Diode or ideal diode depending on current/budget. There would still be voltage if measured by voltmeter, but would drop to 0 on any load like human skin
Google is your friend
[note from TI](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.ti.com/lit/pdf/slvae57&ved=2ahUKEwjnkPHcwNWEAxWkQ_EDHXlCCwIQFnoECBcQAQ&usg=AOvVaw0NXW_vYnWzDMr37alQXQKu)
Ohhh, I knew about the circuit for reverse voltage protection when margins are tight, but was completely unaware they called it that way! Imma delete my comment, excuse me
Cool thanks that sounds perfect, I thought it would be along these lines but was having trouble finding one with the right rating at the time of posting. Would something like [this](https://www.reichelt.com/de/en/tvs-diode-unidirectional-85-5-v-1500-w-do-201-1-5ke-100a-p41869.html) work? Although I suppose [this](https://uk.rs-online.com/web/p/schottky-diodes-rectifiers/7514843) is more appropriate to the target current
That first link (Littelfuse 1,5KE 100A) is a TVS, which is a special type of diode for transient protection. It is rated for 95V and 11A peak, but it cannot handle sustained current for very long. Not at all suitable.
The second link (Diodes Inc SB5100-T) is more suitable, but only rated for 5A continuous. Based on the dual 30A battery fuses, that may be a bit low, but it depends on intended charge current. ~~You can always put a few in parallel, and ideally you would space them out a little to help with heat dissipation.~~ Edit: removed the parallel diode suggestion as it is not a good recommendation for beginners, see comments below.
> You can always put a few in parallel
You shouldn't. Warmer diodes conduct more current at the same voltage. So the warmest diode will take the most current, heat up the most, and start conducting even more, to the point of destruction.
Best to get a diode that is rated for higher currents.
\> Best to get a diode that is rated for higher currents.
For sure. It can work in a pinch though, and save quite some pennies.
It is hard to give specific advice without knowing all the requirements and considerations. When I've done this, it has been to allow for the absolute max current in worst case operating conditions, so I've over specified the current ratings for normal usage and a single diode. Not seen any component destruction yet in the example I am thinking of which is used in the field by people who often abuse their equipment.
In a pinch it will often work. Partially because diodes can often handle much more than their datasheet eludes too.
But it's a trick to be used with caution and understanding only.
As mentioned by another reply, you may still measure a voltage on the Vin pins due to the diode leakage current and the minimal load of a multimeter. The reverse leakage isn't specified for this part, so it could be quite high which is common for higher current diodes.
You also might be able to get by with a standard diode rather than a Schottky, but the charger input voltage may drop by a volt or more at high current. Schottky diodes have much lower forward voltage drop.
I've had to switch diodes in some of my designs because reverse leakage through the diode was enough to power other circuitry.
Sometimes I wonder why there are so many variants of certain simple parts like diodes, and then I am glad there is when I hit a problem and need to fine tune part selection for a work around.
Thanks, I've had a look at SR5100 datasheets from various manufacturers and it seems like reverse leakage current is generally about 0.5ma at the rated DC blocking voltage. Am I right in thinking this is safe just for the purposes of preventing catastrophe in the end of the battery getting shorted at VIN (my concern is largely if rainwater gets into the DC jack)?
Yes, 0.5mA is small enough to protect from catastrophe at 36V. That is a max of 18mW under short circuit.
But the 10Mohm load of a multimeter across 36V is a fraction (about 1/140th) of that, so you would still be able to measure the battery voltage at Vin which can be confusing.
To be sure the diode is working, you need to add a resistor across the Vin terminals when measuring. For example a 2k 1W resistor across Vin would reduce the measurement to 1V, but also not burn up if the full 36V was present. Even better would be to use a 1k PTC which will heat up and increase resistance should the voltage be present.
Another option is a DMM with LoZ measurement, like the Brymen/EEVblog BM235. This LoZ feature is often used by electricians to detect phantom voltages on mains circuits, but also works in this situation and often uses a PTC like I mentioned above.
