Need help deciding an Inverter

beachcombersc

beachcombersc

New Member
May 7, 2007
576
Charleston S.C.
Boat Info
1999 3870
Engines
3116TA
I'm looking at adding an inverter. I have a small modified sine 400watt taking care of the 27 LCD, DVD, HDTV tuner. I want to be able to run the Microwave for some popcorn, and maybe 2-3 hours top with the AC running. I don't have to have both on at the same time. I would remove the smaller inverter if I get a larger one.

I did some tests.
The AC swings up to 30amps for a second starting. Then down to 6.75amps
The Microwave starts out at around 3-4 then slowly peaks at 5ish.
The TV,DVD,tuner draw maybe 1.5amps

I know the microwave will take longer to cook on a modified sine, I didn't know what the true affects are on both the inverter/microwave running on M.S.

I also was wondering if the AC could run on a MS, as well as the affect if any running the A.C. on a MS.

I guess the I should size off the AC then, right?

I know battery size is the big concern, I just want to watch a movie, or have the kids nap for a couple of hours without the genny running. I just want to be able to enjoy the coast silently while the kids are asleep. Plus, in the evenings while eating in the back, to not hear that genny sputtering. It kills the cabin to stop the air conditioning for an hour during dinner.

My batteries are all the same.
105Amps each wet cell group 31

Any help is greatly appreciated!!!
Thanks everyone...
 
You towing a barge for the batteries your going to need for that?

Your Amp numbers are for 120 volts... not 12 volts.
 
By my math, I should get almost an hour and a half. Thats all I need....

But yes, your right. I did, and meant to quote you 120volts... I measured right from the panel. These are without the loss to the panel from the power source.
 
My point was you are not considering how many Amps you need the 12 volt battery to output for it to deliver the Amps you need at 120 volts. You don't have enough juice in your battery to even start the AC unit...
 
Im looking at a true sine, 2000 watt cont. 5000 watt peak.

Thats 41.6 amps peak @ 120v ac
16.6 amps cont. @ 120v ac

Whats the issue? That would mean,using 70+ amps an hour for full Air. It would cycle quite a bit. Maybe lowering that 10-15% of total usage. 315 amp @12v avaliable. Half is 157.amps@12v...
 
I'm not real good with electricity stuff... I'll admit that... especially the conversions between AC and DC and when you have to use RMS of the voltage etc etc..

BUT..

Amps * Volts = Watts

If you are running 30 Amps for an AC system at startup at 120 volts, that's 3600 Watts. Put that 3600 Watts on a 12 volt circuit (i.e. a battery driving an inverter with no loss) and you need 300 Amps... Not the 30 Amps you assumed. Throw in some loss due to the inverter and you are probably talking 350 Amps needed from the battery. AND... batteries are such lovely creatures that as try and suck all this juice out of it, the voltage will drop thereby needing more Amps causing the voltage to drop more etc etc... battery catches fire... air conditioner doesn't start.

Basically, you need to add a "0" to your amp requirements when going from 120v to 12v... it's 10x.. one of them laws of physics things. Now if you had a Tiara...

Now someone will pipe in and say I didn't convert something to AC from DC or some other nonsense... but the concept is right.
 
Last edited:
From my research Gary that sounds right. First and foremost let's say all you wanted to run was the AC. A golf cart battery typically has a 250-350 AMP/HR capacity.
That'll get that AC running for 1 HR and completely drain your batt.
You really want your bank twice the size of what you want to use....hence the barge.
Also, most decent inverters have a 15A output limit. You obviously found a bigger one than I've been eyeing but that will draw more from your Batt Bank and generate more heat and fires aren't good.
Here's some reading
 
Actually, I think a golf cart batter is usually 6 volts isn't it? That means even a 350 Ah golf cart battery won't start his air conditioner unit... Trying to suck that much juice out of a battery that fast and for any length of time is going to cause serious heat... just go drop a screw driver across the terminals.
 
Four Suns said:
Actually, I think a golf cart batter is usually 6 volts isn't it? That means even a 350 Ah golf cart battery won't start his air conditioner unit... Trying to suck that much juice out of a battery that fast and for any length of time is going to cause serious heat... just go drop a screw driver across the terminals.
Click to expand...

