Cranking Battery for Boat Motor

gguertin

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Aug 30, 2021
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Looking at the DIY options for batteries and fairly interested. I have a need for a 12v battery with more capacity my current agm cant support the electronics load on the boat plus working as a starting battery its only 80ah I think at the end of an 8 hour day it is basically dead. I see some of the factory options list dual purpose for cranking batteries.

Do I need a special type of cell to do this?

Does anyone know of a thread that I can read that shows pictures or examples of building a battery that could be used in situations with vibration or water? Like a build thread?

Thanks!
 

wholybee

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Feb 11, 2021
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Cranking and House (electronics) are different uses and different requirements and typically use separate batteries. A starting and a deep cycle. Dual Purpose do both rather poorly.

Tell us a little about your boat, what electronics are on it, and how you use the boat. For a basic fishing boat that you don't stay overnight on, I would suggest looking at either:
1. Add a second battery, just like your first, connected in parallel to your first to double the capacity. You should replace your current battery at the same time so they are the same age.
2. Look at Blue Seas add a battery kits. These would include a switch so you can switch between 2 batteries, and use one for starting, and one for everything else. The house (everything else) battery could be a larger dual purpose, or a deep cycle. It is more complicated to install, but you would not need to replace your current battery. The advantage is that if you run the house battery dead during the day, you can switch to the other and start the engine.

If you have a bigger boat, that you sometimes stay overnight on, or if you are planning a refrigerator or electronics that need more power than a basic fishfinder etc, then I would recommend something bigger and more sophisticated than what I just suggested.
 

gguertin

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Aug 30, 2021
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I have a 6 cylinder older mercury motor to start on a bass boat with 8ah draw of electronics 3 lowrance hds, 2 extra modules and a couple gps pucks.

Weight is a concern and that is why I was looking this route instead of just installing a 31g battery. I recently went up to 29's for the whole boat 1 for start/electronics and 2 for trollingmotor and I can really feel the difference in the time to plane during tournaments. I typically fish for 8 hours with maybe 9 hrs of total run time a day, with ability to recharge in the evening at a hotel or something else.
 

gguertin

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Aug 30, 2021
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Also my question was initially around what the difference is in components for a lithium starting or cranking battery vs deep cycle? I see companies that sell them to purchase but I was watching the videos on diy and was interested in possibly trying something (Maybe a winter project)
 

wholybee

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Feb 11, 2021
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Sorry, didn't realize Lithium was the question. You need to know how much current (called CCA by engine manufactures) is needed for starting your engine. Then make sure the Lithium battery or BMS can support that discharge current (simply called Amps) by battery manufactures. Is your engine an outboard or inboard?

If it takes more than 100 or 150 amps, then you will be out of the range of a simple installation, and more discussion will be necessary. If less than that, then pretty much any 4 100Ah or 200Ah cells will work, paired with a suitable BMS. The key is that you will build a compression frame, typically 2 pieces of plywood and 4 threaded rods, to squeeze the cells together. Vibration shouldn't be a large concern then.

Bonus, a 200Ah Lithium will weigh less than a 100Ah Lead, and have 4 times the usable energy.

If starting current is less than 50A, then a drop in would work, and be very simple. I don't think a 6 cylinder will qualify.
 
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