nicknorman

There is no issue with discharging Li below zero. Down to about -20 anyway, and you are never going to get below that. The internal resistance will be a bit higher at low temperatures and this will cause a bit of heat to be produced which will raise the battery temperature a few degrees under fairly heavy load.

You will probably use less that 100Ah a day in summer or maybe less as you don’t need to have an inverter on if you are on shore power. Over 24hrs that is less than 5A. So a 5A charger is likely to meet your needs. More powerful chargers are useful when you are eg running a generator to charge the batteries, in order to minimise running time. But for 24hr use, 5 or 10A is adequate.

You don’t need an isolated negative. You would use the isolated version if you wanted to charge a battery whose negative is not at the same voltage as the negative of the source battery, for example if it was the second 12v battery in a 24v string - ie its negative at around 12v and it’s positive at around 24v. It is unrelated to the concept of using the hull as a negative return.

A problem with charging lithium optimally and not too hard, is that one really needs to know the charge current. Unlike lead acid, Li will charge at the maximum current the charge device can provide, until it is very nearly full. When the voltage finally becomes constant, and at the sorts of charge currents we are talking about, the current drops off very quickly perhaps only taking 5 mins to drop to the 5% of capacity one is looking for to terminate charge. The nominal “5 minutes” obviously varies somewhat depending on the charge rate as a % of capacity. Without knowing the charge current, any charge device has to guess how long to hold the absorption phase before going to float. What is best avoided is a long period of holding the charge voltage up near the maximum, with very little current flowing into the battery. Golden rule for charging Li is charge to the specified voltage, up to 3.65v/cell, and then STOP charging as soon as the current decreases to 5% of capacity! Of course if the charger goes to float and the float voltage is such that it matches the resting voltage of the battery (around 13.2 or 13.3v) then fair enough - the battery doesn’t realise there is a float voltage being held. But IMO a good design is one that doesn’t hold the voltage up at 13.8v or more once the charge current has fallen off.

In my limited experience (limited because I mostly do my own stuff) the list of “not so good” would be much longer than the “good” list. Which can’t be too surprising bearing in mind anyone can set themselves up as a “marine engineer”. The best indicator is that the “good” have a long waiting list whereas the “not so good” can come out this afternoon.

Yes it’s very often because they come onto the forum, ask “rhetorical questions” ie questions to which they already think they know the answer, or at the very least know what answers they want in order to reinforce their opinion, and then get a sulk on when not everyone spits back the answers they want. And then their narcissistic attention-seeking gene kicks in and instead of just quietly going off and buying a tape recorder so they can be entertained hearing the sound of their own voice giving the answers they want, spend considerable effort telling everyone how unfair it all is, how everyone is wrong and they are right, stamping their feet moments before flouncing off in a dramatic exit stage left. Or at least I think that’s it because to be honest they are forgotten very shortly afterwards.

The NE bond relay only operates in inverter mode when there is no external ac input. If it did operate with “normal” mains input then this would routinely blow eg the shore bollard RCD.

Yes definitely. We have a Mastervolt Masterswitch which does it automatically but a manual switch is a cheaper option.

No this is not correct. We connect our travelpower to our Combi no problem. It doesn’t care whether the neutral is 0v or 115v. And it’s quite happy to provide “power support” by merging the TP supply with inverter output.

Well these two posts can’t both be right!

I am a bit sceptical about that graph. It says zero output for the travelpower at idle. If that really is the case it’s poor design. There must be a very big difference in the pulley ratios compared to the 3.5kw - if it’s correct. And if it is correct, why?

I don’t think it’s unreasonably cheap. I paid the same (in proportion) for my bare cells from china, so you are getting a BMS and case chucked in. Something like a heating element doesn’t add much to the build cost. It’s always a bit of a gamble but no red flags for me there. However the discharge is limited to less than 1/2C which is a bit restrictive, but might not matter if you don’t intend to run a big inverter load.

We’ve got a 3.5kw travelpower on a Beta 43. It makes about 2kw at idle, but that is very bad for the engine. By about 1200rpm it can make full output, although a bit higher rpm is better in order to reduce belt tension / crankshaft side load. If I was expecting 5kw output I think I’d want around 1400rpm. If you let the revs get too low the voltage dips at first, then I think eventually it would trip off though I’ve never had that happen. To organise the source of power to the Victron you could use a manual ac changeover switch, or do as we do and have an automatic changeover such that shore power gets connected if it’s available, otherwise the travelpower is connected.

Well let’s be quite clear, boats move forward going up in locks due to Bernoulli’s effect. Which states that the pressure in a fluid decreases when its velocity increases. Water flowing fast into front of lock = pressure reduction = lowering of water level at front of lock = slope created = boat slides down slope.

I think contemplating blacking a boat outside in early march is a bad idea!

