reduce voltage drop between battery and controller

Sparfuchs

1 kW
Joined
Nov 22, 2020
Messages
305
Hello EV friends,
thanks to all your help in my thread "Sabvoton SVMC 72150 doesn't do full power" (link below),we/I now think that the most plausible problem is, that the Sabvoton SVMC 72150 has a "protection" that limits the current if the voltage drop gets to a (unknown) value that can't be changed. So no, not talking about "Current Limiting Voltage" or "Lack Voltage" :)

As I wanna get more than just 100A for a few ms I'm looking for a way to reduce the voltage drop (regardless of the voltage drop in the battery itself) between my 19s4p Samsung 21700 40T battery pack and the sabvoton at max. loads of up to 200 amps (always just for a few second bursts) and have thought of using one or more capacitors and if necessary diodes between the battery and the controller, to pretend/give the controller stable voltage... or do you have any other ideas?


More precisely: I built a battery pack from 19 serial and 4 parallel connected Samsung 40T 21700 lithium-ion cells that theoretically could handle short-term pulse currents of up to 280 amperes (70A per cell) and a continuous discharge of 100A (even 180A with temp. monitoring). What you could expect is that the voltage drops under load (specifically by about 10v at 100A discharge in my case).
But what I didn't expect is that the controller seems to have a protection that limits the maximum current at a certain voltage drop (much earlier than necessary). Since this cannot be changed in the software and it seems too complex to me to access the firmware, I had the consideration with the capacitors/diodes.

I'm not sure how it behaves with the simultaneous/alternating charging and discharging of the capacitor, but I am considering a strong diode might be required in my 8AWG wire to avoid equalizing currents (the battery can be charged with max. 24A) between the capacitor and the battery, which shouldn't increase my resistance much.

It should also be mentioned that the same wire is used for charging and discharging, which usually happens very quickly and alternately through regenerative braking.

I'm not that well versed in terms of potential, but I wonder if equalizing currents occur at all if the capacitor for charging has a lower potential to the battery, and as soon as the voltage of the bat. suddenly drops due to heavy consumption and it's potential increases, probably the lowest potential should be to the consumer/controller and not to the battery, which means that the capacitor discharge only takes place in one direction (cap. to controller) anyway?

Unfortunately, I don't have the know-how, which components are available and whether/how my idea can be implemented... or whether there is even a better solution and I would be very grateful for your ideas !

Thanks for your Help
Best regards
Sparfuchs



Sabvoton SVMC 72150 doesn't do full power: ://endless-sphere.com/forums/viewtopic.php?f=30&t=118808
 
Have you measured the voltage drop while at full power in the battery wires & components? This would be your starting point. Doing a voltage drop check at each item in the battery loop will show you where problems are. This includes battery to the buss connections. You should check the BMS as well if you have one.

If voltage drop is minimal in the wiring you'll need to look at beefing up the pack. Capacitors help with quick power changes, not typically long term power needs unless you have a huge capacitor bank.

If the battery wires / components don't have much voltage drop and it's mainly from the battery pack sag, you'll likely need to add more parallel cells to prevent that.
 
999zip999 said:
35amp cell at 4p 170amp ??
No BMS ? Did you make the pack ?

He just wants to save some cash, but go for 5P --> math 5x35A equals 175Amps :bolt: :bolt:
The best way to reduce voltage drop between battery and controller is to use zero 0AWG wire. the lower better. or use parallel times AWG 8
 
999zip999 said:
35amp cell at 4p 170amp .
Are you running a BMS ? What kind ? Did you make the pack ?
Jap, according to datasheet 35amps continuous and 45 amps with temp cut off, according to independent testers like "mooch" it's 25 continuous and 35 with temp cut.

Yes I do, it's a 300amp Ant BMS and works fine.
Yes, I did.. so I know for sure that all the components are connected with the lowest resistance "possible" :wink:
 
Jrbe said:
Have you measured the voltage drop while at full power in the battery wires & components? This would be your starting point. Doing a voltage drop check at each item in the battery loop will show you where problems are. This includes battery to the buss connections. You should check the BMS as well if you have one.

