nicknorman

Ah one other general point I meant to make: although charging will happen at idle, it is not a good way to charge the batteries either for the engine, for the drive belt, or for the alternator. It is better to run the engine at a faster speed, say 1000 to 1200 rpm because: There is less tension on the belt and less side force on the engine pulley / crankshaft The engine is happier when there is load, to run a bit faster than idle (in the same way that it is kinder to change down a gear in a car going uphill, rather than having the engine labouring slowly in a higher gear) If the alternator is spinning faster, its fan is spinning faster and thus the cooling air is moving faster and thus the alternator runs cooler. Probably the maximum alternator output is not achieved at idle anyway, even with a good alternator you would get more charge current if you run the engine above idle. You could try increasing the rpm slowly and gradually, noting the charge current on the BMS App, and stop increasing the rpm shortly after the current reaches a maximum %.

I would say this is as close to categoric proof as one can get, that the alternator is defective.

This is not too inexplicable. First a bit of theory for bedtime reading ... Alternators work by having a spinning magnetic field (the rotor) adjacent to some stationary windings (coils of wire) called the stator, and this induces ac current in these windings. The spinning magnetic field is an electromagnet, ie current has to flow through the windings to create the magnetic field. The type of alternator you have is a "nine diode machine", there being 6 diodes that rectify the ac from the stator windings and convert to dc to output to the battery. The other 3 (smaller) diodes, often called "field diodes", rectify a small portion of the generated ac which is used to power the field windings that are on the rotor (the bit that goes round!). These generate a rotating magnetic field which induces the ac current into the stator windings. The regulator adjusts the current in the rotor windings to control the alternator output. So in summary the alternator works by spinning windings containing self generated current that produces a magnetic field, near the stator windings, and this causes power to be generated in the stator windings, some of which is used to power the spinning rotor windings. You can see the chicken and egg situation - when the alternator first starts, there is no current in the stator windings, so no current in the rotor windings, so no current in the stator windings etc etc. It is never going to work! To get around that problem, when you put the ignition on it connects battery power to the alternator warning light and the other end of that warning light is connected to the alternator to feed a small amount of current into the rotor (field) windings to get things going. Once the rotor is spinning fast enough, enough power is generated in the stator to fully power the rotor and off it goes happily generating power without any external input. So in other words, if you gradually increase the alternator rpm, at first nothing much happens, then suddenly there is enough power to increase the rotor current and this increases the stator current which increases the rotor current etc etc - the thing is now "self sustaining" and no external current is needed. rpm could be reduced and the alternator is still working. Normally the speed at which this happens would be, with a modern engine and alternator, below idle speed. So once the engine is started and at idle, the alternator will be working. However if there are blown diodes or other problems (melted wiring in the windings etc) then the efficiency of the alternator is much worse and thus it will take a lot more rpm to get the alternator to self-sustain. But once this is happening, the rpm can be reduced and still there is enough current being generated to self sustain. In other words, the rpm necessary for the alternator to cut in is much higher than that needed to sustain it. So again, the high revs needed to get the alternator going points to a damaged alternator. In your case, once you had revved the engine, the system is charging at around 40A but since you have no means to monitor battery state of charge, and since the voltage of a lithium battery remains very static during much of the mid-charging period, it looks as though nothing is happening. But it is, albeit slowly, Bear in mind that at 40A it will take 8 hrs or so to charge the battery. Only in the latter part of that will the voltage gradually rise. And of course if you are using power on the boat, it will take longer than 8 hours.

Weird, but I suspect it's a red herring.

OK well I think that demonstrates that the domestic and starter alternators are not connected in parallel. So there must just be some leakage current from somewhere that is raising the voltage on the alternator input to the BMS. I can't think of a scenario to explain it since the domestic alternator should be completely isolated from the starter engine system, save for a relay feeding the domestic alternator with power when the ignition is on. And if anything that would result in an increase in voltage on the alternator BMS terminal, not a reduction. It is odd, but probably inconsequential to the problem at hand. Is the 1.27v just with the ignition on, or just with the heaters on, or both? And does it go back to the 13.5v or whatever, when you turn the ignition off?

do you mean 1.27v or 12.7v? Screenshot?

It does fit with a starter battery being in circuit. A fully charged lead acid battey will hold well above the nominal fully charged lead acid voltage of 12.7v for a long time if there is zero load. So 13.44v is not too surprising as the starter battery voltage. You could try one thing to prove the point. Does the engine have a separate heater position on the ignition switch? If so, turn on the ignition and operate the heaters for about 10 seconds but don't start the engine. Then, with the ignition still on but the engine not started, have another look at the voltage on the BMS app. If we are seeing the starter battery voltage on the BMS alternator input, then draining some juice by using the heaters should make the voltage reduce to around 12.7v

Then I don't understand where the 13.5v is coming from!

Is this a new built boat, or a second hand one that originally had lead acid batteries and you then had modified for the lithium system? If the latter, it is conceivable that previous owner decided to connect the starter and domestic alternators together, either directly or via some sort of relay, to increase the maximum charging current. And when your Lithium installer came along, he wasn't aware of that and didn't realise they were already connected together. I am just hypothesising of course!

Nevertheless you had a valid point. It was something I had thought about earlier, if you remember the original problem was that the green "allow alternator charging" LED on the BMS was not illuminated when it should have been, due to a fault in the BMS which was then replaced, and now the green "allow alternator charging" LED is on, but the alternator is only producing limited power. Both the BMS failure and the alternator problem occurred at the same time, which suggests they are linked. Once scenario is that a fault in the BMS caused a sudden interruption of alternator current which generated a transient which blew diodes. The only contrary indication to that is the presence of 13.5v on the alternator input terminal of the BMS, a voltage higher than the Li battery. And thus I can't see how that could come from anything other than a lead acid battery (the starter battery) being in parallel with the alternator. Alternators, since that would suggest both alternators were paralelled. What Mr hoopdriver could do is to get his mate to check the voltage on the starter battery, the starter alternator main positive terminal, and the domestic alternator main positive terminal - with the engine switched off. If they are all exactly the same voltage (to within 0.1v) it tells us that the alternators and starter battery are all paralelled. Which would not be a bad way to install the system, it's just not the way we were expecting.

His alternators are not directly connected to the Li battery. He has a lead acid "buffer" battery and then B2Bs to push the current into the Li battery. The rating of the B2Bs is such that the alternators never work more than around 60% of max output, to avoid overheating.

Anyway all this chit chat is speculation! What we need is a categoric check and (almost certainly) repair of the domestic alternator. The rest is of less consequence other than perhaps the much smaller starter alternator is doing all the work at present and may thus have a short life if the domestic alternator isn't repaired pronto! There are 2 alternators.

But don't forget that diagram is for a vehicle with a single alternator.

There has got to be a connection to a lead acid battery in there, even if the installer doesn't realise it! If there isn't a domestic lead acid battery, it must be the starter battery. Yes, hence I said "directly". And as you said, current doesn't flow up hill so it can't be the Lithium battery that is holding the alternator terminal voltage up.

But only if the alternator is connected directly to a battery, which we are told it isn't.