# 2 batts in Ev pack showing higher volts



## dougingraham (Jul 26, 2011)

I can tell you why those two cells have a higher voltage. It is because the pack is not balanced. In particular the one showing 4.16 volts is most likely being terribly stressed when charging as it is being overcharged every time you charge the pack. At the minimum I would bleed off some charge from those two cells. Your range is short because the rest of your cells are not getting charged fully. A fully charged voltage after an overnight rest (with no load on the cells) should be somewhere between 3.38 and 3.4 volts. A conservative full charge resting voltage would end up around 3.36 volts. A resting voltage of 3.23 volts is about a 20% state of charge.

As for surging. That could be a lot of things. Check for loose connections but I would guess the motor controller is cutting back for some reason. Maybe on low battery voltage. Is there a BMS of any kind?

If you know anyone who has a higher end radio control battery charger I would try to enlist their aid in charging your pack a cell at a time to get the whole thing in some sort of state of balance. With 160AH cells this will take several evenings to accomplish.

Some specifics might help further as would photos. The kind of charger, motor controller and if there are any other connections to the pack other than a cable to the most positive terminal and the most negative terminal and what that device might be.


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## evmetro (Apr 9, 2012)

I would think that you would have a very short range if your cells are reading so low. They should be charged until the highest one reads around 3.5 volts, and that should trigger your charger to cut off. The cells should settle to around 3.33 volts after an hour or several hours after the charger shuts down. I don't know what kind of BMS you have, so I don't know why the charger is not shutting off when those high cells go over 3.5 volts, but I would look into finding out how exactly your charger shuts off, and drain a little charge off of the high ones.


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## sooze (Dec 31, 2014)

It has got a battery management system made by Electric car company that did the conversion- now bust, hence very hard to get info. We wondered if that was cutting off the charging of the others, when the individual one got too high. How do you bleed off charge? We were looking at getting a charger to individually do cells as we had read about it- can you recommend one or do you think that once we have reduced the overcharged on the bms will allow the rest to charge?


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## dougingraham (Jul 26, 2011)

sooze said:


> It has got a battery management system made by Electric car company that did the conversion- now bust, hence very hard to get info. We wondered if that was cutting off the charging of the others, when the individual one got too high. How do you bleed off charge? We were looking at getting a charger to individually do cells as we had read about it- can you recommend one or do you think that once we have reduced the overcharged on the bms will allow the rest to charge?


BMS can do a lot of bad things if poorly designed including unbalancing your pack. If it controls your charger it could be stopping the charge when it sees that cell with the excessively high voltage. And it could be causing the motor controller to stutter when it sees the rest of the pack voltage so low.

You can bleed off charge from a single cell at a time with a resistive load. A reasonable load is an automotive tail lamp connected to a couple of heavy duty clip leads. You do not want to leave something like this unattended.

Best Wishes!


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## sooze (Dec 31, 2014)

Could we use a 12v dc inspection lamp? Would we have to disconnect the cell we are working on from the others first? And then check the voltage regularly, how low would you suggest to take it?- to the same as the others.
Very grateful for the help.
Sooze


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## dougingraham (Jul 26, 2011)

sooze said:


> Could we use a 12v dc inspection lamp? Would we have to disconnect the cell we are working on from the others first? And then check the voltage regularly, how low would you suggest to take it?- to the same as the others.


As long as it is just a bulb, not LED and no electronics in the lamp this should work. An automotive 12V 1056 lamp draws about 2 amps at 12 volts. Not sure what it will draw on 3.3 volts. Probably around 1 amp but it depends on how hot the filament gets. When white hot the resistance is about 6 ohms. I am guessing it will be about 3 ohms when cold. If that high cell is at a little over 4 volts you can consider it more than fully charged. If the cell isn't hurt and still has 160 AH capacity and the other cells that are at 3.2 volts it should take 120 hours to bleed it down to the same level as the others. I am making a lot of assumptions though so don't just clip it across the cell and then walk away for five days. Instead drain it down until your DVM is showing a voltage just slightly less than the average of the other cells. Disconnect and let it spring back up. If you use a high end RC charger/discharger, those can do a lot higher currents and turn themselves off.