If voltage drop is minimal in the wiring you'll need to look at beefing up the pack. Capacitors help with quick power changes, not typically long term power needs unless you have a huge capacitor bank.

If the battery wires / components don't have much voltage drop and it's mainly from the battery pack sag, you'll likely need to add more parallel cells to prevent that.
I just noted the resting voltage, then reseted the cycle analyst v3 trip data, then hit full throttle, and noted the Vmin and Amax from the c.a. memory. Then repeated this procedure many times to get a average. But the Vmin and Amax values were so short pulses that I can't remember that I've ever seen them live on the screen. Not sure if it also counts as the usual definition of the voltage drop if its just for a few ms but thats not the point here I guess ? :)

And as mentioned.. I used way lower resistance in all the components than needed and I't also temp. monitored but no rising temp. there :)

The voltage drop in my test is slightly higher than it should be according cell tests of other tester.. but lets assume the battery and it's drop is fine and could do more if the sabvoton let's is.

Well my thought was, that this might be what I need for just a few sec. bursts. And there could be almost no current at the cap. because "all" the current should come from the battery and the cap. should just keep up the voltage to "trick" the controller if that's even possible ?

And of course I also could "just" built a new, bigger battery pack.. but that's what I'm trying to avoid for many reasons :wink:
 
batteryGOLD said:
999zip999 said:
35amp cell at 4p 170amp ??
No BMS ? Did you make the pack ?

He just wants to save some cash, but go for 5P --> math 5x35A equals 175Amps :bolt: :bolt:
The best way to reduce voltage drop between battery and controller is to use zero 0AWG wire. the lower better. or use parallel times AWG 8
Nope, it's not about saving a couple of dollars and even if I'd build a new pack I'd get more range. And as I won't throw away the current pack and could use it for other projects it won't be any financial loss at all. But it's a bike with a self designed steel construction for a motocross 2 person seat and as I'm not that experienced with those things it took me more time than I wanna admit to design it so precise around this battery pack. And I will absolutely not start all over again :lol: So I'd have to use the exact same battery case/ cell count for the new pack and more parallel cells is way less serial and less voltage on my already very low kv wind motor.
 
Sparfuchs said:
Well my thought was, that this might be what I need for just a few sec. bursts. And there could be almost no current at the cap. because "all" the current should come from the battery and the cap. should just keep up the voltage to "trick" the controller if that's even possible ?

If you know where the controller measures the pack voltage on the pcb you could trick it for a short time by adding a capacitor into that circuit. But this piece of code is likely trying to limit motor current from seeing battery voltage sag too far so it doesn't damage a battery.

If your battery packs voltage sag is more than expected you could have weak cells, bad connections, or bad components. Don't assume things are good because you used good components and did a good job. Things happen and component quality doesn't always match brand image. It's worth measuring this stuff at each junction if it's something your setup is struggling with. Trying to work around it could damage the battery.
 
Here look at this 40T.

@ 40A it drops all its volts. From 4.2v/cell to 3.17v / cell on a 40A discharge.

2p would drop all its volts from 80A. Pack hits LVC.
4p drops all its volts at 160A... and the pack hits LVC. All the usable volts are gone.

@ 20A, is still dangerously close to LVC territory. 20A brings you to 3.36 v / cell... I consider a 3.2v or less as a cell compiled and compromised and cannot apply any more amps. So even at 20A this cell is almost overused.

@ 20A, this sags the pack almost to LVC. 2p this means a sag to near LVC at 40A.. and 4p, this is almost sag to LVC on 80A.

So you will sag almost to LVC on a 20A per cell discharge. Almost all usable volts are gone.

At 10A per cell, 40A for a 4p, the pack will sag from 4.2v to 3.48v per cell.. ON only 10A per cell. Half the usable volts in the pack are gone. 1s pack, 6s pack, 14s, 20s pack. Half the usable volts are gone a 10A hit.
 

