Batteries and AC - my brain hurts

[redunculous]

Ok so I have the following calculated - please correct me where I am wrong

I have 4 x Trojan T105's at 225 AH and they are in series parallel - this gives me a combined 450AH at 12 volts

I have a 50 Amp Alternator

I wish to run my aircon via an inverter (please don't ask me the whys)

I have come to this conclusion

my aircon requires 11.5 amp via AC Mains, to start to convert this to the DC requirement I have multiplied Volts x Amps (110 x 11.5) which equals 1265 VA

1265 divided by 12 gives me the amps required via DC which equals 105.4

105.4 minus 50 amps (alternator running)

= 55.4 amps remaining

450 (battery bank) divided by 55.4 = 8 hrs runtime

Is this right?

[mitiempo]

The math is correct but the reality is far different.

A battery bank cannot be discharged below 50% capacity or the life of the batteries will be very short.

The inverter would shut down when the voltage dropped anyway.

The alternator likely will not output at its rated maximum. 8 hours at max is not what it is designed for either.

Inverters should not be used for energy hogs like AC, water heaters, and room heaters. The draw is too large.

May I suggest a windscoop in the forward hatch.

[US27inKS]

At first glance your calculations make sense, but it's been many, many years since I've messed around with anything like that so my opinion is not to be trusted.

One thing that jumps out at me is your assumption that the alternator will be putting out 50A. While it may be rated at 50A, under most conditions it will be putting out substantially less. Also, if you do run your engine up to speed so you get maximum output from the alternator, you will likely be buying an alternator after a few hours of this. Most alternators are not designed to operate at maximum output for long periods of time.

You also have to consider losses in your inverter. Many inverters are only 85% efficient, so you have to divide your amp requirement by .85. This means you should be looking for 124 amps instead of 105.

How about a little Honda generator in the cockpit?

[mitiempo]

He is also assuming he can drain the batteries to zero.

[US27inKS]

Originally Posted by mitiempo

He is also assuming he can drain the batteries to zero.

That's not going to work. I'll bet his inverter won't let him get lower than 10 volts. Maybe not even that low. Also, as his battery voltage goes down, his amp usage will go up to maintain the same wattage output.

[mitiempo]

"That's not going to work."

No kidding!


Does remind me of the story of the person who installed an inverter and after plugging his battery charger into it thought his energy problems were solved.

[redunculous]

Originally Posted by mitiempo

He is also assuming he can drain the batteries to zero.

The only assumption there is yours, I was simply doing the maths and putting out the calculations as I saw them.

Thankyou for letting me know the maths was ok, albeit I have to adjust for certain losses (I have yet to pick an inverter so I cannot gauge the loss - nor have I measured the output on my alternator I am simply throwing the maths out as I see them as a start)

At no point did I say that I wanted to run it for the entire 8 hrs that would be ludicrous lol I just finished the calculations

Thanks for the input, US27inKS I have thought about a little generator and its something I am still looking at - plenty of choices makes it difficult for me to decide so I'm researching it.

[Tempest]

A buddy of mine recently purchased a Honda 2000 generator to allow him to run his cruisair air conditioning unit while at anchor. He tested it at home with the air con running off it and was pleased with fuel efficiency and relative quietness of the unit. On water test will follow.

[redunculous]

Tempest

Thank you for that input

[remetau]

If you really want to run an AC away from a dock, then get a generator. We have run our 16000 btu AC with a Honda 2k. Not often, but it does work.