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Difference between a full and empty battery is 1 volt ?

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Prather5

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New to solar charging and closely monitoring battery use for first time and have so many questions but one glaring question is around the use of the battery power and recharging and the difference between a full and discharged battery..............
  1. In my observation and research I notice the difference between a full and empty battery is 1 volt (in the case of flooded lead acid because do not want to go below 50%) – is that accurate?
    1. Monitor on charge controller shows the battery bank at 50% when at 12V and full at above 13V (usually peaks at 13.2V)
  1. If above is true I am to educate myself and understand when you have a 12 volt battery but you only get to use 1 volt before it needs recharging – why do you only get 1 volt? Is there misconception or perspective I am missing and how can I educate myself on this?
  2. Again, trying to gain perspective, how can 1 volt last long enough to power anything for any period of time?
Specific to my situation, in theory, with newer batteries, I should be able to run my RV fridge (using propane) on a medium or low setting off of the batteries for at least a week before recharging (so I am told) and normal use of the RV during the day and assuming ample sun, I should be able to run the fridge at its coldest setting, plenty of low voltage lights throughout, the water pump, a couple ceiling exhaust fans and a stereo/sound system throughout the day and have a full battery to get me through the night until next day – is that accurate (generally speaking)? I do this off of 1.2 volt (draining the battery at night from 13.2V to 12V)?

Any and all information is greatly appreciated - thanks in advance
 
One problem is that the voltage is not a very good indicator of SoC (State of Charge) for lead acid batteries. For flooded lead acid, checking the specific gravity of the electrolyte is considered to be the best indicator. If you don't have flooded lead acid batteries, there isn't such a good way. For all battery types, a coulomb counter (keeping track of the energy going out of and coming into the battery) is the only way to tell how your battery is really cycling.

If the only one volt thing confuses you, wait until you wrap you head around voltage "sag" in a battery. When under heavy load, the battery voltage can drive down quite a bit. Once the load stops, the voltage will likely recover quite a bit. Maybe even back to where it was. If you have your equipment set too conservatively, you may force your inverter to shut down prematurely because of this sag. For that reason the better inverters have a hysteresis / delay setting so as not to shut down unless the voltage remains low for some amount of time.

I am moving from AGM batteries to LFP. AGM batteries - and really all lead acid batteries - are based on a technology that is over 100 years old, and yet it all seems mysterious to me. Battery "experts" say you just have to spend the money on better batteries.
 
12.8-13.6 is a “full battery”. But if you try to pour more than a cup into an 8 oz container it runs out: you can’t put extra in.

So don’t think of volts as volume or something that takes up space. Volts is merely a measure of the available electron collection for electrical potential difference.

So while 11.4V may still be / have electrical potential, the design of how a lead battery is constructed and works is such that 11.4V is effectively 100% “empty” - at zero of ‘full’ it’s just that it will measure ~11.4V. And at 12.04-12.06V you may think of it as “50% full” but drawing the battery below 12.4V begins a range that shortens it’s life.

The 12.8 volts to 12.04 volts issue: as the volts go down the batteries “drain” faster given the same loads. Because the further the volts drop the less watts are there and percentage wise the percent of charge per minute that’s diminished gets bigger.

RV fridge: so I take it yours has a ‘board’ and requires electricity to an igniter to run? Most older and many newer propane fridges require NO electricity…they have a ‘pilot’ function and stay lit.

I hope this straw hat plain language explanation makes sense.
 
Horsefly said:
Battery "experts" say you just have to spend the money on better batteries.
Click to expand...
Well if an expert is one who gets paid for knowledge or services in an area of discovery, I’m an “expert” as I’ve been paid to solve 12V problems occasionally for 20? years or more. I’m not anti new batteries technology but lead acid can be a very dependable consistent option.

Lifepo offers clear advantages for certain criteria for sure. But 1800s technology still works- it didn’t change when the newer tech came along.
 
Horsefly said:
One problem is that the voltage is not a very good indicator of SoC (State of Charge) for lead acid batteries. For flooded lead acid, checking the specific gravity of the electrolyte is considered to be the best indicator. If you don't have flooded lead acid batteries, there isn't such a good way. For all battery types, a coulomb counter (keeping track of the energy going out of and coming into the battery) is the only way to tell how your battery is really cycling.

If the only one volt thing confuses you, wait until you wrap you head around voltage "sag" in a battery. When under heavy load, the battery voltage can drive down quite a bit. Once the load stops, the voltage will likely recover quite a bit. Maybe even back to where it was. If you have your equipment set too conservatively, you may force your inverter to shut down prematurely because of this sag. For that reason the better inverters have a hysteresis / delay setting so as not to shut down unless the voltage remains low for some amount of time.

I am moving from AGM batteries to LFP. AGM batteries - and really all lead acid batteries - are based on a technology that is over 100 years old, and yet it all seems mysterious to me. Battery "experts" say you just have to spend the money on better batteries.
Click to expand...
Thank you for the response and of course answers spawn questions - Is a coulomb counter inexpensive and easy to use and is it practical? I'll look it up too to see what is out there. I have not even gotten to inverters yet / have several questions on that too but will make that a different thread and research it first.
 
12VoltInstalls said:
12.8-13.6 is a “full battery”. But if you try to pour more than a cup into an 8 oz container it runs out: you can’t put extra in.

So don’t think of volts as volume or something that takes up space. Volts is merely a measure of the available electron collection for electrical potential difference.