Attachments

  • sammysung40t.JPG
    sammysung40t.JPG
    71 KB · Views: 250
The sabvototon has six 100V 680 uf capacitors and looks a bit like that already acts as a cap. bank or what else what they do ?
I'll try to check with the volt value with the app (while driving) to see if the sabvoton even notices the voltage drop. I would guess it makes more sense for the sabvoton to measure real time voltage before the caps.. also to notice volt drop. But if so there should be ways to change that.
Thanks for the idea @jrbe
1675066333655.jpg
 
What do the capacitors do? They snub. "Snubber capacitor". What is a Snubber Capacitor? A snubber capacitor is a capacitor that is connected to a large-current switching node for the purpose of reducing the parasitic inductance of electric wiring.




I doubt the wires from battery to controller are a bottleneck. You will not see extreme loss of power in wires like this.

If a great voltage drop is suspected through the wre form battery to controller, measure it.

Measure the input V to the controller and the output V of the battery.

" Total power loss"

(output Vbatt)-(Input Vcontroller ) x ( amps of draw) = watts loss
Example :
Lets say you output 10v with your battery. Under 2A current, you see the controller receive 9.5v.

10(v) - 9.5(v) = 0.5v

0.5(v )x 2(A) = 1 watt loss in the wire.

To further figure the resistance of this circuit, the equation would be 1(watt) / 1 ( wire) and...

P ( in watts)/A^2 = R...

1/(2x2)

1/4

0.250 ohm.

250 mOh. This wire would be 0.25Ohm.


So you have a 58v full charge battery. At battery points.

At controller points you see .. lets say 50v.... at a 40A load. 8v drop at 40A load.

58(v) - 50(v) = 8v drop ( huge drop in wire, would expect to see this drop in a controller under load but not in a wire just conducting)

8(v) x 40 ( A) = 320w waste in wire.

320(w)/ ( 40A x 40A )

320 / 1600

0.20 Ohms.

200mOh.

I doubt the wire is a significant loss of volts and inherent power from. Wire is usually in the range of tenths of ohms.. Milli ohms.

A 10AWG wire is 1mOh per foot.. 0.9989mOh per foot. Negligible.
 
Do you think the controller is " limiting current"? Test this hypothosis.

One way is to do this. Trick the shunts in the controller into seeing ( sensing) less current than they actually are. Shunt mod the output wires.


This might damage the controller though. However, if you suspect the controller is limiting amperage, this would fool it into outputting more amps. So you could test the real amps taken and see if this is changing the number with the modification.

Not recommended and might void warranty. Only for suspect testing.
 

Attachments

  • shuntmodSABVOTON.jpg
    shuntmodSABVOTON.jpg
    11 KB · Views: 246
Here is a picture of my connections and the 8awg wire I used. As the voltage drop isn't really that unexpected to me, I l'd just assume everything is fine.
But if you still think it's useful to do do more tests I will.
So my available measuring instruments are a multimeter and my cycle analyst v3. What exactly do I have to do to check ?Screenshot_2023-01-21-16-29-25-130_com.miui.gallery.jpg
 
Sparfuchs said:
I l'd just assume everything is fine...

Sir, you battery looks very nice! But there are some problems associated to safety around that assembly method/design..

1st
Never glue battery cells without separator holders or a card insulation at middle of serial elements. Hot glue?? Serious? It will get lose soon. causing cells to do friction into next and previous elements --> This soon or later will lead to a fire :warn:
Hi power ebike, high vibration x,y transmitted to battery structure. So imagine what could happen :pancake:

2nd
U need to use top cells card insulator rings. Your design is danger, since copper may will cut into cell plastic and lead to a fire soon or later.. :warn:

3rd
As I see sabvototon controller is 150A peak, doing some math 150A/4P equals 37,5Amps draw from each cell!! This is extreme limit :bolt: But it will work.. Maybe not good ideia to go full throttle all time or to do uphill.. At least add a temperature monitor system to battey to monitor bat temperature while riding ebike.

