# Understanding Depth of Discharge (DOD) for Thundersky batteries

6257 Views 28 Replies 7 Participants Last post by  GizmoEV
Hi all,
I want to fully understand the Depth of Discharge (DOD) for Thundersky batteries. IE: the 200ah cells:

http://currentevtech.com/Lithium-Batteries/Thundersky/Thundersky-200ah-cell-p27.html

4.0 v max charged, and 2.8 v max discharged. So, at 4.0 v it is 0% DOD and 2.8v it is 100% DOD, correct?

To take it t a maximum of 80% DOD that would be: 3.04 volts, right? (4 - (4-2.8)*.80 = 3.04)

If I charge the cells to the maximum recommended 3.8 volts, then that is leaving off nearly 16% of the available energy, correct? I must be missing something…as I have heard that most the energy is stored around the 3.2 volt range. I want to understand exactly what this means.

If anyone has any reference papers, or URLs to refer me to I'll gladly read up on it myself.

I know some of this has been covered before on the forums, but it is hard to google out the stuff that doesn't directly talk about this.

thanks!
corbin
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Corbin,

Be aware I am pulling numbers out of the air to make this illustration easier to understand, I'm learning this as well. the numbers are just for illustration and don't reflect an actual battery (as far as I know).

If I understand this lithuim stuff at all, you are not leaving 16% when you charge to 3.8 volts in a 4.0 volt battery. You have to look at the charge curve of the battery. Lets call this 3.8 volts you give the knee of the curve 90+% of the battery power is contained within the flat area of the discharge curve. From 3.8 volts to 4.0 volts might be only 5% of the total power stored. From 3.8 to (lets grab a figure for illustration purposes of 3.0) 3.0 volts might be the other end of that 90% of the battery power, then you his the low voltage knee where it drops from 3.0 volts to 2.8 volts might be again 5% of the power contained in the battery.

For a crude exercise take a piece of paper, turn it landscape and draw 2 verticle lines 10 inches apart 3 inches tall and call the space between them 100% of your battery power now draw a line from one verticle to the other at the 1.5 inch point of the two verticle lines. we will say that line represents 3.2 volts and 100% of the battery's power. Now erase a half inch of each end of the horizontal line. Then connect the left end of the remaining horizontal line to the top of the left line, then connect right end to the bottom of the right verticle line mark the top of the left line 4.0 volts and the bottom of the right line 2.8 volts.

You now have a VERY rough representation of a lithium charge graph. To be truely accruate the horizontal line would slope from about 3.6 to about 3.0 volts but for illustrations sake this will do.

So from 4.0 volts to the central 96% of your power (that 5% we were talking about earlier) the voltage drops fast then hits what is called a KNEE of the curve then you are in the long haul of the discharge till yo get to that 3.0 volt knee at the other end where a last 5% remains, you will drop fast to 2.8 volts and 100% DOD.

If you push a battery above 4.0 volts or below 2.8 volts you can cause irreversble damage. I don't know what the damage will be but I'm told there will probably be damage.

This is why they tell you should short charge the battery and never over discharge the batter. There is really so little power in the area above and below the knee it isn't worth going there and letting a slip of attention or equipment failure cause damage.

Then you have to worry aput the variation between cells. If you charge your lowest cell to max you might drive your best cell over the top.

Please tell me if what I wrote helped you. I drew that graph a while back and it sure helped me when I next looked a t a battery data sheet.

Jim
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Hi Jim -- thanks, that does help me a lot!

So, one can not determine the SOC based only on voltage. They have to record how many amp-hours they have been drawing out of the pack and use that as a value for how much is left in the pack.

If I understand right: cells are charged to 4.0 volts, and have 100% capacity. One can keep track of how many amp-hours they take out of the pack. For instance, with a single 200ah cell, when I have reached 160ah I have taken the cell to 80% DOD. The voltage should be somewhere above 2.8 volts. If it was taken to 100% DOD, then the voltage should be around 2.8 volts.

corbin
Corbin,

Again I'm learning as we go, but from what I understand the relationship between voltage and state of charge is quite good. It's just that the field is very narrow. You are dealing with 100th of a volt and temperature does enter into the measurements. I gather the charge graphes from the OEM are pretty accurate.

