Question about power consumption for Marine/RV batteries

For a calculation it's necessary to know abut the grade of efficiency from this inverter.

Regards,

Ralf

Well, let's do the math.

First, the TV draws 60 watts, but you'll be running it from an inverter. Assume the inverter is 60% efficient, that means that the load on the

12V electrical system is 100 watts for the inverter and TV.

At 12V (nominal) that means you'll be drawing 100/12 or 8.33 amps

5 hours would be 5*8.33 or 41.65 amp-hours.

Assuming you don't want to draw the battery down any more than 50% (you can go further with deep-cycle batteries, but it costs longevity) then you'd want a battery capacity (in the secondary battery alone) of about

85 amp-hours.

Use a battery isolator, don't just manually switch that battery back in parallel with the vehicle battery after discharging it- you want something that will limit charge current so that you're not sucking up the alternator's whole output into a dead battery when you first start the vehicle up again.

I would be more concerned about the load on the alternator. I have a

130 ampere alternator that normally gets a load of about 85 amperes. Were I to have to run that at full load all the time, I would be concerned about it overheating.

IOW, my *idea,* which may be incorrect, is if you are going to put another battery in there as a load, like a Marine deep cycle, I would probably get a higher output alternator to go in the engine compartment, space permitting, In my Sable, there is no wiggle room for a larger alternator, unfortunately.

Lg

I always figure the physically biggest with the largest reserve that can be anchored down somewhere is the best.

I have had one 12 volt the size that Semi Tractors use strapped into my trunk before. I had a bunch of them for running electric boats. One would push my 21' fishing boat's (square back dory with a small cabin) Mercury Marine twin electric motors for 8 hours every day trolling and be charged up overnight. Had a second on board for reserve.

They make fancy isolators these days that will properly recharge the reserve battery without harming the automotive system.

You do have to figure on higher wear on the alternator, but that is a minor issue. When it wears out, just replace it with the largest that will fit.

I also highly recommend a 12 volt TV. They make them for campers and such and they are normally much more efficient than running an inverter.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

How efficient is your inverter?

The TV draws 60W. So if the inverter is 100% efficient, it will also draw 60W.

This means 5A in, in order to get 60W at 12V.

If you want to run for 5 hours then, you need a battery with 5X5 = 25 Ah.

In the real world, the inverter will be much less efficient than that, and the battery won't live up to its nameplate rating after a couple years of use. So add plenty of safety margin.

--scott

I seriously doubt that your alternator "normally gets a load of 85 amps"

It may see an 80+ amp surge for a few seconds to minutes after starting the engine while the battery recharges but a full normal electrical load for a modern car (once the battery is charged) is not going to be more than 30-50 amps tops. Exceptions would be if you have the headlamps, both heated fronts seats, and the the AC, and the rear-window defoggers all running at the same time.

But back to the OP- a battery isolator will limit the charge current to the reserve battery so that both the battery and the car electrical system will be properly protected.

Feel free to stand in front of my vehicle with a clamp meter wrapped around the alternator output cable. Yes, I live up in the frozen north. It is not uncommon to have everything going...headlights, heater fan on full, rear window defogger, rear mirror defoggers ( heated mirrors ), power windows, windshield wipers, it adds up.

We'll make a video. Don't forget to pan-in for that close up of the ammeter.

Lg

Thanks so much for the responses. This information definitely help and will make my two month cross country trip through the countr much more enjoyable... I can't beleive the cost of good inverters

Maybe so. But my money is on an erroneous ammeter reading. DC clamp-on ammeters are very inaccurate because they have to impress an RF field around the wire and measure how the DC current affects it. Any RF component riding on the dc itself (such as the ripple from an alternator) throws them off big-time.

Well, I am going to measure it now, just for the heck of it, since I found less than ideal connections on my Megafuses and have since corrected that. You remember I had a complaint about having to charge my battery from time to time...it would lose charge for some reason.

IF it is because those Megafuses were not tightened enough ( they were not, IMO ), then I might get some different reading today. Standby for a reading with most of the electrics turned on:

66 Amperes

That's without activating the electric seats. I can't be in the engine bay and in the driver's seat at the same time. I will bet a wooden nickel that if I hit the electric seat motors, I'll be around

80 Amperes.

What I did suspect was the alternator was not providing enough charging current to the battery because it couldn't...through a bad connection somewhere. That turned out to be the Megafuse connections at the inside engine compartment fuse/relay box. Well hidden from view, because it has its' own plastic shuttered door ( for insulation reasons no doubt ).

This special meter I'm using, is supposed to be True RMS, which is applicable to AC, but not DC, as True RMS is the AC equivalent needed to match the work that a given DC current can do. But I'll have a wooden nickel ready for someone if I lose the bet.

Lg

Actual measurements of the TV might show that it's using less than the

60w listed.. I have a 27" TV that measures around 85w. Inverters usually run in the 80%-90% or better efficiency AS LONG AS YOU SIZE THEM CORRECTLY.

Using a 2000w inverter for a 25w load will result in a very poor efficiency number.. you will achieve a much better efficiency when you pull 500w from it. I'd go with a 200w-300w inverter.. you should be able to find them fairly cheaply. Any smaller and you might have a problem with the startup surge.

You might get some noise from the inverter.. also consider a 12v TV.

Remember to properly stow your second battery.. proper venting and all that.

Talk to RV and boating people and see what they do..

Jim

When the kids were small, we did a lot of camping using a Starcraft popup. I installed a second battery in the camper and permanently mounted a battery charger so that we could top it up whenever we were at a convenient site.

We ran 12 volt reading lights, a small 12DC /120VAC volt TV, and even some

12 volt fans off that battery, and never had it run out of juice. (We could have run the same appliances off the battery connection to the tow car, but I had fears of being up in the high Rockies and having my car battery run flat.)

This kind of camping breeds conservation with lights, water, etc, so you can't go hog wild, but you can certainly enjoy some of the amenities of home.

I think the dual voltage TV makes more sense than the battery/inverter/isolator system.