# Thunder Sky (Winston) charging question



## EVfun (Mar 14, 2010)

I was noticing some behavior when parallel charging 6, 60 amp hour, cells. These cells have sat at about 50% SOC (3.29 volts) for a little over a year. I put them in parallel and put them on a 6 volt battery charger that I control with a variac to set the current (set to around 60 vac.) I only charge when I can be paying attention, setting a timer and making regular trips into the garage to check the progress. 

Yesterday when I was done charging for the day I was pushing 6 amps (about 1 amp per cell) and the cells where at 3.35 volts while charging. This morning when I came out the charge some more the cells where all resting at 3.34 volts. I know they had quite a few amp hours to go. I tried charging a little more at 6 amps and they continued to take a charge at 3.35 volts. 

I stopped and made the cells into 2 parallel packs of 3 each. One group I left alone for the rest of the day. The other group I charged at 7.5 amps for 3 hours. The cells where at 3.39 volts when I shut down for dinner. I will check the cells again tomorrow to see where each group is at.

My understanding and general goal has been to charge so the cells will rest at 3.34 to 3.35 volts after 24 hours (with no load) has passed. It seems strange to me that I was seeing 3.34 volt this morning when I know I had to still be at least 10 amp hour shy of full. I added about 8 amps hours to each of 3 cells today and they still didn't reach 3.4 amps while taking about 2.5 amps. I'm getting mixed messages and am scratching my head a little. 

Suggestions? Advice? Similar experience?


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## bjfreeman (Dec 7, 2011)

what are the charging stages of you charger, if it has any.
are all the cells the same batch and chemical make up.
I notice now that winston site says 4 Volt top off voltage.
Not sure if that applies to your cells.
if you series two cells you should not ever reach full charge under normal charging.


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## EVfun (Mar 14, 2010)

My charger is dumb. It is just a 20:1 transformer and bridge rectifier (basic 6 volt battery charger) with a timer, volt meter, and amp meter. I've use a good Fluke DMM to get more accurate voltage measurements. I carry a small digital kitchen timer and set it before leaving my garage. 

The cells are all from the same batch, manufactured in Feb. 2010. They where put into service for a while, in a larger pack together with the cells that are in my buggy now. They have had some limited use since, but never outside the range of 3.25 to 3.35 volts. I have never listened to the TS recommendation of charging to 4.0 volts, I view that more as an absolute max not to be exceeded. 

My usual charging, for the rest of the cells from that batch installed in the buggy, is to take them to 3.50 volts and hold until the current drops to about 0.05C (3 amps.) That is about 98% charged. To integrate these 6 extra cells into the pack I need to charge them to that same point. The pack runs without a BMS so after installing I will have to use some clip on resistors to tweak the balance so they all come up to 3.50 volts together. So far my experience is that once a pack is brought into balance they do not change any noticeable amount. There appears to be no cell drift for LiFeYPO4 cells in good condition. The car has a smart charger and the voltage will be changed from 112 volts for 32 cells to 133 volts for 38 cells.

My biggest head scratcher is why cells that are known to be at least 10 amp hours short of full would have a resting voltage of 3.34 volts. That point is usually only seen when the cells are within just a couple amp hour of fully charged. That is what I see in my buggy a day after charging and I know that is about full charge because I can get just over 60 amp hours out of them before the first cell drops to 3.00 volts.


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## Coulomb (Apr 22, 2009)

EVfun said:


> My biggest head scratcher is why cells that are known to be at least 10 amp hours short of full would have a resting voltage of 3.34 volts. That point is usually only seen when the cells are within just a couple amp hour of fully charged.


My take is that 3.34 and 3.35 VPC is on the flat part of the curve, where you really can't tell the SOC from the voltage.

In my opinion, you have to charge them to 3.4 or even 3.5 V to get them off the flat part of the curve; then you know that you are at at least 90% SOC. A full cell should rest at 3.40 V at least. [Edit: this seems to disagree with others' experiences, so I'm probably misremembering there.] If a fully charged LiFe cell (that has been to over 3.60 VPC, for example) rests after 24 hours at less than 3.40 V, I'd say it has excessive self discharge. [Edit: so this is probably wrong.]

[ Edit: I have been exceptionally lazy and didn't read all of your post; sorry. I agree that charging to 3.50 V till the current drops below 0.05C will be around 98% SOC. If your cells drop to 3.35 VPC 24 hours after that, I'm suspicious that there is high self discharge (or maybe the BMS is discharging the cells a little).

