charging 2 battery banks with 2 engines

[Dave_Seager]

My Princess 32 has a setup with splitting diodes just as shown in the diagram above. The diode packs are mounted on the output terminals of the alternators on a heatsink shaped to match the outline of the alternator.

 

[A_8]

I may be a bit simple but I dont really understand why the bank should be receiving a 30 amp charge. In my experience an alternator may start at that level to replace the starting current, and then it quickly drops back to a few amps to charge the batteries, according to their voltage relatively to the cut off voltage. The fact that diodes or VSR's are in the circuit should not affect this ...right??

yes , that is correct. That is how a reasonably healthy battery will behave. I can only assume paulgouch is talking about a battery that eats energy without storing it (not sure that even happens)...if it does it will create heat and is a much bigger problem than draining another battery bank, it will explode or cause a fire independent of a VSR, diodes or what have you.

 

[PaulGooch]

If your batteries are dead or messed up no charge system will help. I'd say you have bigger problems then the VSR if one battery bank is draining the other as well as a 60Ah+ generator, that bank will explode as long a you keep feeding it 30 amps or more if it does not store it so I suggest you get in the life raft even if you have only one generator and one battery bank.
That aside it is not likely in a situation like that for the voltage to go over 13.8 (or you being able to start the engines in the first place ) as everything will be stressed to the max so the VSR still keeps the batteries separate.

Regarding the 12.8 volts, I can only say that you obviously don't understand how it will work in practice, trust me it will not keep the banks connected for very long. I know, I've used it.

If one battery in a bank fails in a particular way, where is will not retain any of the charge being applied to it, that battery will drain the good battery(s). Ask anyone who runs a Jap 4x4 with twin batteries, where one failed. Ask anyone who runs lorries, where one battery has failed. Ask anyone who has had a single battery in a bank on a boat and one has failed. Ask anyone who has a torch with more than one battery, and one h as failed etc etc etc etc.

Therefore, if you parallel all of your boat batteries by fitting a VSR and one of the domestic bank fails, it will not only drain the other domestic batteries, it will drain the engine batteries too.

If you think such a failed battery will overheat and cause other problems, think again.

 

[grumpy_o_g]

VSR's have no place on a boat. As soon as the engine starts, the VSR will energise and effectively parallel the battery banks. If a single battery catastrophically fails it could suck the power out of all the batteries in both banks. If the battery the VSR is trying to charge develops a fault and won't hold a proper charge, the VSR has to carry all the current that this bank requires from the other bank. It isn't designed to carry such loads, nor is the wiring used. If, for instance, the bank that the VSR is charging has the windlass on it, as soon as you hit the windlass control the VSR or the wiring will be fried (hopefully the wiring won't catch light, but it could easily do so as there is no circuit breaker or fuse specified in the typical fitting instructions.

As a pertinent example, we had a battery fail at the beginning of a 150 nm trip at the end of last Summer. When we arrived at an overnight stop 45 nm into the journey i switched the engine off and all of the electronics went off. The electronics battery was totally flat, despite being charged for the past three hours by the alternator. Had we have been using a VSR, it's almost certain that the dead battery would have sucked the life out of the good engine battery. We hooked up to the shore power and the battery was on charge all night via the 25a mains charger. In the morning it was still flat as a pancake. As soon as the engine was started, everything worked fine and we were able to complete the trip, with the totally knackered battery, no problems at all.

I don't follow that - surely the VSR simply switches the charge from engine start battery to the domestic battery/bank once it senses the start battery's been fully charged? With two alternators wouldn't you just let each alternator charge it's start battery and then switch to the domestic bank?

I'm asking I hasten to add - not contradicting - as I'm used to rag and stick. We tend to prefer a big red switch anyway. It just seems a bizarre way to use a VSR to me. Also sounds like people are fitting under-sized ones as I've used 300A jobbies before on aircraft. I'd hope a fuse somewhere blew before I melted part of my charging circuitry by discharging a battery across a near short circuit.

 

[PaulGooch]

I don't follow that - surely the VSR simply switches the charge from engine start battery to the domestic battery/bank once it senses the start battery's been fully charged? With two alternators wouldn't you just let each alternator charge it's start battery and then switch to the domestic bank?

I'm asking I hasten to add - not contradicting - as I'm used to rag and stick. We tend to prefer a big red switch anyway. It just seems a bizarre way to use a VSR to me. Also sounds like people are fitting under-sized ones as I've used 300A jobbies before on aircraft. I'd hope a fuse somewhere blew before I melted part of my charging circuitry by discharging a battery across a near short circuit.

