NG3 chargers can't current limit on lithium batteries?

60365 Views 144 Replies 21 Participants Last post by jollmo, 1 mo ago

customcircuits

Discussion Starter · Oct 27, 2011

We've been using an NG3 charger for the last several years to charge our lead acid pack. We originally had Electric Conversions install an optional low current mode switch on the NG3 so that we could charge on a standard 15 amp outlet while at work.

We're in the process now of upgrading our pack to lithium. Before we purchased the new batteries we had contacted Electric Conversions regarding modifying our NG3 to handle the lithium batteries. They said they would be able to modify the charging profile for our configuration.

Now, after just receiving the charger back from Electric Conversions we noticed that the low power option had been removed from our unit. They failed to mention this little detail to us. I contacted them and all I was told was we can't current limit on lithium batteries and it would take to much time to do. If you can current limit on lead acid why not lithium? If the hardware of the NG3 supports current limiting on lead acid it should be just as capable of current limiting on lithium. I realize that it might require a separate charging profile or something but it can be done.

My plan now is to figure out the profile programming of the NG3 and modify it myself. I have a lot of professional experience in reverse engineering electronic systems and feel that it can be done. I know a lot of people have been down this path before on here and have found some decent information. If anyone has information about the NG3 charger that would help speed up the process it would be appreciated.

Two lessons here, NG3s don't currently have a current limiting capability on lithium and avoid Electric Conversions. After reading some of the other posts on here, I'm not the only one to have problems with this company.

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customcircuits

Discussion Starter · #41 · Mar 17, 2012

Ok, I have a number of boards built, tested, and ready to go.

If you are interested in purchasing one please send an email to [email protected]. I know some of you already have but please resend so I know who is still interested. I'll then send you an email invoice.

The cost for one of these units is $40. Shipping is an additional $5 for standard addresses in the US. Programming of these units will also require that you purchase a PicKit3 programmer.

Please remember that these are experimental units and the specific profile programming is left up to the end user. Everything (LEDS, relays, fans, charging profile) on the charger is now programmable/customizable so assume nothing. Using one of these units will void your warranty and we assume no responsibility for any damage that may occur from there use.

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Elithion

· #42 · Mar 17, 2012 (Edited)

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).

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.

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.)

Please read The 3 stages of charging, or, in video form.

EDIT: I take this part back: I do see the need for a way to program the Zivan voltage at your convenience. I was too hasty when I addressed just the title of the thread: "NG3 chargers can't current limit on lithium batteries". Sorry.

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customcircuits

Discussion Starter · #43 · Mar 17, 2012

You've missed the entire point of these boards. You should read from the beginning of the post. The guys at Zivan USA are the ones that do not understand their own charger.

GizmoEV

· #44 · Mar 17, 2012

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).

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.

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.)

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.

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.

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Elithion

· #45 · Mar 17, 2012 (Edited)

GizmoEV said:
They are a pain to get reprogrammed to what we want!

Ah! Right you are!

GizmoEV said:
...the pack is balanced it doesn't need to continually be balanced on every charge

I never said it did. You are correct.

The first time you charge balancing takes a while. Once the pack is balanced, in the following charge cycles the balancing phase may last just a minute or so. If you wanted to, you could rig something to skip the balancing phase after the first time, but why go through the trouble?

GizmoEV

· #46 · Mar 17, 2012 (Edited)

Elithion said:
If you wanted to, you could rig something to skip the balancing phase after the first time, but why go through the trouble?

To prolong the life of the pack by not over charging it every time it is charged.

Elithion

· #47 · Mar 17, 2012

GizmoEV said:
why would you remove one of the safety mechanisms in a charging system?

Actually, a "Li-ion profile" is not a safety mechanism. That is because, if the pack is not balanced, and there is no BMS (or the BMS as failed), long before you reach the low current portion of the profile, the charger will still pump full current into the most charged cell, which will become overcharged and may explode.

On the other side, if the pack is balanced, the current will naturally go down, without the need for a "Li-ion profile".

However, setting the correct top voltage of the charger is a redundant safety mechanism, and I am all for it: if the pack is balanced, even if the BMS fails, the pack will not be overcharged.

Elithion

· #48 · Mar 17, 2012

GizmoEV said:
To prolong the life of the pack by not over charging it every time it is charged.