In any case, once they are resting at 3.35 VPC, they are in the flat part of the curve, and you can't tell if they are 50% SOC or 95%. At least in my experience, which has been with Sky Energy / CALB cells. ]


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## tomofreno (Mar 3, 2009)

Coulomb said:


> My take is that 3.34 and 3.35 VPC is on the flat part of the curve, where you really can't tell the SOC from the voltage...
> 
> ...In any case, once they are resting at 3.35 VPC, they are in the flat part of the curve, and you can't tell if they are 50% SOC or 95%. At least in my experience, which has been with Sky Energy / CALB cells. ]


 I'd say that is an exaggeration Coulomb. I can definitely tell the difference in SOC of 50% and 95% based on voltage on my 180Ah CALB cells. On my cells 50% SOC is in the 3.2xx range whereas 95% is in the 3.3xx range (both at the high end of those ranges IIRC). Voltage as an indication of SOC is of course fairly useless when you are driving because the "load" on the cell is constantly varying, though JRP3 says he was able to get a fairly good idea - I think by comparing voltage at a given discharge current. I used to record all cell voltages before and after each charge, so the difference was obvious.


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## Guest (Dec 26, 2011)

One thing that is happening is that your partly charging then allowing to sit. Then charge and sit and charge and sit. They are not all being charged in one sitting. It may be a reason. Just a thought.


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## tomofreno (Mar 3, 2009)

EVfun said:


> I was noticing some behavior when parallel charging 6, 60 amp hour, cells...Suggestions? Advice? Similar experience?


 My CALB cells are usually around 3.39V a few hours after a "full charge" which I define as charging the cell to 3.5V, ending with 3A or less charge current - which I do only occasionally (charging the pack as a whole, so only the highest voltage cells get there) to see how cell voltages compare near end of charge. Based on that I think you still have capacity left. I would expect Winston cells to behave similarly, but I have no experience with them.

Edit: Well, that is what I see now, but apparently not back when they were newer. I attached some data from 12/26/09 when I had been driving the car for about one month. The voltage of this cell was 3.352V after sitting overnight after being charged as shown in the figure the day before.
View attachment Charge Curve.pdf

The cell was a bit overcharged because I was watching what I thought was the highest cell (it was up to near the end, then this one and a couple others pulled ahead), and had no bms at the time.


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## EVfun (Mar 14, 2010)

Coulomb said:


> My take is that 3.34 and 3.35 VPC is on the flat part of the curve, where you really can't tell the SOC from the voltage.





gottdi said:


> One thing that is happening is that your partly charging then allowing to sit. Then charge and sit and charge and sit. They are not all being charged in one sitting. It may be a reason. Just a thought.


It may be some type effect from the partial charging. This morning both groups of 3 cells are resting at 3.34 volts, though I know one group is about 8 amp hours "ahead" of the other (that is 13% SOC ahead.) The cells in the buggy are resting at 3.29 volts, right where I dropped the pack for winter storage, so I think the DMM is working correctly.

My experience was that the big flat land for cell voltage started at about 3.32 volts. It seems a little weird that it seems to be starting a couple hundredths of a volt higher on these "lazy" cells that have sat around. Perhaps it is a temporary artifact of sitting unused. Generally I find myself in agreement with JR on the concept that 3.34 to 3.35 is target for a charged and rested cell. Since adding 8 amp hours to the 3 cells didn't raise the resting voltage I am not concerned about overcharging them. I will continue on to 3.50 volts at 2.5 amps.


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## dtbaker (Jan 5, 2008)

if you do a CV charge to 3.60 or even up to 3.80 for at initial top balance until amps drop to just about nothing..... the cells WILL settle to 3.35 or so when you take'm off the power supply and let them sit a couple hours. 

I am a fan of taking them fairly far up the curve for the initial balance charge because I think it provides much tighter balance and raises your chances of all the cells 'finishing' together at your lower daily charge voltage target.


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## Coulomb (Apr 22, 2009)

dtbaker said:


> ... the cells WILL settle to 3.35 or so when you take'm off the power supply and let them sit a couple hours.


I stand corrected. I've edited my post to indicate that my memory is at odds with others' experiences. I don't have an EV running right now; one has been in progress for coming up to 3 years. It's obviously not the same utility (for this sort of discussion) as charging up a pack several times a week in actual use.


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## EVfun (Mar 14, 2010)

Everything seems normal. I finished charging one block of 3 cells and this morning they are resting at 3.35 volts. The other block of 3 cells still needs to be finished as I shut if off at 3.40 volts at 8 amps (3, 60 AH cells in parallel.) They are resting at 3.34 volts this morning.

My finish charge routine for the first block of 3 was to take them to 3.60 volts and hold that until the current dropped to 6 amps. Then I set the current down to 1.5 amps and held it tight. The voltage slowly dropped to 3.47 volts and I continued holding 1.5 amps until the voltage rose again to 3.50 volts. Based on wire millivolt drop readings the cells varied by less than 0.001 volt with only 1.5 amps flowing.


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