nicknorman

You haven't been following the topic very well, have you. I started the issue talking about the VE.Direct connection. Maybe you don't know what that is? It is a 3.3v TTL level RS232 duplex connection between the BMV 712 battery monitor and something else, in this case my BMS. That cable, being single ended data, quite long and fairly low voltages, is screened with the screen acting as the 0v line. So there would be a direct ground connection to the BMS had I not decided to go for an optoisolator at the interface with the BMS. Then I went on to mention the CANBUS and how I had decided not to connect the ground at all because it wasn't necessary and would again create an alternative ground current path. You have focussed on the CANBUS not having a ground path because I decided not to connect it. Congratulations, you remembered that bit! But I don't think you see the point. Neither does the VE.Direct have a ground connection, thanks to the way it is designed. Otherwise known as having an isolated ground in each case. Which was why I mentioned it by way of illustration - sometimes having an isolated ground is necessary even on a narrowboat. Do you get it now? (probably not).

You are getting quite excited! Maybe a lie down in a dark room? As I have repeatedly said and you repeatedly ignored (so I really have no idea why I’m bothering) when there are alternative ground paths this can lead to problems. In the case of current divided between two alternative paths, one beefy, one weedy, the current is of course divided in proportion to the resistances of the two paths. So if the resistance of the beefy path increases, the current in the weedy part increases. The beefy part generates heat, the weedy part generates heat. The excess temperature reached depends on the ability to dissipate that heat and the rate of temperature change depends on the thermal mass. When there are varying resistances along the weedy path, the generation of heat will vary accordingly. So if you consider let’s say a 1mm^2 conductor from the BMV to the BMS, and a 1mm^2 conductor from BMS to battery negative you might think there is even heat dissipation along the route. But somewhere inside the BMS linking the two conductors there is a short and very thin PCB track - after all, the designer consider it a “signal wire” and knows that the return current from the data lines is miniscule. So now we have most of the circuit resistance (most of the generated heat) confined to a very small pcb track with very poor ability to dissipate heat. But as I also said, the main issue is when there is a disconnect in the beefy battery negative line. Maybe someone decided to change the batteries - “disconnect the negative first” someone once told them. They would expect this to de-power the domestic electrics, but instead all the current is now routing through the thin wire. Disaster waiting to happen. Anyway, I’ve got to go now, I am just putting together my bid for President of USA.

The thing with Gibbo was that he was actually wrong about quite a lot of stuff, but he said what he said with such dogmatism, repetitiveness and conviction (DRC) that the forumerati believed it all as gospel and he created a whole new post-truth. IanD still has some way to go in the DRC stakes, although it has to be said he is trying hard. Or maybe just trying? That is irrational as we can't both be wrong.

Bored with pointless arguing with someone who is not capable of ever being even slightly wrong.

As I explained several times, you made a blanket statement that was flawed. I was merely pointing that out. But I realise my mistake now, it is against the laws of the universe for you to have said anything even slightly wrong or over-generalised. Sorry about that. Yes, as I said in the next sentence!

Electricity is invisible and we are not able to smell it from this distance! In other words, the only option for resolving this is to take some measurements. You have a very large battery bank, perhaps the charging system has never worked and you have only now flattened the batteries. Or perhaps the charging did work but now doesn't. You need to measure the voltage across the battery terminals with the engine running, to see if the problem is that it is not charging, or whether it is that the batteries are knackered ( the former is the more likely because it is unlikely, though not impossible, that the batteries are suddenly completely knackered). What engine? What alternator arrangement? What is the behaviour of the alternator warning light(s) on the panel (should be on when you turn the ignition on, and go out after the engine starts) One easy answer could be that the domestic battery isolator has either been turned off by accident, or is faulty (high resistance, melted plastic) and the 12v systems are being run directly from the alternator. What sort of battery isolator is installed and is it on? If it has a red plastic key it has probably melted and disconnected the batteries

Cite a narrowboat or other small boat that uses screened canbus cables?: "My boat. Therefore that must mean that it is often used on boats." <Vetus installation manual doesn't mention any need for screened cable> Examples given of boats needing isolate gnd B2S: "Yes but my boat doesn't do that so clearly it doesn't need to exist elsewhere" It really is all about you!

Maybe you should get some sleep, it must be incredibly tiring to always be right about everything.

