Understanding Depth of Discharge (DOD) for Thundersky batteries

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

spdas said:

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

Click to expand...

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

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