Voltage level vs. Amp Hours used

[larry2c]

Back in November we went out to one of our favorite spots in the Anza Borrego desert - a boondocking site. We stayed 4 nights and during that trip I observed what seems to me to be a mismatch in regards to the amp hours used and what the battery voltage was.

I replaced the OEM group 24 battery with a pair of 6V golf cart batteries a bit over 2 years ago. The batteries are Duracell 215 AH batteries and they have been treated well - solar on the roof keeps them topped off and I check/top off the distilled water about every other month. I installed a low cost (AKA Made in China) monitor with a 100 amp shunt when I did the battery swap. I followed the instructions for the power monitor to get the zero setting and I set the battery capacity at 206AH (I gave it a knock down as a "cushion"). The monitor seems to work properly - showing appropriate amperage draws when I put it in the Amps mode.

So here's where the confusion part comes in - The first couple days out there the sun was hiding behind clouds all day and for each day we were using a bit over 10AH. We never needed the furnace and the DW's CPAP doesn't use any heater or humidifier so it only takes about 8AH for a full night. The rest of the power was just lights, water pump and intermittently the stereo. Normally I leave the power monitor in AH display and after the first day & night it showed we were down to 196 and after the 2nd day & night it was showing us at 185AH. So to me that says we used about 20% of the "safe" capacity of the batteries. However, when I switched it over to the voltage reading that 2nd morning it showed 12.12V. With so little of the capacity used I expected the voltage to be around 12.4V. One last thing I noticed is that as it warmed up that morning the voltage went up about .5V fairly quickly - even though my solar output was almost zero because of the cloud cover.

So, can anyone help me understand why things seem so much different than I expected? Just for the record, I fired up the little Honda and gave the battery a couple hours of charge time and then the sun came out and between the two I got a bit over 10AH back to the battery that day. Sun was our friend the rest of the trip so I didn't really think about it much more until I got home. Thanks in advance for helping me get my head on straight!

[acadianbob]

Voltage is only an approximate estimate and is never very accurate when there is a draw of current. You are much better off to measure AH used and replaced ( if your monitor measures that ).

[Ballistic]

For FLA batteries.

The lower current draw equates to a higher AH rating, inversely the higher current draw the lower the AH rating. The industry used 5Ah over a 20hr time period, so over the loads are more than 5Ah, then the battery performance will be less than its rating.

Read up on Peukerts law. It’s great that you are monitoring the performance as this will provide real world data.

https://en.m.wikipedia.org/wiki/Peukert%27s_law

Do not base a batteries performance by subtracting the power used from its AH rating because it is a moving target based on applied loads. Voltage doesn’t lie because that is was is currently being actively measured. So for a fact, if you’re at 12.1v then you’re at 50% state of charge. You can read the remaining amperage in a battery. This is the tip of the iceberg for FLA batteries.

If you say the ambient temperature increased and the batteries SOC increased .5v, then that would be a miracle.

ETA: when measuring battery voltage, the battery needs to be at rest for a minimum of 1 hour.

[Ballistic]

Originally Posted by acadianbob

Voltage is only an approximate estimate and is never very accurate when there is a draw of current. You are much better off to measure AH used and replaced ( if your monitor measures that ).

How does one measure the full and partial AH in a battery without measuring voltage?

[Hclarkx]

I agree with Acadianbob. Voltage is only a usable indicator if you let the battery rest for 30-60 minutes with no load and no charging before taking the voltage reading. This "resting voltage" is the voltage you see in SOC vs voltage charts. Without the resting period, the voltage will read high when charging or low when there is load with other variables such as battery temperature thrown in to further reduce accuracy.

A proper SOC monitor tracks Ah in or out at the battery terminals. That's a good thing, but it's not perfect either. A good SOC monitor must correct the Ah number using Peukert's equation to account for losses in the battery. This equation accounts for some of the battery's stored energy being used inside the battery when the battery is delivering current to a load. The current flows through the battery's internal resistance thereby creating heat in the battery and using up some of the Ah in addition to what reaches the terminals. If you take 10 Ah out, the battery may lose 11 Ah (or more if the load current is high). Likewise, when charging, current flows through the battery's internal resistance creating heat and so not all of the Ah going into the battery becomes stored energy. As much as 10 to 15% of the energy you put into the battery goes into heat rather than stored energy.

When you combine these two, you may need the charger to put 5-30% more energy into the battery than you took out. Smaller number for lower load and low charging current. Higher number for heavy loads and higher current charging.

Peukert's equation is fairly accurate if you have constants for it from the manufacturer. Typically approximate numbers are used so its not all that accurate but much more accurate than a voltage reading.

Skim the Victron BMV-712 manual (link below). It's not for the feint of heart but will give you an idea of the complexity of monitoring lead-acid batter SOC.

https://www.victronenergy.com/upload...R-DE-ES-SE.pdf

Then there's the problem of SOC monitors needing a full charge to synchronize to the actual battery SOC. Since lead acid batteries need to be fully charged as often as practical, the SOC monitor typically does get re-synchronized often, but if it doesn't, the approximate Peukert parameters will let it drift away from the actual SOC in a few cycles.