So does this BMS have a wire running to every cell? Does it have a circuit board on every cell? When you get a chance take some photos of the battery pack and BMS.

And if the BMS has a way to display voltages for each cell you should verify it against a DVM because it is easy for badly designed BMS's to have very poor accuracy. One cell can be good and the neighbor can be off by volts.


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## sooze (Dec 31, 2014)

Hi again,
Our lamp has 55 watt bulb so we estimate about 4.5amp?? If we drove the car a bit first it should reduce the voltage so we have less to drain? It reduced to 3.29 before and then we would have less to get it to the same as the others.
The bms does have a wire running to each cell- we will take some photos tomorrow, as we are in the uk and it is 5.30pm and dark here! I'm not sure about the circuit boards and I don't know how to access the bms info- I think you need to plug into and have a special device to read it, again hampered by the defunct ecc, just going from what people say on line. People have replaced their bms in other evies, but we are hoping we won't have to. We have been measuring our voltages on ind cells with a voltmeter, on the front pack of 16. Its hard/impossible without a car raiser- to access the 9 cell pack under the car but we have measured the volts from it and when you divide by the no of cells in it then they are showing the 3.22 that the rest are apart from the faulty (?) ones.
Do we disconnect that cell from the others to discharge it?
Sooze


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## glyndwr1998 (Apr 27, 2013)

Hi sooze,

If you go to a website called batteryvehiclesociety.org.uk and join up, there is a whole section on there on the c1 Evie, and more importantly there is a member called grumpy-b that for sure can help you.

He has obtained all the copyrights and obtained all the spare parts from the electric car company that converted your car, he also has spare bms boards, and can clear your bms of codes so it works properly again, assuming it's not faulty. Apparently the bms stops registering new faults once the memory chip is full and needs to be cleared with the factory software.
Grumpy-b is probably the best expert on these cars in the uk, he has got many mods for the heater, motor bracket and upgrades to the bms software.

Your high cells need to be drained down to match your other pack cell voltages, on high capacity cells like yours this can be a time consuming affair.

What i have done is purchased an enclosed 100watt resistor this is basically a 100 watt resistor concealed in a metal housing, I screwed it to an aluminium heat sink, then soldered 2 4mm cables to the resistor terminals, then on the other end on the wires put crocodile clips ( not this cheap ones though), now you can clip the crocodile clips onto each terminal of the high cells to drain them.
At 100 watts load and a cell voltage of 3 volts you'll be drawing about 35 amps current out of that cell, check the cables and clips for overheating.
Of course, if you left the resistor on the cell for an hour then you would have drained approx 35ah capacity out of the cell. The resistor will get very hot......
So make sure it's on some kind of metal for heat dissipation or you'll burn through your carpet in the boot.

You could of course use 2 50 watt lights or 1. 50 watt light would draw 16 amps.

I would recommend 20 minute bursts, leave the cell recover alittle, if it's still high start draining again.
Once close to all being the same voltage recharge the pack, once the charger trips out, again check for high cells, and drain the highest ones, and so on, until all cells are roughly the same, then hopefully your bms can then balance as they are close, that is if course if it is working correctly.....

Hope this helps, but for sure make contact with grumpy-b as he has the most knowledge on ththe se cars, and has one himself too, he has had one for a few years.

Good luck pal.


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## glyndwr1998 (Apr 27, 2013)

I think grumpy-b is based in Colchester.

If you search ebay for a c1 Evie, he is selling one on behalf of a friend of his, and not sure but I think his telephone number is on the advert, so if you wanted to could speak to him for help and advice..

Thanks, happy new year.


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## dougingraham (Jul 26, 2011)

sooze said:


> Our lamp has 55 watt bulb so we estimate about 4.5amp??


At 12 volts it would be about 4.5 amps when the filament gets up to temp. At 3.2 volts it would pull about 1.2 amps if it ever gets up to temperature. Perhaps as much as twice that if it just glows red.



sooze said:


> If we drove the car a bit first it should reduce the voltage so we have less to drain? It reduced to 3.29 before and then we would have less to get it to the same as the others.