Sorry about bad news. Anyway, U could use that battery anyway at your own responsibility.. But U are warned!
Be on the safe side, because battery cells are extremely flammable & similar to pyrotechnics :flame: :flame:

Have a nice day :bigthumb:
 
Sparfuchs said:
Here is a picture of my connections and the 8awg wire I used. As the voltage drop isn't really that unexpected to me, I l'd just assume everything is fine.
But if you still think it's useful to do do more tests I will.
So my available measuring instruments are a multimeter and my cycle analyst v3. What exactly do I have to do to check ?Screenshot_2023-01-21-16-29-25-130_com.miui.gallery.jpg

This gives some info on doing a voltage drop test. You are measuring voltage between connections to see how bad the connection is, the higher the voltage (voltage drop) the more resistance at that connection - where your meter probes are.
https://www.agcoauto.com/content/news/p2_articleid/331

Dogdipstick did the math out for you on what to expect from your pack. If you have other voltage drop issues in the connections /bms / etc. Voltage will be lower than their math and show up at the controller.

I really don't recommend fooling the controller with the capacitor trick.
 
Jrbe said:
But this piece of code is likely trying to limit motor current from seeing battery voltage sag too far so it doesn't damage a battery.
I'm pretty sure it is what it does. But I'm also pretty sure that my cells are not even close to their limits with a discharge of 20amps for just a few milliseconds! and no change in temperature at all. In tests from others with 45amp "continuous" per cell the temp. got up to 80 degrees and more. So doesn' t a not even war battery pack that dropped down from about 80v to about 70V mean that it's far away from being abused ?
 
DogDipstick said:
Here look at this 40T.

@ 40A it drops all its volts. From 4.2v/cell to 3.17v / cell on a 40A discharge.

2p would drop all its volts from 80A. Pack hits LVC.
4p drops all its volts at 160A... and the pack hits LVC. All the usable volts are gone.

@ 20A, is still dangerously close to LVC territory. 20A brings you to 3.36 v / cell... I consider a 3.2v or less as a cell compiled and compromised and cannot apply any more amps. So even at 20A this cell is almost overused.

@ 20A, this sags the pack almost to LVC. 2p this means a sag to near LVC at 40A.. and 4p, this is almost sag to LVC on 80A.

So you will sag almost to LVC on a 20A per cell discharge. Almost all usable volts are gone.

At 10A per cell, 40A for a 4p, the pack will sag from 4.2v to 3.48v per cell.. ON only 10A per cell. Half the usable volts in the pack are gone. 1s pack, 6s pack, 14s, 20s pack. Half the usable volts are gone a 10A hit.
Well, for me all the voltage means from 4,16v down to 2,5v. So almost 30v from fully charged to lack voltage. Means at 160 amps still 10v left before my lack voltage :)
Of course down to 3,17v would be better for cycle life, but I don't think going closer to 2,5 for a few sec. from time to time won't harm to much and is fair balance between more fun/less cycles if the controller would allow it :)
But I can't even do the 20v drop at 160 amps.
Anyway I agree that my 4p are far away from ideal.
 
DogDipstick said:
What do the capacitors do? They snub. "Snubber capacitor". What is a Snubber Capacitor? A snubber capacitor is a capacitor that is connected to a large-current switching node for the purpose of reducing the parasitic inductance of electric wiring.

I doubt the wires from battery to controller are a bottleneck. You will not see extreme loss of power in wires like this.
Thanks a lot for your explanation, but I'm afraid I don't have enough basic knowledge to really get it in detail. But I guess I don't have to because your "shunt mod" hint might solve my problem.

I used 8 AWG wires with over all about 16 inch length. So I also doubt that there is a problem with it :)
 
Think of electricity as water. A capacitor is like a water tower. Voltage is like water pressure. Amperage is like water flow. Lots of flow (lots of amps) bigger pipes required (bigger wires.)

A capacitor helps a pulsed load receive a more constant voltage supply. They also can absorb some voltage spikes.
 
DogDipstick said:
Do you think the controller is " limiting current"? Test this hypothosis.

One way is to do this. Trick the shunts in the controller into seeing ( sensing) less current than they actually are. Shunt mod the output wires.