Yes they are. So 2 in series let's say making his AMP/HR requirements would get him and hour, then be completely drained. Now twice the space/weight needed.

I want an inverter too, but won't run AC. Mayve the Micro for very limited use. It's low on my priority list and more than likely a project for next year.
 
The deal breaker is that you have to calculate that you only have use of half of the listed reserve for your batteries. Not a good idea to drain your batteries too much below half their capacity. Given that, you'll need twice the number of batteries that you are estimating. You'll need quite a bit of real estate to park your batteries. They need to be parked very close to the inverter for minimum loss to resistance in the cables. I don't know if you have that logistically considering you have a genset on the 280. I don't have a genset and have the ability to put 6 golf cart batteries on that pad. My dilemma is that precious real estate is very useful for essential tools and containers. So my solution is a 2000W inverter and 2 golf cart batteries. Not going to run the AC, but can run the lights, DVD, fridge, etc until the cows come home without a worry about the batteries.
 
I never assumed it was 30amps DC...
After the startup, the load would be 7.75 amps @120VAC. This converts to about 70amp hrs @12VDC.
At startup for less then a second, it would draw 28-30Amps @120VAC. Which would convert to 280-300amps @ 12VDC(for a second)
I am even thinking a capacitor could help. Like the ones for stereo amps...
I still dont see where I'm coming up short...
After starting, 70amp hrs DC, I would have 1.5 hours of half life on just two bats. Leaving one for startup untouched.
 
What kind of battery bank do you anticipate to use and what inverter?
 
So...

They basically make these things already. I've looked at tying in the 2 D batteries I have for the bow thrusters into an inverter to run my fridge and freezer (two separate units). The inverter I liked was the Xantrex Freedom 30 which did 3000 watts continuous and 9000 watts peak/surge. It also has some nice features of low voltage cut off so you don't melt the batteries down. These things cost over $1000 (street price).

I just bought 4 new 8D Deka AGM batteries and here is the spec sheet:

http://www.eastpenn-deka.com/assets/base/0194.pdf

Note they have the amount of time there battery line will run at 75 Amps (and 50, etc. etc.). Now with a big honkin' 8D battery, you get about 2 hours at 75 Amps.

So I don't know... Do your own math... Here's a place Xantrex has that will calculate it for you:

http://www.xantrex.com/support/howlong.asp

If I plug in your numbers of 105 Ah for the battery (which is what they have on the chart on that calculator BTW), I can't put in 2000 watts of power. I can put in 1600 watts and get about 20 minutes (.32 hours). My gut tells me it won't start the AC unit with the smaller battery... But prove me wrong and report back!
 
Nehalennia said:
What kind of battery bank do you anticipate to use and what inverter?
Click to expand...
Again, I have three group 31 105 amp wet cell bats.

I wouldnt want to have to tie all three together, so I would like to just make two work. If thats an hour, Great! If thats less than 40 minutes, Im not sure I would spend the extra cash to get a pure wave inverter. I only need it for the microwave/tv....

105*2 =210

105amp hours till both are at half. 70-80amps being pulled at running, leaves me with 1.5 hours. Am I close?
 
So if I plug in 210 Ah (105 x 2) into the Xantrex calculator and stick in 2000 watts, that gives 0.52 hours (30 minutes).

I assume they are accounting for the degradation of the voltage as time goes on. If you are draining the battery that fast, I bet the voltage drops fast... Those Ah ratings are typically for a 20 hour drain I believe. Something faster than that is going to be a different rating.
 
You have a Genny and want an inverter? I'll buy you 20 batteries if i can have your genny.
 
You talking to me? You have diesel tanks?

I looked at doing an inverter due to being at anchor and turning the genny off... but at the end of the day, I don't think it's worth the hassle... I just run the genny all night... all day.... all night.... The Onan diesel genny I have is a low RPM thing and it'll go a looooong time.
 
No, I don't want a diesel genny, I want Beachcomber's genny.
 
unless you want to give me your genny and i'll sell it and buy a gas one.
 
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