Driving the car to remove charge doesn't help. Since all the batteries are in series you are taking an equal amount of amps from every cell. I would charge until that cell that you found to be the highest reaches 3.6 to 3.8 volts and then stop the charge. Put your load on that cell until it is slightly less than the average cell voltage. And by slightly less I mean 0.01 volts less. It will bounce up a little over a few hour time period when you remove the load. Then charge the whole pack some more looking for another cell that gets to 3.6 to 3.8 volts. Probably this will be that other cell you found. You can repeat this process several times but since you don't have easy access to the cells in the rear pack you don't want to overdo it on the ones you do have access to.



sooze said:


> The bms does have a wire running to each cell- we will take some photos tomorrow, as we are in the uk and it is 5.30pm and dark here! I'm not sure about the circuit boards and I don't know how to access the bms info- I think you need to plug into and have a special device to read it, again hampered by the defunct ecc, just going from what people say on line. People have replaced their bms in other evies, but we are hoping we won't have to. We have been measuring our voltages on ind cells with a voltmeter, on the front pack of 16. Its hard/impossible without a car raiser- to access the 9 cell pack under the car but we have measured the volts from it and when you divide by the no of cells in it then they are showing the 3.22 that the rest are apart from the faulty (?) ones.


I said it in a prevous posting, 3.22 volts as a resting voltage is almost empty. Half full is a resting voltage of about 3.3079 volts at room temperature. Resting voltage is the voltage after 12 hours of doing nothing. Hopefully the BMS does not present much of a load.



sooze said:


> Do we disconnect that cell from the others to discharge it?


You don't need to and I wouldn't. Just remember to keep an eye on it. If you run one of those cells much below 2 volts with a 1-2 amp load you will be damaging it.

If it were mine I would top balance it by charging each cell the same way with one of the high end radio control chargers that know about LiFe type cells. This would require you to have access to the rear cells so for the moment just getting the higher voltage cells knocked down will most likely give you a lot more range.

Looking forward to seeing some pictures.


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## sooze (Dec 31, 2014)

Hi everyone who kindly gave us advice. We have drained down the 2 cells giving us the high readings over 24 hours. Found two dc halogen lamps and butchered them! We did disconnect them from their neighbours as we hadn't got the reply but that seemed ok. I would have thought that you would lower the neighbours if you left them joined? Anyway now when we had got both highs to 3.25 we have joined everything up and after 2.5 hours they are all up to 3.3 and the highers are at 3.6, and bms is still charging. We plan to monitor so as to make sure the highers don't go above 3.75 but see what the rest get to with the bms in charge. And probably bring down the highers a bit more over another 24 hours, so that they are all the same.
Sorry forgot the pictures as spent so long yesterday looking at voltmeters until we accepted that it was going to take hours and went in for a cup of tea. I have to say the car gleams now as I spent so long polishing it!
Susan


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## glyndwr1998 (Apr 27, 2013)

Hi sooze,

It's perfectly fine to leave the cells connected to each other and still use the bulbs to drain the cells individually, it won't do any harm at all.

It may take a few attempts to get the voltages close to each other, then after that monitor closely to see if those 2 cells start to drift again.

Also check the cell voltages on the lower end of the discharge too to see if those cells go lower than the rest, if they do then that for sure indicates those 2 cells having lower capacity than the rest as they go high and low earlier than the rest hence take in less energy than the rest.

For reference, in case you do need spares, or the bms EEPROM needing clearing and resetting, remember grumpy-b from batteryvehiclesociety is very knowledgeable on this conversion and holds all spares.

Keep us posted......

Anthony


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## dtbaker (Jan 5, 2008)

the cells that had very high resting voltages definately needed to be drained a little before attempting to balance or charge in series. Balancing can really only be done at the top or the bottom.... balancing 'resting charge' is a complete waste of time.


