NG3 chargers can't current limit on lithium batteries?

I wonder if the temperature compensation is taken care of in this logic chip? If so, it seems that the pins for temperature input could be used for input to control the charger externally: to turn down the current or shut it off completely.

FWIW, Attached are photos of the logic boards in my 115V NG1 and 230V NG3 both with the same programming. The paper work says it is the ZV curve #7 but the sticker on the chip says ZV6, in any case it is set for 69.3V Saturation and the current just tapers down close to zero until the timer during the "yellow" phase times out.

I don't think there is anything wrong with using a re-purposed lead acid curve except that the proper charge profile for LiFePO4 calls for a CCCV curve which terminates when the current drops to 0.05C. For my 200Ah pack that would be 10A, just barely above what the NG1 can do. I compensate by charging 20 cells to 69.3V or 3.465vpc which may still be a little high.

Here is the NG1 115V control board:

Here is the NG3 230V control board:

https://picasaweb.google.com/lh/photo/X7hGcRCA5ZHTl6CRz91NFNMTjNZETYmyPJy0liipFm0?feat=directlink

When I had my NG3 reprogrammed I was told that it couldn't go much high enough in voltage, it was originally a 48V lead acid version, and they wanted to sell me an Elcon. I told them that they programmed my NG1 and it worked fine and that I had personally seen another NG1, programmed for 48V lead acid, put out 72V so they programmed it the same. The NG1 which I got to put out 72V was originally purchased with a NiZn profile which was reprogrammed for "8 T-875s or equivalent" before I got it. I just dialed in the ending voltage to 72V when I initially started charging a LiFePO4 pack before sending in the ones in the pictures I posted to get properly programmed.

From this I assume, like Brent said, that the basic hardware is set for different voltage ranges like both of my Zivans were built for a 48V lead acid setup, and then they have a 72V range and then maybe a 120 or 144V version. The original sticker on both my current Zivans say that 66V is the max voltage they will allow so it makes sense that they could actually put out a little above that. IIRC, the pots are 15-20 turn pots.

I'm not sure what or how I would need to take measurements on mine but let me know and I'll do my best. I have an Extech EX830 meter so I should be able to measure most anything on these if I'm told what and where.

I do want to get a board, maybe two, at some point.

customcircuits said:

Thanks for the info, it both reaffirms what I was thinking and confuses me even more. I would expect with a 2.7V cell limit and 24 cells that your limit would be 65V as well. If you adjusted your pot I would expect the limit to only go down from there.?????

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I'm guessing that the 66V was a safety limit. I never saw my pack go that high, 62V IIRC. Playing with the voltage pot got it up to 72V on a LiFePO4 pack.

Unfortunately, my logic board is all surface mount and the ones in the pictures are through hole components, making it difficult for me to do a direct comparison.

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I'm sure my chargers aren't the newest. As long as the control logic is the same it shouldn't matter. I do know that I couldn't have different profiles programmed into my NG3 except that the sticker on the front seemed to indicate that there were different options based on jumper settings. I played around with them one time and found I could get different current settings which appeared to correspond to what I would expect with different capacity packs. Maybe it is only the newer surface mount type which had the option of different charge profiles.

I think at this point the best to get a few boards out and just let you guys help run some tests. I'll work on getting some boards ordered and assembled as soon as I can.

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Do you have any idea what could happen in the worst case situation testing the a different logic board? Any chance that something might burn up in the charger due to over current or is there enough in place that maybe the worst that could happen is messed up voltage settings and timeouts? If only the latter then that is easy to deal with. Just babysit the pack when charging and unplug the charger if things go wrong.

For my NG1 I can just plug it into my Kill-a-Watt meter but I might want to get some way to measure the current out of the wall for my NG3. What do you think?

Elithion said:

That's all nice and well, but, I hate to tell you, you don't need to modify the Zivan NG3 to work with Li-ion. On the contrary, the nice thing about the NG3 is that it's a plain old CCCV charger, which is perfectly ideal to use with Li-ion (in conjunction with a BMS).

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They are a pain to get reprogrammed to what we want! I had to ask for a profile for 19 cells so that it was at the right voltage for 20 cells given that the ending current tapers to near zero. Furthermore, it costs about $100 to make a simple change.

During balancing, you do not need to ask the charger to lower the current to match the balancing loads' current. Instead, the BMS will take care of not allowing the cells to be overcharged, by switching the charger on and off, such that the average current matches the balancing loads' current.

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But if you applied that to my system the way the chargers were originally programmed it would have taken for ever to get my pack charged. I went from 48V lead acid to 20 cell LiFePO4. Your method wouldn't have worked very well with my charger the way it was.

After the pack is balanced, with a plain CCCV charger, the current will naturally go down to 0 A as the pack is topped off. That is due to physics pure and simple, not to a charger profile. So, again, there is no need to tell the charger to reduce the current. (This is true independently of a BMS.)

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And my data shows that after the pack is balanced it doesn't need to continually be balanced on every charge. Furthermore, the charge procedure is to charge to 3.6V and end the charge when the current drops to 0.05C which is 10A in my case. If the ending voltage is lower then the ending current can be lower without overcharging the cells.