The VSR switches when it senses 13.8 volts. As the alternator will output more than this, the VSR will switch as soon as the engine starts. If you were using VSR's with twin engines, yes, you would let each alternator charge it's own start battery (if you had separate start batteries) with VSR's on each alternator to the domestic bank. Problem is, as soon as the engines start, all three banks would be paralleled. The VSR does not switch the charge from one bank to another, it parallels the two banks to allow the domestic bank to take it's charge from the starter bank. The key point here is that with the engine running, all batteries are connected together.

The wiring diagrams supplied by BEP, the most common VSR supplier, shows no fuse or breaker in the charging circuit. The VSR and the wiring that effectively parallels the battery banks is unprotected. The installation instructions allow for cable as small as 4mm to be used. It doesn't take much imagination to picture what 4mm cable will do if the domestic bank is flat and you use the windlass (for example). All of the power required for the windlass will need to come from the engine bank, via the VSR and the 4mm cables. If the VSR doesn't melt inside, the wiring most certainly will.

The point of separate banks for engine/s and domestics it to minimise the risk of being stranded with flat batteries. The use of VSR's is therefore inappropriate, IMO.

The exact system that would be best for any given boat will depend on several factors. For boats that aren't away from shore power for more than, maybe, 2 or 3 days at a time split charge diodes offer a cheap and reliable solution. This perfectly suits my own boat and it's use. There is the issue of the voltage drop caused by diodes, but this has never affected me. If the voltage drop was a problem to someone, then there are advanced alternator regulators available that more than overcome the voltage drop losses.

 

[grumpy_o_g]

Thanks Paul - I had no idea that would be how they're normally used. It seems "common wisdom" is not that wise in this case.

 

[A_8]

If one battery in a bank fails in a particular way, where is will not retain any of the charge being applied to it, that battery will drain the good battery(s). Ask anyone who runs a Jap 4x4 with twin batteries, where one failed. Ask anyone who runs lorries, where one battery has failed. Ask anyone who has had a single battery in a bank on a boat and one has failed. Ask anyone who has a torch with more than one battery, and one h as failed etc etc etc etc.

Yes, of course they do as they are always connected, these batteries also share the discharge cycle. Over time all batteries will adapt to the dynamic cycle of the weak battery, this is however a pretty slow process and it gives a lot of warning before it gets really bad. It is not a sudden event or to put it in other words, if you check your batteries every now and then its easy to avoid. Maybe you are not aware but most domestic banks on larger boats have 2, 3, 4 or more continuously paralleled batteries and for the most part they last as long as a single battery. You should not overstate this problem in order prove your position, you are misleading the readers.

, if you parallel all of your boat batteries by fitting a VSR and one of the domestic bank fails, it will not only drain the other domestic batteries, it will drain the engine batteries too.

You are missing a critical point, using a VSR the batteries are only connected when there is a surplus charge and they do not share the full charge-discharge cycle therefor this is irrelavant, it is not a reason to avoid a VSR. The starter battery will just be idling when the "bad" domestic banks is draining itself. If you wire the VSR correctly it will always make sure the starter battery gets charged before it connects to the other battery bank.

If you think such a failed battery will overheat and cause other problems, think again.

What can I say, have you never heard of exploding batteries, how do you think that happens..

 

[A_8]

The VSR switches when it senses 13.8 volts. As the alternator will output more than this, the VSR will switch as soon as the engine starts.

This is misleading, the battery will only reach 13.8 volts if it is close to fully charged, a healthy starter battery will get there quickly but that also means that it is ok and will be able to start your engines the next time you need it, with or without a VSR.

If you were using VSR's with twin engines, yes, you would let each alternator charge it's own start battery (if you had separate start batteries) with VSR's on each alternator to the domestic bank. Problem is, as soon as the engines start, all three banks would be paralleled. The VSR does not switch the charge from one bank to another, it parallels the two banks to allow the domestic bank to take it's charge from the starter bank. The key point here is that with the engine running, all batteries are connected together.

Again this is misleading, wired right it will only connect the battery banks provided your starter battery is reasonably charged. It is not taking charge from the starter battery unless it does not need it. Its a way to use surplus charge capacity.

The point of separate banks for engine/s and domestics it to minimise the risk of being stranded with flat batteries. The use of VSR's is therefore inappropriate, IMO.

The point of separate banks is indeed to minimize the risk of being stranded with flat batteries but NOT because you have them connected when you are charging them, its to keep the starter battery charged incase you accidently discharge the domestic bank using the fridge, heather, lights etc when the engines are not running or you don't have shore power.

The exact system that would be best for any given boat will depend on several factors. For boats that aren't away from shore power for more than, maybe, 2 or 3 days at a time split charge diodes offer a cheap and reliable solution. This perfectly suits my own boat and it's use. There is the issue of the voltage drop caused by diodes, but this has never affected me. If the voltage drop was a problem to someone, then there are advanced alternator regulators available that more than overcome the voltage drop losses.