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## sooze (Dec 31, 2014)

Hi again,
We are still struggling with our battery system. We have managed to get the overall charge a lot higher around 3.28- 3.29/cell but with the one cell still tending to shoot up. This morning we hit a problem with the car acting like it did when it had got no charge but with the above charge in most cells. Could our faulty cell show a charge of 3.5 and above but actually not be able to give any power so making the bms think that it can't draw current, this cell draws current when we bleed it, with our automotive lamp? We also measured resistance across the cells-which were consistent except for the bad cell which was higher, does that tell us anything?.
We have been talking to grumpyb and will prob get him to drain the bms etc, I just am trying to understand the lifepos a bit more. I have a pdf on the lifepos which talks about higher resistance affecting current pull. the resistance usually being affected by temperature.Is this the sort of resistance that I measure between the poles with a voltmeter on ohm setting?
http://www.cse.anl.gov/us-china-wor...ePO4 battery performances testing for BMS.pdf

Any help, as always, I am very grateful for, and any other references to read. I am a vet by trade I find it frustrating to not understand all this enough to work this all out.
Susan


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## dtbaker (Jan 5, 2008)

the BMS may be causing more problems than it solves. If your initial balance is too far off many of the BMS can't handle the shunt load if the pack is still charging at full power. They can also fail, and apply parasitic loads UNbalancing the pack.

Voltage at rest shows you almost nothing.... the only time you can check meaningfully is at the bottom if you bottom-balance, or at the top if you top-balance.

sounds like you need to re-balance either way, and consider removing the BMS. If you remove the BMS, you can top-balance your pack, and let the charger handle the charge.... stopping when all cells hit their final voltage at the same time (if they are balanced).


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## dougingraham (Jul 26, 2011)

sooze said:


> We have managed to get the overall charge a lot higher around 3.28- 3.29/cell but with the one cell still tending to shoot up.


A resting voltage of 3.28 is below half charge. The one that shoots up is either fully charged or is damaged and has low capacity. I would continue to bleed of charge from that cell until the average is above 3.36.



sooze said:


> This morning we hit a problem with the car acting like it did when it had got no charge but with the above charge in most cells. Could our faulty cell show a charge of 3.5 and above but actually not be able to give any power so making the bms think that it can't draw current, this cell draws current when we bleed it, with our automotive lamp? We also measured resistance across the cells-which were consistent except for the bad cell which was higher, does that tell us anything?.


If the cell that had the highest voltage before you bled it down is the same one that has the higher resistance then yes, this does make sense. What has probably happened is because of excessive overcharging that cell has lost capacity. The higher resistance is another side effect of the damage. But saying that must be the problem would be premature since there is so much we don't know.



sooze said:


> We have been talking to grumpyb and will prob get him to drain the bms etc, I just am trying to understand the lifepos a bit more. I have a pdf on the lifepos which talks about higher resistance affecting current pull. the resistance usually being affected by temperature.Is this the sort of resistance that I measure between the poles with a voltmeter on ohm setting?


You cannot measure cell resistance with a voltmeter set on the ohm setting. A voltmeter can measure resistance of resistors only. If you place the meter set on the ohms scale across the battery you can damage the meter although this is unlikely with modern DVM's. Cell resistance can be measured by several of the high end RC charger/dischargers although the readings will be different with every vendor. It can also be calculated by measuring the voltage with the cell under the load of two different currents.


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## dougingraham (Jul 26, 2011)

dtbaker said:


> Voltage at rest shows you almost nothing.... the only time you can check meaningfully is at the bottom if you bottom-balance, or at the top if you top-balance.


I have to disagree with this somewhat. Voltage at rest tells you a great deal about state of charge. But when you are near middle of the state of charge (around 3.30 volts) you need a meter that can read out at least four places to the right of the decimal point. Otherwise you can't see what you are looking for. But cells that are significantly different (by 0.01 volts or more) near the 3.30 volt center are badly out of balance. It is easy to see state of balance near the top or bottom of the state of charge and it may be that this was what you meant.


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## glyndwr1998 (Apr 27, 2013)

Hi sooze,

I am not conversant with the car ev install or if you can monitor each individual battery whilst driving under load or if the bms can display that information.
I do know your bms is not a good one by all accounts and is known the damage cells.

I am not an expert myself, i installed a 76 cells in series lifepo4 battery in a plug in hybrid kit into my prius, and it was a big learning curve for me.

If it were my car, I would continue to top balance manually like you have been doing until all cells are resting at about 3.34. A lifepo4 cell is considered all but full at resting voltage of 3.34.

