[Question] Power battery application (DOD, discharge time)  - Page 1
Forum section: electronics / projects

author: maxmbedded
author_rank: Contributor
timestamp: July 27, 2016, 02:12:02 pm
content: 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)

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 08:21:36 am
content: 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.pdfSo 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 ?

author: MosherIV
author_rank: Super Contributor
timestamp: July 28, 2016, 09:31:01 am
content: QuoteIt 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?

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 10:00:03 am
content: Quote from: MosherIV on July 28, 2016, 09:31:01 amYes ! 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% ?

author: Kilrah
author_rank: Unknown
timestamp: July 28, 2016, 11:19:14 am
content: It says:QuoteSo 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/cellYou plan to use 2.5*6 = 15Ah, so you're discharging about 75%.The tables on the 2nd page are for 100% DOD indeed.

author: MosherIV
author_rank: Super Contributor
timestamp: July 28, 2016, 11:31:27 am
content: QuoteThe 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

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 12:58:52 pm
content: Quote from: MosherIV on July 28, 2016, 11:31:27 amThis is why I want DOD 40%. To keep alive the battery for 3 year before replacing it (as 1 cycle per 1 day).Quote from: Kilrah on July 28, 2016, 11:19:14 amI see more clearly now. Let do it again :I want draw 2.5A during 6 hours.the table show that I can draw2.52Aduring8hand 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% = 8.064AhThe supply time give now : t = 8.064Ah/2.5Ah = 3,22h =3h13!Half time of my requirement ... MmhDoes 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/2.06A5hr, 1.75V/cell, 25°C 18.0AH/3.6AWhich mean 10hr*2.06A = 20.6Ah ? 22AhBut 5h*3.6A = 18Ah

author: MosherIV
author_rank: Super Contributor
timestamp: July 28, 2016, 01:35:50 pm
content: QuoteQuoteThe 2 goals are incompatible !If you only want to allow 40% DOD, then you can only use 22Ah * 0.4 = 8.8Ah /12V = 0.733AIf you draw 2.5A for 6 hour from the battery - you will use 75% DODTo get to only 40% DOD, you can ONLY draw 0.733A over the 6 hours.

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 01:51:19 pm
content: Quote from: MosherIV on July 28, 2016, 01:35:50 pmAh/V = A ?I don't get it. Are you sure of that ?Maybe you did not read my previous post.

author: Kilrah
author_rank: Unknown
timestamp: July 28, 2016, 02:50:54 pm
content: Quote from: maxmbedded on July 28, 2016, 12:58:52 pmYup, 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.

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 03:02:25 pm
content: 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.pdfhttp://www.mpoweruk.com/performance.htm

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 03:26:57 pm
content: 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 itBut 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 ?

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 03:30:50 pm
content: Quote from: tautech on July 28, 2016, 03:02:25 pmGreat ! I check it now !http://www.mpoweruk.com/performance.htm. I found it yesterday night. It is a bible !

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 03:38:35 pm
content: 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_batteryhttp://www.batteryweb.com/manuals/techman.pdf

author: MosherIV
author_rank: Super Contributor
timestamp: July 28, 2016, 03:41:22 pm
content: QuoteYou 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.

author: Kilrah
author_rank: Unknown
timestamp: July 28, 2016, 03:45:27 pm
content: Quote from: maxmbedded on July 28, 2016, 03:26:57 pmIf 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).

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 03:46:02 pm
content: Something like this is perfect for detecting discharge cutoff:https://www.maximintegrated.com/en/products/power/supervisors-voltage-monitors-sequencers/ICL7665.html

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 04:05:55 pm
content: 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.pngI can change charge and discharge parameters. And this is why I want to know to voltage level at xx%DOD.

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 04:07:57 pm
content: It is very powerful tool but for the moment I don't understand what setting I should set.

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 04:12:07 pm
content: Quote from: maxmbedded on July 28, 2016, 04:07:57 pmSee reply #13, the links within and following replies.

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 04:13:18 pm
content: Quote from: Kilrah on July 28, 2016, 03:45:27 pmYes, I suppose that I need replace this by "Dishcarging Limit Volt" or "Low Volt Disconnected" parameters.

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 04:18:37 pm
content: Quote from: maxmbedded on July 28, 2016, 04:13:18 pmYep.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.

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 05:03:23 pm
content: 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 = 11.1VLow Volt Reconnected default parameter = 12.6V

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 05:07:30 pm
content: Quote from: maxmbedded on July 28, 2016, 05:03:23 pmThat seems to be working correctly.QuoteMaybe 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

author: MosherIV
author_rank: Super Contributor
timestamp: July 28, 2016, 05:07:55 pm
content: QuoteDepends.....My understanding is that there are 2 types of Lead Acid battery :Normal - normally used for standby applications, therefore cheaper, eg used in UPSCyclic - 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.

author: Kilrah
author_rank: Unknown
timestamp: July 28, 2016, 05:08:26 pm
content: Because their voltage drops significantly under load, so when you stop drawing current by disconnecting the load the battery voltage will rise quite a lot again. You don't want to reconnect the load at that point since the battery is still empty, hence the hysteresis.

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 05:12:28 pm
content: Quote from: MosherIV on July 28, 2016, 05:07:55 pmMany more in fact.OP is using a GEL cell = SLAEditBut you are correct, charge cutoff will be different for Cyclic or Standby.

author: maxmbedded
author_rank: Contributor
timestamp: July 28, 2016, 05:29:43 pm
content: Quote from: tautech on July 28, 2016, 05:07:30 pmHaha I don't know x).Adjustable ? The Battery ? no use an adjustable power supply to make the test.My current battery is the deep cycle GEL series of the vendor.Quote from: Kilrah on July 28, 2016, 05:08:26 pmOk I see. That make sense

author: tautech
author_rank: Super Contributor
timestamp: July 28, 2016, 05:33:57 pm
content: Quote from: maxmbedded on July 28, 2016, 05:29:43 pmSorry I meant the hysteresis and I should've been clearer, but it would seem you have it all under control.

author: Kilrah
author_rank: Unknown
timestamp: July 28, 2016, 05:34:55 pm
content: BTW you can work that out by looking at the graph in the "Battery charging" PDF again. See how at the same state of charge the voltage is different depending on the load?

author: maxmbedded
author_rank: Contributor
timestamp: July 30, 2016, 12:56:34 am
content: Quote from: tautech on July 28, 2016, 05:33:57 pmYes, I can adjust the hysteresis voltage. Such a good productBy the way thank for your help guys !I will continue my research to find where is the 40%DOD voltage threshold. I will update here when it will be done.CheerMax