By the way if I'm off in left field here, any of you battery experts feel free to come in and squash me like a bug.

Jim
Corbin, take a look at the graphs in the pdf in the link you put in your first post, it shows how voltage is fairly flat during discharge until you get near the end, the knee, and then it drops off. Same thing happens in the other direction when charging.
Corbin, take a look at the graphs in the pdf in the link you put in your first post, it shows how voltage is fairly flat during discharge until you get near the end, the knee, and then it drops off. Same thing happens in the other direction when charging.
Good point. Is there a reason the graphs don't show a charge voltage of 4.0? Or is it simply not necessary? I guess, as others have said, there isn't much power at the top of the cell voltage.

corbin
Yes, not much capacity, better to slightly undercharge for safety, and, mostly, because as soon as you take it off the charger even if charged to 4.0 the voltage will quickly drop down to around 3.4 unloaded, and even lower as soon as you apply a load.
Yes, not much capacity, better to slightly undercharge for safety, and, mostly, because as soon as you take it off the charger even if charged to 4.0 the voltage will quickly drop down to around 3.4 unloaded, and even lower as soon as you apply a load.
This is frustrating me a bit. I am charging and discharging a 400ah TS and I so far get no way near 4.0 or 3.8, or 3.6, so unless I am doing something wrong or have a bad batteries (played with 4-5 so far). The voltage after resting is 3.4 or so MAX, (more like 3.35V) so why all this 4.0, 3.8 B.S and not call it a 3.4 or 3.5 battery?? And discharge down to 2.9 or 3.0 and that will be it??

Francis
G
This is frustrating me a bit. I am charging and discharging a 400ah TS and I so far get no way near 4.0 or 3.8, or 3.6, so unless I am doing something wrong or have a bad batteries (played with 4-5 so far). The voltage after resting is 3.4 or so MAX, (more like 3.35V) so why all this 4.0, 3.8 B.S and not call it a 3.4 or 3.5 battery?? And discharge down to 2.9 or 3.0 and that will be it??

Francis
Most likely your not seeing 3.8 volts because you have not filled up the battery. those are 400AH your playing with. It can take quite some time to charge up. What amperage are you using to charge up the 400AH cells? The 3.8/4.0 are absolute values not to go over. The absolute is 4.0 so the company dropped the end voltage limits to 3.8. Most will charge to even less. That voltage is not resting voltage either. It is charging volt limit. Mine settle into about 3.3 volts there abouts after resting. That is normal. I think the TS settle in at a bit higher. Mine are Hi-Power and will be a bit lower but I can take mine a bit lower too.

I have been charging mine at 15 to 20 amps. You?

Pete
G
By the way it is a 3.2 volt battery.
Most likely your not seeing 3.8 volts because you have not filled up the battery. those are 400AH your playing with. It can take quite some time to charge up. What amperage are you using to charge up the 400AH cells? The 3.8/4.0 are absolute values not to go over. The absolute is 4.0 so the company dropped the end voltage limits to 3.8. Most will charge to even less. That voltage is not resting voltage either. It is charging volt limit. Mine settle into about 3.3 volts there abouts after resting. That is normal. I think the TS settle in at a bit higher. Mine are Hi-Power and will be a bit lower but I can take mine a bit lower too.

I have been charging mine at 15 to 20 amps. You?
Pete

It is a TS 400ah Bench charger set to constant Current of 20.4 Amps. About 18hours from 2.9 to 3.34 Noticibly quicker from 3.34 to 3.4, Now gotta go check every couple of minutes as 3.4 to 3.6 was very fast and climbing past 3.65 in the last 10 minutes. Gotta go set the charger to constant Voltage to watch the amps go down. Very interesting

Francis
G
I think yours are charging just fine. You said resting at 3.4. That is what they should rest at. That last bit is in the knee area and it will quickly jump up if you don't hold it at a specified voltage until the amps drop down. I will be set at 3.6 volts there abouts and CV down to 2 amps then terminate. I have actually 10 different end of charge voltages to choose from in my charger for the car. My bench charger is not a CC CV charger. I have to watch things like a hawk near the end of charge. Your charge times are about right. Lots of time for that many AH. long run times.