Mastervolt's own Masterbus/CANBUS cables are not screened. So I don't know where you are getting your "often screened on boats" from. How many boats with CANBUS are you aware of that use screened cable? Please cite them. And I don't mean eg naval warships! 120 ohm impedance and voltage differentials of several volts between hi and lo means that the chances of some SMPS radiating interferece that affects the data is effectively zero. More so (better than zero😁) if you use a twisted pair. I am personally aware of 2 boats that use an isolated B2B, in both cases they are hybrids with 48v LA drive batteries and 24v engine electrics along with 2 B2Bs for charging the 48v bank from the engine alternator/battery. The upper one at least must be isolated.

What happens when the water has all steamed off?

With CANBUS being a differential system, generally there is no need for cable screening. Certainly not for boat systems. At a pinch you can use a twisted pair but it's not necessary at the 250kB/s rate that Masterbus uses. In fact thinking about it, probably the only reason for Mastervolt including a ground in their connectors/cabling is because some of their devices need to receive power via the cable to run the CANBUS interface. But if I had been careless of thought I might have connected the gnd on the CANBUS system to the gnd on the BMS and Alternator controllers and that would have been a mistake. My point was that was too blanket a statement. Often no, but sometimes yes especially if there are systems at different supply voltages (eg 12v, 24v, 48v systems) which does sometimes happen on boats.

You have to consider the heat dissipated in the “resistor”. A short length of rod with resistance 1 ohm and 100A is going to dissipate 10kw! Apart from anything else, 1 ohm is far too much as it would drop 100v. More realistic would be 0.01ohm which would drop 1v at 100A. But that is still going to dissipate 100w and will get extremely hot. This is why a long wire is the preferred choice - the longer the wire the more scope for dissipating power without getting too hot.

You started the ad hominem attack, but of course when you do it, it doesn’t count. In your head. As I said at the outset, under normal circumstances the stray found current would not be problematic for heating/damage/fire risk. However in the event of a poor or bad connection it could be significant. And of course you would only be one human error away from disaster - someone decides to disconnect the battery negative and then you are left with the thin wires and PCB tracks to pass the entire system current. It is fundamentally bad design to create that sort of alternative current path gotcha. Same applies to alternative current paths created eg by a radio aerial earth connected to hull. It is probably not going to be an issue due to the differing current path resistances, but it could be under unusual circumstances. And while I think of it, I had similar thoughts on the CANBUS interface. In the end I had 2 pads on the PCB, CANBUS ground and system ground. Because CANBUS is a differential system I decided that most likely it would not be necessary to connect the 2 grounds together and this has shown to be correct, so another alternative current path was eliminated. A good design engineer thinks about these things and tries to eliminate these sort of issues that lie only just below the surface of a correctly functioning and correctly operated system.

And there was me thinking you were some sort of design engineer! I did explain the consequences of the differing cable resistances. I also explained the potential for a bad connection to result in excessive current flowing in the thin wires / pcb tracks. What I didn’t explain was that the BMS measures cell voltages with a resolution of 1mV and an accuracy of a couple of mV and even a small current flowing through the BMS 0v wire to the cells is going to introduce a significant error. It would simply be bad design to create an alternative current path. Actually I did think of connecting only the Tx and Rx VE.Direct wires but not the 0v, but decided this was a bit gash so used optoisolators instead.

Well in general terms it can be an issue. For example we have a BMV712 battery monitor. It is connected to 0v at the shunt. We also have a BMS. It is connected to 0v at the cells, so not quite the same place. Then there is a lead taking VE.Direct data from the BMV to the BMS, including a 0v line. So that immediately creates an alternative current path for the 0v current. Certainly the difference in cable sections (and hence resistance) between the battery 0v line (70mm^2) and the BMV data line (very thin) means that the dc current flowing through that thin wire, and thence through another thin wire between BMS and cell 0v, wont be much. But then again “not much” of 200A can still be a fair bit. But if you introduce some slightly corroded or bad connection in the 70mm^2 wire, you could end up with large currents flowing through the thin wires and much smoke and melting ensuing. So I used an optoisolator to ensure there wasn’t a secondary 0v current path.

It allows you to charge a battery with a different negative potential. For example, you have a 12v alternator and starter battery. You have a 24v domestic system (2 x 12v batteries in series). You can use 2 B2Bs, one connected to the 0 and 12v points of the 24v battery as usual, and the isolated one’s negative connected to the 12v battery mid-point, and its positive to the 24v. Thus charging a 24v system from a 12v system.