What i have observed in my 76 cell pack is that during the charge process, whilst monitoring the charge, the cells voltage rises slowly to about 3.45v and after this point the voltage climbs quite fast to 3.5v, in my pack that happens within a minute. I cut my charge voltage at 3.5v to protect the cells alittle as there isnt alot to be gained into the cell beyond 3.5v, maybe an ah at most.
Once all the cells are resting at about 3.33 or 3.4v then all your 25 batteries are full.
Now drive the car to as far as you can to discharge all the cells until the car is now low in charge.
Check all 25 cells and see if any cell is significantly lower than the rest. make a note of that cell and recharge. 
If the same cell goes high voltage first, and also low voltage first it has lower capacity than the rest of the cells.
For examble, i have 2 cells in my pack that are lower in capacity than the rest, maybe by about 10% less. I nw what cells they are and i am monitoring them. When they get too bad i shall replace those 2 weaker cells.

For now i would continue to balance, drain power out of the cells that are getting full charge earlier than the rest, then recharge the whole pack until the next (or same cell) goes high voltage, and so on until at rest the cells are around 3.4v. 

If you can monitor all the cell voltages whilst driving, that too would indicate where your weak cells are as they will be sagging in voltage far greater than the rest.
As an example, my cells at rest are about 3.34v fully charged.
Under driving load they sag immediately to about 3.1v and the weaker ones maybe to about 3.05v, the greater the load on the cells (ie going up a hill and drawing more amps) the greater the voltage sag.
You may have a cell sagging really badly under load the the bms is restricing the power to protect that bad cell.

Whatever i say though is alittle advice from someone wh is relativel new to this too, but if i can help alittle thats ok.

However, grumpy-b is very experienced with this car and conversion so take his advice if he offeres it, and report back to him with whatever he advises you do / test.

I do know of other members on the battery vehicle society that have lost a few of their cells on this EEC conversion, from mind the are thundersky 160ah cells. Grumby-b did also have spare cells to for this car, as theyt are getting harder to find.

Hope this helps.


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## sooze (Dec 31, 2014)

So if it was you and you had bled that faulty cell down below the others a few times and it had jumped past the rest again, would you replace it? Honestly if they were cheap, I think we wouldn't even debate keeping it. I think the bms would balance the rest nicely if it didn't keep shutting off as that cell raced away to above 4volts, when we started they were all 3.22 apart from the bad one, so it does do something. And also the bms is seeming to not let the car run- although the volts look good in total -it hasn't got any current. As the cells are in series, wouldn't a faulty cell block the current. I did get ohm readings from all my cells- 86 on the ok ones 95 on the other, what was I measuring?
Susan


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## dougingraham (Jul 26, 2011)

sooze said:


> And also the bms is seeming to not let the car run- although the volts look good in total -it hasn't got any current. As the cells are in series, wouldn't a faulty cell block the current. I did get ohm readings from all my cells- 86 on the ok ones 95 on the other, what was I measuring?
> Susan


In order to block the current, a faulty cell would have to show essentially negative pack voltage across its terminals when under load. This would almost certainly ruin the part of the BMS connected to that cell. A weak cell would show a marked drop in voltage under load. This alone would not do anything except limit the top speed of the vehicle. But if a BMS is monitoring the cell it could be sensing the low voltage and telling the controller to cut back power in an attempt to protect that already bad cell.

We don't know what is causing the car to shudder or lose power. It could be the BMS is telling the motor controller to cut back power but we don't know if it is set up to do that. It could be the BMS is seeing one cell go below some threshold and telling the controller to cut back power. Or it could be the whole pack voltage is going too low and the controller is seeing that and cutting back power. Or it could be something else entirely.

If you set the volt meter to ohms and placed the leads across the battery you were most likely just measuring the voltage of the cell in a weird way. On the ohms scale the meter puts out a known voltage on the test leads and measures the current that flows and then calculates the resistance from that. Volts/Amps = Ohms. When you try to measure the resistance of a battery with a DVM the voltage the meter puts out is either charging the battery or discharging the battery and while the current flow is measured the calculation and display does not mean much. To calculate the resistance you need to load the battery at two different currents and measure the voltage at both currents. To give a number that you can compare to other cells the test needs to be run at the same state of charge for all cells. An example would be 10 amps load and 3.335 volts and a 20 amp load and a volt reading of 3.327 volts. The voltage difference is 0.008 volts and the current difference is 10 amps so the cell resistance would be 0.0081 ohms. This is just a made up example and not representative of what you would expect to see from those cells. In fact I would hope the IR was closer to 0.001 ohms. As you can see these are tiny differences in voltage so it takes a good meter to read it. Typical DVM's cannot read resistances directly that are less than an ohm.

