[Question] Power battery application (DOD, discharge time)

[maxmbedded]

Hi everyone !

I have here a GEL battery of 12V/22Ah (Ultracell UCG22-12 datasheet -> http://ultracell.net/datasheets/UCG22-12.pdf) and I would like to supply a load of maximum 30W during 6 hours. So it will draw a constant current of ±2.5A.

In the data sheet of the battery, the Constant Current Discharge table figure out that I can draw about 2.85A for a cell of 1.85V during 6 hours. So, it look like that is fine for my application but I would like to fix DOD to 40%.

I guess that a DOD of 40% will decrease the total supply time of my load but how can I figure out with the data sheet ? 

(The battery is managed with a controller)

[maxmbedded]

Ok in the data sheet, Constant Current Discharge table show FV vs TIME. I think FV mean Final Voltage according to this table found on edaboard.com -> http://images.elektroda.net/22_1328278640.png.

The Final Voltage is also know as Cut-off Voltage. Where its definition is : 

The minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.
-> MIT ref : http://web.mit.edu/evt/summary_battery_specifications.pdf

So if I draw 2.85A during 6hours, the empty state of the battery will be define at 1.85V/cell. As there is 6 cell into a 12V battery, my cut-off is 6*1.85 = 11.1V.

But I could say the same thing for a cut-off voltage of 1.6V/cell and thus 9.6V, which it is very low discharge !

I am still confuse. How can I get the relationship between cut-off voltage and DOD for my battery ?

[MosherIV]

I am still confuse. How can I get the relationship between cut-off voltage and DOD for my battery ?

It is not clear what you are asking.

Are you trying to figure out how many charge/discharge cycles the battery can do when the battery is discharged to 40% of capacity?

[maxmbedded]

Are you trying to figure out how many charge/discharge cycles the battery can do when the battery is discharged to 40% of capacity?

Yes ! But this is the next task.

For now, I would like to understand what will be the voltage level of my battery if I set a DOD of 40% and if my cut-off voltage is 11.1V.

Does the cut-off voltage mean a DOD level of 100% ?

[Kilrah]

It says:

Rated capacity:
 10hr, 1.80V/cell, 25°C 22AH/2.06A
 5hr, 1.75V/cell, 25°C 18.0AH/3.6A

So roughly extrapolating that with 2.5A it would mean that 100% DOD would be about 20Ah, and at this point voltage would be about 1.78V/cell
You plan to use 2.5*6 = 15Ah, so you're discharging about 75%.

The tables on the 2nd page are for 100% DOD indeed.

[MosherIV]

The tables on the 2nd page are for 100% DOD indeed.

The graph called "Cycle Life in Relation to Depth of Discharge" shows the range of Depth of Discharge in relation to the number of cycles the battery should be able to achieve.

Based on the 75%, it looks like the battery will only last around 650 cycles

[maxmbedded]

Based on the 75%, it looks like the battery will only last around 650 cycles

This is why I want DOD 40%. To keep alive the battery for 3 year before replacing it (as 1 cycle per 1 day).

You plan to use 2.5*6 = 15Ah, so you're discharging about 75%.

I see more clearly now. Let do it again :

I want draw 2.5A during 6 hours.

the table show that I can draw 2.52A during 8h and after this time, the cut-off voltage will be at 1.75V/cell (10.5V).

2.52A during 8h give a rated capacity of 20,16Ah (this relation seem not to be linear in reality but whatever, see note *). With a 40%DOD, the usable energy is 20.16Ah * 40% = 8.064Ah

The supply time give now : t = 8.064Ah/2.5Ah = 3,22h = 3h13 !

Half time of my requirement ... Mmh 

Does it seem correct ?


And how can I find the voltage level of my battery after drawing 8Ah ?


note *:

Rated capacity:
 10hr, 1.80V/cell, 25°C 22AH/2.06A
 5hr, 1.75V/cell, 25°C 18.0AH/3.6A

Which mean 10hr*2.06A = 20.6Ah ? 22Ah
But 5h*3.6A = 18Ah

[MosherIV]

I have here a GEL battery of 12V/22Ah (Ultracell UCG22-12 datasheet -> http://ultracell.net/datasheets/UCG22-12.pdf) and I would like to supply a load of maximum 30W during 6 hours. So it will draw a constant current of ±2.5A.

This is why I want DOD 40%. To keep alive the battery for 3 year before replacing it (as 1 cycle per 1 day).

The 2 goals are incompatible !

If you only want to allow 40% DOD, then you can only use 22Ah * 0.4 = 8.8Ah /12V = 0.733A

If you draw 2.5A for 6 hour from the battery - you will use 75% DOD

To get to only 40% DOD, you can ONLY draw 0.733A over the 6 hours.

[maxmbedded]

If you only want to allow 40% DOD, then you can only use 22Ah * 0.4 = 8.8Ah /12V = 0.733A

Ah/V = A ?

I don't get it. Are you sure of that ?

Maybe you did not read my previous post.

[Kilrah]

This is why I want DOD 40%. To keep alive the battery for 3 year before replacing it (as 1 cycle per 1 day).

Half time of my requirement ... Mmh 

Does it seem correct ?

Yup, your battery is way too small to satisfy your requirements, that's very obvious... Forget all the non-linearity (the usable capacity decreasing the more curent you draw) for now, and do a VERY simple calculation:

You want to supply 2.5A for 6h. That's 15Ah.
Given you want those 15Ah to be less than 40% of the battery's capacity, you can already see that you need a battery that's at least 15/0.4 = 37.5Ah.

