Newbie..... Batteries?

[philo111]

I am desperate to be able to run a 1000 watt Microwave oven for a total of 15 minutes before needing to recharge the golf cart batteries. The inverter is 2000 Watts.

How many 6 volt batteries will I need?

THANKS!!

[pianotuna]

Hi philo111,

I can do 15 minutes on the microwave. It draws 1581 watts. But I do have surplus acid 12 volt AGM jars that total 556 amp-hours. I do have a 3000 watt Magnum inverter. It shows a draw of 160 amps IIRC. That works out to 40 amps per cell. A Peukert calculation shows that as 270 amps or 68 amps per cell. Weight of the battery bank is 420 pounds.

Before the AGM's I had 875 amp-hours of marine batteries. They too when they were new could power the microwave for your specified time. I did manage to get 9 years from 4 of them, and 6 years from 3 of them. I still use one as the starter battery, but this year there was corrosion around the positive post, so I expect its days are numbered. I've cleaned it up--and may replace it with an AGM. I can not use a starved acid jar there--they are too tall, unfortunately.

To expect four golf cart batteries to do 135 (peukurt adjusted )amps per cell for 15 minutes is asking a great deal of them. Normally 25 (real) amps per cell is a good target.

philo111 wrote:
I am desperate to be able to run a 1000 watt Microwave oven for a total of 15 minutes before needing to recharge the golf cart batteries. The inverter is 2000 Watts.

How many 6 volt batteries will I need?

THANKS!!

[MEXICOWANDERER]

Dont'Cha know GOLF CAR BATTERIES is the new buzzword around RVs.

For DEDICATED short cycle microwave usage a STANDARD AUTOMOTIVE engine starting battery outperforms a 5% antimonial battery by a long short.

This is exactly why I had to settle for 24 cells of flooded batteries to operate high loads for moderate duration. Each binary requires a forklift to move.

For multi-purpose usage it's impossible to beat AGM as far as dollars expended is concerned. A pair of GC220's barely can meet BCI CCA or MCA at 500 ampere test. That's FAR TOO GREAT A DISCHARGE LEVEL. It would take EIGHT 3-cell batteries to take a sustained 1,500 watt load discharge comfortably for ten minutes. My Trace MSW inverter sucks 2,230 watts, to power a 1,500 watt commercial grade microwave. The generated inverter and microwave excess BTU's are a crime.

Reality Is...

A pair of Group 31 starting batteries, and a PSW inverter would add 10-20 percent to the efficiency. This means FAR LESS amperage draw on the batteries and BETTER inverter performance.

I have a suspicion the inverter in question is NOT pure sine wave. And a six minute single baked potato will in reality be a ten minute baked potato.

How's THAT for "an educated guess"?

[landyacht318]

Having learned the tendencies of voltage over many sets of flooded batteries, and now having a Northstar AGM battery I've cycle hard but recharged properly since day one, I am dang impressed by the AGM's ability to hold higher voltages under load at lower states of charge compared to a larger capacity of 12v marine flooded group 27 and 31 batteries.

I imagine gc-2s and thier dense thick plates struggle even more under higher loads than the 12v flooded marine batteries.

Me thinks people in general put too much faith in the weight and price of Lead when stick and brick loads are tried on wheels.

[georgelesley]

Why not just use a small portable generator if your RV does not have one? I suspect you would find many other uses for it without the cost and weight of a huge battery pack.

[MEXICOWANDERER]

Precise math for electrochemical

You freakin' made my day!

Good on you!

High draw short duration, mild to intermediate low total amp hour use leads to only one intelligent answer ABSORBED GLASS MAT BATTERY.

High Amp Draw and GC220 battery performance is an oxymoron. Otherwise you'd see 50 mph 9-hole golf cars.

You mention microwave. You said nothing about total daily hotel amp hour requirements.

THAT leads to UNeducated guesswork...

[Almot]

philo111 wrote:
Thanks everyone but it sounds like a bunch of "educated guesses".
Isn't there some precise math?

People's guesses are based on education and experience. This is all you are going to get here.