As was suggested I would not worry about a bad cell until you have a better state of balance. A cell with a low state of charge could cause all of what you are seeing.


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## glyndwr1998 (Apr 27, 2013)

Hi sooze,

I agree with Doug who is far more expienced than i am at this, i have had my kit now for about 18 months so my knowledge is 18 months and limited to my system, i have no bms or controller to worry about too.

I would visit the battery vehicle society web page and the evie section, and read through previous evies that have ahd the same issues, im almost sure most are atrributed to a weak cell. I am nt 100% sure but i think the bms does monitor invidiual cell voltages and the pack voltage and does tell the controller to limit power when it sees something it doesnt like.

On my pack a cell at 3.22v is nigh on empty, it very quickly sags to around 2.8v under load and then my system stops working a cells hits at 2.6v under load, when ff load the cells bounce back to around 3.1v but they are in fact nearly empty not much gained in ah (amp hours) by running much lower voltage than that.

If you can try to attach a multimeter to the suspect weak cell and maybe have a friend sit in the back seat reading the multimeter to see what the cell voltage sags to when your driving, then as a reference, put the meter on what you think is a good cells and do the same and then compare readings, but the cells need to be close to the same state of charge to do this, again comes back to balancing the cells so they are all resting around the same voltage, in my case 3.4v, maybe higher with the thundersky batteries though, im not sure on the specs on those.

keep us posted


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## glyndwr1998 (Apr 27, 2013)

Hi sooze this is a quote from grumpy-b in the eec Evie section of the battery vehicle society website

Joined: Sat Dec 01, 2007 12:06 pm
Posts: 845	
The early versions of the Evie had a BMS that can best be described as a battery killer. This version had to have each cell voltage adjusted individually by using a set of multiturn Pots , by all accounts they were a terror to set up. They drift. And as a result you can get the car shutting down, because the BMS believes a cell has gone low voltage. This part of the system didnt do the active balancing, which was a separate board, with a second cable set. This also had to be manually setup. I have seen a number of these cars having both cut out problems and batteries that are being bled down by the BMS. 
Since I now have the spare cable sets , and also new BMS units, I have now worked out a mod to take out the original BMS , add the new one into the spare wheel well, (As per later cars) put in new BMS / Cell cables and re wire a lot of the Curtis controller inputs and outputs. Once this is all done the software in the Curtis controller has to be changed, and I have now modified a version to work with this set up. Hey presto, the cars now have the later BMS, and drive much better with less pack destruction. 
The first one took 6 weeks lots of sleepless nights and grey hair to sort out the software issues, but I can now convert a car in about three days. This gives a new lease of life to the early cars, and also allws me to download the BMS data for any future diagnostics.


Report this post


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## glyndwr1998 (Apr 27, 2013)

In another post he has supply of 160 ah thunder sky batteries at a cost of £140 plus vat each plus another £15 per cell to carry out the cell initial charge at 20 amps cc to 4.1v, this is must with thunder sky apparently.


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## glyndwr1998 (Apr 27, 2013)

Just read that a fully charged thunder sky will rest to about 3.5v when full and left a few hours after the charge has stopped.


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## sooze (Dec 31, 2014)

We haven't got the early version of the bms when we spoke to grumpyb but the later one, however this does need clearing and then running to get any readings that will help diagnose what is happening. we have to get grumpyb to send his netbook to do that. We have a heater that loads the circuit quite well and that you can put on whilst testing the cells, so i might use that to see if any drop as you describe. I am not sure what these particular evie cells are aiming for but grumpyb did say resting at 3.3 after an hour of standing so it is possible that with most now at 3.28, 3.29 that the 4.something one takes the total above what the bms is comfortable with. I think we will continue to try and bring the high one down and see if we can balance. i will speak to grumpyb tomorrow. Unfortunately he is a long way from me even though he is also in the uk.


