Understand Battery Charging

[The_Breeze]

I bought 4 Everstart 12v 109ah wet cells 3 days ago. Twice, I was able to dump 13v+ in them on a 30min and 50min gen run. Today I ran the gen 3 times for 3.5h with a 10min rest in between runs and taking readings and could only get it to 12.91.

In the last row, the readings went from 12.43v to 12.89v to 12.91v. So the last 30m only got me .2v I checked it 10min after the last gen run to let it expel surface capacity later and it read 12.91. 30min later they read 12.84. That's a change of .07. Where is going in that short period of time?

These batteries shouldn't be failing me already, right?

[16ACE27]

How/when are you measuring the voltage?

Any voltage measured when charging does not reflect the battery voltage - it reflects the charging voltage.
Any voltage measured immediately after charging reflects the capacitive voltage (surface charge) not the capacity voltage (chemical charge).
Any voltage measured above 12.7 volts is also a capacitive or surface charge, not reflective of the chemical charge.

[Bob Denman]

Ace,
So what you're saying is that when my batteries charge to 12.8 volts: I'm good to go?

[The_Breeze]

Originally Posted by 16ACE27

How/when are you measuring the voltage?

Any voltage measured when charging does not reflect the battery voltage - it reflects the charging voltage.
Any voltage measured immediately after charging reflects the capacitive voltage (surface charge) not the capacity voltage (chemical charge).
Any voltage measured above 12.7 volts is also a capacitive or surface charge, not reflective of the chemical charge.

Any Gen time is followed by a 10min rest before readings are taken. Sorry if that wasn't clear before.

"Any voltage measured above 12.7 volts is also a capacitive or surface charge". You're kidding me?! Man oh man. Learn something new everyday. That was an extra 30min of time and gas wasted; maybe more.

[Pete'sMH]

Originally Posted by The_Breeze

Any Gen time is followed by a 10min rest before readings are taken. Sorry if that wasn't clear before.

"Any voltage measured above 12.7 volts is also a capacitive or surface charge". You're kidding me?! Man oh man. Learn something new everyday. That was an extra 30min of time and gas wasted; maybe more.

But now you know something you didn’t know a short time ago! Love this forum.

[The_Breeze]

Originally Posted by Pete'sMH

But now you know something you didn’t know a short time ago! Love this forum.

Got that right

[The_Breeze]

I've been watching these batteries for a few days now and have some observations I'm hoping someone has some insight on.

1) During the day, I'll run the gen for an hour. The batteries will get in the high 12.7's, even an occasional 12.8.They sit for an hour and lose .1v or more doing almost nothing but running the onboard stuff.

2) They'll drop into the 12.4's and stay pretty much in that range for the rest of the day.

3) They seem to pick up .02 or .03v during the night.

Questions:
1) Is there a problem brewing with the batteries to lose that much juice in such a short period of time?

Why, when doing nothing, do they lose that much voltage, yet at night when doing almost the same thing, they gain .02 or .03 volts?

I know .02 and .03 is like splitting hairs but I'm trying to understand the observations more than the numbers. The numbers contribute to the reason for asking the questions.

[Beau388]

Chemistry says the hotter the batteries the faster the reaction. With flooded and AGM lead acid batteries, the fully charged voltage varies with temperature. The standard used is 12.73 volts @ 25 C (77 F). With temps above 25 C, the fully charged voltage will be lower and for temps below 25 C the fully charged voltage will be higher. A three state charger will compensate for temperature. Better chargers actually monitor the battery temperature; cheaper ones assume the batteries are the same temperature as the charger.

[Pete'sMH]

Originally Posted by Beau388

Chemistry says the hotter the batteries the faster the reaction. With flooded and AGM lead acid batteries, the fully charged voltage varies with temperature. The standard used is 12.73 volts @ 25 C (77 F). With temps above 25 C, the fully charged voltage will be lower and for temps below 25 C the fully charged voltage will be higher. A three state charger will compensate for temperature. Better chargers actually monitor the battery temperature; cheaper ones assume the batteries are the same temperature as the charger.

I concur and I think you might be overthinking the whole thing. A variation of a few hundredths of a volt seems perfectly normal.

[16ACE27]

You said:

Originally Posted by The_Breeze

They sit for an hour and lose .1v or more doing almost nothing but running the onboard stuff.
They'll drop into the 12.4's and stay pretty much in that range for the rest of the day.

Then you asked:

Originally Posted by The_Breeze

Why, when doing nothing, do they lose that much voltage, yet at night when doing almost the same thing, they gain .02 or .03 volts?

So is there a load on the batteries or not?

When determining the state of charge of the battery bank the batteries have to be at rest - no load at all on them. Your batteries have internal resistance so if there is a load on them their supplied voltage will be less than their state of charge would indicate. As current increases the supplied voltage decreases.

So you're trying to do science experiments with your batteries but you aren't adhering to established data taking techniques; therefore your data is flawed.

If you want to compare daytime voltage vs nighttime voltage all other variables, like temperature, SOC, and current draw have to be the same.

If your current draw was almost nothing when you measured 12.4 VDC then you could almost say that voltage indicated your batteries' SOC.

I mentioned before that any voltage measurement above 12.7 VDC would reflect a capacitive charge not a chemical charge but I didn't add FOR A FULLY CHARGED BATTERY.

If the true SOC of your battery is reflected by 12.4 VDC then any voltage measured above 12.4 VDC would also be due to a capacitive charge and would quickly dissipate down to the voltage for the batteries' true chemical SOC.