This might damage the controller though. However, if you suspect the controller is limiting amperage, this would fool it into outputting more amps. So you could test the real amps taken and see if this is changing the number with the modification.

Not recommended and might void warranty. Only for suspect testing.
Awesome, that's easier, cheaper, better than a cap bank !
Now I feel kinda stupid because I started a thread called "Controller tuning or mods for more volt or amps" and this shunt mod was discussed a lot and even you yourself replied to it :lol:
Why the hell did I not think of it myself ?! Maybe because it was a while ago and because I left the whole topic behind me by buying unlocked controllers.
Anyway, thanks a lot !
If you say "This might damage the controller" do you mean its more likely than with a unlocked one ? Is there a difference in risk between a unlocked controller set to 200 dc amps or a shunt moded set to 100 dc amps ?
If that doubles my dc current (without the controller knowing that) how do I know how exactly it influences the phase amps ?
 
batteryGOLD said:
Sir, you battery looks very nice! But there are some problems associated to safety around that assembly method/design..

1st
Never glue battery cells without separator holders or a card insulation at middle of serial elements. Hot glue?? Serious? It will get lose soon. causing cells to do friction into next and previous elements --> This soon or later will lead to a fire :warn:
Hi power ebike, high vibration x,y transmitted to battery structure. So imagine what could happen :pancake:

2nd
U need to use top cells card insulator rings. Your design is danger, since copper may will cut into cell plastic and lead to a fire soon or later.. :warn:

3rd
As I see sabvototon controller is 150A peak, doing some math 150A/4P equals 37,5Amps draw from each cell!! This is extreme limit :bolt: But it will work.. Maybe not good ideia to go full throttle all time or to do uphill.. At least add a temperature monitor system to battey to monitor bat temperature while riding ebike.

Sorry about bad news. Anyway, U could use that battery anyway at your own responsibility.. But U are warned!
Be on the safe side, because battery cells are extremely flammable & similar to pyrotechnics :flame: :flame:

Have a nice day :bigthumb:
Thanks for your recommendations,

But there are insulation rings, they are just under the insulation wrap in my case. Hot glue is a common method for space saving builds but in my case it was mainly to keep the cells into place while spotwelding. I made sure the pack can't move in the case at all. I got to admit the contacts are quite close to each other... that's why I did many layers of spray insulation varnish on it for the unlikely case that something comes loose at a crash. But if it really leads to fire, the crash must have been so fatal that the fire would be my smallest problem :wink:
I guess however you built it there are always ways to make it more safe (but also with
disadvantages) so If I wanna make 100% sure there is no chance, in any scenario that something goes wrong I should just power my bike with muscles :p

and there are three temp. sensors on different spots in my pack to monitor :)
 
Sparfuchs said:
Thanks for your recommendations :thumb:

Sir Fox, your battery is great!! don't worry about anything and just enjoy many power from it :bolt: :bolt:
those cells give high current output, so your ebike will go very power for sure! :thumb:


your battery has 4Px 4000mAh= 16Ah battery for a power e-bike ok. but if you charge up to 4V means lose 20% cap, so your battery will only put out 12,8Ah but able to output up to 35Ax4= 140A or maybe 40Ax4= 160Amps..
recommended for less than 20Km rides full throttle, but can go up 30Km-40Km if you go 25Km/h (EU ebike limit), maybe perfect for one day of e-cycle fun!..

A good idea, give a lot of spray insulation varnish between cells top positive and copper sheet, means a extra layer of safety.
Hot glue could be nice for filling all battery air spaces, between cells. This way you create battery 3D plastic holder using many hot glue.. This trick could give Ur battery a safety certificated! :pancake:
Battery temperature monitor is recommended for first rides. to know battery thermal behavior (dont go more than 60celcius :lowbatt: )

PS (out_of_topic funny) I've seen some China batteries for citycoco scooters adverted as 12Ah.. but China people used configuration 4P 18650 2000mAh. so means total pack 8Ah and sell as 12Ah (with nice label telling 12Ah.. citycoco scooter needs 30A peaks or 30A continuous at uphill, so 30A/4P = 7,5A per cell.. ) battery goes on a big plastic suitcase bag style,but when you open it.. U just see a small battery half size of battery case!.. anyway it works great at citycoco for 20Km ride flat road enjoying riding slow speed, near beach, under great sun! here at PT we call that battery 4P "cuxi" means cute for a square shape battery surrounded by foam and nice wrap China sells many!!)