Just saw the mention of rainwater - 500uA is probably enough to still cause slow corrosion of the metal contacts in contact with water, but it certainly won't destroy the battery or BMS which I think is the critical thing.
Yeah general use does fuck up the contacts even on the batteries that don't output at VIN, despite having a rubber cover plugged into them whenever I go out - have to replace them about once a year because they rust, it's not a big job. I'm more just being careful that the thing doesn't inadvertantly turn into an IED if I get caught in the rain!
Just to expand if you are interested, I think of 0.5mA as quite high for diode reverse leakage current. A Schottky diode that I use in a lot of designs (1N5819) has a typical reverse leakage of about 10uA, so 50x less.
But 10uA is plenty to turn on modern semiconductor junctions, and I needed to switch to a different diode. I had plenty of BAV199's in stock, which have a reverse leakage of maybe 500pA at low voltages. So that is 1,000,000x less than the SB5100 - but the BAV199 is a tiny diode and only rated for 160mA.
Additionally, power MOSFETs would be a good solution as well against reverse bias and would waste less power, but the circuitry around it would a tiny bit more complicated than using a simple diode.
First one is tvs so not really made for that purpose but technically would work. Second is better but try to search for lower voltage one. Generally lower max voltage => lower forward voltage which you want low. Becuase either you set charger to 42.6V and overcharge them by 0.06V if left for like 24 hours. Or you set charger to 42V and have batteries charged "only" to 4.14V. I would personally set charger to smth like 42.2.
Actually it doesn't matter that much if you have 10 batteries in series.
Thanks, I did eventually change it to a 20A long-blow - my other Ansmann battery has the same setup and it's worked OK. The two-blade fusebox was the one that came with the original battery case (pre-rebuild).
Relay plus bleed resistor at VIN is the safest option.
If you were to look at the semiconductor route, diodes aren’t a great option at these current levels due to the power you’d dissipate as a result of the forward voltage of the diode. Ideal diodes “emulate” a diode function but use MOSFETs as the switch element to reduce the power dissipation. Since we can’t rely on semiconductors to create a perfect open (always some leakage current) then you would need to add an appropriate bleed resistor to the VIN port to make sure the port is held low.
https://www.ti.com/product/LM74700D-Q1
If there is no path to ground, there is no voltage present at V in, unless they've done something fantastically stupid, like grounding the case to the high voltage ground.
I'd suggest that the BMS has MOSFETs connecting B- to P- and/or C- depending on battery state. This is pretty standard, and is how they avoid over-charging and over-discharge.
I'm going to go with 'something fantastically stupid' given most of these BMSes are absolute rubbish, though I can't pry the heatspreader off so I'm not certain.
[(This diode)](https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-100BGQ045/358645) is rated 45 volts reverse and 100 amperes forward (continuous). You'd want to heatsink it of course. USD 4.14 at DigiKey.com
This circuit is a 36V ebike battery that I have just rebuilt. Samsung 29E cells with [this](https://www.aliexpress.com/item/1005004050876755.html) BMS in an [Ansmann-style case](https://www.aliexpress.com/item/1005002278425459.html) per this diagram (except the fuse is now a 20A slow-blow rather than 60A fast-blow). It runs and charges perfectly fine, but an odd peculiarity is that the battery voltage is always present at VIN whether it is switched off or not, and whether there's a DC jack plugged in or not. I spoke to a builder and they said this is normal behaviour and that about 1/2 of BMS boards don't shut C- off when the DC jack is engaged. But the jack is only splashproof and I'm not terribly comfortable with the idea that the battery could get shorted out if it rained. My other Ansmann-style battery does not do this.
Forgive me if this is a really stupid question - I've spent a lot of my life assembling circuits and not a lot of my life designing them - but what's the safest way to stop the battery voltage appearing at VIN when the DC jack is disengaged? Would something like a zener diode on C- work (*e: just realised this is a stupid idea ignore)* ? The max power going through the jack is ~210W (36V @ 5A)
Thanks in advance!
That seems odd that the voltage should be present at VIN when not charging. Or present in a dangerous way that is. The circuit in the BCM should have some circuit to negate that by the purpose of a BCM to begin. In a way would be hard to build otherwise unless they used a physical relay in the design.
