Battery overvoltage protection - dump load

Discuss hardware related to the VESC such as the NRF nunchuk.
Nordle
Posts: 37
Joined: 23 Mar 2016, 17:04
Location: Italy

Battery overvoltage protection - dump load

Postby Nordle » 10 Jul 2018, 15:39

Hello,
because i run several times into overvoltage cutoff and have no other brakes than my motors on electric boards, i tried to make a circuit to solve this problem.
It should switch a dump load in parallel to my battery and ''v''esc whenever the voltage in said lines goes above a set value 0,5 volts less than cutoff is set in vesc), and switch it back off when voltage is aprox. (depends a bit on set voltage) 0.5v lower than the set point.
here is my circuit:
circuit (1).png
circuit (1).png (107.51 KiB) Viewed 658 times

any toughts?
how big should the dump load be? the same size as i set my battery regen current? and any ideas what i could use, these resistors are big...

please try to keep it simple, i'm relatively new to this^^
got a lot of help here -> https://forum.allaboutcircuits.com/threads/braking-chopper-overvoltage-dumb-load.149820/ just in case someone wants to see my noobish errors :)
Last edited by Nordle on 15 Jul 2018, 16:46, edited 1 time in total.

ThierryGTLTS
Posts: 86
Joined: 09 Aug 2017, 11:10

Re: Battery overvoltage protection - dumb load

Postby ThierryGTLTS » 11 Jul 2018, 09:16

Good idea :!:

The resistor should dumb the "regen" power, so you have to calculate it.

It depends mostly of your hardware, of the downhill you're doing, and how "agressive" you're braking.

Have a Nice Day.

Thierry

pf26
Posts: 305
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Battery overvoltage protection - dumb load

Postby pf26 » 12 Jul 2018, 14:38

I doubt you scheme will work. When the dump load is switched on, the internal resistance of the battery will cause the main voltage to drop, and depending upon the hysteresis of your comparator, will switch off the load or not. Anyway, you need some capacitance somewhere to have your dump load switched at a reasonnable frequency.
If you dig into this forum (search term "dump" ?), there has already been some dump load circuits proposed and some tested I think.

TilmanBaumann
Posts: 15
Joined: 15 Feb 2018, 16:50
Location: Germany

Re: Battery overvoltage protection - dumb load

Postby TilmanBaumann » 12 Jul 2018, 18:19

I guess it could work, Though I suspect it could be quite susceptible to oscillations.
As long as the load cuts in progressively and let's go when the voltage is below acceptable again, I think it should work in practice.

I wonder, does it need to have a 'load'? Why not drive the current straight into the FET and dissipate the heat there?

Nordle
Posts: 37
Joined: 23 Mar 2016, 17:04
Location: Italy

Re: Battery overvoltage protection - dumb load

Postby Nordle » 13 Jul 2018, 07:38

thanks for the feedback

pf26 wrote:I doubt you scheme will work. When the dump load is switched on, the internal resistance of the battery will cause the main voltage to drop, and depending upon the hysteresis of your comparator, will switch off the load or not. Anyway, you need some capacitance somewhere to have your dump load switched at a reasonnable frequency.

where would you add said capacitance? 1uF between opamp +input and ground?

TilmanBaumann wrote:I wonder, does it need to have a 'load'? Why not drive the current straight into the FET and dissipate the heat there?

because i think that's not healthy for the fet or battery, and voltage would drop even faster, thus make oscilation even worse

i think ideally there would be a variable load wich is always the same as my braking current
if the load is bigger as my braking current it would oscilate
and if the load is smaller than my braking current it would still go into overvoltage cutoff at some point
are these thoughts correct?

TilmanBaumann
Posts: 15
Joined: 15 Feb 2018, 16:50
Location: Germany

Re: Battery overvoltage protection - dumb load

Postby TilmanBaumann » 13 Jul 2018, 20:32

I'm no expert. But I would say the idea is to burn exactly as much energy on the FET, respectively the load, as it takes to pull the voltage below limit.
It's not just on/off. Effectively a constant current load proportional to the voltage error.

pf26
Posts: 305
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Battery overvoltage protection - dumb load

Postby pf26 » 15 Jul 2018, 16:06

Did you have a look at this one: http://vedder.se/forums/viewtopic.php?f=15&t=586 ? (dump, not dumb..)

Nordle
Posts: 37
Joined: 23 Mar 2016, 17:04
Location: Italy

Re: Battery overvoltage protection - dump load

Postby Nordle » 15 Jul 2018, 16:50

thanks, i'm a bit dumb sometimes:) still have to read that topic

TilmanBaumann
Posts: 15
Joined: 15 Feb 2018, 16:50
Location: Germany

Re: Battery overvoltage protection - dump load

Postby TilmanBaumann » 16 Jul 2018, 11:31

Oh yes pf26, I like that one. Simple and straightforward.

arvidb
Posts: 234
Joined: 26 Dec 2015, 14:38
Location: Sweden, Stockholm

Re: Battery overvoltage protection - dump load

Postby arvidb » 17 Jul 2018, 00:14

Nordle wrote:still have to read that topic

If you don't want to read all of it, you can head straight to the working schematic. Also see rew's suggestion to use a power LED as an "automatic dump load". Very convenient, if you find power leds with the right forward voltage.


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