There is a precise math - Peukert spreadsheets. But you don't have a precise data, so we are back to guesses again. MW current is unknown - unless you really measured those watts, or Peukert coefficient for your batteries. 1000W oven draws around 150A DC at high setting.

My educated guess - assuming Peukert 1.4 for average flooded, and DC current with inverter 150A, gives 420A Peukert corrected current for 2 GC. Meaning, you will be down to 50% after 15 minutes run. Sounds like possible - in theory - but your 2 GC are not going to like 150A.
Get 4.

Btw, I don't understand a "desperate" need for MW without shore power, unless this is something that is really, really difficult to do on a stove top. Very few tasks are, baby food heating maybe.

[philo111]

2oldman wrote:

philo111 wrote:
Isn't there some precise math?

Not really, not with the Peukert effect, soc, temperature, wire size, type of inverter... We've been through this before with you.

Can you answer my question?

"We've been though this before with you".
Your hilarious! I completely forgot about the Peukert Effect. Thanks for scolding me!

[YC_1]

CAtravler is dead on. His 100 amps is probably closer due to losses etc. With four batteries it will have no trouble providing solid power for 15 minutes.

Here is my rough calculations:

1100 Watts used to allow for some losses.

1100/12.5 volts DC = 88 amps DC. Divide that by four and you get 22 amps.

I often use a rough simple method.
Using AC take the 1000Watts and divide by 120 volts. You then get 8.3 amps. If you multiply the 8 amps times ten you get 80 amps of course and that is close to amount needed to run it via DC.

You could do this with two decent 6 volt batteries if rated properly.

[CA_Traveler]

Four fully charged batteries is more than adequate.

I've done this on 2 rigs with 4 6V batteries.

But batteries when boondocking are rarely fully charged unless you have solar. So typically the batteries are already discharged to a varying degree

So there is not nearly enough information on your requirements to do much more than guess.

Here's some information: 4 GCs have 460Ah capacity. Your MW uses 100A for 20 minutes or 33Ah total. So that is less than a 10% discharge.

[dahkota]

We have a 900W coffee maker that takes 10 minutes to brew a pot of coffee using a 1200W inverter. It uses 11 amps over that 10 minutes. We typically make coffee in the morning, when the batteries are about 70 percent full. The battery voltage drops from 12.5 down to about 12 during the brewing and typically returns to 12.5 when it is done. We have 4 6V 220AH AGM batteries.

So, my guess would be it will take at least 25amps and will be possible with at least 4 batteries that are 3/4 charged.

Would I do it with my set up? Yes. But we try not to make pots of coffee when the batteries are less than 65 percent full. The current draw would take the batteries below 11.9V - the cut off point at which I set my inverter.

Hope that helps.

[time2roll]

philo111 wrote:
Thanks everyone but it sounds like a bunch of "educated guesses".
Isn't there some precise math?

Sure

2 is weak and marginal at best for a 2000 watt inverter. Minimum recommended by Xantrex is 400 amp hours.

3 don't go there.

4 is about minimum and should work fine. They work fine for me. 15 full minutes will still be a heavy discharge especially in one swoop. My MW draws about 1650 watts and 150 to 170 amps from the battery. So the 1000 should be easier. But is that cooking power or input power?

6 if space and money are easy this is the way to go.

[2oldman]

philo111 wrote:
Isn't there some precise math?

Not really, not with the Peukert effect, soc, temperature, wire size, type of inverter... We've been through this before with you.

Can you answer my question?

[coolmom42]

1000 watts/12 V = 84 amps

84 amps X 0.25 hours = 21 amp-hours

21 amp-hours/0.8 inverter efficiency = 27 amp-hours

As you can see it's not a huge total amount of energy. BUT it is a huge current drain in a short period of time. You need a huge battery bank and super-heavy wiring to pull 84 amps in 15 minutes.

Listen to those with experience in such matters.

I would find some other way to heat the item in question. Try the range top or oven. People have cooked without microwaves for thousands of years, you can manage it too.

[philo111]

Thanks everyone but it sounds like a bunch of "educated guesses".
Isn't there some precise math?