Overvoltage protection

wali1

10 µW
Joined
Oct 21, 2020
Messages
6
Hello people,

I have a rather special question regarding electrical engineering:

Two years ago i welded a Long John style cargo bike together, build myself a 13s9p VTC6 Battery Pack safely controlled by a lithiumbatterypcb BMS and hooked the thing up with a Kelly KLS7212S to a MXUS 3k Turbo 5t.

The bike is riding like a charm and i haven't had any problems since.

But i didn't want to stop there, so i added a lighting system based on a Arduino nano. It drives two power Led's as headlights (harvested from headlamps and build into the frame tubes) as well as a strip of 80 ws2812b LED's at the back of the cargo frame which enables me to display a back-light, brake-light as well as turn signals. The whole lighting system is consuming around 50W at full light and braking. This is the reason why I power it with a cheap but beefy DCDC step-down from Aliexpress that is rated for 15A 200W and 60V. ( https://www.aliexpress.com/item/32970433581.html?spm=a2g0s.9042311.0.0.26be4c4d4dsDu0 )
The stepdown is hooked up in parallel between the battery and the motorcontroller and provides the Arduino and LED's with the required 5V.

After a year of flawless riding the following problem occurred:

I charged my battery-pack completely over night and started riding down a little hill the next morning. When i started using the brake, i recognized a little pop and smoke coming out of the electronic box.
I immediately disconnected the battery and realized that the whole 5V circuit was fried. The caps from the Stepdown exploded, the fuses from the Arduino were blown as well and that not being enough, every single one of the 82 LED's were burned through as well.

My guess for the reason of the whole fiasco is, that the battery was so fully charged, that the BMS closed to protect from overcharging and the generated power from the braking didn't had anywhere else to go. Is this conclusion realistic?
Does somebody have another answer to why I had the problem?

I found the following possibilities to protect the stepdown and in conclusion my 5 V circuit in the future:

  • Crowbar circuit
  • Zener-Diod with fuse
  • some fancy comparator circuit that requires a steady voltage that i can maybe get from the kelly controller?
  • regulating the input of the Stepdown with a power MOSFET controlled by the arduino that senses the voltage

Thanks in advance for any helping advices and i hope i didn't screw up big time with my first post in this forum.

Some pics of the project are attached,

all the best,

WALI
 

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Another possibility might be measuring the battery voltage and somehow setting the controller to only use regen braking when under a certain battery voltage.
 
wali1 said:
I found the following possibilities to protect the stepdown and in conclusion my 5 V circuit in the future:
The $5 Aliexpress converter was 60V absolute max, not nominal, and definitely not "beefy".

Just get a proper 72V-max converter and never worry about it again.
 
wali1 said:
Another possibility might be measuring the battery voltage and somehow setting the controller to only use regen braking when under a certain battery voltage.
This is indeed configurable in most regen-capable controllers.
 
fatty said:
wali1 said:
I found the following possibilities to protect the stepdown and in conclusion my 5 V circuit in the future:
The $5 Aliexpress converter was 60V absolute max, not nominal, and definitely not "beefy".

Just get a proper 72V-max converter and never worry about it again.

Alright, getting quality products seems to be the best solution once again :)
I will have to check, if the big DCDC converter from meanwell fits into my electronics compartment...

Thanks for the advice!
 
fatty said:
wali1 said:
Another possibility might be measuring the battery voltage and somehow setting the controller to only use regen braking when under a certain battery voltage.
This is indeed configurable in most regen-capable controllers.

great, i will try to find the configuration for the kelly kls7212s.
 
wali1 said:
Alright, getting quality products seems to be the best solution once again :)
I will have to check, if the big DCDC converter from meanwell fits into my electronics compartment...

Haha, it always is.
I remember the Meanwell dimensions weren't too far off from the Aliexpress converter, assuming you built an enclosure for the Aliexpress or left equivalent clearance.
 
fatty said:
wali1 said:
Alright, getting quality products seems to be the best solution once again :)
I will have to check, if the big DCDC converter from meanwell fits into my electronics compartment...

Haha, it always is.
I remember the Meanwell dimensions weren't too far off from the Aliexpress converter, assuming you built an enclosure for the Aliexpress or left equivalent clearance.

Oh, the dimensions are pretty different, in fact i just checked if I can fit the meanwell converter in my box and it does not work at all, so eighter I put it somewhere else or I come up with another idea.
Dimensions ali-express converter: 70 x 38 x 31mm
Dimensions meanwell converter: 160 x 100 x 40mm
My electronics compartment is a rhombus of about 200 x 180 x 70mm (see picture) that contains the controller, the main relay and some connectors as well as the converter + the Arduino .