Keep UP your project! :bigthumb:
 
You could also add a fast or very fast charger to your system!! :bolt: :bolt: Butt.. who needs fast charging??.. :pancake:
40T supports charge rate of 6A up to 4,2V, but if U charge to max 4,0V U could charge @ rate 8A or 10A..

soo 10Ax4P = 40A charge rate
this means U could go to a 40A 75,2V charger (3000W charger!!) maybe AC plug can't hold soo much power draw.. incandescent AC wires :twisted:

Anyway if U charge a 4000mAh cell at 10A , charge time goes to only 30min for a full charge (2,7V to 4,2V) :bolt: :bolt:
or only 10min to charge 50% (allways carry your charger with you on any ebike ride)

But U could go to datasheet standard recommended value of 2A charge per cell, this means 2A x 4P = 8A charger. maybe a 10A charger is a good ideia 10A x 75,2V equals 750W charger, now charge time goes to 60-80minutes full or 20-30minutes to charge a 50% energy packet
Better to charge Your battery in small packets instead of a full time charge.Battery last longer :thumb:

HOW to calculate charging time:
Imagine U have 10Ah battery and using 2A charger, soo using math 10Ah/2A equals 5hours, but this is ideal case. At reality we have a factor multiplier for charging losses, this factor maybe 0,8 value
Soo now you need to take that into consideration, means your charging time from 0% to 100% is now 5 divided by 0,8 equals 6hours,15min total time to charge 0->100% a 10Ah battery with a 2A charger or maybe up to 7hours, because final charge current decreases slow at the end of any charge.

Its good to know all math behind tha scenes :bigthumb:
 
*edited
hope U people learned some math today! :bolt: :bolt:

At this moment after U people read this , some of U are now doing math calculations for your own battery system charging time calculation, maybe.. :thumb:

Compare real life charging time hours results vs math theoretical value calculated. Does it match?? :pancake:
 
batteryGOLD said:
You could also add a fast or very fast charger to your system!! :bolt: :bolt: Butt.. who needs fast charging??.. :pancake:
40T supports charge rate of 6A up to 4,2V, but if U charge to max 4,0V U could charge @ rate 8A or 10A..

soo 10Ax4P = 40A charge rate
this means U could go to a 40A 75,2V charger (3000W charger!!) maybe AC plug can't hold soo much power draw.. incandescent AC wires :twisted:

Anyway if U charge a 4000mAh cell at 10A , charge time goes to only 30min for a full charge (2,7V to 4,2V) :bolt: :bolt:
or only 10min to charge 50% (allways carry your charger with you on any ebike ride)

But U could go to datasheet standard recommended value of 2A charge per cell, this means 2A x 4P = 8A charger. maybe a 10A charger is a good ideia 10A x 75,2V equals 750W charger, now charge time goes to 60-80minutes full or 20-30minutes to charge a 50% energy packet
Better to charge Your battery in small packets instead of a full time charge.Battery last longer :thumb:

HOW to calculate charging time:
Imagine U have 10Ah battery and using 2A charger, soo using math 10Ah/2A equals 5hours, but this is ideal case. At reality we have a factor multiplier for charging losses, this factor maybe 0,8 value
Soo now you need to take that into consideration, means your charging time from 0% to 100% is now 5 divided by 0,8 equals 6hours,15min total time to charge 0->100% a 10Ah battery with a 2A charger or maybe up to 7hours, because final charge current decreases slow at the end of any charge.

Its good to know all math behind tha scenes :bigthumb:
I do use a adjustable charger and charge it to 4,1v per cell with 24amps. When it's done I charge it to 4,16v per cell with just a few amps. Seems safe and good for for cycle life to me and is definitely fast enough :)
 
Back
Top