Inverters won't work in boat

rangoneer · Sep 9, 2009

I'm tuning our boat on the water and need a laptop on board to stay charged so we can make changes to the fuel without the laptop dieing shortly into the day.

The 300/600watt inverter will charge the laptop in my truck, but when I try to use it in the boat, the 'low battery' indicator on the inverter lights up and shuts down the inverter. We tried a new 400/800w one and it does the same thing at idle and up to speed with the volts reading at 14.

Boat has two batteries controlled by the either-or/both switch. Doesn't have an issue starting.

Any thoughts? Thank you.

jlinder · Sep 10, 2009

Re: Inverters won't work in boat

By any chance did you check the voltage at the inverter with a meter? Wonder if you really have 14 volts being supplied to it.

fishrdan · Sep 10, 2009

Re: Inverters won't work in boat

^ my thoughts too.

I assume this is plugging into a lighter socket. Does that socket have clean and tight connections and the socket itself not rusted or corroded. Could be bad wiring going to the socket, a poor crimp further down the wire, or wire that is too small for the current draw.

rangoneer · Sep 10, 2009

Re: Inverters won't work in boat

Thank you for the help. I did not check the volts at the inverter because even after I commected it directly to the battery it said the same thing but yet the boat starts fine.

But I will do as you suggest and try and get out today and see what the volts are. What else would you recommend checking while out today?

Silvertip · Sep 10, 2009

Re: Inverters won't work in boat

You have a weak battery or bad connections or wire size issues. I run an 800 watt inverter from a garden tractor battery with no issues. The low voltage indicator is telling you that there really is low voltage. You may have a battery that measures 12.6 volts with no load on it. Connect the inverter, let it stabilize as some of them draw a large amount of current for a short period. Then measure the battery voltage.

rangoneer · Sep 10, 2009

Re: Inverters won't work in boat

When I plug the inverter in and wait, the inverter does power up/spins the fan, until I plug the laptop in, then it beeps and shuts down.

Thank you. I'll check each of the batteries and see what they read without the inverter; and with the inverter.

What do you suggest they should read? And would putting a trickle charger on each of them overnight solve anything or is it a matter of an old battery? Thanks again.

Silvertip · Sep 10, 2009

Re: Inverters won't work in boat

I suggest that the age of the battery has nothing to do with the voltage. The battery is either good, going bad, or shot. Only a good battery will measure 12 volts (static or with a moderate load). Which battery are you operating the inverter from? The START battery, the AUX battery, or BOTH. In other words where is the switch set when you try running the inverter. Switch to BAT 1 and try it. Then switch to BAT 2 and try it. Then switch to BOTH and try it. You can draw conclusions from the results you get with that test. If you are in the habit of leaving the switch set to BOTH, you essentially have the batteries in parallel. The weakest battery in a parallel system will suck the life out the better battery until both are equal (and that will not be 12 volts).

Splat · Sep 10, 2009

Re: Inverters won't work in boat

Could be too small of wire running to the cigarette lighter plug as well. The truck cigarette lighter is expected to run a cigarette lighter so they will typically be rated for 20a with like a 16-14 gauge wire.

The plug in you boat may have smaller gauge wire causing higher resistance/line loss.

I would also rty connecting directly to the battery with the inverter. When I run mine in my truck from the cigarette lighter, I can run much more either wired directly to the batter with alligator clips, or using the plug I made that connects through the charging wire in my tow plug, this plug is wired with 14gauge in my truck.

Bill

jlinder · Sep 10, 2009

Re: Inverters won't work in boat

Connecting directly to the battery is a very good idea, with caveats.

Consider that if you are running 100 watts you are talking about 10 amps. A 400 watt inverter could draw up to 40 amps (about). You really don't want to do that using a cigarette lighter connection. A solid connection will help out all around.

The caveat: Of course you need to make sure everything is fused and if you have dual batteries how does the inverter work? You may want to keep it off the engine battery to keep from finding your self out on the water with dead batteries.

Silvertip · Sep 10, 2009

Re: Inverters won't work in boat

He's running a laptop - no 40 amp draw involved. The wattage on the inverter (800 watts for example) is at 117 volts on the "output", not 12 volts. Thats about 7.5 amps on the output. The inverter is about 85% efficient so the input would be 15% above that or about 9 amps ignoring the initial high current draw at initial connection (lasts for a few seconds). Many inverters are also fused on both the hot and ground sides.

jlinder · Sep 11, 2009

Re: Inverters won't work in boat

Silvertip said:
He's running a laptop - no 40 amp draw involved. The wattage on the inverter (800 watts for example) is at 117 volts on the "output", not 12 volts. Thats about 7.5 amps on the output. The inverter is about 85% efficient so the input would be 15% above that or about 9 amps ignoring the initial high current draw at initial connection (lasts for a few seconds). Many inverters are also fused on both the hot and ground sides.

Silvertip,

You are right, but I was referring to the capacity of the inverter, not the laptop draw. My thought was if you were installing a 400 watt inverter might as well wire it for that.

If you use 25 watts for the draw of the laptop power supply/charger it comes to about 2.4 amp draw on the battery.

One shortcut I use to approximate the draw is to use a simple rule of thumb - take the output draw in watts and divide by 10.

