AMP hour rating and voltage. When is a battery dead?

[Checkthisout]

If I have a flooded lead acid battery with a 240 amp hour rating or whatever, what will the terminal voltage be after pulling 240 AMP hours out of the battery?

In other words when a manufacturer is calculating AMP hours to put on the sticker, they start with a fully charged battery and place a load on it.

Once the voltage hits "X" the amount of energy that was pulled is measured and that is the AMP hour rating.

What is "X" voltage?

 

[robby]

12 Volts is when its flat.
So X=12 but some companies may cheat and say X=11.5V

 

[mikefitz]

Most manufactures specify the Ah capacity at the 20 hour rate, thus your 240 Ah battery will provide 12 amps for 20 hours before its considered 'flat'. The voltage used for this test to determine a fully depleted 6 cell battery is 10.5 volts. Due the Peukert's effect, if you discharge at a higher current than the 20 hour rate current, the effective capacity will be less, and if you discharge at a current lower, the effective capacity is greater.

for example if your battery load was 1.0C, 240 amps, the battery would be completely discharged in 40 minutes, giving an effective capacity of 160 amp hours.

In practice the battery will be unhappy being completely discharged, so around 11.5 volts could be regarded as the absolute limit. For long life the battery should not be discharged below 50% of capacity, corresponding to around 12v, depending on load current.
The actual recommended depth of discharge (for long service life) will depend on the type of battery, but in general the lower the depth of discharge the longer the service life.
The graph below indicate the number of cycles obtained with respect to depth of discharge for a quality deep cycle battery under lab test conditions. In real life the number of cycles is usually considerably less.


Mike

 

[Checkthisout]

12 Volts is when its flat.
So X=12 but some companies may cheat and say X=11.5V

Ok so why is 12.2 listed as 50% SOC?

Edit: Sorry, late reply!

 

[chrisski]

I can’t tell you a voltage for 0%, but this is the voltage chart which shows 11.51 votls as 10%:\
A Hydrometer reading is more accurate. The votlage would need to sit to stabilize. Right after it was on load, the battery would be slightly lower in volts but recover with time. Hydrometer won’t change.

 

[mikefitz]

so why is 12.2 listed as 50% SOC?

perhaps for your battery , rested under no load and no charge that corresponds to 50% SOC.
At different temperatures and as the battery ages, this voltage will change.
There is also a slight difference between types and manufactures of lead acid batteries, flooded, AGM, and GEL.
Table below for AGM and flooded batteries,

Mike

 

[Checkthisout]

Ok. So the reason I am asking is simply because I am wanting to have an idea of how to monitor health from battery voltage reading.

It's at my cabin 4 hours away but I can monitor the charge controller etc over the internet. I had to use the backup generator to keep voltage topped up for the last couple months but now the longer days and better weather, the solar system keeps up no problem.

I have a 37 watt draw constant. Overnight battery voltage drops from 12.7-12.8 to 12.35 or sometimes 12.2.

During the day the charge controller puts about .9KWh back into the batteries. It surprises me that 37 watts draw for 14 hours can deplete 3 8D 240 amp hour batteries by 30%. That is why I wanted to know what the voltage "floor" was for an amp hour test. I bought deep cycle batteries, not specific for solar and they are about 10 months old.

Going to check, equalize and water this weekend and check SG for the first time ever.

Thank you for your help.

 

[chrisski]

This situation sounds perfect for a shunt. I have the Victron 712 Battery Monitor, which is really their smart shunt with a display. That could be hooked up to give you a SOC for the battery. I’m sure there are others.

With the Victron shunt, it takes the guess work out of State of CHarge of the Battery.

 

[robby]

Ok so why is 12.2 listed as 50% SOC?

Edit: Sorry, late reply!

I think you already got your answers from others but my post was based on 50% SOC as anything below that is not going to be good for the battery. As for a 0% SOC I would never tell anyone to go down into the 10V+ level. Flat to me with FLA is at 50% SOC.

 

[time2roll]

BCI rating of minutes (RC minutes) defines dead as 10.5 volts at 25 amp load.
Hydrometer might be better. I really dislike pulling out the hydrometer.

Also the 240 rating is commonly at the 20 hour rate so max 12 amp load for that rating. Sometimes the rating is the 100 hour rate or 2.4 amps.
This is due to the Peukert effect.

