Transient voltage suppressor

Hello everyone..again.

I have a situation that may or may not be unique, so here goes:

The setup: I have a battery bank of 20 LiFePO4 batteries, charged by 6, 420 watt solar panels feeding a 100 amp MPPT charge controller. The panels are mounted in an array that tracks the Sun in 2 axis, and is controlled by an Arduino Mega. The Mega is fed with a power supply that can take from 8 to 40 volts and step it down to 5 volts. There is also a 6KW inverter connected to the batteries.

The problem: As long as the batteries are charging, everything is fine. The problem arises when the last battery's BMS shuts off, and the charge controller is now powering an open circuit, and the voltage spikes high enough to reset/damage? the arduino power supply, and trips the inverter's overvoltage alarm. It only lasts a few milliseconds before the charge controller shuts it off, but the damage is done, and everything has to be reset.

Solutions tried: I added this TVS diode across the battery leads from the charge controller, and now the arduino works fine, but it still exceeds 32 volts and thus trips the inverter overvoltage shutdown/alarm. BTW, Tech support for the controller is useless. I speak Chinese about as well as I do Aramaic. I'm on my own, apparently. LOL

Two weeks ago, I had never heard of a TVS diode, and thought that a diode with a breakdown voltage of ~31 volts would work. Wrong. Digging into the data sheet, I see that 31.28 is the MINIMUM breakdown voltage, while the max is over 34 volts. A diode with the maximum BD voltage below 32 volts puts the standoff voltage less than the highest battery bank voltage of ~29 volts, and the last thing I want is to create a situation where there is essentially a dead short across the battery bank and it's 2000 amps.

I hate to reinvent the wheel here, so am open to suggestions from folks more knowledgeable than I (almost everyone here, I suspect).

Sorry for the long post, and as always, if the mods feel this is in the wrong section, please move it. A hearty thank you to anyone who has read this far!

You select a TVS diode AFAIK only by stand-off voltage, not by breakdown voltage.
So use a TVS with the same (not higher) voltage as maximum continuous circuit voltage.

Minimum breakdown voltage is where the TVS (zener on steroids) must start to conduct.
Maximum is where it releases the magic smoke.
Leo..

Wawa:
Minimum breakdown voltage is where the TVS (zener on steroids) must start to conduct.
Maximum is where it releases the magic smoke.
Leo..

I'm still pretty green on this topic, but the spec sheet lists a maximum clamping voltage for this part of 50 volts. Here are the specs:
Reverse standoff voltage: 28
Breakdown Voltage: Min 31.28 Max 34.41
Maximum peak pulse current: 606 Amps
Maximum clamping voltage: 50

My battery bank runs around ~27.5 - 28 volts, and I need something that prevents the transients from exceeding 32 volts. As usual, it seems I have a lot to learn.....

JohnDeere630:
My battery bank runs around ~27.5 - 28 volts...

Then a TVS with 28volt stand-off seems ok.
They unfortunately don't start to conduct significantly until voltage reaches about 33volt.
You can put more than one in parallel...

Maybe you could try to slow down the rise-time of that voltage with a large capacitor.

The BMS I know use shunt regulation, and don't disconnect the battery.
Does the charge controller have a 'load dump' connection?
Leo..

Wawa:
Then a TVS with 28volt stand-off seems ok.
They unfortunately don't start to conduct significantly until voltage reaches about 33volt.
You can put more than one in parallel...

Maybe you could try to slow down the rise-time of that voltage with a large capacitor.
Leo..

How large would you suggest, or better yet, how would I figure that out myself? I assume an eletrolytic cap?

Wawa:
Then a TVS with 28volt stand-off seems ok.

The BMS I know use shunt regulation, and don't disconnect the battery.
Does the charge controller have a 'load' connection?
Leo..

You are probably correct, since the inverter is always powered on. Nonetheless, when they finish charging, and the controller current drops to zero, there is a voltage spike. I'll readily concede that my understanding of just what is going is less than optimal.

As an aside, I wired 2 small 12 volt UPS batteries I had into 24 volts, and connected them to the system more as an experiment than anything, and the voltage spike went away. I assume they act as a big capacitor to simply absorb the transient.....?

JohnDeere630:
You are probably correct, since the inverter is always powered on. Nonetheless, when they finish charging, and the controller current drops to zero, there is a voltage spike. I'll readily concede that my understanding of just what is going is less than optimal.

As an aside, I wired 2 small 12 volt UPS batteries I had into 24 volts, and connected them to the system more as an experiment than anything, and the voltage spike went away. I assume they act as a big capacitor to simply absorb the transient.....?

I do not see where you ever quantified the voltage spike. IS it equal to the open circuit voltage of the solar panels or is it greater? If greater, then there is an inductance somewhere with a collapsing magnetic field that is generating the spike. Also, is the spike + or -.

All batteries are capacitors and resistors in series. So, yes, your UPS batteries are effectively electrolytic capacitors.

Paul

Paul,
the reason I assumed there was a voltage spike was because the inverter's overvoltage alarm was being tripped. I have no way to tell how much it spikes, nor the duration. I have an oscilloscpe, but have not figured how to really use it for this purpose. I bought it to diagnose a problem in an i2c circuit.