For example, take case of running a 100w light bulb off the inverter.

At 120 volts this would be .8333 amps.

On the input side, if you were feeding it 12 volts and had a 100% efficient inverter you would need 8.333 amps. If you use 85% for efficiency you get 9.58 amps

100 watts divided by 10 would give you 10 amps. Not 100% correct but pretty close and good for quick calculations.

BTW - if the input drops to just 11 volts the current goes up to 10.45 amps, almost an amp more.

Silvertip · Sep 11, 2009

Re: Inverters won't work in boat

I just happened to have my high current ammeter, 850 watt Cobra inverter, a battery, a 75 watt trouble light, and my jig saw all within steps of each other so here are some real numbers.

The 75 watt bulb draws .625 amps on the inverter output. On the 12 volt side the ammerter showed 5.75 amps @ 12 volts. In this particular case your rough calculation using the number 10 works quite closesly.

The tag on the jig saw says 3.5 amps which equates to a 420 watt draw at the output. On the battery side, my ammeter showed 22 amps at 12V with the saw running wide open. While this example does not support the factor of 10, it is in reality probably close. A motor tag that says 3.5 amps is at its stall or near stall state. I do not normally try to stall a saw by grabbing the moving blade so in reality, the motor was not under load so it was very likely drawing somewhere in the 2 - 2.5 amp range so again, the factor of 10 would apply.

jlinder · Sep 11, 2009

Re: Inverters won't work in boat

Silvertip said:
I just happened to have my high current ammeter, 850 watt Cobra inverter, a battery, a 75 watt trouble light, and my jig saw all within steps of each other so here are some real numbers.

The 75 watt bulb draws .625 amps on the inverter output. On the 12 volt side the ammerter showed 5.75 amps @ 12 volts. In this particular case your rough calculation using the number 10 works quite closesly.

The tag on the jig saw says 3.5 amps which equates to a 420 watt draw at the output. On the battery side, my ammeter showed 22 amps at 12V with the saw running wide open. While this example does not support the factor of 10, it is in reality probably close. A motor tag that says 3.5 amps is at its stall or near stall state. I do not normally try to stall a saw by grabbing the moving blade so in reality, the motor was not under load so it was very likely drawing somewhere in the 2 - 2.5 amp range so again, the factor of 10 would apply.

Silvertip,

I think you are getting some bad readings on the DC current measurements. You don't say how your DC ammeter works, whether it is a shunt type or magnetic, but magnetic readings of DC current are difficult to make accurately.

Consider:
On the 120V side 120v X .625A = 75 watts

But on the 12 volt side 12v X 5.75A = 69 watts.

That would mean you are drawing 69 watts from the battery but getting 75 watts to the bulb.

If the inverter were 100% efficient you would need 6.25A to get 75 watts. With 15% loss you would need 7.18A

Again, the rule of thumb is not 100% accurate but fairly close.

Silvertip · Sep 11, 2009

Re: Inverters won't work in boat

Not a scientific study to be sure but the point here is I'm agreeing with you. However, we are not certain that particular 75 watt bulb is actually drawing 75 watts. Nor do we know exactly how much current the jig saw was drawing. These are the numbers I observed based on label data and the numbers are close enough that the factor of 10 works (as well it should). 12 volts in vs 120 out = 10. Close enough for me. As for the meter, it is an industrial quality Western Electric. It is a sealed unit so I can't see the shunt if it has one. Normally they are external but not always.

jlinder · Sep 11, 2009

Re: Inverters won't work in boat

If it passes the current through the meter then it is a shunt type. The magnetic I was thinking of worked as a clamp on. you put it around the wire and just like the clamp on AC ammeters it measured the magnetic field.

You probably don't have that.

And you are right, just cause i says 75 watts doesn't mean it is right on at 75.

I guess what I was trying to say is that I have all too often seen people try to run things and not consider how much power is really needed. Like the guy bragging about his 300w sound system in his car with all the fancy looking hardware and he has some dinky wire feeding it 12v. Spends thousands on the system and saves $10 on wire.

Splat · Sep 11, 2009

Re: Inverters won't work in boat

You know a whole other though process comes to mind here.

Unless your running a very high quality power inverter with a true sine wave, your going to be causing damage to your transformer that's inline with the power cord for your lap top. Basically most inverters "cheat" and create a square sine wave by rapiditly switching the power on and off. This is how they create the AC or alternating current. The problem is this rapid switching can create vibrations and severe damage to the components in the power supply.

An easier proposition here may be to read the power supply and see what the output voltage. Chances are its going to be either 9 or 12vdc. If 12volts you can simply run to your local radio shack with the laptop and get a cigarette plug, with the correct size jack on it that will plug right into your laptop eliminating the need for the step down power supply.

If the output is less like 9 or even commonly 6volts dc, radio shack sells a cigarette adapter with a stepdown circuit built right in. The only word of caution with these is to make note of the current output of the original power supply, and purchase accordingly.

Bill

jlinder · Sep 12, 2009

Re: Inverters won't work in boat

Sometimes we miss the obvious. Did a quick check on the internet and found a number of car adapters for laptops.

Inverters won't work in boat

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