Designing a simple DC system from scratch.

[exMaggieDrum]

Originally Posted by fguptill

I would make two important changes immediately:

2) ditch the engine start battery. Install one battery system only. This will save money, weight, and space. It also makes your entire electrical system simpler, cheaper, and easier to maintain. No 1-2-both switch - just an on-off switch. In the rare event that you have allowed your battery to discharge to the point that it cannot start your engine (in 50+ years of boating, this has never happened to me), you simply pull out the portable lithium battery pack that you keep on board for such emergencies. These things are compact, lightweight, cheaper than a regular starting battery, and are available at most automotive suppliers. (E.g. Canadian Tire, here in Canada). It's also handy for troubleshooting, or taking ashore for picnics, or for charging cell phones, or whatever. You can also use the space and weight saved to increase the size of your battery bank.

I understand where you are coming from. It's good to continuously challenge the "standard - old school" thought. I just put on a big lithium bank myself with a huge solar bank and other backups. But I will not ditch my start battery and put in the relatively minor extra work and one more battery switch to use it if I need to. If I have an emergency and need to up anchor or start the motor to get away from a reef coming up, I like the ability to go turn a switch and get going, and not have to dig out a lithium battery (that you do have to check and recharge occasionally somehow) and hook its clamps on to the engine starter in a seaway.

Or like three weeks ago when I had a dirty fuel issue and had to bleed my engine multiple times which brought my start battery down too much by turning it over multiple times. If my main battery were down I am not sure a lithium backup pack would have lasted as long as my start battery did that day.

I like the little backup starter packs. Great technology and could be very handy. But I don't think they are the same thing as a backup battery system on board that is ready to go.

But just like a lot of things on a boat, how much safety gear, spares, and backup systems to have is an individual choice. A particular issue is certainly not as black and white as some might think.

[SecondBase]

Originally Posted by fguptill

I would make two important changes immediately:

2) ditch the engine start battery. Install one battery system only. This will save money, weight, and space. It also makes your entire electrical system simpler, cheaper, and easier to maintain. No 1-2-both switch - just an on-off switch. In the rare event that you have allowed your battery to discharge to the point that it cannot start your engine (in 50+ years of boating, this has never happened to me), you simply pull out the portable lithium battery pack that you keep on board for such emergencies. These things are compact, lightweight, cheaper than a regular starting battery, and are available at most automotive suppliers. (E.g. Canadian Tire, here in Canada). It's also handy for troubleshooting, or taking ashore for picnics, or for charging cell phones, or whatever. You can also use the space and weight saved to increase the size of your battery bank.

+1. This is also an easy refit on boats with an separate banks and existing 1-2-both switch. Buy a jump box with a USB charging port.

I'd like to hear from knowledgeable commentators on "using a very large house bank used to start a small 2 or 3 cylinder diesel" beyond the rote recitation of beware the starting load. At some point can a giant house bank overcome the stater load issue/wear of a small diesel?

[john61ct]

Absolutely, any decent House bank can crank the starter on any sized engine of any type, until the cows come home.

Originally Posted by exMaggieDrum

a backup battery system on board that is ready to go.

I agree there should be redundancy, but IMO batteries purely dedicated to nothing but Starting are a waste, dead lead.

From a pure design POV anyway.

http://www.sailnet.com/forums/showthread.php?t=298497

[john61ct]

However I see those little lithium jumpstarter packs as "belt and suspenders" only.

I personally want all essential nav / comms safety gear covered by my redundant Reserve bank, not just cranking.

[SecondBase]

Originally Posted by john61ct

Absolutely, any decent House bank can crank the starter on any sized engine of any type, until the cows come home.



I agree there should be redundancy, but IMO batteries purely dedicated to nothing but Starting are a waste, dead lead.

From a pure design POV anyway.

Design eliminating Starter batt(s) and trad 1/2/B switch - SailNet Community

It kinda feels that way to me also. Let me ask it a different way. How many times could I start a small Diesel engine from a 6 x 6volt deep cycle say Trojan T-105 house bank before damaging the bank? Assume well-maintained and fully recharged between starts.

[john61ct]

It would be necessary to measure total AH consumed per time.

Because of Peukert's Law, higher amp currents like that pull down % capacity more quickly than the 20-hour rate.

There is no "damage" involved until voltage gets to say 10.5V, long as you soon get it charged back up.

And once you go below 50% of the rated capacity you're reducing cycle lifetime a bit more quickly, but that's happening all the time, most boaters really abuse their banks.

Even though amps are high, each attempt is just a couple of seconds, I would say a few dozen tries wouldn't hit 50%?

In any case, a normal Starter batt would be flattened much more quickly.

However note we're talking about starting out with a fully charged bank.

The usual concern is that House may already be depleted close to 50% when cranking is needed, so I'm **not** advocating only having that one bank for everything.

My point is to consolidate all "not House" into a single Reserve bank that is not depleted at all in daily use.

In contrast to many boats, with several separate Starters (two engines + genset), a separate Nav systems batt, another bank for the windlass and/or bow thrusters, none of which are connected except perhaps while being charged.

To me that's just crazy wasteful, although I do recognize that long distances and very high currents may sometimes make the required large gauge wiring impractical.

[GILow]

Originally Posted by john61ct

..... but IMO batteries purely dedicated to nothing but Starting are a waste, dead lead.

Except when your engine is 24 volts and your house bank is 12 volts.

Guess why I know this..

[gbr]

A lot of good advice here. One thing I would consider (and I would definitely do) is have the house system 24V. Most marine electronics already will cope with it, and what it will do is enable you to reduce your wiring size for the heavy duty items, or if you keep it the same reduce your losses significantly. I'm particularly thinking of things like windlasses and winches (if you go electric). if you had a 1200W windlass, it will draw 100A at full load which requires very chunky wiring. If you have some resistance in one of your connectors or say the main on/off switch you will notice it gets warm. By doubling the voltage, you halve the current and reduce your losses to 1/4.
In that example, say you have a 0.01 Ohm resistance in your on/off switch, you will end up trying to dissipate 100W in your switch and it will melt. However at 24V it would only be 25W. Big difference huh?

It scuppers the plan above of only having 1 battery bank, but then you could just get away with a very small 12v start battery. Of course you will need a separate 24V alternator, but that will run cooler and more efficient as well.

Maybe its overkill for your project, but that is what I would do.

Best

GBR

[john61ct]

If the engine is not used for very long periods, or its Alt is not a significant charge source for House, check out Yandina's TrollBridge.