[The_Breeze]

Originally Posted by 16ACE27

cut...

So is there a load on the batteries or not?

cut...

Did it again. When I wrote 'nothing' I meant nothing in excess of the standard parasitic draw of whatever's going on in the coach to keep stuff alive.

Thank you for taking the time to explain what you understood from my post.

I consider anything above and beyond as excess demand and try to account for it (mentally) in my readings- ran the inverter to charge the laptop and phone and that's where the 'draw' came from.

The batteries have been at rest no less than 10 minutes when making observations. Sometimes an hour or more. I have data if you want it.

When we were talking about the 12.7, Bob posed a question about being at 12.8 as being 'good to go'. If I understand this statement correctly, I have to know the gravity more than the voltage? I have a hydrometer. (re: "If the true SOC of your battery is reflected by 12.4 VDC then any voltage measured above 12.4 VDC would also be due to a capacitive charge and would quickly dissipate down to the voltage for the batteries' true chemical SOC.")

If there's any info I can supply that will help you help me better, don't hesitate to ask. If I can't give it to you, I'll say so.

Take care.

[The_Breeze]

Originally Posted by Pete'sMH

I concur and I think you might be overthinking the whole thing. A variation of a few hundredths of a volt seems perfectly normal.

Possibly. The upturn overnight and downturn during the day confused me so I wanted to throw it out there and not be confused

[sanjaun2]

There are so many variables with your batteries and load put on them from your coach to try and fully determine what is going on. Battery charging is very complex. simply put if your batteries get discharged 50% of capacity which is 12.06 volts in a rested battery. ( no draw for several hours) and you try to charge them with your generator it starts charging quickly (bulk charge) The voltage goes up but it does not mean your batteries are fully charged. As the battery charges it accepts less and less current. so, The last 10 % or so takes many hours. Like 8-12 hours or more. You will just be wasting gas running your generator. Your lead acid batteries should be charged at 13.2-14volts before going into float voltage around 13.2 volts. It sounds like you are not achieving these voltages and your batteries are not getting fully charged. You need to put in a disconnect switch to isolate your batteries or find a way to hook up electrical power to charge your batteries in storage, or Add solar panels like I did. My batteries are fully charged for free every day. Best way to kill a battery is to discharge below 50% frequently, and not fully charging it often.

[The_Breeze]

I found an online calculator that translates 12.24 into 50% (re: https://www.random-science-tools.com...id-battery.htm)

I'm not calling it right but it's what I'm using until I find find something better.

[Hooligan2]

Checking a battery State of Charge, the battery should rest for 1 to 2 hours at no load,to let voltage stabilize and ideally for 6-24 hours. If you are reading above 12.7 shortly after charging you are probably still reading surface charge.



More information here: https://modernsurvivalblog.com/alter...-charge-chart/

[The_Breeze]

I'm not sure there's a way to be at 'no load' in an RV without flipping breakers, pulling fuses and disconnecting everything at the source. At a minimum, the control boards will operate.

What are you really trying to suggest?

[sanjaun2]

Originally Posted by The_Breeze

I'm not sure there's a way to be at 'no load' in an RV without flipping breakers, pulling fuses and disconnecting everything at the source. At a minimum, the control boards will operate.

What are you really trying to suggest?

Breeze, You are correct in the fact that it is very difficult to achieve a no load scenario in a motorhome. As myself and others has tried to say battery systems are much more complicated than some lead in a plastic box.
You need to analyze your usage in your head and monitor your volt gauge accordingly. On my boat my house bank was over 1000 pounds and I would be away from shore power for weeks at a time. I had a system on the boat that counted amps going out and amps coming in. That made it easy. A bit of an investment but worth it with a large bank of batteries.
If you boondock a lot you will need solar power. Your onboard generator is great forerunning air conditioning and electric heaters but is overkill for charging your batteries. I bought a 2000 watt Yamaha generator at Costco for 450 bucks for the times I may need to charge my batteries. A gallon of gas lasts all day and its super quiet. A small solar panel will keep your batteries charged while in storage if there is no power available.
Running coffee makers and other high power items should be kept to a minimum on battery power.

[16ACE27]

Originally Posted by The_Breeze

I'm not sure there's a way to be at 'no load' in an RV without flipping breakers, pulling fuses and disconnecting everything at the source. At a minimum, the control boards will operate.

What are you really trying to suggest?

No Load for battery voltage checking means no load on the battery. If only they made a way to easily disconnect RV house batteries from the DC buss.......

Oh, yeah, salesman switch, USE/STORE switch, battery disconnect switch.

[sanjaun2]

Your readings seem normal to me. During the day and evenings you are using more battery power via lights, awning, charging batteries on laptop etc. That usage drops your voltage. At night you have very little draw going on because your sleeping. So if your not running your furnace your battery voltage gets it's rest period and will show closer to its actual voltage. This can be slightly higher than you read previously. Very normal.

[The_Breeze]

Originally Posted by 16ACE27

No Load for battery voltage checking means no load on the battery. If only they made a way to easily disconnect RV house batteries from the DC buss.......

Oh, yeah, salesman switch, USE/STORE switch, battery disconnect switch.

If you're being humorous. Nice . Disregard the rest and thanks.

My understanding wasn't that the U/S shuts down everything. Supposedly CO2 and propane monitors are still active? I have no idea what else.

I posed a question out here on parasitic draw. Got few replies and that's ok. I asked to see if anyone knew.

Safe Travels.