Only someone who has not had Li on their boat would say that.

Yes I get the cost thing and that was my point really - the intrinsic cost (component cost) of a fancy smart alternator regulator is less than £100 so it is a shame that they cost nearly 10 times that. The only reason why they do cost that much is because the manufacturer and the retailer both need to make a profit, and a lot of up-front man-hours go into software development, testing etc. But there is a lot of “open source” stuff out there, people give freely of their time to create stuff and publish it on the internet, which eliminates most of the one-off costs above. So it’s a shame that this doesn’t happen for an alternator controller project. Well it did, as we know, but it then went commercial.

Bluetooth is easy to implement at the regulator end. It’s is just a serial interface and a £3 BLE module from China, plus the software to compose the appropriate sentences. iPhone and Android App development is beyond my pay grade and probably harder work, but I question why a phone app is really necessary for a well designed alternator controller that “just works”. Yes very interesting to see the field current etc for the first 5 minutes, but after that what are you going to do with it? Ok maybe configuration too, but again that is a one-shot usage. I disagree, obviously! An automotive chip is designed to quite a high standard, much higher than stuff for domestic use etc, and of course it will have been “field-tested” in millions of vehicles world wide. It has to be 100% robust and with several layers of fail-safe. In using one, all one is doing is specifying the voltage to regulate to and the maximum field current, data which would normally have come from the vehicle’s ECU. In the event of the vehicle ECU ceasing communication with the chip (wire come off etc) the devices go into a failsafe mode, in the case of the one I use, a default voltage of 13.5v which is vehicle-safe but also of course is lithium-safe. There are separate hardware (ie not software dependant) protections within the chip eg a hardware over-voltage shutdown. It would be a massive and pointless task to repeat all that design and testing work and I can see no reason other than masochism to follow that route!

All this shenanigans with trying to interface a LA alternator to Li by means of heavy current electronics, when the solution is to convert the alternator to Li-friendly charging. End of problem! It is a shame that currently available alternator regulators are so expensive, but good to see the Zeus appearing. Maybe some other manufacturers will produce something, and with a bit more competition prices will drop. The crazy thing is that it can be done cheaply in terms of component cost, I think mine might be over £50 but certainly less than £100, and that is with CANBUS and a VE.Direct interface for the BMV712. All the extra cost is in the software development, testing etc. It would be good if there was another open source project such as the Wakespeed was originally, but with a better initial concept - the Wakespeed design was far too complicated in that it reinvented the wheel and all the regulation was done from scratch, whereas there are several digital automotive alternator control chips on the market for a few £ that mean 90% of the hard work is already done.

Easy to say, not easy to achieve. It is easy to have a solid state switch that is either on or off, because in either state there is very little power dissipation. But a switch half-on dissipates a huge amount of power as heat. So you need to add the sort of technology found in B2B converters. Switch mode buck converters. Which can be done, but at the sorts of currents involved is going to be very expensive and bulky. All of which is why this would be a crazy way of doing it. The correct way to do it is to control the charge sources. Which is quite easy by comparison. No heavy current electronics required.

Old dogs flock together.

Very true. For example in the certification of airliners carrying hundreds of passengers at 35,000’, the certification requirement is not that the wings can never unexpectedly fall off resulting in everyone plummeting screaming to their deaths, rather it is that the probability of this happening is “extremely improbable” and they go on to put a number on it, 1 times 10 to the power -9. What the unit of that event is doesn’t seem to be specified - an hour, a flight maybe. So although a one in a thousand million sounds quite improbable, and you would be pretty unlucky if it were you, a lot of flying goes on around the world and so we can’t be too far off such an event becoming a near certainty. And if a bad event could happen once in 1,000,000,000 flights that does not mean it will happen on the 1,000,000,000 flight, the laws of probability say it could happen on the first flight, or anywhere in between, or after..

The problem arises due to arse covering and not really understanding the detail. I would draw a likeness with installing a new gas appliance, as you know the overriding requirement is to install it in accordance with the manufacturer’s instructions. If for some reason you decide to install it in contravention to the installation instructions, even if it is completely safe, you face a severe uphill battle trying to convince a bureaucrat that it is safe despite the apparent contravention. So it is with the various Li battery and cell manufacturers who state “do not mix with batteries of other chemistries”. Even though they probably mean to not mix with other Li chemistries, the statement is taken at face value by people who have no hands on experience.