Anyway, assuming the spike may be as much as the OC voltage of the panels, which could be as high as 150 volts, I'd need an electrolytic cap good for maybe 200 volts, but how many uf? How would I calculate that value?

Without know anything about the "voltage spike", you cannot pick a proper capacitor. Get several of one value and add them in parallel to increase the capacity until you get what you want.

Paul

I'll try to quantify what's going on with my oscope....as soon as I figure out what settings to use. Stabbing at a moving target in the dark seems counterproductive, LOL.

JohnDeere630:
I'll try to quantify what's going on with my oscope....as soon as I figure out what settings to use. Stabbing at a moving target in the dark seems counterproductive, LOL.

You are looking for the voltage peak and the time duration of the spike.

Paul

Paul_KD7HB:
You are looking for the voltage peak and the time duration of the spike.

Paul

I put my scope on it today, as well as my digital MM that captures maximum and minimum values, and learned a few things. First, the TVS diode seems to be working, as voltages didn't rise above 32.21 volts, nor was the inverter's OV alarm tripped, although it probably would be, sooner or later. (The manual doesn't specify how fast the voltage is polled.)

Nothing shouted out from the scope, at least not to my untrained eyes. The ripples seem pretty small, and I had the set the trigger down to 29.2 volts to capture them. I attached a pic of the waveform, but I don't think this is causing any problem. I did catch something else going on that I will detail in the next post.

Paul_KD7HB:
You are looking for the voltage peak and the time duration of the spike.

Paul

I put my scope on it today, as well as my digital MM that captures maximum and minimum values, and learned a few things. First, the TVS diode seems to be working, as voltages didn't rise about 32.21 volts, nor was the inverter's OV alarm tripped, although it probably would be, sooner or later. (The manual doesn't specify how fast the voltage is polled.)

Nothing shouted out from the scope, at least not to my untrained eyes. The ripples seem pretty small, and I had the set the trigger down to 29.2 volts to capture them.

The next 2 pics show what I think is really going on. The entire line rises past the trigger point, and the digital MM showed a peak of 32.21 volts. This happens every few minutes

This shows the normal waveform:

And the rise:

I have a pair of small (8 amp/hr) UPS batteries wired for 24 volts, and when they are connected, this intermittent rise goes away. Could I use capacitors to replace the small batteries? It seems the voltage doesn't get much above 32-33 volts, so a 40 or 50 volt electrolytic cap seems indicated, but I have no idea how to determine how many uF are needed.

Unless I missed it somewhere, it is time for a schematic drawing of exactly what you have there.

Paul

Paul_KD7HB:
Unless I missed it somewhere, it is time for a schematic drawing of exactly what you have there.

Paul

Paul, I certainly will if you think it will be helpful, but it is just 20, 24 volt lithium batteries, all connected in parallel, with a 6KW inverter and a 100 amp MPPT charge controller connected to it. The scope and MM were connected to the inverter leads.

JohnDeere630:
Paul, I certainly will if you think it will be helpful, but it is just 20, 24 volt lithium batteries, all connected in parallel, with a 6KW inverter and a 100 amp MPPT charge controller connected to it. The scope and MM were connected to the inverter leads.

Ok, then the reason for the voltage rise is likely to be caused by one or more of your 20 batteries not being identical as far as age, chemistry, manufacturer date, or even different manufacturer.

Paul

Paul_KD7HB:
Ok, then the reason for the voltage rise is likely to be caused by one or more of your 20 batteries not being identical as far as age, chemistry, manufacturer date, or even different manufacturer.

Paul

They are all the same batch from the manufacturer, but I would not be surprised in the slightest if there were some small variations among them. There is nothing I can do about that that I know of, however. The small batteries/TVS combo do a good job of stabilizing the overall system voltage, but I dislike the kludginess of having these batteries connected if a capacitor or capacitors could do the same thing. Having said that, I also may have to accept that capacitors won't work, and I'm stuck with what I have.

JohnDeere630:
They are all the same batch from the manufacturer, but I would not be surprised in the slightest if there were some small variations among them. There is nothing I can do about that that I know of, however. The small batteries/TVS combo do a good job of stabilizing the overall system voltage, but I dislike the kludginess of having these batteries connected if a capacitor or capacitors could do the same thing. Having said that, I also may have to accept that capacitors won't work, and I'm stuck with what I have.

That is a good start, but how old are the batteries?

Also begin a program of monitoring the temperature of each battery. One or more will likely be hotter than the others.

Paul

Paul_KD7HB:
That is a good start, but how old are the batteries?

Also begin a program of monitoring the temperature of each battery. One or more will likely be hotter than the others.

Paul

The whole system is brand new. Each battery has an LCD on it, and all 20 battery temps are within 0.1 degrees C of each other, all read 100% charged, and all read 27.8 volts.

Unless a battery was defective, I doubt I'd ever see much temperature rise, since they are being charged and discharged far below their maximum rate of 100 amps. At most, each battery is only getting 5 amps of charging current, and at the inverter's maximum surge rating, they are discharging at only about 35 amps each.