# LifePo4 charging



## dimitri (May 16, 2008)

Bruce,

how many cells are in the pack? What type and size of cells?
Are your MiniBMS modules 3.6V or 3.8V ?

At the end of charge, do all red LEDs light up? Does the charger reach the end of charge on its own or is it stopped by BMS via REGBUS or AC relay?

MiniBMS is shunting even after charging is done, so the pack voltage will stabilize at lower level that final charging voltage, plus surface charge tends to disappear after some time as well, so its perfectly normal to see voltage dropping, as long as its not dropping below normal fully charged resting voltage of 3.4V per cell.

Can you also elaborate on this


> The more charge I put on the pack, the _LOWER_ the voltage.


I'm not really following what you are saying here.


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## bruceme (Dec 10, 2008)

dimitri said:


> how many cells are in the pack? What type and size of cells?
> Are your MiniBMS modules 3.6V or 3.8V ?


44x100Ah Thundersky, not sure on the voltage, but I know I ordered the one for Thundersky.



dimitri said:


> At the end of charge, do all red LEDs light up? Does the charger reach the end of charge on its own or is it stopped by BMS via REGBUS or AC relay?


Not that far yet, I was just running tests, saw the inverted voltage and stopped.



dimitri said:


> MiniBMS is shunting even after charging is done, so the pack voltage will stabilize at lower level that final charging voltage, plus surface charge tends to disappear after some time as well, so its perfectly normal to see voltage dropping, as long as its not dropping below normal fully charged resting voltage of 3.4V per cell.


My current resting voltage is 3.2v/cell. What I'm seeing is a drop to like 3.0v/cell when I initiate charging.



dimitri said:


> Can you also elaborate on this
> 
> I'm not really following what you are saying here.


[/quote]

Yes, if I charge, I see voltage dropping (not by much), but definitely not rising. 

BTW, all BMS modules have a solid green light now, and none have every shown any other indication either charging or resting.

Does any of this make sense?!?


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## bruceme (Dec 10, 2008)

So maybe i'm not making this clear enough... When I charge, the voltage drops, this scared me, so I stopped charging.

resting voltage ~140, under charge it dropped to 137.


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## EVfun (Mar 14, 2010)

Interesting... I am running 40, 60 amp hour cells with a PFC-20 and EVworks regulators (3.62 volts) with a mini-BMS control board. 

I wanted to point out that there is little increase in voltage from charging, compared to no load, until the cells are nearly done. That is roughly the point where they head over 3.4 volts. The drop is strange, but an increase of less than a volt wouldn't surprise me at your lower state of charge because it is barely 1 volt when the resting voltage is 3.3 volts per cell.

Hopefully someone has answers. I haven't seen what you are seeing but have noticed the charge voltage profile is very different than lead. In my limited experience it is very flat until the end, then it scatters quite a bit, but the next morning (hours after the charge is complete) they are back to reading within 1 or 2 hundredths of a volt of each other.


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## dimitri (May 16, 2008)

> BTW, all BMS modules have a solid green light now, and none have every shown any other indication either charging or resting.
> 
> Does any of this make sense?!?


Bruce,

I don't think you have anything to worry about. You are quite a long way from a full charge, its actually unusual that your cells are at such low voltage from the factory, they usually come at least half charged and have resting voltage 3.3V per cell.

A small drop in voltage can be explained by reduced internal resistance when you start charging and cells warm up as they pass current.

I assure you that BMS is not doing anything while green LEDs are lit and not red LEDs.

Since your cells came with unusually low resting voltage, its also possible they have mismatched state of charge, so you need to pay attention to initial pack balancing and watch for red LEDs coming up at the end of charge. End of charge will be soon after cells start climbing the top curve, after 3.5V per cell.

Have you tested ability of BMS to shut off the charger when signaling loop is interrupted? If so, then you have nothing to worry about, just let the charger do its job until red LEDs start to come up. When you see red LEDs come up, you can gauge how well the pack is balanced. If only a few reds come up while the charger is still in CC phase, then those cells are ahead of others and one of them will likely trigger HVC and not allow the rest to catch up. This would require you to manually balance the pack by either draining high cells or boosting low cells via single cell charger or 12V charger on groups of 4 cells. If most red LEDs come up within few minutes of each other and charger goes into CV phase, then your pack is well balanced.


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## dimitri (May 16, 2008)

Of course CC and CV phases on Manzanita are user adjustable, so we don't know what you have CV voltage set to. In your case of 44 TS cells and 3.8V BMS modules I would recommend CV voltage of 163V - 165V.

Remember that Manzanita is not isolated, so don't touch cell terminals while charging, unless you enjoy a little zap to keep you alert


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## bruceme (Dec 10, 2008)

All great posts... I'll press on and let you know what I find.


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## zwmaster (Nov 23, 2009)

bruceme said:


> All great posts... I'll press on and let you know what I find.


Check batery connections.
Bad connection could have such effect on current passing thru it.

Keep us posted.


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## dimitri (May 16, 2008)

zwmaster said:


> Check batery connections.
> Bad connection could have such effect on current passing thru it.
> 
> Keep us posted.