[aircommuter]

A similar thing to keep in mind is when charging those batteries there is absorption time, typically 2 hours, so it wont be fully charged when you might think it is.

[TitanMike]

Originally Posted by Ballistic

If you say the ambient temperature increased and the batteries SOC increased .5v, then that would be a miracle

Actually no miracle involved.

Lead acid batteries are electro-chemical devices and as temp goes up so can voltage. Down will lower voltage. SOC is a function of voltage and specific gravity of electrolyte so temperature is a factor.

That said it's not usually a huge factor unless colder temps are involved.

[Ballistic]

Originally Posted by TitanMike

Actually no miracle involved.

Lead acid batteries are electro-chemical devices and as temp goes up so can voltage. Down will lower voltage. SOC is a function of voltage and specific gravity of electrolyte so temperature is a factor.

That said it's not usually a huge factor unless colder temps are involved.

Correct, but you missed the critical data the OP provided. Do you feel .5v (aka: 50% SOC) is due to ambient temps rising. Are you suggesting the next time a battery is at 12.1v (50% SOC) it should be placed in the sun or a warm area and it will be charge back up to 12.6? I can see .2v max and that would require the battery temperature to rise along with ambient, which they do not. It takes a lot of heat to warm 40-60lbs of lead.

[Larry-NC]

Originally Posted by Ballistic

How does one measure the full and partial AH in a battery without measuring voltage?

With a hygrometer.

A fully charged lead-acid battery is filled with sulfuric acid. A fully discharged battery is filled with water which has a different density (weight per fluid ounce).

With this hygrometer, you suck out some fluid, measure the density, and then squirt it back in. Then look at a table, correct for temperature, and you have the state of charge.

Batteries have internal series resistance. Thus the output voltage changes depending on how much load is present--internal voltage drop across the resistance per Ohm's Law. If you don't want to turn off everything each time you want to determine the state of charge, forget voltage and get a hygrometer.

(Easy to turn off everything on the golf cart. Not so easy on a 38' destination trailer.)

[Ballistic]

Originally Posted by Larry-NC

With a hygrometer.

A fully charged lead-acid battery is filled with sulfuric acid. A fully discharged battery is filled with water which has a different density (weight per fluid ounce).

With this hygrometer, you suck out some fluid, measure the density, and then squirt it back in. Then look at a table, correct for temperature, and you have the state of charge.

Batteries have internal series resistance. Thus the output voltage changes depending on how much load is present--internal voltage drop across the resistance per Ohm's Law. If you don't want to turn off everything each time you want to determine the state of charge, forget voltage and get a hygrometer.

(Easy to turn off everything on the golf cart. Not so easy on a 38' destination trailer.)

It didn’t do a good job making my statement rhetorical. That is THE only method to determine an accurate SOC. Not many people (some do though) carry and temperature compensating hydrometer with them in the field.

[Tabasco_Joe]

Lead acid battery temp correction is about 0.0004 V per deg F from 80 deg F.


Larry, how long did you run the generator to recharge?
Can we assume you had the generator powering the camper and thus the convertor was doing the charging?
Or did you use a different charger?

It appears you believe you only used 20AH or so before recharging?
If you ran the generator much of the day it would indicate your batteries were much further discharged or they are in poor condition.

[Larry-NC]

Originally Posted by Ballistic

It didn’t do a good job making my statement rhetorical.

Sorry. I'm a very literal person. I sometimes respond to rhetorical questions and miss sarcasm. (That's why they invented smileys and other emoticons.)

Originally Posted by Ballistic

Not many people (some do though) carry and temperature compensating hydrometer with them in the field.

We can only wonder why. $10-12 at Amazon isn't exactly a fortune.

[larry2c]

Hi all and my apologies for dropping offline for a while - it's been a bit hectic here lately.

Thanks for all the info and I will admit, digesting the info on Peukerts law and the Victron linked info is a bit daunting for me but I think I get the gist of it.

Regarding running the generator, yes, I just plugged the trailer into the genny and let the WIFCO do its thing. As far as using a hydrometer to get the specific gravity of the batteries, I don't usually carry one on our camping trips but maybe I'll do that next time we go out. I didn't realize/think about the specific gravity not needing a resting period like the battery voltage measurement would. Adding one more data point to equation would be a good thing. Oh, and next time I'll make all my notes while we are out there camping instead of trying to remember the specifics when I get home.

[Larry-NC]

Originally Posted by larry2c

Oh, and next time I'll make all my notes while we are out there camping instead of trying to remember the specifics when I get home.

Welcome to "advanced middle age." I can't do the memory thing anymore, either. But I've gotten really good at using the notepad in my phone, and occasionally even voice-dictating to it.

[larry2c]

Originally Posted by Larry-NC

Welcome to "advanced middle age." I can't do the memory thing anymore, either. But I've gotten really good at using the notepad in my phone, and occasionally even voice-dictating to it.

I do take notes now - even have a little notebook just for the trailer. Unfortunately I didn't put anything in there on the battery variances I saw.

CRS wins again!