Pete
Yeah, 400ah sure is a big bucket to fill ! I bought them used and now selling them as I decided to go with 180calb or 200TS. They test up to or over 400ah and are made in March 2008. Would be great for a larger build, not my Yaris. Even the 180's might be a bit heavy, but i am looking for 80-100mile range.

Francis
By the way it is a 3.2 volt battery.

I was trying to find out my average voltage with "typical" current and I thought about 3.3v. Now I am more inclined to call it a 3.1 volt battery as at 40mph and about 1/2 charge, it seems to be about 3.1v. Anyway its my theory and I'm sticking to it. !! <g>
G
Yeah, not bad for your guess. My Bus will have two 120 volt packs of 100 AH batteries. That way I should get my distance. I won't be hot rodding the Bus so these Hi-Power should do me just fine. For a hot rod I'd like CALB or GBS 200 AH size.

Pete
But I will give way and call them 3.2v, so as not to encourage a lesser important debate.

Francis
Yeah, 400ah sure is a big bucket to fill ! I bought them used and now selling them as I decided to go with 180calb or 200TS. They test up to or over 400ah and are made in March 2008. Would be great for a larger build, not my Yaris. Even the 180's might be a bit heavy, but i am looking for 80-100mile range.

Francis
Hi Francis,
I didn't realise they had been making the big format LiFeP04 batteries that long. Any idea what their previous life was? Apparently they start to sag after a lot of cycles at heavy discharge but the get 400ah batteries to that high a discharge rate would be a huge load. What load rate did you test the 400ah capacity at and down to what voltage? I haven’t been able to find a lot of long term uses information over here because no one seems to have had their’s for long enough. The only deaths have been by neglect, abuse or faulty manufacture, these died very early in their career.
As far as recharging, the last 5% to 10% of capacity can be very slow, especially in a big bank because some batteries will go high voltage and require BMS equalising will others are still excepting charge. The biggest balancing cell bridges I’ve seen available over here are rated at 2 amps, if the bank of 400ah cells is 10% out of balance that could take 20hrs to get all the cells back up.
I'm guessing you are somewhere in the states, pity, I would have been interested in some of those big batteries for my house powerpack in the motorhome I'm building. The cost of freight out of the states for anything bigger than a letter is virtually prohibitive, especially to Australia.

T1 Terry
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Aloha, I am onl testing at 20amp draw and just judging the total capacity. All the used cells i got were 3.30+ after sitting on a shelf for 6-8 months. I have no way to draw down that 400ah big boy as I am just bench testing. Every cell I tested that I did not "top off" came in at rated or slightly above rated capacity. I am topping off some TS 160ah cells now to see how much over 160 they are. My next test later today is to get 3 cells at varying SOC and series them and carefully watch and see how they behave. (why can't I just put them in the car and leave well enough alone ! !)

Francis
Francis,

I hope you are recording the data on your tests. It may come in useful.
Hi Francis,
thanks for the welcome. I think you are going to need to make a heavy discharge rig like a copper wire wound around a piece of bakelite (sp?) or something non flammable or conductive and put it in a big tub of water for the tests so it can dissipate the heat.
I hope I've got the picture attachment part right.
http://www.diyelectriccar.com/forums/attachment.php?attachmentid=9057&stc=1&d=1296428142

These are the charts from the newest Thundersky 400ah batteries with Yidium (sp?) that is supposed to improve their low temp capacity and cycle life but until someone has actually used them to confirm or deny the claims it's about the best we have.
The graph shows that even with a 200amp discharge (0.5C) the capacity of a good battery would still be over 100%. You need to be testing at the peak discharge rate your EV will draw to see if the batteries are going to be suitable or if they have passed their use by date as EV batteries. They could still be a candidate for home power batteries because the rate of discharge is much lower.
I would love to get my hands on some large capacity used cells to see what they are still capable of delivering.

T1 Terry

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