High-output low Ah battery

[hwse]

On my sailboat, the in-rush current for the starter on the diesel engine far exceeds the discharge capacity of most BMS's, but the cells can handle the current without problem.

I have been thinking that I could build a pack with small cells [50Ah to100Ah] with a BMS and a High output normally open contactor relay for connecting to the starter. The power lugs for the relay would attach between the battery negative and the BMS but it would get its energizing current from after the BMS. That way all cell management would be handled by the BMS, but the actual high load would not.

That way I would not be limited by the BMS FETS. The contactor would need to be a naturally open and get a control current from the BMS. Then if the BMS shuts off, it would open the contactor due to the loss of voltage.

Does this sound like a workable solution or am I missing something?

 

[glandpuck]

Yes, you maybe:

Can I use Lithium Engine Starting Battery for my Marine Diesel?

Thinking of using a Lithium Engine Starting Battery in your sailboat? Read the pros and cons of Li-Ion cranking batteries for small diesels.

saltwaterdiesels.com

 

[yabert]

On my sailboat, the in-rush current for the starter on the diesel engine... or am I missing something?

It's a plan. After all, every EV battery use contactors and no fets BMS.
A better plan can be to ditch the diesel engine and add an electric powertrain ?

 

[hwse]

It's a plan. After all, every EV battery use contactors and no fets BMS.
A better plan can be to ditch the diesel engine and add an electric powertrain ?

That just does not work in my cruising grounds. I am in the Salish Sea and have 500-nm of inland water with thousands of miles of channels to play in. It is often flat calm which means that the sailboat needs to cover many miles in a day under power when the wind is not blowing. I would like to have a hybrid system where I had a small diesel DC generator and a large 48-volt propulsion battery that would also supply power to the house loads.

 

[hwse]

I have a simular post on a boating forum and several have expressed a concern that cells in the 50Ah - 100Ah range cannot put out enough amps to start a small diesel engine. This engine is currently starting fine with a 14-year-old group 30 AGM with a 950CCA rating. Would that be too much power from a 50Ah pack? The specs on my EVE 280K cells, say pulse current of 2C for 30-seconds. but this would be more like 5C to 10C for 1-2 seconds.
Given that we need a class T fuse with an interrupt current of 20,000A to protect from a dead short, starting an engine does not seem like a big ask.

 

[hwy17]

I think it would work. You can use a contactor BMS like Orion or you can use a FET BMS and a manual bypass switch around the BMS for starting.

But my question would be why when a lead acid is both cheap and proven solution for the application.

Even with Orion, it is common to use a lead acid battery for the BMS and contactors.

 

[mikefitz]

engine is currently starting fine with a 14-year-old group 30 AGM with a 950CCA rating.

Then keep this system, it's the most reliable for a marine application, get a new AGM battery.

With lithium you have the following issues, cannot interface to engine alternator directly, need specialised alternator control and protection, current is limited with most LFP cells, need contactor based BMS, BMS protection shutdown leaves you with no power.

You can get LFP cells with high output short term, or use an alternative lithium type battery, or an off the shelf solution,

Lithium Car and Boat Batteries

The best lithium car batteries for starting cars, trucks and boats. #PowerYourPassion with 1,000 cold cranking amps of Dakota Lithium power

dakotalithium.com

 

[yabert]

I have a simular post on a boating forum and several have expressed a concern that cells in the 50Ah - 100Ah range cannot put out enough amps to start a small diesel engine. This engine is currently starting fine with a 14-year-old group 30 AGM with a 950CCA rating. Would that be too much power from a 50Ah pack?

You can stop to look at storage cells and take a look at high discharge rate cells.
One example here of a 74ah cells able to output 600A: https://batteryhookup.com/products/new-case-with-6x-3-2v-74ah-lifepo4-prismatic-cells

 

[hwse]

Then keep this system, it's the most reliable for a marine application, get a new AGM battery.

With lithium you have the following issues, cannot interface to engine alternator directly, need specialised alternator control and protection, current is limited with most LFP cells, need contactor based BMS, BMS protection shutdown leaves you with no power.

You can get LFP cells with high output short term, or use an alternative lithium type battery, or an off the shelf solution,

Lithium Car and Boat Batteries

The best lithium car batteries for starting cars, trucks and boats. #PowerYourPassion with 1,000 cold cranking amps of Dakota Lithium power

dakotalithium.com

I already have the specialized alternator with external regulator which has all of the protection and works fine. I have looked at the Dakota Lith dual purpose batteries but given that they are a black box there is no way to tell if the cells are any different or it is just the BMS. My thought is it make a contactor based BMS from a regular old JK where the FET controlled output from the JK is supplying the signal to the contractor for the starter loads. If the JK shuts down power, that loss of power opens the contactor [normally open]. That way, the high load is bypassed around the BMS but still controlled by it.

 

[hwse]

I think it would work. You can use a contactor BMS like Orion or you can use a FET BMS and a manual bypass switch around the BMS for starting.

But my question would be why when a lead acid is both cheap and proven solution for the application.

Even with Orion, it is common to use a lead acid battery for the BMS and contactors.

It is true that lead start and LFP house works very well together and that is the current system I have. My high-output externally regulated alternator goes to an ArgoFET battery isolator which sends power to both battery banks. The only problem with this system is that when cruising, I often do not run the engine long enough to get the large house bank to full charge to the max charge voltage is around 13.3v and that is too low to fully recharge the AGM. My current workaround is to turn off charging to the LFP in App for the first 12-minutes after starting to let the AGM get topped back off at 14.2v. After that, I allow the LFP to get charged and the voltage drops back down to whatever the house bank is (13.3v)