Now add an approximation of the non-linearites again, round it to 40Ah min and you have it. If you want to be precise find the datasheet for the 40Ah version and validate the estimation,but that's already overkill. 2.5A on a 40Ah battery will now be so small of a load that the effect will be pretty much negligible.

[tautech]

Discharge curves will give you better understanding and yes there is a relation between capacity, current draw and DOD.
Some quick Googling:
http://www.arttec.net/Solar_Mower/4_Electrical/Battery%20Charging.pdf
http://www.mpoweruk.com/performance.htm

[maxmbedded]

Ok, thank you to confirm my though.

I checked data sheet for a battery grader than 37.5Ah and 40Ah is nearest value I found at Ultracell ( -> http://ultracell.net/datasheets/UCG40-12.pdf ). Indeed, I need to double the capacity of my battery if I want to keep for my requirement.

But the price for a 22Ah VS 40Ah is ±45€ VS ±100€ respectively (without transport). So I could also decrease the cycle life by ±2 with a 80%DOD with 22Ah one and plan to replace it every 1,5 year.

Or re-evaluate my load. Need to think about it 


But my question still not solve (sorry  ). It about the voltage level when I reach the %DOD. I would like to disconnect my load when this case occurred.

How can I know ?

[maxmbedded]

Discharge curves will give you better understanding and yes there is a relation between capacity, current draw and DOD.
Some quick Googling:
http://www.arttec.net/Solar_Mower/4_Electrical/Battery%20Charging.pdf
http://www.mpoweruk.com/performance.htm

Great ! I check it now !

http://www.mpoweruk.com/performance.htm . I found it yesterday night. It is a bible !

[tautech]

There's plenty of info about discharge cutoff voltages, IMO you want to stay above 2V/cell.
More reading from Google:
https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery
http://www.batteryweb.com/manuals/techman.pdf

[MosherIV]

But my question still not solve (sorry  ). It about the voltage level when I reach the %DOD. I would like to disconnect my load when this case occurred.

How can I know ?

You will need a circuit to detect the battery voltage under load. When the voltage reaches a threshold, the load must be turned off or disconnected.

[Kilrah]

But my question still not solve (sorry  ). It about the voltage level when I reach the %DOD. I would like to disconnect my load when this case occurred.

How can I know ?

If you look at the PDF tautech linked you'll see that at C/10 and 20% you'll ve at about 11.5V, that's when you'd want to cutoff (and make sure it doesn't turn on again once voltage goes back up).

[tautech]

Something like this is perfect for detecting discharge cutoff:
https://www.maximintegrated.com/en/products/power/supervisors-voltage-monitors-sequencers/ICL7665.html

[maxmbedded]

Great docs guys ! 


I use a solar charge controller which I can set parameters with a PC.

This link is a screen shoot of the app.
http://www.hostingpics.net/viewer.php?id=311279Capturede769cran20160728a768175903.png

I can change charge and discharge parameters. And this is why I want to know to voltage level at xx%DOD.


 

[maxmbedded]

It is very powerful tool but for the moment I don't understand what setting I should set.

[tautech]

It is very powerful tool but for the moment I don't understand what setting I should set.

See reply #13, the links within and following replies. 

[maxmbedded]

If you look at the PDF tautech linked you'll see that at C/10 and 20% you'll ve at about 11.5V, that's when you'd want to cutoff (and make sure it doesn't turn on again once voltage goes back up).

Yes, I suppose that I need replace this by "Dishcarging Limit Volt" or "Low Volt Disconnected" parameters.

[tautech]

If you look at the PDF tautech linked you'll see that at C/10 and 20% you'll ve at about 11.5V, that's when you'd want to cutoff (and make sure it doesn't turn on again once voltage goes back up).

Yes, I suppose that I need replace this by "Dishcarging Limit Volt" or "Low Volt Disconnected" parameters.

Yep.

However one thing SLA's don't like is deep discharge, it shortens their life dramatically and I would stay on the side of caution when setting the discharge cutoff.
Low Volt Disconnected is the hysteresis setting and should be set above cutoff.

[maxmbedded]

Ok understand for SLA.

I make some test with Low Volt Disconnected parameter. If I set it to 12.20V and I plug 12.00V source. Then the load is automatically disconnected and I am not able to control it until the source voltage pass above Low Volt Reconnected parameter (which set as 12.30 for the test).

The question is now, why the factory setting for a GEL battery have this wide hysteresis ?

Low Volt Disconnected default parameter = 11.1V
Low Volt Reconnected default parameter = 12.6V

[tautech]

Ok understand for SLA.

I make some test with Low Volt Disconnected parameter. If I set it to 12.20V and I plug 12.00V source. Then the load is automatically disconnected and I am not able to control it until the source voltage pass above Low Volt Reconnected parameter (which set as 12.30 for the test).

That seems to be working correctly. 

The question is now, why the factory setting for a GEL battery have this wide hysteresis ?

Low Volt Disconnected default parameter = 11.1V
Low Volt Reconnected default parameter = 12.6V

Maybe they want to sell more batteries. 

Those settings might be fine for a deep cycle LA battery but I wouldn't use them for SLA.

Did you say they were adjustable?

Edited

[MosherIV]

However one thing SLA's don't like is deep discharge, it shortens their life dramatically

Depends.....

My understanding is that there are 2 types of Lead Acid battery :
Normal - normally used for standby applications, therefore cheaper, eg used in UPS
Cyclic - these are designed to be charged and discharged over and over, eg the batteries used in electric fork lift trucks. These are more expensive than the other type.