Battery life and electronics

[Delaware Valley Tackle]

I have two Lowrance hook units wired to the ignition battery. How long should they be able to run before the motor (40 hp 4 stroke) won’t turn over?

 

I fish lots of small lakes and don’t run the big motor a lot

[TOXIC]

Need more info.  What size and type of battery and do you charge off the water? 

[Delaware Valley Tackle]

It’s an interstate marine cranking battery  24m—rd. I usually charge both batteries between uses but I might have missed one. This made me curious to post the question.  I charge each separately no onboard charger. 

[TnRiver46]

Bout 5 days, I accidentally left mine turned on in the carport once ha ha

[Tennessee Boy]

I have two Humminbird Solix,  360 imaging,  an ethernet switch, an GPS puck,  and a heading sensor puck running off my cranking battery.  The Solix are touch screen fish finders and use a good bit more power than most fish finders.  I've fished hard for 14 hours before and didn't have any problems.  I have an X2Power 27M battery so it's a good one.

 

My guess is you will run out of juice before your battery does.

[Bankc]

The Hook 9 says it draws about .75 amps.  The Hook 5 says it draws 12.2 watts, which would be about an amp, though I don't know how much I trust those numbers, because one would think the smaller 5" screen would draw less juice than the larger 9" screen.  Anyway, let's say we're looking at about 1 amp per hour for the fish finder.  

 

The Interstate Marine 24m-rd has a reserve capacity of 75, which gives us about 31.25 amp hours.  I don't know how many volts your motor needs to start, but let's guess it's around 11.5v.  Many starters don't need more than 10v, but at 11.5v, you're going to be down to around the 10-20% battery life remaining.  So let's say 20% for the upper limit.  That means you can run 80% of the battery for the fish finder and you should still be able to crank the motor.  So there's 25 amps (80% of 31.25 total amps).  At one amp per hour, that's 25 hours.  Now, every time you start the battery, and anything else running off that battery, will take away from that.  

 

However, you're not supposed to drain a cranking battery below about 5%, as it'll shorten the lifespan.  They're not like deep cycle batteries that can be drained down to 50% on the regular.  So 5% of 31.25 is 1.5625, or about an hour and a half.  But all that means is that you'll have to replace the battery sooner.  How much sooner?  Who can say?

 

So, the math says 1.5-25 hours, depending on how many years you want your battery to last.  And I can say from experience with this type of stuff, that the math doesn't usually give you a good real world estimate. The health of the battery and the type of charger can effect their capacity.  Also can any number of variables you may not consider can come into play here, like the quality of wires and connectors (and any corrosion) and if there's any weak connection to ground that will slowly drain the battery.  Not to mention the health of the motor and starter, and how much it actually need to start.  Plus, who knows if the figures the math is based off of are even accurate?  It's just hard to consider every possibility.

 

Personally, I'd just run it as is and not worry about it and bring a paddle.  In a small lake, you can always paddle back if an emergency arises.  When the battery finally gives up the ghost, or starts giving you trouble, you'll have a better, real-world idea and can make an changes at that time if it seems like it might be a problem.

 

EDIT:  I forgot you said 2 Lowrance Hook units.  So make that 45 minutes to 12.5 hours.  

[InfantryMP]

52 minutes ago, Tennessee Boy said:

My guess is you will run out of juice before your battery does.

I agree here as well. 

 

It does not take a lot of power to crank the motor over. If your motor typically fires right up, it can literally be on its last leg of power, and still give you just enough to crank it. Worst case scenario keep a set of jumper cables to jump off another battery if need be!

[TOXIC]

Sorry, You lost me at Interstate.? Bad experience with that brand.  I can’t add any more than what’s been said but just for reference, I have a Sears PM1 (no longer made) AGM with like 1100 mca’s and 200 RC.  I need the extra juice to crank a Yamaha 250 HPDI and 2 units, a gen 1, 8 inch and a gen 2, 9 inch both with downscan and sidescan.  Even with the battery being a few years old, I have never had a starting problem.  

[Junger]

Grab one of those battery jumper power packs that go on sale for like $30, should be no prob for 40HP. Then just use your electronics without worrying too much.

[Team9nine]

Running a similar setup on my boat (24M battery with 2 units hooked to it and small engine). Fishing 4-5 hr trips with limited big engine running, I’m getting about 5-6 trips before risking starting failure. That is with keeping both units on full time, every trip. Used to go months without issues when I just left the big engine idling all the time (backtroll type setup). 
 

I’ve now set up one unit to always display voltage on the screen so I can monitor. I don’t seem to have any issues until I get below 11.0 volts.