 

[Checkthisout]

BCI rating of minutes (RC minutes) defines dead as 10.5 volts at 25 amp load.
Hydrometer might be better. I really dislike pulling out the hydrometer.

So the manufacturer is guaranteeing that I will get 240 amp hours out of the battery between full charge (~12.8) and dead @ (~10.5)?

So 10.5 is "X"?

 

[time2roll]

So the manufacturer is guaranteeing that I will get 240 amp hours out of the battery between full charge (~12.8) and dead @ (~10.5)?

So 10.5 is "X"?

That is the RC (reserve capacity) and is a rating given in minutes. This is most common at the 25 amp rate and due to the high load will yeild a lower amp hour. Golf batteries have a 75 amp RC rating as 75 is a common draw of a golf cart. Again higher load will yield lower total energy due to the Peukert effect.

Yes I would always consider 10.5 volts fully dead discharged. Going this low will significantly reduce the cycle life of the battery.

 

[time2roll]

Here is a chart to see the change of capacity depending on the draw rate:

 

[Checkthisout]

Ok so. 240 amp hours x 3 (3 12v 8d batteries @ 240 ah each)

8640 Wh.

12.8 minus 10.5 equals 2.3 volt swing between full and dead.

8640 Wh divide by 23 equals 375

I should be getting 375 Wh per .1 volt drop. Gross approximation because of Voltage drop.

So 12.8 to 12.3 is equals 5. 5 x 375 equals 1875.

Charge controller is putting .9 kwh total into system including charging per day.

Looks like someone needs to equalize their batteries.

 

[Mike Mas]

It's impossible to extract 240 amps from a 240 amp hour battery. For the most part amp hour ratings are for smoke and mirros to confuse consumers. At best you'll only get 50-60% - any more destroys the battery chemistry.

You should look into a lithium pack!

Thanks Mike

 

[Checkthisout]

It's impossible to extract 240 amps from a 240 amp hour battery. For the most part amp hour ratings are for smoke and mirros to confuse consumers. At best you'll only get 50-60% - any more destroys the battery chemistry.

You should look into a lithium pack!

Thanks Mike

I was looking for a guideline into gauging battery health based on daily Amps vs Voltage.

Not to be too jumpy but do you think people go with lead acid instead of Lithium because they don't understand the benefits of Lithium?

Or is it because Lithium costs way more?

"How do I make my Honda civic handle better?"

"Oh you should just look into getting a Ferrari"

 

[Mike Mas]

Hello and thanks for the reply - I just helped a friend install a 300 ah battery in his coach and the cost which was $1,039 for the 300 ah lithium was less than it would be to replace his 6 flooded batteries. Of course the biigest advantage is you can draw lithium down to 10-15 % where a flooded battery you're limited to around 50%. So a 100 ah lithium battery provides the same power as two 100 ah flooded batteries. With that said lithium is finally cost competitive to AGM and lithium lasts 10-15 years.

You can check out my personal (non-sponcered) web site at www.rvvolt.com for some lithium installations I did on some RV's.

Be safe - Mike

 

[Checkthisout]

Hello and thanks for the reply - I just helped a friend install a 300 ah battery in his coach and the cost which was $1,039 for the 300 ah lithium was less than it would be to replace his 6 flooded batteries. Of course the biigest advantage is you can draw lithium down to 10-15 % where a flooded battery you're limited to around 50%. So a 100 ah lithium battery provides the same power as two 100 ah flooded batteries. With that said lithium is finally cost competitive to AGM and lithium lasts 10-15 years.

You can check out my personal (non-sponcered) web site at www.rvvolt.com for some lithium installations I did on some RV's.

Be safe - Mike

I assume that's a chins or ampere time. Do you trust those batteries and bms to last 10-15 years?

 

[Mike Mas]

If you take reasonable care of a lithium battery (providing its good when you buy it) there's no reason it won't last 10-15 years. Lithium is really an investment. Thanks Mike

 

[Checkthisout]

If you take reasonable care of a lithium battery (providing its good when you buy it) there's no reason it won't last 10-15 years. Lithium is really an investment. Thanks Mike

Agreed. I like the price of the 300ah models.

Is that what you're using?

My personal experience is that what something is made doesn't matter. It's the quality of materials, build etc. Lithium does not equal quality.

Are the cells used in those cheaper lithium batteries been used elsewhere with good success? In other words what are the cells in those battery for?