If you have access or want to go and find shore power every few days there is absolutely no point in considering a VSR for a twin engine install. I initially posted to share a possible low cost solution for more charging capacity when you needed it, if you don't then it is sort of pointless..

 

[PaulGooch]

<snip>



You are missing a critical point, using a VSR the batteries are only connected when there is a surplus charge and they do not share the full charge-discharge cycle therefor this is irrelavant, it is not a reason to avoid a VSR. The starter battery will just be idling when the "bad" domestic banks is draining itself. If you wire the VSR correctly it will always make sure the starter battery gets charged before it connects to the other battery bank.

Absolute rubbish. The VSR is triggered at 13.8v, the alternator outputs more than that so the VSR gets triggered as soon as you start the engine. Every person reading this who has a volt meter fitted will know this to be the case, as they will see the voltmeter rise when they start their engines.

What can I say, have you never heard of exploding batteries, how do you think that happens..

Exploding batteries are almost always caused by gasses escaping during charging being ignited by a spark or a naked flame. A battery that will not hold any charge rarely gives off gas.

 

[PaulGooch]

This is misleading, the battery will only reach 13.8 volts if it is close to fully charged, a healthy starter battery will get there quickly but that also means that it is ok and will be able to start your engines the next time you need it, with or without a VSR.

You seriously need to pay attention. It's getting tiresome pointing out the same few FACTS over and over and over. THE VSR WILL TRIGGER AS SOON AS THE ENGINE STARTS, AS THE ALTERNATOR OUTPUTS OVER 13.8v. Look at your voltmeter when you start your engine.

Again this is misleading, wired right it will only connect the battery banks provided your starter battery is reasonably charged. It is not taking charge from the starter battery unless it does not need it. Its a way to use surplus charge capacity.

Wrong. When the VSR is triggered the banks are paralleled, with the banks paralleled the flatter battery will draw charge from the other bank. The VSR isn't some magical device that only allows alternator output to be transferred, it isn't even connected to the alternator, like a diode is. The VSR is connected across the isolators or directly to the battery banks.

The point of separate banks is indeed to minimize the risk of being stranded with flat batteries but NOT because you have them connected when you are charging them, its to keep the starter battery charged incase you accidently discharge the domestic bank using the fridge, heather, lights etc when the engines are not running or you don't have shore power.

So what happens when you get a sudden failure of one of the domestic banks whilst under way ? Let me give you a clue, the bloody batteries all go flat and you get stranded. I've had it happen, see my earlier post. Lucky for me i wasn't using a VSR.

If you have access or want to go and find shore power every few days there is absolutely no point in considering a VSR for a twin engine install. I initially posted to share a possible low cost solution for more charging capacity when you needed it, if you don't then it is sort of pointless..

The FACT remains that using a VSR is inappropriate for boats. If the resulting voltage drop from using diodes means you don't have enough charging capacity, look at a proper, safe alternative, such as advanced alternator regulators etc. Don't compromise safety by fitting second rate, ill thought out equipment. I've explained how it works, why it's flawed and why it can be dangerous. If you choose to ignore that advice, that's your choice. Why not drop an email to Fairline or Princess and ask them why they don't fit VSR's ?

 

[A_8]

I have given up on PaulGoosh and will not respond to any of this. I am also not advocating the use of a VSR as neither good or bad however I am concerned when it is dismissed based on ignorance.

One part is what happens at 13.8 volts. The pic below is a diagram of Battery state of charge vs voltage while battery is under charge, the C/5 to C/40 represents different charge currents.
It clearly shows that a battery do not reach 13.8 volt unless its more then 80% charged, in a typical installation it will be 90% charged when it reaches 13.8 volts.
The screen dump is from this document, page 2 http://www.arttec.net/Solar_Mower/4_Electrical/Battery Charging.pdf

 

[PaulGooch]

I have given up on PaulGoosh and will not respond to any of this. I am also not advocating the use of a VSR as neither good or bad however I am concerned when it is dismissed based on ignorance.

One part is what happens at 13.8 volts. The pic below is a diagram of Battery state of charge vs voltage while battery is under charge, the C/5 to C/40 represents different charge currents.
It clearly shows that a battery do not reach 13.8 volt unless its more then 80% charged, in a typical installation it will be 90% charged when it reaches 13.8 volts.
The screen dump is from this document, page 2 http://www.arttec.net/Solar_Mower/4_Electrical/Battery Charging.pdf

Of course the battery does not instantly go u p to 13.8 volts, but as the alternator is charging circa 14.4 volts this will trigger the VSR.