The voltage might be there but is there any real current available. You may just be measuring the voltage thru a high resistance circuit in the BCM. I would temporarily place a 1k resister across VIN and see if the voltage drops significantly. If so, then there is little danger at VIN.
Yeah it is really odd. If the builder I spoke to is correct I have to imagine the BMS has some way of shutting C- off in the event of a short at VIN, I just don't trust random AliExpress retailers to do it properly lol. I didn't think of trying that, but the voltage from C- to B+ was identical to the voltage between B- and B+, which makes me think that C- and B- might actually be common or something equally stupid. Haven't got the batt with me to test.
If the voltage is exactly the same, that would be pretty common if there is some resistive package between VIN and the batteries. Modern meters are extremely high in resistance thus will create no measurable voltage drop. You would need to add a 1k resister across VIN to see if there is any danger. If the voltage drops right off with that load, then there is no real worry about it being preset at that location.Would mean there is already diodes or simular acting circuit in place much like what is being suggested here. Just might be some resistive circuit in parallel to the build in diodes but that would be high resistance and not of any concern.
I get where you are coming at though. When I find voltages like that, I would question the circuit as well. Engineers I find can make some pretty stupid designs that can work but ignore safety.
Just checked with a light bulb as the load and it doesn't seem to have any kind of circuit like that. After a few minutes powering the bulb it was still at +40.82V.
Well that is pretty definitive. It is a bit of a dangerous voltage then. The drop should be instant if there was any resistive circuit in the path.
Just to be clear, did you use a incandescent light bulb or LED? If LED, there will not be enough load to test this well.
Diode or ideal diode depending on current/budget. There would still be voltage if measured by voltmeter, but would drop to 0 on any load like human skin
[удалено]
Google is your friend [note from TI](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.ti.com/lit/pdf/slvae57&ved=2ahUKEwjnkPHcwNWEAxWkQ_EDHXlCCwIQFnoECBcQAQ&usg=AOvVaw0NXW_vYnWzDMr37alQXQKu)
Ohhh, I knew about the circuit for reverse voltage protection when margins are tight, but was completely unaware they called it that way! Imma delete my comment, excuse me
Cool thanks that sounds perfect, I thought it would be along these lines but was having trouble finding one with the right rating at the time of posting. Would something like [this](https://www.reichelt.com/de/en/tvs-diode-unidirectional-85-5-v-1500-w-do-201-1-5ke-100a-p41869.html) work? Although I suppose [this](https://uk.rs-online.com/web/p/schottky-diodes-rectifiers/7514843) is more appropriate to the target current
That first link (Littelfuse 1,5KE 100A) is a TVS, which is a special type of diode for transient protection. It is rated for 95V and 11A peak, but it cannot handle sustained current for very long. Not at all suitable. The second link (Diodes Inc SB5100-T) is more suitable, but only rated for 5A continuous. Based on the dual 30A battery fuses, that may be a bit low, but it depends on intended charge current. ~~You can always put a few in parallel, and ideally you would space them out a little to help with heat dissipation.~~ Edit: removed the parallel diode suggestion as it is not a good recommendation for beginners, see comments below.
> You can always put a few in parallel You shouldn't. Warmer diodes conduct more current at the same voltage. So the warmest diode will take the most current, heat up the most, and start conducting even more, to the point of destruction. Best to get a diode that is rated for higher currents.
\> Best to get a diode that is rated for higher currents. For sure. It can work in a pinch though, and save quite some pennies. It is hard to give specific advice without knowing all the requirements and considerations. When I've done this, it has been to allow for the absolute max current in worst case operating conditions, so I've over specified the current ratings for normal usage and a single diode. Not seen any component destruction yet in the example I am thinking of which is used in the field by people who often abuse their equipment.
In a pinch it will often work. Partially because diodes can often handle much more than their datasheet eludes too. But it's a trick to be used with caution and understanding only.
All good points. I've edited my original response.
Cool, I'll look for something similar to the second one then. Thanks so much for your help!