Bad connection causes voltage to go up, not down. What he describes looks like reduced internal resistance when charging, which is not unusual due to cells warming up a little. It can also be caused by charger trying to regulate its output and varying its current, causing voltage to swing.


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## zwmaster (Nov 23, 2009)

dimitri said:


> Bad connection causes voltage to go up, not down. What he describes looks like reduced internal resistance when charging, which is not unusual due to cells warming up a little. It can also be caused by charger trying to regulate its output and varying its current, causing voltage to swing.


Yes. My mistake. 
I had similar effect with dead (near death) cell in the past. 
Put them on charger and checking individual cell voltage could be a way.
If all miniBMS boards are signaling OK, then cell voltages are in green range, right? 
Would BMS identify the cell that would drop to 0V? Probably all LED’s off?


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## dimitri (May 16, 2008)

zwmaster said:


> Yes. My mistake.
> I had similar effect with dead (near death) cell in the past.
> Put them on charger and checking individual cell voltage could be a way.
> If all miniBMS boards are signaling OK, then cell voltages are in green range, right?
> Would BMS identify the cell that would drop to 0V? Probably all LED’s off?


Correct, if BMS has ability to shut the charger off when any cell goes outside of valid range, either too low or too high, then you can relax and let the charger do its thing. Observing LEDs at the end of charge is just a simple way to tell how well the pack is balanced.

I have many MiniBMS customers running into initial balancing issues because they get mismatched cells from vendors, some are quite severe cases requiring lots of time to do initial balancing. This is the most common support question I get all the time.

I am thinking of writing up a user guide just for initial pack balancing, because I am getting tired of typing the same thing over and over to different customers  ( no offense to those customers, I never expected you to sift thru dozens of forum threads, some of which are buried mile deep, where all this has been discussed ).


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## bruceme (Dec 10, 2008)

dimitri said:


> I am thinking of writing up a user guide just for initial pack balancing, because I am getting tired of typing the same thing over and over to different customers  ( no offense to those customers, I never expected you to sift thru dozens of forum threads, some of which are buried mile deep, where all this has been discussed ).


That would be awesome... and consider perhaps re-word the warning about not connecting the last loop until the batteries are connected. It took about five reads before it totally made sense and I knew what you where saying.


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## dimitri (May 16, 2008)

bruceme said:


> That would be awesome... and consider perhaps re-word the warning about not connecting the last loop until the batteries are connected. It took about five reads before it totally made sense and I knew what you where saying.


Can you propose a better worded version of that part of the manual? I'd be happy to update it.


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## bruceme (Dec 10, 2008)

UPDATE:

I charged and the voltage did drop slightly initially, but it climbed steadily. I'm used to my weak PbA which would dramatically jump 5+ volts on charge and soar way above float voltage. These nice young LiFePo4's are just soaking up the current. I did the math, at 150v/20A, it will take 4hrs to charge... that's a lot of charging.

I don't have the head unit installed yet, so I cut the charging short after an hour, I was getting sick of closely watching the green lights. After charging for an hour I went from 140v to 150v (44 cells, you do the math), so all is right.

I found I hadn't installed a lock washer under one battery bolt, the screw bottomed out in the threads and that caused the BMS' connector to be loose and intermittent in a not so obvious way. That BMS started humming oddly and eventually it went red very early (3.3v). On the red light, I cut the charge and immediately saw the problem and put the lock-washer back under it... Fixed!


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## dimitri (May 16, 2008)

bruceme said:


> I found I hadn't installed a lock washer under one battery bolt, the screw bottomed out in the threads and that caused the BMS' connector to be loose and intermittent in a not so obvious way. That BMS started humming oddly and eventually it went red very early (3.3v). On the red light, I cut the charge and immediately saw the problem and put the lock-washer back under it... Fixed!


Bruce, I'm glad my Blue Elephant Gun is working for you 

Such loose connection could easily melt the terminal and ruin the cell at the minimum when motor load is applied. I won't speculate what could happen at the maximum....


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## EVfun (Mar 14, 2010)

dimitri said:


> Bruce, I'm glad my Blue Elephant Gun is working for you
> Such loose connection could easily melt the terminal and ruin the cell at the minimum when motor load is applied. I won't speculate what could happen at the maximum....


It would work better because you don't have a regulator attached to that cell, right


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## magchicago (Jun 24, 2010)

Have you seen Jack Rickard's you tube videos? also his website http://web.me.com/mjrickard/

He seems to know a lot. He is totally against BMS systems. He does manage his batteries but does it with one display and access terminals. I have watched many of his videos and he has explained and answered every question I had about EV conversions. He is long winded and takes a bit of time to get to the point. I believe every Ev er should watch all his postings that relate to their project.


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## O'Zeeke (Mar 9, 2008)

magchicago said:


> Have you seen Jack Rickard's you tube videos? also his website http://web.me.com/mjrickard/
> 
> He seems to know a lot. He is totally against BMS systems. I believe every Ev er should watch all his postings that relate to their project.


