35 Amp Alternator on a 115HP Merc 4 Stroke

[Big Bobber]

Is that enough to keep the accessory battery charged if running all the accessories all day?

Thanks

[marine_man]

How much are you traveling (motor running) during the day, and what accessories are you running?

For the "typical" user, I'd say yes assuming you're talking about a depth finder, livewell pump on auto & a radio here and there, running the big motor to move between spots.

marine_man

[Big Bobber]

I will put on about 16 miles per day. I use the radio most of the day, and will be running 2 live wells and bait well.

[marine_man]

I think you'll be ok... how long is a "typical" day?

marine_man

[Big Bobber]

6:00 am-10:00 pm

[delcecchi]

Looks like a good estimate for a live well pump is around 5 amps, likewise for other live well and bait well. Finder is maybe 3 amps. Radio is maybe 1 amp.

You are out for 16 hours. If live well and bait well run half the time that is 120 amp hours(3*5*8). Finder is 48 amp hours, Radio is 16 amp hours.

Total is 224 Amp hours. That may be a little heavy but gives a little margin.

(I used http://www.rulepumpsupply.com/Bilge_Pumps.html for pump current).

So you would have to be running the big motor at a high enough speed to get full output for around 7 hours. 16 miles is probably less than an hour, maybe half an hour.

I think this estimate says you have a problem. Feel free to mess with the numbers any way you please, I didn't find them carved on a stone tablet.

[Big Bobber]

I am debating between going with a group 29 or 31.

[delcecchi]

I am debating between going with a group 29 or 31.

If you believe my back of the envelope calculation, it won't make that much difference. You might use 240 amp hours, the motor makes maybe 40 amp hours. That means the battery would have to supply 200 amp hours plus have enough left to start the big motor when needed.

Unless your accessory usage can be reduced considerably you could be rowing home. On the other hand, you probably don't need to run two live wells 8 hours out of the day. When mine are on the timer they run for a few minutes and then turn off for several times longer.

Good Luck.

[Big Bobber]

I think I will buy a jump pack.

[marine_man]

Well, 6 AM to 10 PM running all that will be a problem... you really meant a full day .

Splitting hairs a bit, but my guess is your baitwell runs off the same pump as your livewell? That would drop the amp rating down a bit there, but in this case, not enough to make it all day.

If these are the requirements, you'll need a second accessory battery and an on-board charger to recharge both at night.

marine_man

[Big Bobber]

If I put in a separate battery for accessories is it fair to say I should get by without having the starting battery on the 3 bank charger? On my present boat I had 2 batteries hooked together and with the E-tec I did not need to put the charger on the rear batteries for 5 days.

[delcecchi]

marine man, that brings up a question I have. Is the "35 amps" for the alternator net of the draw from the motor itself? Since the injectors and (probably) the fuel pump and computer take juice do they come out of the 35 or is that all available to charge the battery?

[delcecchi]

If I put in a separate battery for accessories is it fair to say I should get by without having the starting battery on the 3 bank charger? On my present boat I had 2 batteries hooked together and with the E-tec I did not need to put the charger on the rear batteries for 5 days.

So you will have two accessory batteries and a cranking battery and a battery or two for the trolling motor? Is that what you are thinking of doing?

[Big Bobber]

1 accessory, 1 start, 2 trolling.

[delcecchi]

1 accessory, 1 start, 2 trolling.

If accessory and start are separate it won't run what you said for as long as you said.

This is one of those cases where you need to be realistic about what you actually will be using for how long.

You said you wanted to run a bunch of stuff for 16 hours. Takes way more juice than is reasonable, so you either need to rethink exactly how much stuff will run for how long, or fill the boat with batteries, and chargers.

[Big Bobber]

That is one of the things that has me uneasy about the Merc. I tied 2 batteries together with my Evinrude and never had a problem. I believe it has an output of 75 amps. Also on the new boat I will be running an additional live well and bait well that are all tied together. I don't think they can be split apart very easily. Won't know until I get the boat. The dealer assured me there shouldn't be a problem, but I think I will feel better if I have a jump pack on boat as a belt for my suspenders if I tie both of these batteries together. Thanks for all of the help the posters have offered. It is much appreciated. It is nice to receive sincere advice.

[Kyhl]

The other option is to idle the main motor intermittently throughout the day. I do that when night fishing to make sure the battery stays strong enough to start the main motor.

Keep an eye on the voltage reading on the depth finder and start up the main motor to recharge the batteries when they start dropping.

My night fishing includes powering running lights, HDS8, HDS5, stereo, and sometimes interior lights. I usually start the motor at 11.8 volts and let it idle or troll until the starting battery is back to 12.5.

Curious, does this boat have multiple switches for the bait wells vs live wells and multiple switches for the live wells? I don't have a bait well but my two live wells drive off the same pump. One switch runs both.

I never use the front well anyway and close the nozzles.

[Big Bobber]

Sadly, only one switch for the live wells. One of my biggest concerns all along has been that I will have a bigger trolling motor and troll less with the big motor. Nothing would be worse than not having a motor start when the wife is out and being at the far end of the lake.

[h8go4s]

Separate the starting and accessory batteries and add a battery combiner. That will switch alternator power to the accessory battery ONLY when the starting battery is fully charged. If your accessory battery goes dead, you still have a full charge in the starting battery.

[delcecchi]

Sadly, only one switch for the live wells. One of my biggest concerns all along has been that I will have a bigger trolling motor and troll less with the big motor. Nothing would be worse than not having a motor start when the wife is out and being at the far end of the lake.

If there is only one switch, there might be only one pump which would help considerably when estimating current draw. I am surprised there is only one switch for two live wells, however.

[Big Bobber]

I did look at a display boat and only one switch.

[marine_man]

marine man, that brings up a question I have. Is the "35 amps" for the alternator net of the draw from the motor itself? Since the injectors and (probably) the fuel pump and computer take juice do they come out of the 35 or is that all available to charge the battery?

My understanding is the 35 amps is the total output of the charging system, not the net output after powering the electrical needs on the motor.

If you truly need to power all the accessories listed for the entire time, you will need to do the following:

Connect the main motor battery cables to the "starting battery".

Install a battery charger isolator that does exactly as an earlier poster mentioned - starts charging the accessory battery after the starting battery is fully charged.

Connect the accessory battery to the accessory leads on the boat & any other accessory leads you may have.

This will really be your best bet to accomplish what you listed as requirements. You should be ok with not connecting a charger to the starting battery, and the third bank of a charger to the accessory battery.

marine_man

[Kyhl]

If there is only one switch, there might be only one pump which would help considerably when estimating current draw. I am surprised there is only one switch for two live wells, however.

I should have been more clear. In my rig, one switch powers one pump that operates both livewells with fresh water. There is a T in the line after the pump that sends water to each well.

Closing the nozzles effectively diverts the water to only one well.

I'm not sure how the front recirculate operates or if there is one. The rear recirculate system on my rig has it's own pump seperate from the fresh water pump.

[delcecchi]

OK, that's different from my boat that has two pumps with two switches, and the recirculate function uses a valve that has "fill" "drain" "recirculate" positions for each live well.

One pump probably uses less current than two, although size differences matters too.