Balancing does not harm cells. At least not the way balancing is done by all the BMSs I am familiar with.

On the contrary, balancing maximizes pack capacity.

GizmoEV

· #49 · Mar 17, 2012

Elithion said:
However, setting the correct top voltage of the charger is a redundant safety mechanism, and I am all for it: if the pack is balanced, even if the BMS fails, the pack will not be overcharged.

And that is why this NG mod is being made. It is being made to provide multiple charge current options, a user settable ending voltage, and the option to properly terminate the LiFePO4 charge procedure which includes the ending voltage and ending current. The last item, ending current, is what so many seem to miss/ignore continually.

Elithion

· #50 · Mar 17, 2012

GizmoEV said:
The last item, ending current, is what so many seem to miss/ignore continually.

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

GizmoEV

· #51 · Mar 17, 2012

Elithion said:
Balancing does not harm cells. At least not the way balancing is done by all the BMSs I am familiar with.

On the contrary, balancing maximizes pack capacity.

Do you have data to back that up? After balancing, disconnect the cells and let them sit for a few hours. What voltage do they rest at? If it is over 3.4V then the cells were over charged and their life shortened.

As for max pack capacity, have you done your own tests to see how much capacity is really above 3.4V? I have. I started with a TS-LFP100AHA cell resting at 3.397V and charged it 4.00V. The cell accepted only 0.6588Ah. Why over stress the cell for less than 1% of its capacity. I'm giving up less than 1 mile of range for a longer life pack.

Elithion

· #52 · Mar 17, 2012

GizmoEV said:
If it is over 3.4V then the cells were over charged and their life shortened.

Yes, absolutely.

GizmoEV said:
As for max pack capacity... I started with a TS-LFP100AHA cell resting at 3.397V and charged it 4.00V. The cell accepted only 0.6588Ah.

Yes, absolutely.

GizmoEV

· #53 · Mar 17, 2012

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

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.

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Elithion

· #54 · Mar 17, 2012

GizmoEV said:
Naturally the idea is to be able to fully charge the cell without overcharging....

Yes, I see where you're coming from, and I fully agree with your understanding. Thank you for taking the time.

GizmoEV said:
Last I checked your BMS...

Actually, that is not quite how the Lithumate BMS works. But that's besides the point. The point is that I fully agree with all you said about how cells should be treated.

bgeery

· #55 · Mar 21, 2012

Well, I receive the chip today. Thanks! Can't do much until I also receive the programmer I found on eBay.

octagondd

· #56 · Mar 21, 2012

GizmoEV said:
A fully charged LiFePO4 cell rests at 3.4V, if it rests above this voltage it is overcharged.

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

customcircuits

Discussion Starter · #57 · Mar 22, 2012

bgeery said:
Well, I receive the chip today. Thanks! Can't do much until I also receive the programmer I found on eBay.

Let me know if you have any questions. I realize there isn't a user's manual yet.

My recommendation is to get familiar and run the device outside of the charger first. You can use the PicKit to power it with 5V and run through the code. You'll want to get familiar with the debugger and your watch variables.

Once you have an understanding of how to operate the device and how to use the debugging environment, you can install the device in the charger. You'll then want to disable the power provided by the PicKit and allow the charger to provide the 5V power to it.

bgeery

· #58 · Mar 22, 2012

customcircuits said:
Let me know if you have any questions. I realize there isn't a user's manual yet.

It'll probably be a couple of weeks to receive the clone Pickit 3. Hopefully David already has the debugger in hand and will have a lead on me.

Have you experimented with getting more than the rated 10 amps out of your charger? Using the trim pot and the stock chip, I'm able to adjust it up to 13.9 amps with no ill effects so far.

customcircuits

Discussion Starter · #59 · Mar 22, 2012

bgeery said:
Have you experimented with getting more than the rated 10 amps out of your charger?

Can't say that I have. I've usually been more concerned with turning down the output. We're lucky to even find a 15A outlet around here.

bgeery

· #60 · Mar 22, 2012 (Edited)

customcircuits said:
Can't say that I have. I've usually been more concerned with turning down the output. We're lucky to even find a 15A outlet around here.

I forgot you'er running around 160V. My GEM NEV peaks out at about 85V while charging. 85V * 13.9A = 1181 Watts. Well below the 1800 Watts a standard 120V 15A outlet will handle. That extra 40% is appreciated.

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