Do I really need a separate starter battery

[Bowdrie]

^Hank, that is a nice set-up, I like the DC>DC gizmos, when my system went together things like that weren't invented,, so the boat is pretty "old school", maybe "old fashioned" is a more likely term,

[smac999]

Lithuim is still highly Experimental at this point. It’s on maybe 0.1% of boats. Bms’s and cells fail leaving you with no power.

I’d sure as heck have an agm start battery with a lithium house bank. And I’d have switching in place so I could run the house off it when the house battery failed. Even with a trusted agm house I’d still have a starter battery. It’s 70lbs. That’s nothing.

[Dockhead]

Originally Posted by Gudgeon

Lets say you house bank failed in any number of ways. Having a dedicated start battery allows you to start your engine.

That's it.

You could carry one of those emergency starting packs (or are they fire hazards?).

But do you want to faff with that at o dark hundred when your anchor is dragging and you can't start the engine?

I go, personally, with completely separated engine start battery and charging system, separate alternator, not touching house system in any way. Not even same voltage. Necessary? No. Prudent? You bet your hind parts.

[Dockhead]

Originally Posted by Bowdrie

I too am in the separated banks crowd.

Although, I went one step further.

I use 2 alternators and 2 batt chargers, the house and engine systems are totally separate from each other,, they share nothing, not even the grounds.

The lead-antimony starting batts have different self discharge and float characteristics than the lead-calcium deep cycle house batts,, hence the 2 chargers, each set at different float levels.

The engine lives in its own little world, free of being the "Ground Central Station" for the boat.

If I had to use the house bank to start the engine?, well, that's what the jumper cables are for.

It works for me, never had any issues.

To my mind, this is clearly Best Practice.

I especially advocate jumper cables instead of some kind of interconnection switch for emergency jumping. Nothing to fail or go wrong or to be set incorrectly. "A B Both" switches are from the devil.

[rslifkin]

Originally Posted by Dockhead

I especially advocate jumper cables instead of some kind of interconnection switch for emergency jumping. Nothing to fail or go wrong or to be set incorrectly. "A B Both" switches are from the devil.

1/2/both switches have their place, but if they're used correctly, you never move them in normal use. They're basically treated as high speed jumper cables. On my boat, I have 3 of them for starting power source selection. But with 2 engines and a generator, I have 2 start batteries. So each engine and the generator have a switch to select which start battery they use. Normal positions for me are port engine on "1", stbd engine and generator on "2".

House bank is just an on/off switch and only connects to start via ACRs (ignition interlocked so they're only used for alternator charging and they stay separate for shore / solar charging). Only way to tie the house bank to a start bank manually is via jumper cables.

[Dockhead]

Originally Posted by rslifkin

1/2/both switches have their place, but if they're used correctly, you never move them in normal use. They're basically treated as high speed jumper cables. . .

To each his own, but I'll take the low speed, Old Skool jumper cables over any "A B Both" switch.

[mvweebles]

The reason lithium are not recommended for starter is due to high amp draw that is beyond the BMS setting of a single lithium battery. Parallel several lithium together and you can build-up delivery of required current. Also, voltage drop across lithium is much less than FLA so electronics should be fine.

As Mark Twain once said "if you put all your eggs in one basket, make sure you really watch that basket!" More or less what the OP is proposing. With modern day monitoring electronics, it's much easier to set a custom low-voltage alarm. That said, lithium voltage drop happens very quickly. The difference between 90% SoC and 30% is only 0.3v.

It's a question of risk. I agree that with lithium and modern monitoring, makes sense to revisit the traditional wisdom of separate engine start battery. I'm not sure I'm there yet, but the gap is definitely smaller than it was in the pure FLA/AGM days.

Good luck

Peter

[Frankly]

I must be missing something here. Does anybody make a single 900AH lithium battery (Tesla excluded). Assuming that there will be at least two batteries in parallel the probability of simultaneous failure gets pretty small with the ability to switch out at least one side.


Frankly

[Dockhead]

Originally Posted by Frankly

I must be missing something here. Does anybody make a single 900AH lithium battery (Tesla excluded). Assuming that there will be at least two batteries in parallel the probability of simultaneous failure gets pretty small with the ability to switch out at least one side.

Frankly

Most people are using stacks of cells controlled by a single BMS. With or without cells paralleled.



I would think that a well engineered system would have a very low chance of failure, but then -- these systems do have low voltage and high voltage cut offs and there are scenarios, hopefully very rare, where the system can go offline.


Do you want all your eggs in that one basket? Depend for engine starting on a system which serves all your electrical needs and could get taken out in case something somewhere in the whole electrical system fails?


As many have argued here, it's very good to keep engine starting system completely separate from everything else. Dead simple, single lead acid battery, own alternator, own charger, nothing connected to it except the engine. That's a recipe for robustness and reliability, which you want in a life safety critical system like main engine starting.

[noelex 77]

Another vote for retaining a separate start battery. Redundancy is one of the mainstays of a reliable and safe cruising boat. A separate start battery with an engine that can also be started from an independent house bank is one of the first requirements towards this goal.

This is even more important with a lithium house bank. While lithium batteries have many advantages, the system is more complex with numerous points of failure.

Lead acid start batteries are easily and inexpensively replaced even in remote areas, as most are simple car batteries. Removing a simple, reliable, and cheap system is not wise in my view.

[realem]

We do keep a starter battery isolated and have on many occasions found the need to use the starter battery. We recently purchased a starter pack for the car and when sailing we take it along for the boat just in case. The just in case happened and the starter pack worked perfectly, more than enough power to start the Yanmar 2gm20f. Connected it to the starter battery and it did its job. The size of the jump starter packs are so small and inexpensive that it is worthwhile to have one and they recharge on a usb port

[Stewie12]

No way would I have one bank of batteries for the house and engine. Not being able to start your engine could cost you dearly. Look on it as a safety issue.

[Frankly]

Those that carry a spare starter motor/ solenoid please raise your hand.


Frankly

[AndyEss]

Another reason to have a lead acid type start battery separate from your LFP house bank is to protect your alternator from hi or lo voltage BMS disconnects. With no battery to absorb alternator output, you will burn out the alternator’s diodes.
My alternator output goes to a simple (and easy to replace anywhere in the world) lead acid battery. Wind and solar feed the house bank. An ACR parallels all the charge sources when its set point voltage (13:7Vdc) is reached. I do have the two battery banks wired through a A/B switch, but because of the ACR, I never touch the switch.

[rslifkin]

Originally Posted by Frankly

Those that carry a spare starter motor/ solenoid please raise your hand.

I don't, but for a reason: I have 2 engines. They're counter rotating, so they use different starters. So I have redundancy in the engines, plus I'd need 2 spares or I'd only have a 50/50 shot of actually having a useful spare in the event of a failure. Spare solenoid would be an option though, I just haven't looked at carrying one.