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scott_fx

Amperage During Cranking

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scott_fx

Hey guys,

I'm planning on making a controller to switch between two of my batteries, like the manual ones but with a little bit of intelligence added. I was wondering if an 80 amp relay would handle the current during cranking or would i need to double up on the relays?

this is for a 98 sunsetter vlx

thanks

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martinarcher

Not even close. Most good batteries are rated at 600+ CCA (cold cranking amps). CCA is the amount of current in Amps the battery can supply at 0 degrees F for 30 seconds without dipping below 7.2V. CA or MCA (Cranking Amp or Marine Cranking Amps) is typically 20% higher than the CCA since the measurement is done at 32 degrees F.

Granted, your boats wiring is not typically going to see anywhere near that amount of current, but it's good to know what the battery is capable of producing.

High torque starts can draw 250+ amps so I would be leery of running any relay in line between my battery and starter. Also keep in mind as voltage drop (during cranking), current rises. That is one reason heavy duty, high current rotary switches are used. Heck the terminal posts on the back of those switches are probably 5/8"!

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scott_fx

Not even close. Most good batteries are rated at 600+ CCA (cold cranking amps). CCA is the amount of current in Amps the battery can supply at 0 degrees F for 30 seconds without dipping below 7.2V. CA or MCA (Cranking Amp or Marine Cranking Amps) is typically 20% higher than the CCA since the measurement is done at 32 degrees F.

Granted, your boats wiring is not typically going to see anywhere near that amount of current, but it's good to know what the battery is capable of producing.

High torque starts can draw 250+ amps so I would be leery of running any relay in line between my battery and starter. Also keep in mind as voltage drop (during cranking), current rises. That is one reason heavy duty, high current rotary switches are used. Heck the terminal posts on the back of those switches are probably 5/8"!

thanks martinarcher. after i posted this i actually did look at the rating on my perko switch. it was rated at 250 rms /350 max

would you still advise against a continuous duty solenoid rated at 250 amps?

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martinarcher

thanks martinarcher. after i posted this i actually did look at the rating on my perko switch. it was rated at 250 rms /350 max

would you still advise against a continuous duty solenoid rated at 250 amps?

I would think you would be OK with that. Thumbup.gif I over engineer everything.....for wire size and current capacity bigger = better = safer. Clap.gif

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Earmark Marine

Scott,

One little issue that still worries me on this.

Let's say that the cranking battery is a little low for some reason. As you engage the starter the overall system voltage will certainly sag. If the voltage sags momentarily below the 'close' threshold of the solenoid then the solenoid could momentarily open. And that could leave you stranded. So I would check out what that threshold is and verify that it is below what the boat needs to start. You can still start a boat with a fairly low voltage.

David

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scott_fx

Scott,

One little issue that still worries me on this.

Let's say that the cranking battery is a little low for some reason. As you engage the starter the overall system voltage will certainly sag. If the voltage sags momentarily below the 'close' threshold of the solenoid then the solenoid could momentarily open. And that could leave you stranded. So I would check out what that threshold is and verify that it is below what the boat needs to start. You can still start a boat with a fairly low voltage.

David

i thought about that too and figured i'd leave the perko switch in there as a manual override. in case of an emergency.

I wonder though, could i add a cap to buffer the sag in voltage in that situation?

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Earmark Marine

Scott,

A capacitor will only stiffen the voltage for a very, very short period of time. It might be effective in consuming a peak or weathering a rapid dip but I don't think it would help in this situation since the comparatively sustained starter draw would also deplete the capacitor charge and the maximum capacitor charge can be no higher than the static voltage supply. You could possibly use a diode as a one-way valve so the capacitor could not be depleted by the starter. The diode has an insertion loss too. So I would keep it simple with a switch for an emergency bypass. And the switch could also engage the stereo bank if the starter battery had a major failure like a dead cell, etc.

David

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scott_fx

yeah, good point. i've been dealing with caps to buffer analog inputs on my microcontroller and forgot that i'd be dealing with seconds, not milliseconds and a lot higher current.

the perko is already there and installed, may as well use it.

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scott_fx

im going through with this but just have one question. would it be to my advantage/disadvantage to have both batteries turn on during cranking?

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Earmark Marine

Scott,

It depends.

If the starting battery is always isolated from the stereo battery when at rest then the starting battery should always be more than sufficient to turn over a small block in warm weather.

If the stereo battery is too seriously depleted then it represents a significant load on the starting battery in addition to the starter so you may not want it in parallel at that moment.

If the starting battery fails for any reason its nice to have the ability to use the stereo battery for starting in an emergency. It takes less voltage to start a boat than to run a stereo so the stereo battery is usually going to have enough charge for a quick start.

In the switching scheme it is fine to introduce a second battery while the engine is running but potentially damaging to the alternator if you switch from battery to NO battery in the course of selecting another battery or dual battery combination. Often BOTH batteries are selected before starting to eliminate the risk of this happening.

David

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scott_fx

Scott,

It depends.

If the starting battery is always isolated from the stereo battery when at rest then the starting battery should always be more than sufficient to turn over a small block in warm weather.

If the stereo battery is too seriously depleted then it represents a significant load on the starting battery in addition to the starter so you may not want it in parallel at that moment.

If the starting battery fails for any reason its nice to have the ability to use the stereo battery for starting in an emergency. It takes less voltage to start a boat than to run a stereo so the stereo battery is usually going to have enough charge for a quick start.

In the switching scheme it is fine to introduce a second battery while the engine is running but potentially damaging to the alternator if you switch from battery to NO battery in the course of selecting another battery or dual battery combination. Often BOTH batteries are selected before starting to eliminate the risk of this happening.

David

thanks for the input (as usual). As far as the setup. it's really open right now. i can do anything i want, what is the best way to set up a battery system like this? It was a bit confusing because at the begining you said that the second battery could represent a load. then you said it's risky to not start with both (in case one switches off).

how does this set up sound:

while cranking: battery 'a' on, battery 'b' off

after crank, while engine running, battery 'a' on, battery 'b' on

engine off: battery 'a' off, battery 'b' on

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Earmark Marine

Scott,

That scheme you outlined works for me.

If you had a mammoth stereo bank you could make that particular solenoid voltage sensing to protect the alternator from the sudden introduction of an excessive load. But if the stereo bank is limited to a single battery then there is little concern.

The 'risk' mentioned strictly relates to alternator protection and the possibility of switching through a total OPEN while the engine is running.

All dual bank battery systems are loaded with contradictions, especially in the sense of what is most healthy for the batteries can be unhealthy for the alternator and the inverse. I'm afraid a little imperfection just comes with the territory.

David

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scott_fx

Scott,

That scheme you outlined works for me.

If you had a mammoth stereo bank you could make that particular solenoid voltage sensing to protect the alternator from the sudden introduction of an excessive load. But if the stereo bank is limited to a single battery then there is little concern.

The 'risk' mentioned strictly relates to alternator protection and the possibility of switching through a total OPEN while the engine is running.

All dual bank battery systems are loaded with contradictions, especially in the sense of what is most healthy for the batteries can be unhealthy for the alternator and the inverse. I'm afraid a little imperfection just comes with the territory.

David

thank you.

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