Do I really need a separate starter battery

[Zzmeyer]

I started sailing when a sailboat had one battery. You woke up in the morning and the engine wouldn't start. Someone figured out to separate the engine start battery from the house bank. My new boat will have 900 amps (12.8 volts) lithium and 1200 watts solar with a Victron battery monitoring system. So why do I still need a separate starter battery? It is very unlikely I'll ever run the house bank down both because of its size and monitoring capabilities and if I do, wait a couple hours for the solar to recharge. While redundancy is always nice, the complexity of multiple battery types and their charging requirements seems to offset the advantage. It would give me more space, less weight and less complexity if I just skipped the starter battery. Help me with my idiocy.

[Muaddib1116]

For some reason I was under the impression that it wasn't great for lithiums to use them as starter batteries. I don't know if this is true or not, or if I made it up at some point.

[Frankly]

I made the no starter battery decision 15 years back. Never looked back. Need to keep an eye on things but "you should keep an eye on things". 6ea GC2s with switch on each bank. On few ocassions when really discharging the bank just switch out a set and they become starter bank by definition.


Frankly

[Gudgeon]

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

[rslifkin]

Personally, I like a separate start battery for a couple of reasons: No concern for voltage dip to electronics during engine cranking, and less chance of leaving yourself without starting power. Having the ability to also start from the house bank is good though, that way you have redundancy if a battery fails.

[sailingabe41ds]

sh*t happens. tried that on a hunter 36....then one day low voltage...no start. got saved by honda 2000...... maybe keep one of portable jump starter for dieael trucks.
abe

[Zzmeyer]

Originally Posted by Muaddib1116

For some reason I was under the impression that it wasn't great for lithiums to use them as starter batteries. I don't know if this is true or not, or if I made it up at some point.

I've heard that too and that is the only reason I could see having a non lithium start battery if true. If my lithium house bank is dead and can't be charged I'm in a world of hurt. I guess I could always sail.

[Zzmeyer]

Originally Posted by rslifkin

Personally, I like a separate start battery for a couple of reasons: No concern for voltage dip to electronics during engine cranking, and less chance of leaving yourself without starting power. Having the ability to also start from the house bank is good though, that way you have redundancy if a battery fails.

Voltage drop would be an issue. Do you know if lithium has a voltage drop when starting?

[Muaddib1116]

What's interesting is that a lot of the small form-factor emergency jump packs to keep in your glove box are lithium, so I really don't know.

[Zzmeyer]

Originally Posted by sailingabe41ds

sh*t happens. tried that on a hunter 36....then one day low voltage...no start. got saved by honda 2000...... maybe keep one of portable jump starter for dieael trucks.
abe

I'm thinking my solar in a couple hours would do it, but like the idea of a simple portable jump starter. Would take care of the redundancy issue although I'm not really too worried about that with the lithium and monitoring capability.

[CarinaPDX]

A few years back I bought one of those compact lithium jump starters, because my car (3rd gen MDX) is known to work fine for years and then, with no prior symptoms, just fail to start when the battery is EOL. I haven't needed it (yet) but likely soon as the battery is more than 6 years old. I have jump started other vehicles, including big V-8s (van, performance Camaro), Rotax engines, etc. It turns them over faster then they ever did before - very impressive. So yes, get one that is large enough to start a diesel engine, and keep it aboard and charged, and you should be good.

That said, the reasons given by others for a separate starting battery such as isolating electronics from starter motor and windlass loads still applies - turning on and off those big DC motors can cause nasty voltage spikes. With a sufficiently large main battery I don't think this will be a problem with lithium technology but it is something to look out for - worth checking if you have access to an oscilloscope.

Greg

[Philtao]

I have kept both as I would not want to be in an emergency situation and not be able to start the engine because the BMS has shut down the lithium lithium battery for some valid reason.

It is all managed by the BMS and I never have to think about it.
The BMS commands a relay to connect the alternator to the lithium battery when ignition is ON and to disconnect it when the lithium battery is full. The same relay is commanded by another BMS output to connect the lead-acid battery to the loads just before the BMS disconnects the loads or isolates the lithium battery. Like that there is no risk of being blacked-out.

[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.

[HankOnthewater]

Although I have no lithium on the boat, I believe lithium can deliver high currents for starter motors, but.... the BMS is the limiting factor!!
For the moment I remain in the camp of two separate batteries.

I have a similar setup as Bowdrie in the previous post, but I am now contemplating to go simpler, doing away with one (the smaller) alternator, and the starter battery is exclusively used by only one thing : the startermotor, even the instrument panel is powered by the housebank. BTW, I have the RV wired like that for over 10 years, like that solution.
The remaining large alternator just charges then the house battery. Starter battery to be charged by small DC to DC charger, 2-5 amp is ample.

[Pitchondesign]

As others have said, lithium can deliver very high currents without voltage sag. The issue, as others have said is the BMS. As an example, if you have 9 100ah 12v drop in batteries, each having a BMS capable of 100 amp discharge, you would have 900 amps for cranking. That would be sufficient for most engine starters in a perfect world. In an imperfect world, with slightly unbalanced cells, some batteries might provide more current than the others. It such a case, it might be possible to trip a BMS on some batteries. If you trip a BMS you need to know how to reset it because it has to be done manually. Much depends on the specifics.

If you have a system with external control through a CAN bus, it would act differently, depending on the programing of the BMS.

Convential LA batteries don't have the BMS issue so they send as much current as the batttery will allow. LA start batteries are rated in amp hrs and CCA, or cold cranking amps, and the numbers are vastly different. A LA battery rated at 100ah might have 800 CCA. That is why boats with a lithium house bank will have a LA start battery charged with its own DC-DC charger.