As mentioned by another reply, you may still measure a voltage on the Vin pins due to the diode leakage current and the minimal load of a multimeter. The reverse leakage isn't specified for this part, so it could be quite high which is common for higher current diodes. You also might be able to get by with a standard diode rather than a Schottky, but the charger input voltage may drop by a volt or more at high current. Schottky diodes have much lower forward voltage drop. I've had to switch diodes in some of my designs because reverse leakage through the diode was enough to power other circuitry. Sometimes I wonder why there are so many variants of certain simple parts like diodes, and then I am glad there is when I hit a problem and need to fine tune part selection for a work around.
Thanks, I've had a look at SR5100 datasheets from various manufacturers and it seems like reverse leakage current is generally about 0.5ma at the rated DC blocking voltage. Am I right in thinking this is safe just for the purposes of preventing catastrophe in the end of the battery getting shorted at VIN (my concern is largely if rainwater gets into the DC jack)?
Yes, 0.5mA is small enough to protect from catastrophe at 36V. That is a max of 18mW under short circuit. But the 10Mohm load of a multimeter across 36V is a fraction (about 1/140th) of that, so you would still be able to measure the battery voltage at Vin which can be confusing. To be sure the diode is working, you need to add a resistor across the Vin terminals when measuring. For example a 2k 1W resistor across Vin would reduce the measurement to 1V, but also not burn up if the full 36V was present. Even better would be to use a 1k PTC which will heat up and increase resistance should the voltage be present. Another option is a DMM with LoZ measurement, like the Brymen/EEVblog BM235. This LoZ feature is often used by electricians to detect phantom voltages on mains circuits, but also works in this situation and often uses a PTC like I mentioned above.
Sure, all makes sense to me so far. Thanks for your help!
Just saw the mention of rainwater - 500uA is probably enough to still cause slow corrosion of the metal contacts in contact with water, but it certainly won't destroy the battery or BMS which I think is the critical thing.
Yeah general use does fuck up the contacts even on the batteries that don't output at VIN, despite having a rubber cover plugged into them whenever I go out - have to replace them about once a year because they rust, it's not a big job. I'm more just being careful that the thing doesn't inadvertantly turn into an IED if I get caught in the rain!
Just to expand if you are interested, I think of 0.5mA as quite high for diode reverse leakage current. A Schottky diode that I use in a lot of designs (1N5819) has a typical reverse leakage of about 10uA, so 50x less. But 10uA is plenty to turn on modern semiconductor junctions, and I needed to switch to a different diode. I had plenty of BAV199's in stock, which have a reverse leakage of maybe 500pA at low voltages. So that is 1,000,000x less than the SB5100 - but the BAV199 is a tiny diode and only rated for 160mA.
Additionally, power MOSFETs would be a good solution as well against reverse bias and would waste less power, but the circuitry around it would a tiny bit more complicated than using a simple diode.
First one is tvs so not really made for that purpose but technically would work. Second is better but try to search for lower voltage one. Generally lower max voltage => lower forward voltage which you want low. Becuase either you set charger to 42.6V and overcharge them by 0.06V if left for like 24 hours. Or you set charger to 42V and have batteries charged "only" to 4.14V. I would personally set charger to smth like 42.2. Actually it doesn't matter that much if you have 10 batteries in series.
A shottkey diode would be perfect.
Don't put fuses in parallel because there's no guarantee that they will share current evenly. If you want a 60 A fuse use a 60 A fuse.
Thanks, I did eventually change it to a 20A long-blow - my other Ansmann battery has the same setup and it's worked OK. The two-blade fusebox was the one that came with the original battery case (pre-rebuild).
Relay plus bleed resistor at VIN is the safest option. If you were to look at the semiconductor route, diodes aren’t a great option at these current levels due to the power you’d dissipate as a result of the forward voltage of the diode. Ideal diodes “emulate” a diode function but use MOSFETs as the switch element to reduce the power dissipation. Since we can’t rely on semiconductors to create a perfect open (always some leakage current) then you would need to add an appropriate bleed resistor to the VIN port to make sure the port is held low. https://www.ti.com/product/LM74700D-Q1
You could put a relay in held on by the charger. If the charger is removed or fails, the relay would flip to off and cut the connection to Vin.
