Engine and house 24v, Genset and instruments 12v - dedicated starting batteries?

[MondayNever]

Originally Posted by ranger42c

IMYep, I was taking that into account. Just thinking that maybe another pair of Fireflys might not hurt, if you have the space... and if your wallet can stand the gaffe... especially if you have fridge issues. Adding another pair later (if you decide to) maybe wouldn't necessarily be as good as adding them up front.

-Chris

I hear ya. I am torn on that. It’s kind of annoying that expanding the 24v bank by default happens in increments of 2 batteries. I am not ultimately constrained by the cost or space, but to paraphrase Rod Collins/Mainesail, if I find that my usage profile does not discharge the fireflies all the way down to 20%, I am wasting my money. If after the summer i find that I’m running the genset more than once a day at anchor or twice a day offshore, I’ll add more. I do fret about introducing new batteries to a “used” bank, but hopefully 50-100 cycles will make this trivial.

[Dockhead]

Originally Posted by TheoWjr

. . . But note that you will not have exactly the same redundancy as with the simple boat you describe above. With the simple system you could start the engine immediately from the house bank if needed. In your proposed setup you might need to wait some time for the start battery to get enough charge before it could start the main engine. Maybe that's fine for you... maybe not?. . .

You can easily jump a 12v starter from a 24v battery bank by connecting the cables to the terminals of just one 12v battery in the 24v bank. You don't even need to disconnect the battery. No need to wait for charging up the dead start batt.

[lady sophie]

I have a similar situation. All house loads including refrigeration, windlass, autopilot, etc. are 24v. Only the engine and a few instruments are 12v. I have a dedicated 12v battery for the engine starting and instruments. I also have a 2000 W inverter which runs a small 110v smart charger which keeps the 12v battery up. The system seems to work very well. If I ever loose the 12v battery I can always jump 12v for the starter off 24v batteries.

I initially had both 24vand 12v alternators but got rid of the 12v.

[skenn_ie]

I would change to 24V. 12V generation will be rectified AC, probably a single-phase one which could be boosted with a transformer or a DC-DC converter. I run my 12V genset starter off 24V, but the heaters and solenoids are run off 12V DC-DC buck converter. For radios etc, use a DC-DC converter close to the load.

[Pizzazz]

It seems that you have only one big 12V load and that is the genset starter. I would select the best four/six batteries that you have (test them first) and wire them in a 24V house bank. Then tap out 12V from the bank for the instruments, the SSB and the genset starter. You get the benefit of efficient 24V charging from the alternator.

Every couple of months you can switch the batteries, so that the 12V supply for genset starter alternates to the other pair. You still have redundancy but a much cleaner system. As long as you do not have regular heavy loads on the half bank it would not be a problem.

If you end up with six good 12V batteries you will have plenty of redundancy if one or two of them fail, you will just need to be ready to rewire them. I advise any boater with more than five batteries to come up with a regular way to check the health of individual batteries regularly.

SV Pizzazz

[skenn_ie]

A 12V starter-motor will just turn faster with 24V. As long as it isn't run for several minutes at as time, it won't be damaged. Solenoids and particularly heater plugs, won't fair so well. Heater plugs will blow in a couple of seconds.

[Dockhead]

Originally Posted by Pizzazz

It seems that you have only one big 12V load and that is the genset starter. I would select the best four/six batteries that you have (test them first) and wire them in a 24V house bank. Then tap out 12V from the bank for the instruments, the SSB and the genset starter. You get the benefit of efficient 24V charging from the alternator.

Every couple of months you can switch the batteries, so that the 12V supply for genset starter alternates to the other pair. You still have redundancy but a much cleaner system. As long as you do not have regular heavy loads on the half bank it would not be a problem.

If you end up with six good 12V batteries you will have plenty of redundancy if one or two of them fail, you will just need to be ready to rewire them. I advise any boater with more than five batteries to come up with a regular way to check the health of individual batteries regularly.

SV Pizzazz

Why in the world would you do this if you already have a completely segregated and redundant system? You are destroying the highly failure-resistant architecture of the system, creating multiple single points of failure which did not exist before, and then on top of that, you are introducing imbalances into the one remaining battery bank, which are extremely harmful. This is just awful design. There is NO redundancy in a single battery bank -- when anything fails, in 99% of cases, all the batteries are dead and you are without power. Furthermore it is very bad to rely on a house bank which is being deep discharged, for engine starting. These don't even have to be completely dead, but in many cases just drawn down to the planned DOD, in order to fail to start a diesel engine. And if you only have one alternator, then any failure means you have no power. If you have two, then you ALWAYS have power if one goes down.



Main engine starting ability is something which can be a matter of life and death, so it's a system which should be kept very carefully as resistant to failure as reasonably possible.


Fundamental design principles to achieve this are -- Redundancy, Diversity, and Segregation. See for example: https://www-ns.iaea.org/downloads/co...ctive%20v3.pdf


Redundancy avoids the effects of random failure. Diversity and segregation avoid the effects of common cause failure.



The OP's original electrical system design fulfills all of these criteria. The segregated engine starting battery system totally prevents common cause failure. No possible failure in the house battery system can affect the engine start system, nothing from the alternator to the batteries to anything else. This is extremely desirable. Then, he has complete redundancy because he is generating and storing electrical power by an entirely separate means, which can be used in case of need by jumping from the house bank.



These kind of shade tree baling wire cobbled up solutions should be strictly avoided in such an important system as engine starting. Just add the two Fireflies; no one ever complained about having too much battery power. After a certain amount you may be getting diminishing returns, but more is ALWAYS better, even if after a certain point it may be only a little better. It certainly does not harm except to your wallet; you can't say that about these other proposed solutions.

Originally Posted by lady sophie

. . .I initially had both 24vand 12v alternators but got rid of the 12v.

Have none of you people ever experienced an electrical failure which led to the loss of your ability to start an engine, just when you needed it most? Maybe the young generation of cruisers never experienced those awful 1 2 Both Off switches that would kill your whole system if you left it in the wrong position.



Maybe all it takes is one time waking up in sudden overnight violent squall, with your anchor dragging (a CQR in those days), and you can't start the main engine because you forget to switch back to "1" before going to bed. Combining engine start and house banks permanently is even worse than that -- it's like the 1 2 Both Off switch permanently set to Both