Yes why not? Schottky diodes drop less voltage, but they will still drop around 0.6 or 0.7v at high currents. You don't say what the alternator max. output is (or rather, the input to the Li you get with the long cable) but let's say it is 100A, that means the diode is disspating 60 or 70 watts and so good heat sinking would be essential. You then have to factor in the max ambient temperature (engine bay temperature) etc in order to work out how much heat sinking is needed. But it certainly could be done. If the alternator regulatates to 14.4v then losing 0.6v will significantly decrease the charge current - but that can be compensated for by reducing the length of the "long cable". A Schottky diode at low currents will drop around 0.3 to 0.4v so that does limit the max charge voltage to around 14 to 14.1v but perhaps that is no bad thing. It will prevent any significant amount of charge being removed from the Li, but that is not the issue. Starting an engine probably uses less than 1Ah anyway. It is the momentary peak current that might be the issue, damaging the MOSFETs in the BMS. But that depends on the BMS and how well designed it is.

However the disconnection happens AFTER the peak current is taken by the starter activating. So there will be a short period when the Li is supplying a lot of current. This might only be milliseconds but that could be enough to damage the BMS, depending on how much current and what the BMS rating is

We had our boat blasted and 2packed in 2011. We took it out of the water in 2016 but, having jet washed and generally cleaned it, you couldn’t see where the waterline had been. So we really didn’t need to have bothered. We did apply a further coat of the 2-pack though (having abraded it) because the biggest problem was that the stuff had gone very grey. We are going to take it out of the water again this year (so that’s 12 years later!, and 7 since we last had it out), in part because we recently rather overdid the ice breaking and there is some shiny steel (now no doubt rusty steel!) showing at the bow. I suspect the waterline will still be invisible elsewhere. When we were on the Thames last summer, the clear water showed that there was no significant rust below the water line. It really is great stuff, there is a bit of rust on the rubbing strakes but that’s it. I think with bitumen it can allow water to seep in between the bitumen and the hull in places where it is not well adhered - bitumened boats often look a bit rusty around the waterline even when most of the coverage is still sound.

Obviously the best solution is grit blast and new 2-pack. However that is expensive! If the bitumen is mostly well adhered to the previous layer, one possible answer would be SML Ballastic epoxy 2-pack which they claim can be applied over existing bitumen. Not sure how good an idea that is, but if the manufacturer proposes it then presumably it works reasonably well.

The long cable introduces resistance which introduces voltage drop when current flows. So there will be a balance between current drawn from the starter battery and current drawn from the Li - more current from Li drops more voltage which means the voltage at the LA battery dips and makes the LA supply more current until equilibrium is reached. The proportion of current supplied by the Li will therefore depend on the resistance of the long cable and the other infrastructure between the Li and the LA. I don’t have any practical experience of commercial BMSs but if this one is the type with a separate charge and discharge connection, does it in fact allow discharge via the charge connections (or charge through the load connection)? It could be that the charge input is protected by a “perfect diode” bit of electronics that only allows current to flow one way - into the battery. And if that is the case, there is no possibility for the Li to discharge into the starter battery.

Bearing in mind that when the VSR is on, the voltage on the Li and the starter batteries will by definition be (more or less) identical. And the Li will sit well over 12.8v even well quite well discharged. So the strategy isn’t going to work, as you have discovered! What you want is for the batteries to be connected together when the engine is running, and disconnected when not. A good engine running indicator is the alternator D+ terminal (assuming it has a 9 diode alternator). So one solution would be a motorised battery switch that is turned on by the presence of 12v on the D+ terminal, and turned off when that voltage drops to near zero. Or of course a manual battery switch, but that is prone to human error. I know you aren’t asking about protection of the Li during charging using the BMS, but I will mention that the devil is in the detail. What you want is for the batteries to be charged until the voltage reaches some limit, say 14 to 14.2v (max 14.6v if you are sure they are perfectly top balanced) and the current has dropped to about 5% of capacity, then charging to stop, and not restart until there has been significant discharge. Presumably your BMS has separate connections for charge and discharge? What you want to avoid is charging stopping when voltage hit say 14.4v and then restarting when it drops to say 13.8v. With that, you will be repeatedly ramming the maximum amount of charge into the batteries, which isn’t good for longevity. But I also suggest that the Sterling B2Bs are pretty horrid things, very expensive and very inefficient.

They are a game changer!

But in that case, they don't know whether you have a licence or not. For an unregistered or mis-registered beacon I think it will depend on the false alarm rate, which is probably quite high. Are they really going to launch a search when the beacon is obviously out of its remit - eg registered to a yacht in america but pinging from the centre of birmingham? And if they do, they can prosecute you for not having it registered correctly whereas they would be unlikely to ask to see your radio licence unless you made a fool of yourself on the radio.

If a battery has been on float for a long time, it seems to get "lazy" such that the first cycle is disappointing. It then gets better for subsequent cycles.