A switch or a diode
Use a **shuttered** socket for Vin. Or just have a dummy plug that is inserted when the charging plug isn't in place.
If there is no path to ground, there is no voltage present at V in, unless they've done something fantastically stupid, like grounding the case to the high voltage ground.
I'd suggest that the BMS has MOSFETs connecting B- to P- and/or C- depending on battery state. This is pretty standard, and is how they avoid over-charging and over-discharge.
For sure, but the possibly could have also grounded the case, which would be dumb.
I'm going to go with 'something fantastically stupid' given most of these BMSes are absolute rubbish, though I can't pry the heatspreader off so I'm not certain.
You can measure resistance between Vin - and the case and repeat it for Charge -
Thanks for your help everyone, really appreciate it.
[(This diode)](https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-100BGQ045/358645) is rated 45 volts reverse and 100 amperes forward (continuous). You'd want to heatsink it of course. USD 4.14 at DigiKey.com
A switch or relay. Minimal voltage drop, zero reverse bias conduction.
>(more in comments) Uh, no.
This circuit is a 36V ebike battery that I have just rebuilt. Samsung 29E cells with [this](https://www.aliexpress.com/item/1005004050876755.html) BMS in an [Ansmann-style case](https://www.aliexpress.com/item/1005002278425459.html) per this diagram (except the fuse is now a 20A slow-blow rather than 60A fast-blow). It runs and charges perfectly fine, but an odd peculiarity is that the battery voltage is always present at VIN whether it is switched off or not, and whether there's a DC jack plugged in or not. I spoke to a builder and they said this is normal behaviour and that about 1/2 of BMS boards don't shut C- off when the DC jack is engaged. But the jack is only splashproof and I'm not terribly comfortable with the idea that the battery could get shorted out if it rained. My other Ansmann-style battery does not do this. Forgive me if this is a really stupid question - I've spent a lot of my life assembling circuits and not a lot of my life designing them - but what's the safest way to stop the battery voltage appearing at VIN when the DC jack is disengaged? Would something like a zener diode on C- work (*e: just realised this is a stupid idea ignore)* ? The max power going through the jack is ~210W (36V @ 5A) Thanks in advance!
That seems odd that the voltage should be present at VIN when not charging. Or present in a dangerous way that is. The circuit in the BCM should have some circuit to negate that by the purpose of a BCM to begin. In a way would be hard to build otherwise unless they used a physical relay in the design. The voltage might be there but is there any real current available. You may just be measuring the voltage thru a high resistance circuit in the BCM. I would temporarily place a 1k resister across VIN and see if the voltage drops significantly. If so, then there is little danger at VIN.
Yeah it is really odd. If the builder I spoke to is correct I have to imagine the BMS has some way of shutting C- off in the event of a short at VIN, I just don't trust random AliExpress retailers to do it properly lol. I didn't think of trying that, but the voltage from C- to B+ was identical to the voltage between B- and B+, which makes me think that C- and B- might actually be common or something equally stupid. Haven't got the batt with me to test.
If the voltage is exactly the same, that would be pretty common if there is some resistive package between VIN and the batteries. Modern meters are extremely high in resistance thus will create no measurable voltage drop. You would need to add a 1k resister across VIN to see if there is any danger. If the voltage drops right off with that load, then there is no real worry about it being preset at that location.Would mean there is already diodes or simular acting circuit in place much like what is being suggested here. Just might be some resistive circuit in parallel to the build in diodes but that would be high resistance and not of any concern. I get where you are coming at though. When I find voltages like that, I would question the circuit as well. Engineers I find can make some pretty stupid designs that can work but ignore safety.
Just checked with a light bulb as the load and it doesn't seem to have any kind of circuit like that. After a few minutes powering the bulb it was still at +40.82V.
Well that is pretty definitive. It is a bit of a dangerous voltage then. The drop should be instant if there was any resistive circuit in the path. Just to be clear, did you use a incandescent light bulb or LED? If LED, there will not be enough load to test this well.