But now, I am afraid that you're better off leaving the NG3 charger as is, and let your BMS do its job.

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And that is where many of us disagree with you. Besides the BMS vs no BMS debate, why would you remove one of the safety mechanisms in a charging system? What you are proposing is that a BMS never fails. It is made by humans so it is not if it will fail it is when it will fail. By having a properly programmed charger to shut off when it should, entirely on its own, you stand a better chance of your pack being protected when the BMS fails to do its job. Why wouldn't you want this redundancy?

Given the data I have on my pack so far, and that of others, the pack does not have to be balanced on every charge provided that there are no bum cells in the pack. Having a user programmable Zivan means that I can do a balance charge and then easily reprogram the unit to not charge to such a high voltage until the BMS signals that a cell is getting out of line and needs to be balanced again. My data suggests this is easily a year or more. I believe others have gone even longer.

Elithion said:

I am one of those people. Please do explain. (Seriously.)

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Naturally the idea is to be able to fully charge the cell without overcharging. A fully charged LiFePO4 cell rests at 3.4V, if it rests above this voltage it is overcharged. One option is to charge to 3.4V and just hold it there but then it would take a very long time to fully charge the cell. To compensate for the ESR of the cell, which raises terminal voltage while charging, the charging can be to a higher terminal voltage than 3.4V but then the ending cutoff current needs to be increased the farther above 3.4V the ending voltage is set. The most common charging procedure I have seen is to charge to 3.65V and end when the current drops to 0.05C. For my 200Ah 20 cell pack that would mean I should charge to 73.0V and hold that voltage until the current drops to 10A and then terminate charging. When the cells settle down they will be resting at just under 3.4V or 68.0V for my particular pack. Continuing to charge below the cutoff current of 0.05C can/will overcharge the cells. Last I checked your BMS doesn't balance at or above 10A which means you have to continue charging until the current drops to the programmed shunting current (or the BMS tells the charger to begin throttling back the current) and hold that current for a time to maintain the voltage of the highest cell at 3.65V or the shunting voltage until all cells have matched up. Even with a balanced pack this 3.65V is held with a current significantly lower than 0.05C which means that the cells are being over charged.

Test it out like I said in a previous post. "Balance" the cell and disconnect all loads from it and let it sit for a few hours then measure the terminal voltage. If it is resting at over 3.4v it was overcharged.

octagondd said:

David, I am curious if you could point me in the right direction for the data regarding the resting voltage for LiFePo4 cells. This is a very interesting discussion and I want the specs and/or data to look at with regard to what voltage a fully charged cell sits at. Are SE cells different than TS or any of the newer ones in regards to resting voltage?

Thanks,
Dave

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Sorry for the delay in responding. I've been dealing with two family emergencies at once so I'm a bit behind.

This is really a bit OT for this thread but I'll post a few things to investigate. Unfortunately I haven't found a single definitive source for this information so I have had to glean from multiple sources including user's experience (and mine), user reports, technical papers and the like.

Watch this video for a good overall understanding to LiFePO4 cells. I recommend watching it more than once if you want to get a better understanding of things: http://chargecar.org/blog/main/Battery_Tech_Talk

This paper is very technical. I originally read it to learn why manufacturers say not to charge LiFePO4 cells when they are below 0°C. If you know your physics and/or chemistry you can get some info out on voltages. It is not for the "faint of heart". http://www.tinhoahoc.com/Battery/cr030203g.pdf

There used to be a CALIB spec sheet which showed 3.4V as the "float" voltage but I can no longer find it. I also had an email from someone who posted on the EVDL, they worked for a LiFePO4 battery manufacturer, that in testing that they could fully charge a cell if it were held at 3.4V long enough. I gathered that this was quite a long time but he didn't say how long. He also wouldn't disclose which manufacturer he worked for.

Jukka Järvinen has been testing and using LiFePO4 cells for many years. Search for his name on the EVDL and you will come up with posts by him. Basically he says that if a cell rests above 3.4V then it is evidence that all the Lithium has been moved from one side to the other indicating a fully charged cell. I do know that it takes very few ions to be transferred for the voltage to go from 0V to 3V so it makes sense that a similar thing happens at the fully charged end which my testing agrees with. For the older TS cells he said that they only need to be pushed to near 4V once in a while and at low currents. Charging them to lower voltages between them is definitely beneficial for life span.

Jack Rickard (evtv.me) has done quite a bit of battery testing but it takes a while before you can glean enough info from him to understand why he makes the claims he does. Over the past 2-3 years I have pieced together enough info from him to agree with him on the 3.4V number.

The documentation which came with my TS cells in 2009 say to stop charging when the current drops to 1% of what it was to start with where 0.5C was recommended. This would be an ending current of 0.005C which is quite low. Using this method left cells sitting above 3.5V for several days. I don't know how much of that was due to capacitance of the cells and how much was due to actual Li charge storage. Remember, these LiFePO4 cells are also big capacitors with their hundreds of plates separated by a dielectric.

Finally, the best thing you can do is get some cells and equipment and play with them.

HTH, now I'm going to keep watching this thread because I want to be able to program my Zivan chargers for LiFePO4 charging with both a voltage and current cutoff.