Monday, September 25, 2023

# RVelectricity â€“ Just Ask Mike (J.A.M.): How many batteries equal a propane tank of energy?

Dear Mike,
I’m curious just how much battery power is needed to equal a 20-lb. tank of propane? For example, my 3-way fridge can run for a few weeks on propane mode, but when I try to power it from my batteries and inverter, it won’t even last a day before the battery is dead. Can you explain why this happens? â€”JP

Dear JP,
Ah, yes, it’s the energy density thing. And while modern lithium batteries can store a lot of power, they pale by comparison with a tank of propane. Let’s compare by first calculating how many kWh (Kilowatt Hours) of energy is in a gallon of propane.

### Do the numbers

You can easily look up that the available energy in 1 gallon of propane is equal to 27 kWh (Kilowatt Hours) of electricity. Now, this is assuming a 100% conversion efficiency, but we’ll ignore that variable for now.

A 20-lb. tank of propane is right around 4.8 gallons, since each gallon of propane weighs 4.1 lbs. Because they never fill these tanks all the way to the top, let’s assume a maximum of 4.5 gallons in a 20-lb. tank. To find the total energy we just do the multiplication and find that 4.5 gallons x 27 kWh = 121.5 kWh in a 20-lb. tank of propane.

This is even easier to calculate. We already know how many amp-hrs of storage each battery has. And you also should know that lead-acid batteries (flooded or AGM) should only be discharged down to 50% SoC (State of Charge) or battery life will suffer and you’ll be buying a new battery (or batteries) next season.

Let’s use a common 100 amp-hr (or Ah) battery as an example. If it’s an AGM battery, you should only use 50 amp-hrs of its capacity. So 100 Ah x 12 volts = 1,200 watt-hrs (Wh) of energy. But since it shouldn’t be discharged below 50%, that’s 600 Wh of available energy. Here’s what the equation looks like: 100 Ah x 12 volts x 50% = 600 Wh.

It’s the same calculation of 100 Ah x 12 volts x 100% = 1,200 Wh of available energy. As I’ve written many times, a 100 amp-hr lithium battery has twice the available storage of a lead-acid battery. Plus, they charge faster and weigh a lot less than a lead-acid battery. So a win-win if you have the budget for them.

### Final calculations

We already calculated there’s around 120 kWh of energy in a 20-lb. tank of propane, which is around 120,000 watt-hrs for a 100 amp-hr AGM or flooded lead-acid battery

If we divide propane energy by battery energy, then you need 120,000 Ah / 600 Ah = 200 lead-acid batteries of 100 Ah capacity each to equal the energy in a single 20-lb. tank of propane.

Similarly, for lithium batteries, we see that 120,000 Ah / 1,200 Ah = 100 lithium batteries of 100 amp-hrs each to equal one 20-lb. tank of propane energy.

### Wow, that’s a while lotta energy!

While it will be a long time before there’s a battery no bigger than a propane tank that can store 120,000 Wh of energy, all is not lost. I’m studying fuel cells that do a direct conversion of propane to electricity. They come in up to 1,500 watt modules that can be stacked in parallel.

If these fuel cells work out as planned, you could have the best of both worlds. Since the conversion efficiency is close to 90% compared to the 30% conversion efficiency of a generator running on propane, you could power an RV with an air conditioner for nearly a week on a single tank of propane with a fuel cell. Stand by, as I’m working on getting a demo fuel cell to experiment with.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Letâ€™s play safe out thereâ€¦.

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent bookÂ RV Electrical SafetyÂ is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert,Â click here.

Mike Sokol wrote this when he was a columnist for RVtravel.com. He has since moved on and is therefore unable to respond to comments.

##RVDT1931;##RVT1067;##RVDT2192

1. Very good response, Mike. But my question for JP is, why are you running the inverter if you have a 3-way fridge? Just turn off the inverter, run the fridge on 12vdc, and save a lot of battery amps!

2. At 80 yrs young I donâ€™t have to worry about the change over to EVs. My ashes will be blowing in the wind long before anybody has to be concerned about getting rid of their ICE vehicles. By that time gas will be back down to 29 cents/gal.

3. Will fuel cells follow the lithium cast model. At first to costly for most of us to afford and then settle to a still expensive floor level cost.

4. A minor but important detail. The energy available in a battery is equal to capacity (in Ah) x nominal voltage. The nominal voltage of a battery is defined as the voltage at 50% capacity. A LA battery is 12.0V, a LFP battery is 12.8V so the capacity (energy) in a 100Ah LFP battery is 2.8V x 100Ah = 1280Wh. For a LA battery, your calculation is correct. The reason there are so many errors in energy calculations from capacity in amp-hours is that all energy calculations need to use the nominal voltage for the specific battery chemistry.

• You are correct, but Iâ€™m trying to keep the concept as simple as possible. This is what I call a SWAG (Scientific Wild A## Guess). When I was calibrating nuclear missile guidance systems back in the â€˜80s we had to be accurate down to Parts Per Million (PPM). Good timesâ€¦

5. Really looking forward to your fuel cell experiments. Guess they will be pretty expensive though

6. Hope you’re feeling better Mike. The trouble with going electric is the electric companies are known to find ways for us to pay more for electric. Watch for surcharges to upgrade the electric service. Electric cars will be more expensive than ICE cars to drive. My wife bought a new electric stove and I hate it. Difficult to regulate the burners temperature. Old electric stove top worked great. If something happens to her, the electric stove is gone and a gas stove will replace it.

• Interestingly, Iâ€™m proposing an experiment to determine if more or less propane is used to boil water directly over a flame compared to a propane fuel cell generating electricity thatâ€™s powering an induction cooktop boiling the same amount of water.

• Nothing happened to my wife, but we now have a dual fuel kitchen range. Gas burners on top for me and electric oven for her. Canâ€™t seem to find anyone interested in buying the electric range for half of what we paid. The F150 hybrid is now in the junkyard. Totaled it in July 23. Bought a GMC Sierra. Had too much problems with the F150 hybrid and too much electronics. GMC Sierra isnâ€™t as complicated.

7. Don’t know if anyone else read the article but governments want to ban all gas and diesel modes of transportation. Doubt they will succeed, but what if? No ones paying attention to the cost and damage being done to create electric alternatives.

• We really do need to switch to non-fossil fuels for vehicles, but currently the electrical grid and power generation infrastructures aren’t up to the task. However, the latest generation of EVs do offer great performance with simple use.

It will take a combination of wind, solar, hydro and nuclear to meet the energy needs, plus a power grid upgrade. And we need new battery technologies that will increase energy density while reducing costs and the environmental impact of mining raw materials.

I believe that the USA has the best scientists, engineers and entrepreneurs in the world, and we are up to the task. After all, it took only 66 years from the Wright brothers first flight in 1903 until we landed a man on the moon in 1969.

But all consumer vehicle changes have to be phased in, so we’re not all switching to EV driving at the same time. It will likely take around 20 years before EVs are dominant. In the meantime, I think that plug-in hybrid vehicles are a great option.

• We need a viable way to recycle all spent electric car batteries before we even think about this, Also, my two vehicles that are 2008 and 2009 models. On fixed income, I can’t afford to replace them now, and thanks they are low mileage and very good condition, but what if they were electric. Would they still be operative at their age and 70 and 80 thousand miles? Costing me very little operating upkeep or costing \$10,000.00+ for new batteries 2x’s already?

• I believe thereâ€™s a big push to recycle all EV batteries. Interestingly, Tesla has been finding it difficult to get used Tesla batteries turned in for recycling since theyâ€™re still working great for solar energy storage. As I learn more Iâ€™ll report on it.

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