently drained dead, the normal voltages of a converter/charger are not going to be able to return the specific gravity to the maximum potential even if plugged in for an excessive amount of time..
So saying voltage is 12.X volts, well this means much less than a hydrometer reading 1.275, or 1.225.
With a 'full charged' resting coltage of 12.5, I highly doubt the specific gravity was up in the 1.275 green range especially on an AGM.
Guessing at battery health by resting voltage is not wise.  just as guessing at battery state of charge by a non resting voltage is unwise.    
AGMS, one cannot dip a hydrometer, they need to be held at absorption voltage until amps taper to very low levels, and some would benefit from a little higher voltage applied after amperage tapers to very low levels.
I know this raises the hackles of those who say AGMS will be killed if they are overcharged as they will pop their vents, but at their half life, when abused, an overcharge and a little offgassing ensures they are back at their full remianing capacity potential.
Overvolting an AGM when they are still depleted and accepting high amperage is more dangerous. When they are nearly fully charged, but not quite, and resisting that last few %, a few more hours at more elevated voltages, as long as battery temperature is not allowed to rapidly start rising, is not a death sentence and is likely very beneficial in restoring the battery to its maximum remaining potential capacity.
Plugging in for a week is no guarantee the batteries are fully charged. Abused batteries need to be held at higher voltgaes for longer and automatic smart 7 or 12 or  33 stage ultra smart chargers that try and fellate you afterwards, usually can't  and won' t hold the high voltages required to actually fully truly fully charge the abused battery, as the lawyers might smell blood if 1 battery in 1000 is overcharged and someone decides to check it with a lit match.
Charger marketers go to great lengths to convince the mases otherwise, and far too successfully in my opinion.
Enter the 'works just fine' brigade.
Beware of marketing. Few plug in charging sources can truly fully charge abused batteries, as this ideal requires massaging and effort, and higher voltages held for a somewhat precise amount of time, and that time is variable depending on sevral factors that no smart charger is going to figure out.