Split diode or VSR
- Thread starter oakleyb
- Start date
geem
Well-Known Member
There are other ways as well. Dual solar regulator that charges domestic and engine batteries separately. With the same set up on the Duogen the engine battery never needs anything else on our boat. We have a 1,2 both battery switch that never gets moved from the domestic battery position. It can be in an emergency but it's not happened yet. We have 380w of solar so the engine battery gets priority then the solar reg sends the charge to the domestic bank. Has worked for us for several years now and we wouldn't change it
neil1967
Well-Known Member
We have a Sterling Pro-R splitter for our battery bank of a red flash start battery and 3 trojan wet batteries (390AH) , which works very well - install and forget. It is not cheap - about 120 GBP - but it has less than 0.1v drop and prioritises the start battery over the domestics, until the start battery is charged. Worth a look.
Last edited:
findus
Active Member
Use a battery to battery charger, check the Adverc website
ex-Gladys
Well-Known Member
That's what I thought, but I have a VSR as well, and basically had an issue with the lot being paralleled when using the domestics, so that I've disconnected the engine battery from Solar all together. However if someone can recommend a dual-sensing VSR to replace my old simple one, then I'd be grateful
sailorman
Well-Known Member
this is the vsr i now have a BEP
nigelmercier
RIP
- Joined
- 20 Jun 2007
- Messages
- 16,234
- Location
- Live in Kent, boat in Canary Islands
Indeed, same problem here. Simple to fix though, route the negative lead to the VSR via a NO relay activated by the ignition.Mm - I have quite a fancy charger that can be programmed separately for starter and service banks with different chemistry and capacities. Which facility is completely pointless in my installation since as soon as it fires up the SmartBank parallels the two banks anyway...
prv
Well-Known Member
Not that simple with the SmartBank system - if it decides the relay ought to be closed, but it isn't, then it will flag an error.
In any case, it would be a daft arrangement to put in from scratch, completely defeating the VS part of the VSR. You might as well just use a dumb relay in that case.
Pete
sailinglegend420
Well-Known Member
Split Diodes have problems if a battery volts sense cable is used.
Diodes were the first automatic solution but they have a voltage drop of 0.7 volts across each diode, maybe double this at high charge currents. This causes a huge power loss, but this loss can be compensated for by having regulators that sense the service battery voltage and boost the charger output. This can produce the right voltage at the service battery but can produce too high a voltage on the starter battery. Consider a heavily depleted large service bank - and an almost full smaller starter battery. The voltage drop across the diode feeding the service bank increases with the current so may well reach 1.4 volts or more. The alternator senses the voltage at the bank and raises the output by 1.4 volts to say 15.8 volts to get 14.4v at the service battery. The voltage drop across the diode feeding the starter battery is only 0.7 volts because the smaller service bank is taking a much much smaller current. So the starter battery is sitting at 15.1 volts for maybe several hours! This is almost an equalizing charge - so not very good for any sealed, Gel, and AGM batteries. Modern split diodes use newer technology with virtually zero voltage drops, but they are much more expensive.
The above example represents the extreme case of overcharging, but also consider if the diode fails open circuit on the battery with the voltage sensing wire. As the battery voltage falls with increase load the alternator will raise its voltage to its maximum to compensate. This will very quickly cook the other battery.
Split diodes failing “open circuit” is a very common problem. So, never connect a voltage sense cable to a battery with split diodes unless there is a high voltage alarm to warn of failure.
Diodes were the first automatic solution but they have a voltage drop of 0.7 volts across each diode, maybe double this at high charge currents. This causes a huge power loss, but this loss can be compensated for by having regulators that sense the service battery voltage and boost the charger output. This can produce the right voltage at the service battery but can produce too high a voltage on the starter battery. Consider a heavily depleted large service bank - and an almost full smaller starter battery. The voltage drop across the diode feeding the service bank increases with the current so may well reach 1.4 volts or more. The alternator senses the voltage at the bank and raises the output by 1.4 volts to say 15.8 volts to get 14.4v at the service battery. The voltage drop across the diode feeding the starter battery is only 0.7 volts because the smaller service bank is taking a much much smaller current. So the starter battery is sitting at 15.1 volts for maybe several hours! This is almost an equalizing charge - so not very good for any sealed, Gel, and AGM batteries. Modern split diodes use newer technology with virtually zero voltage drops, but they are much more expensive.
The above example represents the extreme case of overcharging, but also consider if the diode fails open circuit on the battery with the voltage sensing wire. As the battery voltage falls with increase load the alternator will raise its voltage to its maximum to compensate. This will very quickly cook the other battery.
Split diodes failing “open circuit” is a very common problem. So, never connect a voltage sense cable to a battery with split diodes unless there is a high voltage alarm to warn of failure.
lw395
Well-Known Member
This kind of thing can be a problem.
The worst scenario is often when running an anchor windlass or bow thruster with the engine running.
That's a lot of current, hence a lot of drop, so the engine battery can get a hard time.
Not so bad if it's a wet battery you can top up, but not good with sealed starter batteries.
ex-Gladys
Well-Known Member
Cheers sailorman...
john_morris_uk
Well-Known Member
Sorry but this is not altogether true.
The problem you describe was a real worry for me when I first set our system up, but as someone explained, the reality is that the batteries sort themselves out. The theoretical problem seems to make sense, but disregards the fact that lead acid batteries are very good at NOT accepting a charging current. The alleged problem of the engine battery being forced to accept more charge than it will just doesn't happen in practice.