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## glyndwr1998 (Apr 27, 2013)

For sure keep on draining the full cell and recharging etc...... Until all cells are resting about 3.3 3.4v we know then they they are all very close to full capacity. Once there then drive or load the battery and see the voltage sag and record a good v bad cell.
It may take some time and patience to get it balanced but from that point of full charge accurate tests and data can be collected


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## glyndwr1998 (Apr 27, 2013)

Hi sooze,

im not sure how many amps the heater will load on the system to see any significant voltage drop.

ie, my cells are 33ah capacity and when i am travelling along a flat road at 45mph the amps draw from the pack is about 50 amps, and when i go up a hill the amps from the battery increase to about 70 amps for a short period.

so 70 amps current draw from the battery is over 2C (2 x the battery ah capacity of 33ah) my cells are rated 2C continuous or 5C max for short periods ( 30 seconds)

you have 160ah cells so to draw only 1C capacity you would need to draw 160 amps load from the battery, and those will gladly run at 1C all day i would have thought.
The battery would need loading about 2C to get the battery to do some hard work (otherwise they are cruising so to speak) the harder you work them the more strain they are under and the more voltage sag you will see.
Im not sure 100% but lets say your motor is 30KW, and you have 25 batteries at 3.4v resting so thats 25 x 3.4 makes a pack voltage of 85v.

Power = volts x amps or amps = power / volts,

then 30000 / 85 = 353 amps draw at max load,

that equates to over 2C load on the battery.

So to get a load of 2C on the battery you need to work the car up hill or pull off from a standing start quite hard if your bms will allow it in its current state.


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## sooze (Dec 31, 2014)

Hi again everyone,
Continuing with my efforts to balance the pack, a slow process with life and work intervening. I am starting to realise that i may have to individually charge some of the cells to top balance the pack. I was looking at buying a charger discharger but am a bit flumoxed-
Is this one the right sort of thing?
http://www.ebay.co.uk/itm/iCharger-...r-lipo-battery-charger-250-Watt-/121510386862
I would be grateful for any links to ones that I can buy in the uk or europe.
Thanks again
Susan


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## glyndwr1998 (Apr 27, 2013)

It's your bms function to balance the pack, but as your too far from balance the bms cannot drain the out of balance cells fast enough before the charger trips out, at least that's how I'm assuming it works.

You have 25 cells, and it's much quicker and easier to drain energy out rather than put energy in with a hobby charger. I have one of those and a meanwell 3.3v psu, and I do use them sometimes to add small amounts of energy to top balance, maybe an amp hour at most and the most they'll put out is about 4 amps, so that's 4 amps per hour.

Whereas, with a 100w enclosed resistor you can drain approc 30 amps out so in an hour that's 30 amp hours drained, it's a much faster process. You can leave all the cells connected, just connect onto the cell you want to drain.
I would continue with draining out your high cells but at a much faster rate than your doing.
When I bought my first pack I was back and forth with a small bulb for a few minutes at a time, but with alittle experience, can now judge roughly how long the resistor has to be attached to drain out a measured amount of energy.

See if you can borrow a dc clamp meter, and when you are draining energy use the clamp meter to measure the current being drained out of the battery at that time, then you can estimate how long to leave the bulb connected to get out the energy you need to be removed, do that to all your high cells,
Then recharge, again looking to see which cells get high first, then do it all again until they are all about 3.33 resting voltage.

If you have drained too much out of the cell, then you will need something like what you asked for, I personally have one, plus a meanwell 3.3v psu, as they can be manually adjusted to your preferred target voltage when full, in my case 3.5v.


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## glyndwr1998 (Apr 27, 2013)

That charger is expensive too, go to a site called hobbyking much cheaper there if you really want one.
I paid about £20 for mine, but I prefer using the meanwell psu, paid less than £10 for that on eBay. Search for meanwell 3.3v enclosed psu, they have an adjustment pot at the output that can be adjusted to about 3.8v so ok for lifepo4 I set mine to 3.5v, when the battery is low it pumps in at max output, but closer to 3.5v it's more slow, a few milliamperes.


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