Besides, it isn't especially important whether the VSR triggers 10 seconds after the engine starts, or 2 minutes. The facts remain that with a VSR :

a) all batteries are paralleled whilst the VSR is working
b) the circuit that parallels the batteries is unfused and likely to catch fire in certain circumstances.

Carry on doing whatever you want on your boat, as is your right. But you don't even have a basic understanding of 12v circuits and charging systems, particularly as they relate to marine use.

 

[grumpy_o_g]

A question for anybody to answer. Assume the alternator is producing 14.4 volts o/c and assume the battery is discharged to the extent that the o/c voltage across the terminals is 12.4 volts (an arbitrary figure). Assume that any wiring between the alternator o/p and the battery terminals is zero resistance.

What happens to the voltage across the battery terminals and alternator when they are connected? There is no load on the battery - the only connection is between the alternator and the battery.

Have a made a false assumption with the zero resistance (OK, it would be something due to the wiring but it would be low) - is there something that has to sit between the two?

Next question - what voltage is the alternator sensing between (i.e. the voltage difference between which two points)?

Please don't let a discussion about battery charging get personal - it's really not worth it and I'm learning if no-one else is. It's an honest question - I can easily work out the answer for resistive or inductive loads but i want to understand how the chemical charging of the battery affects things.

 

[PaulGooch]

A question for anybody to answer. Assume the alternator is producing 14.4 volts o/c and assume the battery is discharged to the extent that the o/c voltage across the terminals is 12.4 volts (an arbitrary figure). Assume that any wiring between the alternator o/p and the battery terminals is zero resistance.

What happens to the voltage across the battery terminals and alternator when they are connected? There is no load on the battery - the only connection is between the alternator and the battery.

I'm not 100% certain that i understand what you're asking, but here goes. If you connected a voltmeter to the battery, the voltmeter would read 12.4 volts, with the engine not running and no load from any boat systems. Cable resistance and voltage drops would be so minimal as to not be worth factoring in, for the purposes of this thread. When you start the engine, the voltmeter will display the voltage being output by the alternator, with a modern alternator this is typically 14.4 volts. This is where the VSR sales blurb is extremely misleading, people think that the VSR will only activate when the battery reaches 13.8 volts. That is not the case, the VSR senses the same voltage as the voltmeter, therefore, as soon as the engine starts, the VSR is activated by virtue of the 14.4v alternator output. It's very easy to prove my claim here, ask anyone who has a voltmeter on their boat what it reads when the engine first starts.

Have a made a false assumption with the zero resistance (OK, it would be something due to the wiring but it would be low) - is there something that has to sit between the two?

Not sure what you mean here.

Next question - what voltage is the alternator sensing between (i.e. the voltage difference between which two points)?

Good question. As the alternator is directly connected to all of the batteries, as soon as the VSR activates, logically, it should be more influenced by the battery with the lowest voltage, otherwise in an installation with multiple batteries in a bank, the alternator would reduce it's output when one battery was fully charged and the other batteries would never get charged.

The theory/claim that a VSR will only allow "surplus" charging capacity to be "directed" to the domestic bank is totally, 110%, incorrect. It's almost certainly going to be the exact opposite. This is very easy to test for in any installation where a single alternator is charging more than one battery bank, no matter if that system is using blocking diodes or VSR's. With the domestic bank discharged more than the engine bank and as little load on the circuits as possible, measure the amperage between each charging cable and it's battery bank. In my experience, there will be a higher reading on the bank with the flattest batteries. But, as we're talking mobo's here, that doesn't really matter in most cases, as we've usually got plenty of alternator power and will be running the engines all the time we're under way.

There isn't a single solution that would suit all mobo's though, never mind sailboats. My own boat has a single engine and uses a simple split charge diode to keep two, single battery banks charged. We can go away for long weekends without shore power and not have any power problems. I've no idea how long we could stay away for, as we've never had any shortage of power, but it we did, i'd only have to run the engine to charge the batteries up. If we had constant shortages of power, i'd look into fitting extra domestic batteries and/or increasing charging capacity, most likely by fitting an advanced alternator regulator.

Twin engines, with two starter banks and a domestic bank would perhaps be best served with two split charge diodes. Adding advanced alternator regulators or management systems as required.

Twin engines and a single starting bank might be better served by having one alternator purely charging the domestic bank and the other charging both banks via a split charge diode. This does away with any voltage losses, associated with the diodes, on one alternator.

Bigger boats, or boats with extensive electrical installations, may well require different/additional solutions but two facts remain :

1) There is only so much power available from the alternators, if the voltage losses caused by diodes are a real issue, fit something to overcome them.

2)VSR's, whilst they will allow all the batteries to be charged without any of the voltage losses associated with diodes, DO introduce the possibility of serious or catastrophic problems.