I agree chicago, it's too bad he don't come around here no more


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## EVfun (Mar 14, 2010)

magchicago said:


> He is totally against BMS systems. He does manage his batteries but does it with one display and access terminals.


So he is pro *man*ual BMS. I asked him once to find my any commercially available product with a rechargeable Lithium battery (battery = more than 1 cell) that didn't have some type of BMS. Not all have shunt regulators (I think most do not) but cell level management of charge and discharge cutoff points seem to be the norm.


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## MN Driver (Sep 29, 2009)

EVfun said:


> So he is pro *man*ual BMS. I asked him once to find my any commercially available product with a rechargeable Lithium battery (battery = more than 1 cell) that didn't have some type of BMS. Not all have shunt regulators (I think most do not) but cell level management of charge and discharge cutoff points seem to be the norm.


Even single cell devices have cutoffs, your cell phone is a good example. It will shut down when it drops below a specific voltage and under an even lower voltage it will prevent any power from being drawn at all, and if it gets to a point where the voltage is so low that it will be unstable if charged again, it will permanently cutoff the cell and prevent itself from accepting a charge by the device.

I have a cell phone that uses a lithium polymer cell, it was an older phone that provided an hour of Youtube watching or web surfing if I was bored and wanted to use the phone for that purpose if I was waiting for something and had nothing better to do after a few months of doing this daily, it suddenly, in less than a month dropped from still providing a full hour to not even giving 5 minutes of power to do the same high-load task. Yet it would still last a day waiting for a call but if I took that call, it would be dead in a moment. The voltage sag increased dramatically causing the device to shut off. After I got a new battery for it, I noticed it was swollen, lithium polymer should be discarded if this happens, I just didn't notice it because it wasn't too obvious as it hadn't popped off the cover yet. 

Almost no consumer devices have shunt devices as far as I know but some might have ways to selectively charge the cells but I'm sure most do not. I took apart a laptop battery with cylindrical lithium cobalt cells in it. The battery was at 100% SOC on the laptop when I disassembled it and when I charged the cells I took down what they all accepted and what they discharged to. The whole thing was unbalanced the lowest capacity cell was realistically at 50% state of charge. The one that had the least real SOC in comparison to the whole pack when I ended up discharging it was the second best cell in terms of capacity. One had really bad internal resistance and was likely the cause of the laptop only providing 3 minutes of power running a video card benchmark and 30 minutes surfing the web. It seemed pretty clear that the battery had no way of balancing and it seemed evident that it was out of balance. In the end it seems the cell with the worst internal resistance is what caused the laptop to shut off in the end.

My opinion is if you can keep the batteries from going beyond a safe charging voltage or below a safe discharge voltage and not exceeding safe temperatures or charge/discharge current limits you are at the very least preventing a sudden premature failure. Running on the edge of those limits often or constantly will reduce the lifespan but it all depends on the expectations of that lifespan. I'm looking at a 1.5C constant discharge on LiFePO4 with bursts of 1.25C charge and 2.5C discharge into them. Do I expect them to last as long as a .5C charge and discharge, no I don't. Am I taking a risk? Sure, but I'm not putting that many cells on the line and my application allows for 4 extra cells to be installed but it is possible to run without those 4 extra without much consequence other than lost capacity. Will I be monitoring them for temperature, over/under voltage, and keeping them from being discharged below 80% via some means or another, absolutely. Will I be shunting, no, but I have over and undervoltage checks in place that disable the system if something is out of range and I can correct it manually with an RC charger to add or remove whatever I determine is needed.


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## EVfun (Mar 14, 2010)

MN Driver said:


> My opinion is if you can keep the batteries from going beyond a safe charging voltage or below a safe discharge voltage and not exceeding safe temperatures or charge/discharge current limits you are at the very least preventing a sudden premature failure. Running on the edge of those limits often or constantly will reduce the lifespan but it all depends on the expectations of that lifespan. I'm looking at a 1.5C constant discharge on LiFePO4 with bursts of 1.25C charge and 2.5C discharge into them. Do I expect them to last as long as a .5C charge and discharge, no I don't. Am I taking a risk? Sure, but I'm not putting that many cells on the line and my application allows for 4 extra cells to be installed but it is possible to run without those 4 extra without much consequence other than lost capacity. Will I be monitoring them for temperature, over/under voltage, and keeping them from being discharged below 80% via some means or another, absolutely. Will I be shunting, no, but I have over and undervoltage checks in place that disable the system if something is out of range and I can correct it manually with an RC charger to add or remove whatever I determine is needed.


I was just out removing a few amp hours at up to 5C this afternoon. I have a 60 amp hour pack of TS cells and the Zilla is set for 300 battery amps and 600 motor amps. Standing on the throttle from 0 to 40 mph (0 to 5200 rpm in second gear) resulted in a minimum voltage of 117.8 (2.945 vpc average.) After a few blasts around the block all 40 cells where resting at 3.32 volts in the garage. I've only taken out a few amp hours at a time right now and I've been recharging to 146 volts and hold for 1 hour. I have EVworks regs operating stand alone currently (3.62 volt on, 1/2 amp max load.)

I'm liking these things!


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