2 cycle battery questions

[Tacrolimus]

My boat is a 76? Crestliner with a 70hp merc 2stroke. It’s our first boat, and it is plain jane: no gauges, no radio, 2 lights, and a depth finder.
Does anyone know off hand how much electricity it put out by a 2 stroke or how I can measure it? I would like to add a stereo and lights to the boat but I don’t want to kill the battery when we are anchored and not be able to start it back up.

thanks!

 

[Chris1956]

A 70HP Merc of 1976 vintage will charge about 9 Amps at high throttle setting.

Of course, not being able to start your motor has nothing to do with how much it charges. It is a function of battery size and power usage of the stereo. Do you plan to use lights at anchor? If not, they are of no consequence to the equation.

If you plan to use a powerful stereo, a group 27 battery is a good choice. A normal stereo, and you can get by with a group 24.

 

[Captain Caveman]

I would check to ensure that your Mercury has an alternator on it. Putting lights and a radio on it shouldn't be much of a problem, as long as you don't run the radio for hours. You could add a second, deep cycle battery that would be dedicated to all of your electronics.

 

[Texasmark]

I agree with Chris on a 9 amp. On a 70 hp engine you can usually expect a charging circuit. Charging circuits are designed to "keep a charged battery charged". They aren't high capacity devices like one would find at a service station to jump start a dead battery on a car.

I agree with Chris on a large battery for your boom boom box and don't expect the engine to keep it charged. Running a 100 watts worth of boom box at 12v is 8 amps. A 200 watt box eats 16 amps. As Chris said, the alternator on the engine only puts out 9A at max rpms, otherwise it's a lower amperage and really isn't expected to start working below 1k rpm since that where the service manual starts listing amperage output as a function of rpms.

If you sit running your box for a couple of hours with the engine below replacement rpms, or not running at all, you will have to run your engine at full rpm for 2 hours to put the charge lost back in the battery.

Other thing is if you have only a charging battery and use it for boxes and such, or trolling motor type things, when you get ready to pull the anchor and head for home you may need to get the manual crank rope out and pull the engine off by hand as your battery won't have enough juice to roll the engine......and as a side problem, you can overheat your charging system and possibly burn up some components on your way back to the dock.

 

[achris]

I generally find that the my 22w system on about 1/3 volume is plenty enough. Better to get a lower power output and couple with better efficiency speakers than try to get a high powered amp drive into rubbish (low efficiency) speakers. Remember, 10W driving into high efficiency speakers has the same SPL as 100W driving into rubbish speakers. And uses 1/10th the power from your battery.

I often do overnight trips, and part of that is having an Engel fridge on board. At night, I leave the fridge on, and the nav lights, and we usually stay up a while with a light or 2 on in the cabin, and the radio playing (at a sensible low volume). I always switch one battery off (I have 2) so I know I have a full battery in the morning, if I need it. I haven't yet run the single battery down far enough to need the second battery to start the engine.

Chris.......

 

[mike_i]

If you have room install a second battery and a switch so you can have a batt dedicated for starting.

 

[Texasmark]

I generally find that the my 22w system on about 1/3 volume is plenty enough. Better to get a lower power output and couple with better efficiency speakers than try to get a high powered amp drive into rubbish (low efficiency) speakers. Remember, 10W driving into high efficiency speakers has the same SPL as 100W driving into rubbish speakers. And uses 1/10th the power from your battery.

I often do overnight trips, and part of that is having an Engel fridge on board. At night, I leave the fridge on, and the nav lights, and we usually stay up a while with a light or 2 on in the cabin, and the radio playing (at a sensible low volume). I always switch one battery off (I have 2) so I know I have a full battery in the morning, if I need it. I haven't yet run the single battery down far enough to need the second battery to start the engine.

Chris.......

I just pulled the speakers out of a recently purchased Crestliner. I was amazed at the amount of magnet, aka weight of those marine quality speakers. Magnets make the difference. Sound energy (dynes/cm exp2) comes from the "electro-magnetic" circuit. Just like ft-lbs. If you want a solution, you can change either and get the same answer. Heavy magnets for a quantity of boom boom means less amperes.