250W heater & Dual Batteries

[Raspy]

Mike,

Good post.

In the quest for the simplest way of understanding energy loads of electric heating, I try to measure it this way:

The amp draw of the load. Say 1200 watts at 120 volts (typical electric heater), looks like this: Watts over volts = amps. So 1200 watts over 120 volts = 10 amps. 1200 watts = approx. 4095 BTUs

12 volt to 120 volt conversion, including inverter losses of 10% results in a 12 volt battery amp draw of the 120 volt amps X 11 for the DC battery load. So, 10 amps at 120 volts X 11 = 110 amps load on the 12 volt battery.

A 220 amp hour battery should only be drawn down to 50% of it's rated amp hours. So, 220 amp hours divided by 2 (50%) =110 amp hours available.

So, from the previous calculation that the load was 110 amps, and now seeing that the available total capacity of the battery is 110 amps, the load can only be applied for one hour max. This means the heater can only be run for one
hour, and that might be too much because battery capacity is reduced even further when a heavy load is placed on it. You might only get 45 minutes from that battery to reach 50% discharge. This is why running an electric heater on batteries is impractical. Propane tanks have much more energy stored.

To get an idea of how much heat BTUs represent, connect (4) 75 watt 120 volt incandescent light bulbs (300 watts total), which give off about 90% of the energy they consume in heat. That'll give you about 270 watts of heat, or about 920 BTUs. This will give you an idea of how much practical heat it represents in the trailer. Not much.

The electric heater in the example = 4095 BTUs continuously. The 12 volt driven electric heater = 920 BTUs for one hour.

Bottom line: Not much heat, or heat for only an hour. Impractical.

[k corbin]

A heater one can buy for a very small amount of money and then power essentially for nearly free using low voltage and a small amount of storage.


If it were possible then most would be using it and you would not have to be asking because it would already be widely known as the best solution to keeping an RV warm.

[Mike_L]

Battery power is stored energy and this storage capacity is very limited so batteries are only suitable in low energy consumption applications. Heating is a high energy consumption application. This makes electric heating suitable only in situations where there is sufficient electricity to power it, such as shore power (AC) sources.
Fossil fueled heating applications can access much more energy and are capable of liberating sufficient energy to meet the heating requirements. That's why battery powered heaters are not viable for trailer application. Batteries just don't have enough energy to meet the demand.

[Civilguy]

All true!

I was particularly interested in the the difference expressed as a ratio. It's common for people to say you'd need a "huge" battery bank and solar array, but this boils it down to a 10:1 ratio of a common-sized battery per pound of LP gas.

So about 200 each 100AH batteries would be required to match the energy in a single 20 pound cylinder of LP gas.

To bring this all home to the cardinal forum themes here, that's going to really mess with the tongue weight, and I can't even begin to think about the tire pressures we are going to need! And then I am going to have to figure out how I'm going to tow it with a Smart car and whether I will need a sway bar.

[Civilguy]

Originally Posted by Glenn Baglo

I'm not sure watt you're saying.

Ah Glenn, wire you insulate?

[Raspy]

Just get a big rig, tractor-trailer to follow you to your camp site with the 200 batteries on board. Presto! Your own private utility company. Then plug in and enjoy the solitude! Electric warmth all night. Or, just put the Casita up on the semi-truck trailer, with the 200 batteries, and you are good-to-go and self contained! Plus, you never have to worry about the condition of the Casita tires, or sway, or the condition of the axle, or a possible breaking frame. Now this is starting to sound very practical.

[Raspy]

ooops, double post

[Glenn Baglo]

I don't have the energy to respond.

[ShelbyM]

Well I'm just disappointed that nothing has changed since Jon gave the definitive answer one year ago, post #15.

[Raspy]

Originally Posted by ShelbyM

Well I'm just disappointed that nothing has changed since Jon gave the definitive answer one year ago, post #15.

It's so annoying that physics and chemistry still follow the same rules after a whole year!

Seriously though, since propane tanks and batteries both store energy, the only real game changer, if there is going to be one, would be an extreme new battery design that everyone could switch to. It will have to be small, quickly re-charged and cheap. Just as propane tanks are today. Lead acid batteries work for some purposes, but they are terrible things and very limited in capacity.

[Jon Vermilye]

Originally Posted by Raspy

Just get a big rig, tractor-trailer to follow you to your camp site with the 200 batteries on board. Presto! Your own private utility company. Then plug in and enjoy the solitude! Electric warmth all night. Or, just put the Casita up on the semi-truck trailer, with the 200 batteries, and you are good-to-go and self contained! Plus, you never have to worry about the condition of the Casita tires, or sway, or the condition of the axle, or a possible breaking frame. Now this is starting to sound very practical.

Of course you still have the problem of recharging the battery bank.

[k corbin]

no energy left for a response...


That means the built in forum "creative energy spark arrestor" finally began working. And it was just in time to prevent another out of control wild fire.