So while 11.4V may still be / have electrical potential, the design of how a lead battery is constructed and works is such that 11.4V is effectively 100% “empty” - at zero of ‘full’ it’s just that it will measure ~11.4V. And at 12.04-12.06V you may think of it as “50% full” but drawing the battery below 12.4V begins a range that shortens it’s life.

The 12.8 volts to 12.04 volts issue: as the volts go down the batteries “drain” faster given the same loads. Because the further the volts drop the less watts are there and percentage wise the percent of charge per minute that’s diminished gets bigger.

RV fridge: so I take it yours has a ‘board’ and requires electricity to an igniter to run? Most older and many newer propane fridges require NO electricity…they have a ‘pilot’ function and stay lit.

I hope this straw hat plain language explanation makes sense.
Click to expand...
Actually very helpful and makes me feel better that this is not simple and I don't get it but rather complex!! Thank you for your response as well. I honestly do not know the answer of the fridge pilot question and need to research for sure but I know that before I recently replaced the batteries the old batteries were not lasting hardly any time at all and when the batteries were dead the fridge was not cooling. I just assumed there was some electricity required for a circulation fan or other component of the fridge. I'll confirm in the manual. Unfortunately I am new to RVing as well and the RV is the reason for my new interest in 12Volt and solar.
 
time2roll said:
Yes that is how it works. What is lacking is what battery capacity you have and what solar wattage you have.

Here is some basic RV electric information: The 12 Volt Side of Life
Click to expand...
WOW AND THANKS - "The 12 Volt Side of Life" is very, very good for the basic education I needed. I( printed it and saved it to refer back to often. Among other things, the primary thing I learned is not to get caught up in volts and measuring them which I believe was the advice I was given on here. The real measuring "cup" is Amp hrs and figuring out your needs vs your supply and restocking that supply through recharging the battery bank. That being said and using the calculation examples given in "12 Volt Side of Life", I should be in pretty good shape.

My set up is (4) 6V Deep Cycle 225 Ah in Series/Parallel, so that gives me a rating of 12V 450Ah. On a typical day (not using HVAC) I don't see me using more than 250-300Ah and 75% of any usage would during the day when solar is keeping the batteries charged.

My solar setup is qty 2 100W panels in parallel with a 30A charge controller so even with my location only getting about 4 hours of sun that should be plenty of amp replenishing for my usage.

I, of course, have onboard Generator as needed.

I am starting to get it now - thanks again for the tip to the information!!!
 
IMHO 200W will replenish the batteries fine but the batteries will have longer lifespan if you have another 200W of solar to recover them more quickly. Hitting the batteries hard (for lead acid) on the SCC’s weekly equalize routine is better with more amps/volts, and with four panels you can go 24V from the panels (2S2P)

just a thought.
 
12VoltInstalls said:
IMHO 200W will replenish the batteries fine but the batteries will have longer lifespan if you have another 200W of solar to recover them more quickly. Hitting the batteries hard (for lead acid) on the SCC’s weekly equalize routine is better with more amps/volts, and with four panels you can go 24V from the panels (2S2P)

just a thought.
Click to expand...
Thanks, under normal or daily use I would consider adding more but in reality I am using the RV once, maybe twice a week and typically only for about 3 or 4 hours at a time and rarely spending the night (at my property / hunting preserve) when using solar for power and charging. The 2 100W (P) are probably overkill. When I take trips in the RV I won't be relying on solar (panels are currently mounted on the ground and not the RV).
 
Prather5 said:
Thank you for the response and of course answers spawn questions - Is a coulomb counter inexpensive and easy to use and is it practical? I'll look it up too to see what is out there. I have not even gotten to inverters yet / have several questions on that too but will make that a different thread and research it first.
Click to expand...
This a a really good pair of articles that explain what is a pretty complex topic.
marinehowto.com

Under-Load Battery Voltage vs. SoC - Marine How To

After 20 Hour Capacity Test = 95.69 Ah Capacity For this test I wanted to: Show the relationship of loaded voltage to SoC on a popular AGM lead acid battery. Utilize a battery not in perfect health to represent real
marinehowto.com
marinehowto.com

Making Your Battery Monitor More Accurate - Marine How To

Program The Ah Capacity This article features the older Victron BMV-602. The 602 has been replaced by the BMV-7oo series (700, 702 & 712). Everything in this article is still relevant to the current series of BMV-7xx battery monitors. WARNING: This article
marinehowto.com
 
And now you too can cringe when you hear people talking about how long their lead acid lasted and "only went down to 10.5 volts!"

Thanks to those that posted by the way. I'm still learning about lead acid as it seems to me to be more complex than lithium for some reason.

I'm not sure I'm sold on the 80% discharge for a deep cycle though just yet.
 
Last edited:
Short_Shot said:
And now you too can cringe when you hear people talking about how long their lead acid lasted and "only went down to 10.5 volts!"

Thanks to those that posted by the way. I'm still learning about lead acid as it seems to me to be more complex than lithium for some reason.

I'm not sure I'm sold on the 80% discharge for a deep cycle though just yet.
Click to expand...
Lithium is *way* simpler than lead!
 
My rig has a volt reading which isn't super accurate. Full batteries are 12.9 and it settles down to 12.8. 75% discharge (half of the usable power) is at 12.4v. 50% discharge is at 12v.

It will drop down to 11.6 under load but then it rises back to 12 to 12.4 after.

Voltage doesn't tell the whole story and each setup is different
 
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