We've had a battery sense with a diode splitter system on our last two boats. The current one has been in use for the last eleven years and we've never had a problem. The engine start battery is the one I fitted eleven years ago and the domestics have been changed once. We do use wet batteries though and keep the levels topped up. Good practice is to never deplete the domestics below 50% of their capacity. We rarely let ours go below 70%.
Our Sterling 'smart alternator charge regulator' has a high voltage cut off/alarm. It has never operated. What happens in reality is that the smart charger senses that the domestics are low and ups the voltage to force charge into them. The engine battery is already nearly fully charged and accepts a bit and then settles down and perhaps gasses slightly. (In practice I've never seen it gassing very much.)
Anyway, the VSR's that automatically link the domestic and engine start batteries potentially have exactly the same issue.
Last edited:
sailinglegend420
Well-Known Member
I'm sorry but you clearly don't understand the "theory" of battery charging. Your split diode system might have worked well if the voltage "sense" wire was on the starter battery not the service batteries.
You are right to say that batteries only accept charging current based on their state of charge, but any "extreme case of overcharging" that I was describing always refers to a battery that has excess voltage applied to it - it won't take excess current. It''s the high voltage for extended periods that causes excess gassing, which is why chargers should drop back to a lower Float voltage when a battery is fully charged.
I hope that makes sense now.
john_morris_uk
Well-Known Member
Actually I think I have a reasonable idea about the ways that wet, AGM and gell batteries charge, their charging regimes and possibilities, charge acceptance rates and voltages necessary. That's why we use wet batteries as the possibility of excess charge damaging the starter battery is minimised. If the voltage sense wire was on the starter battery then the house domestic batteries wouldn't stand much chance of getting to 100% The charger would quickly wind itself back to float charge rates and there would only be a trickle charge ending up in the domestics. Exactly the opposite of what we want to achieve.
Besides which if what you say is true, then VSR's wouldn't charge batteries very well either.
Of course I quite prepared to admit that I might be totally wrong, but so far your explanation hasn't persuaded me.
I suggest that the gold standard is two separate alternators. Each with its own alternator charge controller and one dedicated to the engine start battery and the other charging the domestic battery bank. You put a conventional charge regulator on the engine start alternator and a smart charge regulator on the domestic banks's alternator.
Last edited:
Robin
Well-Known Member
That is the system we had on 'Heartbeat' but both alternators had Adverc regulators, however the engine battery alternator was the standard output (50A?) Yanmar versus the two separated service battery banks one which was a high output one. We inherited the boatwith 2 service banks that kept the high users ( SSB, radar, pilot and fridge) separated from the essentials like nav lights, plotters, GPS sets . Currently Roxanne has one Balmar 150A alternator with a smart regulator and VSR plus a 40Acharger/ come 2KW inverter for when shorepower is connected or not or we use the little Honda genset. Personal preference was for a simple spilt diode setup as I find the logic easier to understand but I was overruled by the ex-spurt installation man. Having just installed a little digital voltmeter on the eng battery in addition to the one on the service battery, I can see now that the main bank ends up a tiny fraction higher volts ( Yesterday 0.1) than the engine one. Our batteries are all simple deep cycle wet golf cart ones. We have no solar or wind charging currently. oh I forgot we still have a 25A smart charger for when we run the little 2Kw genset
Quite probably there are grounds for critique but that is what our 'spurt' said is optimum and cost effective for what we do nowadays and using what we inherited..
Last edited:
lw395
Well-Known Member
Possibly. But so many boats are using either a fair bit of solar, aux generator or shore power, it soon gets very complicated.
And quite a few boats have more than two banks for various reasons.
I don't believe there is a universal best solution.
But having a good smart charging system (mains, solar and engine alternator) for the house bank, then using a battery to battery charger to keep the start battery topped up has a certain appeal.
Main thing is understanding the system you have and its limitations. That does not seem to be getting any easier judging by some of the threads on here!
john_morris_uk
Well-Known Member
Fair point and my suggestion of 'gold standard' was just for the engine driven alternator charging system. Provided the engine and domestic battery banks are kept completely isolated then all the other charging options can go straight to the domestic batteries. If you're worried that you might lose the engine start battery either wire an 'emergency link' switch or get a set of jump leads to jump the engine off the domestic battery bank. Frankly, you shouldn't ever need to use either option, but at sea you never know...
I think a good battery monitoring system is also essential. Our measures Ah in and out of the domestic bank, along with voltage and current being drawn or going in of either battery.
lw395
Well-Known Member
Tend to agree with that, but some people on a yacht with simple needs and a modest budget might do better to have just a voltmeter and prioritise spending money on batteries or a bit of solar...
A mate of mine bought his boat in Holland. He asked the broker if there was any way to start the engine with the house battery. The response was a mime of how to move the house battery to the start battery's leads...
ghostlymoron
Well-Known Member
I had that system on Anemone and wouldnt recommend it! Not something you want to be doing when you need your engine in a hurry.
I agree with you that its better to spend hard earned cash on more/bigger/better batteries than fancy monitoring stuff. As they say round here "you don't fatten a pig by weighing it".
john_morris_uk
Well-Known Member
I knew someone who had fitted a fancy chart plotter to his boat and set sail across the channel. Three quarters the way across the lights went out which confused him. He said to me, "But I knew the batteries were charged, I'd motored down the river from my mooring." For some bizarre reason he thought half an hour of motoring would charge his batteries 100%. His answer was to fit a new battery which solved the problem for a few days.... and then the lights went out again and so he thought he'd bought a duff battery. And so he replaced it and the whole sorry tale carried on.
You might not fatten a pig by weighing it, but checking that it's putting on weight might help you know if you're feeding it enough.
Last edited:
