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Old 25-01-2022, 09:05   #1
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Getting rid of the start battery

I am completely rewiring my boat, and I was thinking last night - why not get rid of the start battery entirely?

I would keep something like https://no.co/gb40 on the boat instead, it's small, light and capable of jumpstarting my boats engine if the house bank failed or got low enough not to crank the engine properly.

I'd save space/weight on a bunch of wiring, an echo charger, can ditch the selector switch etc.... am I missing something?

I'd lose the redundancy from having a spare start battery, but those tend to be small anyway.
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Old 25-01-2022, 09:21   #2
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Re: Getting rid of the start battery

In my mind, it's a bad idea. I personally consider engine starting power to be safety critical on any boat. I don't mind sharing a start battery between multiple engines, or an engine and a generator. But I prefer to have a dedicated source of starting power that can't ever be accidentally drained (absolutely nothing should be powered from this battery). And a switch to select an alternate battery as a backup source of starting power (house bank is fine for this).



I also don't like starting from the house bank as a matter of practice. Depending on the engine, state of house bank charge, and size of the house bank it can cause enough voltage dip to reboot electronics. That's not ideal if you need the engine on short notice and now have lost your chartplotter for a minute while it reboots.



As far as switches, if the wiring design is good, you should never need to touch a selector switch in normal operations. There should be an on/off (or selector) for the house and a selector for the engine. It should be designed so everything pulls from the desired places (and charges) with switches in the normal positions, so the engine selector is only for turning things off or for selecting backup starting power if the start battery fails.
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Old 25-01-2022, 09:34   #3
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Re: Getting rid of the start battery

I like rslifkin's answer. I take it a step farther. Separate engine start, genset start, house battery, and instrument panel backup battery. Engine alternator and battery charger for the house bank, with battery charger powered by shore power or the genset. The two start batteries have their own altenators. Then, the whole thing can be cross connected with a jumper cable. Just one cable, because all the battery negatives are already bonded together. Save the lithium ion jump start battery for really unusual failures.

Actually, I do use the Li-ion jump starter for our house power failure genset. It gets used so rarely that keeping a lead acid battery charged for it is a nightmare.
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Old 25-01-2022, 09:42   #4
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Re: Getting rid of the start battery

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Originally Posted by tkeithlu View Post
I like rslifkin's answer. I take it a step farther. Separate engine start, genset start, house battery, and instrument panel backup battery. Engine alternator and battery charger for the house bank, with battery charger powered by shore power or the genset. The two start batteries have their own altenators. Then, the whole thing can be cross connected with a jumper cable. Just one cable, because all the battery negatives are already bonded together. Save the lithium ion jump start battery for really unusual failures.

Actually, I do use the Li-ion jump starter for our house power failure genset. It gets used so rarely that keeping a lead acid battery charged for it is a nightmare.

Mine isn't quite as simple as what I was mentioning either. I've got 3 banks on my boat (2 engines + generator). 2 start banks (1 for port engine, 1 for stbd + gen, but any of them can be started from either battery). Then a single house bank for everything else. Engine alternators charge the start batteries first, then ACRs feed power to house for charging. The ACRs are ignition interlocked, so non-alternator charge sources won't activate them and charging of each bank is independent with the engines off.
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Old 25-01-2022, 09:51   #5
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Re: Getting rid of the start battery

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Originally Posted by alctel View Post
I am completely rewiring my boat, and I was thinking last night - why not get rid of the start battery entirely?

I would keep something like https://no.co/gb40 on the boat instead, it's small, light and capable of jumpstarting my boats engine if the house bank failed or got low enough not to crank the engine properly.

Imagine you're sailng, turns out your house battery bank has gotten low, and for some emergency reason you need to start the engine RIGHT NOW!

-Chris
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Old 25-01-2022, 13:02   #6
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Re: Getting rid of the start battery

Unpopular opinion, but I think it is possible to go simpler and still have mission critical redundancy. Usually with dedicated Starters, the best advice if at all possible, is to go with just one big House bank. If you are eliminating the Starter batts, then a secondary "Reserve" House bank can fulfill that function. As critical as it is, cranking is still a trivial load, does not pull Ah / SoC% down appreciably, just need to ensure cranking POWER.

If you feel you have a need for a dedicated windlass bank, or for thrusters, bilge pump, whatever, then that could also be incorporated into the design without requiring a third bank.

But relying on complete automation, so the captain can remain heedless of bank SoC% requires greater complexity, thus is not a good approach IMO.

So the larger capacity, constantly used "Primary" bank is the one gets depleted, thus gets directly charged by all significant energy sources. The Reserve bank gets a DCDC if not the same chemistry, but ideally an ACR or Echo Charger will do, just a lower current rate.

Critical load circuits, aka "Essentials": engine, safety including navigation, comms etc are segregated from "Auxiliary" non-essential loads, entertainment of course, with refrigeration in between, each of these could have different voltage cut-offs to preserve the energy reserved for Essentials.

If energy on demand, genset is aboard, then simpler is better. Alternator(s) on the propulsion engine(s) should be optimized as effective sources and additional redundancies, as also the Lithium jumpstart packs, belt and suspenders.

Reserve must be sized appropriately, big enough not only for cranking but many hours' runtime of all Essentials circuits - hence usually much higher capacity than Starters need to be. It may be not frequently used, but must still be regularly load tested via Cranking and Essentials circuits, easily switched over with two make-before-break 1-2 switches (no Off position). Note these are only switching / choosing the LOAD circuits, not charge sources - KISS as much as possible!

Note that this design concept give many safety / redundancy advantages, it's probably not worth implementing just to eliminate Starters.

...

Additional wrinkle, keep both Primary and Reserve connected in normal operations, using only 10-20% of total capacity per cycle, all the benefits from Peukert "one big bank" Effect. But once it hits 80% SoC, have an adjustable LVC isolate the Reserve "sub bank" so energy is kept for cranking/essential loads.
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Old 25-01-2022, 14:11   #7
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Re: Getting rid of the start battery

QUOTE "Imagine you're sailng, turns out your house battery bank has gotten low, and for some emergency reason you need to start the engine RIGHT NOW!"

Actually, take it a bit farther. Run all the scenarios that could leave you sucking your thumb, and design a system that covers all the "What ifs."

I added the spare battery behind the instrument panel when I realized that seawater coming in would immediately short out my house bank, leaving me no way to even call for help on the VHF.
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Old 25-01-2022, 15:30   #8
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Re: Getting rid of the start battery

If saltwater "immediately" shorts out a battery, how do you explain the Samurai that got stuck in the mud flats of San Filipe when the 20' tide came in. All night long you cold see the 4-way flashers glowing through the water. I'm sure the Samurai was junk, but the battery still worked.
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Old 25-01-2022, 16:01   #9
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Re: Getting rid of the start battery

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If saltwater "immediately" shorts out a battery, how do you explain the Samurai that got stuck in the mud flats of San Filipe when the 20' tide came in. All night long you cold see the 4-way flashers glowing through the water. I'm sure the Samurai was junk, but the battery still worked.
The conductivity of seawater is around 5 S/m.

For 2 X 3 in^2 terminals 1 foot apart on a sealed gel cell, or Lithium 12v battery completely charged before it was submerged, that ends up about 3 amps of discharge, or about 4 ohms of resistance.

It will take several hours, (maybe days), for a 200amp hour battery to go completely dead to the point it couldn't power a light.

I wouldn't try to start an engine on a submerged battery though.
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Old 25-01-2022, 16:07   #10
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Re: Getting rid of the start battery

Quote:
Originally Posted by alctel View Post
I am completely rewiring my boat, and I was thinking last night - why not get rid of the start battery entirely?

I would keep something like https://no.co/gb40 on the boat instead, it's small, light and capable of jumpstarting my boats engine if the house bank failed or got low enough not to crank the engine properly.

I'd save space/weight on a bunch of wiring, an echo charger, can ditch the selector switch etc.... am I missing something?

I'd lose the redundancy from having a spare start battery, but those tend to be small anyway.
I don't have a problem with it. Done it with two house banks or a house bank and starting battery.
I like the two house banks better. You need to remember to switch to all for charging and switch off all so you have a backup though.
Not too fond of just a bank and a starting device though. They are notoriously unreliable if they sit unused.
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Old 25-01-2022, 18:07   #11
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Re: Getting rid of the start battery

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I like the two house banks better. You need to remember to switch to all for charging and switch off all so you have a backup though.
Exactly what combiners, VSR/ACR are designed to automate. Senses from either side charge source is active, joins, not active isolates.

The Blue Sea ML latching version is designed to sustain 500A, can combine manually for jump starting

Literally indestructable, never fail
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Old 25-01-2022, 18:26   #12
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Re: Getting rid of the start battery

Operating for 16 years with house bank only, 3X 2GC2 in series with ability to switch each set. Simple and has not left me stranded yet. On a couple of ocassions when the bank was getting uncomfortably low just switched a set off and they became the starting battery. Several times eash year start the engine on each set. When a single set start shows any hesitation, get 6 new batteries.


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Old 25-01-2022, 19:28   #13
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Re: Getting rid of the start battery

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Originally Posted by ranger42c View Post
Imagine you're sailng, turns out your house battery bank has gotten low, and for some emergency reason you need to start the engine RIGHT NOW!

-Chris
He has that lithium battery starter battery which is probably better and more reliable anyway, so it has a starter already, and you can still hand-crank, or raise the anchor up the mast and drop it to start the engine anyway.

But then again, in an emergency starting an engine is not needed or helpful as I have found my self in bad situations but never once wanted an engine.
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Old 25-01-2022, 21:26   #14
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Re: Getting rid of the start battery

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Originally Posted by PaulCrawhorn View Post
Unpopular opinion, but I think it is possible to go simpler and still have mission critical redundancy. Usually with dedicated Starters, the best advice if at all possible, is to go with just one big House bank. If you are eliminating the Starter batts, then a secondary "Reserve" House bank can fulfill that function.
So, in a nutshell, you seem to be saying that he can eliminate the starter battery by replacing it with a reserve house battery. Perhaps I am not quite grasping your intent, but how does that actually simplify his system by eliminating a secondary battery?

On my old C&C 27, I had two Series 27 Deep Cycle batteries that acted as both house and starter batteries. On any given day, only one battery would be in use, with the other held as a reserve. On odd-numbered days of the month, use Battery No. 1, on even days use Battery No. 2. All of my crew understood this rule and it worked very well for 15 years, evenly distributing the wear and tear on the batteries.

HOWEVER, as lsrifkin so correctly pointed out, this approach did have its downsides. Whenever I started the engine, the GPS Chartplotter would reboot and take several minutes to re-establish its position. This was usually a minor annoyance, but in the case of a man overboard situation, or critical maneuvering in an obstructed channel, it could have had serious consequences. My efforts to overcome this behaviour with voltage stabilizers, etc. were unsuccessful and I was never comfortable with the situation.

On my current boat, I have both the space and adequate reasons to maintain separate house and starter banks, albeit with a combiner switch to allow cross-over between banks in case of an emergency. As well, I am adding another battery switch which will allow me to isolate either of the batteries in the house bank, should that become desirable. Over my decades of sailing and racing, I have seen too many situations in which someone 'forgot' to switch off the reserve battery and found themselves in the position of not even having enough power left to make a radio call for help. At least with a dedicated starter battery you can always get the engine going and generate the needed power. I want to be sure that in the case of an MOB, or otherwise needing instant maneuverability, I will always be able to start my engine, or restart it. Redundancy is a prudent sailor's security blanket.
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Old 25-01-2022, 22:01   #15
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Re: Getting rid of the start battery

Yes, jf your system works fine off two single 12V lead batts, then it cannot be simplified further without increasing risks.

I am not suggesting would be an improvement for everyone, just responding to the OP with food for thought.

My scenario is more along the lines of a 600Ah and 300Ah, ideally both LFP

and the Reserve being larger than a Starter, is adding Redundancy for more than just cranking functions.

Also, when there are already multiple batteries for Starter, or the other functions mentioned, these get consolidated, reduced to two in total, which is IMO the minimum safe count
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