Regarding to the initial question, can you tell my why the stepdown only blew up when the battery was fully charged?
Doesn't the higher voltage from regen braking also appear when the battery is not fully charged?
Why doesn't it harm to the Converter in that condition? (Just so i can improve my understanding of electronics :) - I want to improve the system anyways )

Do you maybe know how to set a max battery voltage for regen braking for a kelly controller?
If I understood correctly something like that exists.
I didn't find the setting in the Kelly manual.

Thanks a lot,
Wali
 

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A battery has a low internal resistance, so during regen braking your voltage shouldn't increase much.
If the BMS indeed shut off the battery due to overvoltage protection, it's a different story.
On some controllers you can limit the maximum amount of regen current, but the voltage?
Isolated step down converters have the advantage that in case of failure the output voltage becomes 0V.
step down converters like the one you use have the output voltage equal to the input voltage if the switching fet's become shorted.
 
obcd said:
A battery has a low internal resistance, so during regen braking your voltage shouldn't increase much.
If the BMS indeed shut off the battery due to overvoltage protection, it's a different story.
On some controllers you can limit the maximum amount of regen current, but the voltage?
Isolated step down converters have the advantage that in case of failure the output voltage becomes 0V.
step down converters like the one you use have the output voltage equal to the input voltage if the switching fet's become shorted.


Ok, so here is how I imagine it :
When the battery isn't fully charged all the current is going into the battery, since it has the lower resistance.
The DCDC-converter never the less, is still connected in parallel so it has to experience the same voltage, correct?
But due to having a much higher resistance than the battery almost no current flows through the converter branch.
I try to understand it with the help Kirchhoff's circuit laws. :wink:

So limiting the regen current will not help in my case, since every reasonable current that would charge the battery is to much current for the converter in case the BMS closes.

Since any isolated converters I could find so far, deliver either to less power or are to big for my enclosure: Whats about the crowbar methode? (https://laptrinhx.com/crowbar-circuit-design-using-thyristor-working-3838592984/)
Since the problem seems to occurs only very rarely, i would be ok with changing a fuse every now and then.
Do you think a fuse (I know, there are specifications on how fast fuses blow) triggers fast enough so the converter and the whole 5V circuit are safe?

Or do you think the only real safe way is an isolated converter?

Thanks in advance !
 
The problem is you can't tell for sure it was overvoltage that killed your step down converter.
They are rated for 15A peak and 10A continously with an output voltage set to 12V.
In your case, the output voltage is set to 5V. A power consumption of 50W is 10 Amps.
For 80 leds, this would mean 125mA / led. Assuming those are RGB's that would be 42mA / individual led.

That thyristor circuit is nice, but thyristors can also fire (start to conduct) when there is a fast voltage peak between the anode and kathode. In this circuit, you will have such if you connect the battery, so it's possible that the circuit will destroy the fuse even when there is no overvoltage.

You could use a P mosfet in serie with your supply line to switch off the voltage in case of an overvoltage.
The arduino might survive a short time of a higher voltage, but I have no ideas how well those leds will handle it.
I'll check their datasheets first.
 
Would a dual stepdown be an option?

https://nl.aliexpress.com/item/4000777481955.html?spm=a2g0o.productlist.0.0.71cf7ea8HqUgNE&algo_pvid=null&algo_expid=null&btsid=0bb0624716210857290074984e41c6&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

This module can convert 12 - 24V to 5V. So you could put your other module in front of it and set it to 15V

It should have over voltage protection.

Your first converter would only need to output 3-4A instead of 10.

You could add a little circuit on the input of the second module to detect over voltage there. A voltage divider made of 2 resistors connected to an analog input of your arduino could already do the trick.
 
obcd said:
The problem is you can't tell for sure it was overvoltage that killed your step down converter.
There is pretty clear temporal evidence that overvoltage did kill the converter. Full battery voltage + regen = overvoltage condition.

obcd said:
They are rated for 15A peak and 10A continously with an output voltage set to 12V.
In your case, the output voltage is set to 5V. A power consumption of 50W is 10 Amps.
For 80 leds, this would mean 125mA / led. Assuming those are RGB's that would be 42mA / individual led.
If they all worked fine before the supply overvoltage, current draw is unlikely to have caused the failure.
 
wali1 said:
Ok, so here is how I imagine it :
When the battery isn't fully charged all the current is going into the battery, since it has the lower resistance.
The DCDC-converter never the less, is still connected in parallel so it has to experience the same voltage, correct?
But due to having a much higher resistance than the battery almost no current flows through the converter branch.

So limiting the regen current will not help in my case, since every reasonable current that would charge the battery is to much current for the converter in case the BMS closes.
Limiting regen current will not help, but this is because the controller current limit is limiting what the attached current sinks (battery and converter) can draw. The controller doesn't "push" current -- the current sinks pull it.
You would need to limit regen voltage.

wali1 said:
Or do you think the only real safe way is an isolated converter?
Yes: do it right with a real isolated converter so you never have to worry about it again. Just expand your enclosure if necessary.
 
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