Determining battery needs

Hellosugaree

Almost A Member
Sep 6, 2019
236
272
36
San Francisco
Name
Timbo
Boat
13' Boston Whaler
Hi,

I'm planning on getting a new dual purpose battery for my boat running a 40hp mercury elpt 4 stroke. It currently has a group 21 size AGM dual purpose battery which has been working fine for starting and running a fish finder.

In addition to the fish finder, I plan on also adding a radio and 500gph livewell pump (with an intermittent run timer so it's not running constantly), and a USB port for occasionally charging a phone.

Obviously in a perfect world I'd get 2 batteries and keep the starting battery separate from all the electronics, but it's not a perfect world and I have a 13 foot boat with limited space and weight capacity.

My 5inch screen fish finder says 13W max, so that's ~1A at 12v.

The radio says 0.3A standby or ~1A receiving. Transmit draw isn't a huge factor since I don't plan on transmitting much beyond a radio check and emergency. Let's just use 1A for some margin.

The livewell pump draws ~2A, but I would have it running intermittently, so let's say 50% of time time it draws average 1A.

If I'm charging a phone that's ~2A

With everything running (but no phone charging): 1A fish finder, 1A average pump @50% running time, 1A radio, that's ~3A. Over 10 hours that would draw ~30 Ah. If I had a dual purpose 90Ah battery (about the range I'm seeing for group 27) running all electronics on 10 hours drifting I'd drain 1/3 of the brand new manufacturer stated battery capacity. Is this outside of what these batteries would be designed to do, or would this be a reasonably safe setup? And by safe I mean will this leave me a solid margin to get my motor started when I need it, and also not damage the battery through my use pattern.

In the real world, I'd be motoring around a bit so the battery would be charged a bit in between. Then again, in the real world capacity is probably less than stated and declines with age. Also, I'd occasionally like to charge a phone for a bit here and there (an hour or two over the course of a day). And of course, most importantly, I want the engine to start when I turn the key because my fish finder won't drive me back to the launch ramp.

In theory I could put in two smaller batteries and separate the electronics from starting battery, but I'd rather not unless it's completely necessary. Anyone else run a similar setup that can comment?

Thanks
 
Upvote 0

plj46

I Post A Lot But I Can't Edit This
Jan 7, 2008
8,605
11,329
Socal
Name
john
Boat
24 ft grady white
I'd bet that 5" screen fishfinder pulls more than 1 amp.Bait pump also.Look at what fuse they require.Just go to a group 24 size battery.
 
Upvote 0

Hellosugaree

Almost A Member
Sep 6, 2019
236
272
36
San Francisco
Name
Timbo
Boat
13' Boston Whaler
I'd bet that 5" screen fishfinder pulls more than 1 amp.Bait pump also.Look at what fuse they require.Just go to a group 24 size battery.

I'm basing draw for fish finder off of manufacturer listed specs. Yes the pump draws 2 amps but if I'm running it on an intermittent timer half the time then it should be 1Ah per hour.

The 90Ah battery figure was based on a group 27 battery. Group 24 is more like 75Ah range. I can fit a group 27 in the same spot without putting in new tie down points, so I might as well.
 
Upvote 0

Fisha

skipper
Aug 3, 2006
693
129
Orange
Name
Dave
Boat
24' sea ray flybridge, 12' w/outboard
I would run the bait pump always when you have bait in the tank, so it doesn't die.
 
  • Like
Reactions: Spoons
Upvote 0

40Grit

Member
Sep 9, 2009
738
358
Newport Beach California
Name
Steve Brown
Boat
Radovich 34
When calculating amp hours. You use half of the batteries capacity. Most charge regime will only want to consume to fifty percent. As the completely discharged batt will take a charge at a much lower rate.
Purchase Nigel Calleders book he explains all these questions
 
Upvote 0

Hellosugaree

Almost A Member
Sep 6, 2019
236
272
36
San Francisco
Name
Timbo
Boat
13' Boston Whaler
When calculating amp hours. You use half of the batteries capacity. Most charge regime will only want to consume to fifty percent. As the completely discharged batt will take a charge at a much lower rate.
Purchase Nigel Calleders book he explains all these questions

Thanks for the book suggestion. I'll check it out.
 
Upvote 0

karlow

Twins 2017
Apr 29, 2004
1,880
806
63
Duarte/Covina
Name
karl
Boat
20' skippy Topless /17' whaler Wet Ride (sold) /18' Outrage
Wait,
The first question is how much boat do you have?
When you fish in the ocean, you should always have two batteries. You should always have reserve power to start the boat. I use two GP27 batteries, they are in the console. When I go out I use one battery all day. I should not need to use the other one. They are both the same, one is used and one is held in reserve. They are separated by a battery switch. If you can only fit one battery on your boat, I would also bring one of the Li jump start batteries as backup. They are around 100 bucks. Two things to remember, never use the switch when the motor is running. When you run down your batteries out there on the water. Turn everything off for about 40 min. Then turn on the battery all of the batteries and try to fire up the engine. If that does not work, I would shut everything off for about 40 min and call for help on the radio. After that you will take more care with your batteries.
 
Upvote 0

miaf

I Should Upgrade My Account
Apr 14, 2013
1,136
1,100
.
Name
.
Boat
.
I would make a two battery configuration work. The batteries don’t have to be positioned right next to each other. You could put one in the bow and one in the stern. I know you said you can’t fit two, but I’m guessing you could.

Put up a picture of your boat and I’m sure some guys could give some ideas on how to make it work.
 
Upvote 0

MYNomad

Heading South
Dec 12, 2007
4,099
4,615
Pacific Northwest / West Coast Mexico
Name
Rick
Boat
Yes
When calculating amp hours. You use half of the batteries capacity. Most charge regime will only want to consume to fifty percent. As the completely discharged batt will take a charge at a much lower rate.
Purchase Nigel Calleders book he explains all these questions
Actually, the conventional wisdom of using only half of the battery's capacity is based on the idea that if you go below about 25% the battery gets damaged, and that the fuller the battery is, the slower it charges, so only expect to fill it up to the 75% level. This is for guys with an on board generator. Trailer boats can plan to use 75% if the intention is to run the battery down to 25% then put it on a charger and keep it there until fully recharged. But, when a battery is heavily discharged, it accepts current (ie, recharges) at a much faster rate than when it is almost fully charged.
 
Upvote 0

Hellosugaree

Almost A Member
Sep 6, 2019
236
272
36
San Francisco
Name
Timbo
Boat
13' Boston Whaler
Thanks for all the replies and input. Considered my options for a bit. I debated going with two batteries but I need to be realistic... It's a 13ft boat and space is precious. I'm gonna keep it simple and bump up to a single dual purpose group 27 and get an emergency jump starter to keep on board just in case. I always keep my battery on a battery tender at home so the battery should stay healthy. There's an emergency pull cord in the cowl just in case too. I'll save the dual battery battery and wiring option for when I get a bigger boat ;)
 
Upvote 0