12-Volt to 110-Volt Conversion

JDHarrin · 10-07-2020, 11:08 AM

Okay, I'm trying to figure out how much power my house batteries can supply iif I used a 12-to-110 Volt Inverter. I have two AGM Batteries that are rated at 210 Amp-Hours. From what I understand, you're not supposed to take them below a 50% charge. Does this mean I have 105 amps to use (I don't believe this is the case)? Is there a calculator or equation that can tell me how many watts are available at 110 Volts before the batteries get to 50% capacity? Let's say I have a 3000 watt Inverter and plan to load it at half it's rated capacity (1500 watts). How long would my batteries last (assuming the inverter uses a minor amount of power on its own)? I don't understand the relationship between the 12-volt side and the 110 volt side. Anyone have a better understanding of all this, that can explain it in layman's terms? :-)

**ThomB** · 10-07-2020, 11:35 AM

Maybe plugging your numbers here?

https://www.inchcalculator.com/ah-to-kwh-calculator/
I have to be honest, I rarely use battery power long term or while we're not moving so I can only rely on what I google.

Winterbagoal · 10-07-2020, 11:49 AM

Quote:

Originally Posted by JDHarrin

Okay, I'm trying to figure out how much power my house batteries can supply iif I used a 12-to-110 Volt Inverter. I have two AGM Batteries that are rated at 210 Amp-Hours. From what I understand, you're not supposed to take them below a 50% charge. Does this mean I have 105 amps to use (I don't believe this is the case)? Is there a calculator or equation that can tell me how many watts are available at 110 Volts before the batteries get to 50% capacity? Let's say I have a 3000 watt Inverter and plan to load it at half it's rated capacity (1500 watts). How long would my batteries last (assuming the inverter uses a minor amount of power on its own)? I don't understand the relationship between the 12-volt side and the 110 volt side. Anyone have a better understanding of all this, that can explain it in layman's terms? :-)

The 50% warning for drawing them down is more to preserve the longevity of the batteries. You can take AGMs lower, but it is supposed to shorten the number of lifetime charge/discharge cycles they're good for.
With any lead acid batteries, the problem (in my opinion) isn't how far down they can go, but how do you efficiently replace what you've used?
I would say 105 Ah is plenty for a couple of nights without recharging, but there are so many other factors involved that can affect that guess, it's hard to say for sure. You could probably use lighting, watch TV, recharge electronics, run a laptop, and probably a water pump, and furnace, but without knowing the average Ah draw from that group, I coudn't say how many nights you'd get from the 105Ah available. If you threw in a CPAP machine, all bets are off.
I would suggest getting out in the real world, with them fully charged, and experiment. AGMs are also among the hardiest of battery types, and I don't think you can do too much damage to them if they get below the 50% state of charge threshold a few times.

TomC · 10-07-2020, 01:34 PM

If you are pull a constant 1500 watts you only have about 8 hours of battery usage. If you google you can find a conversion for using a 12V dc inverter to 120V ac. However I doubt you will be pulling 1500 watts continuously. For the conversion I plugged in 120V ac, 1500 watts and 12V dc. It said you would be pulling 138 battery amps. So in 10 hours you would use 138 amp hours dc.

DavidM · 10-07-2020, 02:42 PM

The answer is somewhat complicated. First the easy part:

Let's use your 105 usable amp hour case above (and the answer about why you don't want to draw down below that above is correct). 105 amp hours is 12 x 105 = 1260 watt hours. If you drew 1,500 watts continuously the battery would last about 1260/1500 = 0.84 hours.

Then the hard part:

But those 105 usable amp hours are based on drawing down at a rate of 1/20 of the total amp hour rating or about 10 amps or 120 watts, and 1,500 watts is many times that. If you could find your battery's rating at 1/1 of the total amp hours it would result in roughly half of the 1/20 rating.

You would be lucky to get 1/2 an hour at 1,500 watts.

And as you can see from the foregoing, working in watts and watt hours is the way to cross over from 12V to 120V systems. Watt hours is energy and watts is the rate of energy used no matter what the voltage or the amperage are individually.

David

TomC · 10-07-2020, 04:29 PM

Quote:

Originally Posted by TomC

If you are pull a constant 1500 watts you only have about 8 hours of battery usage. If you google you can find a conversion for using a 12V dc inverter to 120V ac. However I doubt you will be pulling 1500 watts continuously. For the conversion I plugged in 120V ac, 1500 watts and 12V dc. It said you would be pulling 138 battery amps. So in 10 hours you would use 138 amp hours dc.

Loss of brain. In one hour based on 12V side of inverter pulling 138 amp you would use 138 amp hours in one hour. Sorry for misleading.

JDHarrin · 10-12-2020, 05:13 PM

So based on what DavidM said, (if I interpreted him correctly), you get the same amount of power (wattage) from either side of the system - the 12-volt side and the inverted 110 volt side. Using a power calculator I found online, these two statements below are equal - it shows that the number of amps drop as the voltage increases, however, the total amount of power remains the same (minus whatever small draw the inverter uses). Does this sound right?

105A / 12V / 1260W = 11.45A / 110V / 1260W

DavidM · 10-12-2020, 05:20 PM

Quote:

Originally Posted by JDHarrin

So based on what DavidM said, (if I interpreted him correctly), you get the same amount of power (wattage) from either side of the system - the 12-volt side and the inverted 110 volt side. Using a power calculator I found online, these two statements below are equal - it shows that the number of amps drop as the voltage increases, however, the total amount of power remains the same (minus whatever small draw the inverter uses). Does this sound right?

105A / 12V / 1260W = 11.45A / 110V / 1260W

You are correct. The first law of thermodynamics says in effect that the left side of the equation equals the right side less heat losses (5-10%) in the inverter, ie energy is conserved (equal) when it is changed from one form to another.

David

Pianotuna · 10-14-2020, 09:52 AM

Quote:

Originally Posted by JDHarrin

Okay, I'm trying to figure out how much power my house batteries can supply iif I used a 12-to-110 Volt Inverter. I have two AGM Batteries that are rated at 210 Amp-Hours. From what I understand, you're not supposed to take them below a 50% charge. Does this mean I have 105 amps to use (I don't believe this is the case)? Is there a calculator or equation that can tell me how many watts are available at 110 Volts before the batteries get to 50% capacity? Let's say I have a 3000 watt Inverter and plan to load it at half it's rated capacity (1500 watts). How long would my batteries last (assuming the inverter uses a minor amount of power on its own)? I don't understand the relationship between the 12-volt side and the 110 volt side. Anyone have a better understanding of all this, that can explain it in layman's terms? :-)

105 x 12 = 1250 watt/hours

So a 1500 watt load could be powered, in theory for about 50 minutes.

In real life, due to voltage drop, you might get 30 minutes.

NeilV · 10-15-2020, 08:22 PM

Then you have to subtract 10% to 20% since most inverters are at best 90% efficient under "Optimum Conditions" I believe is how many spec sheets put it.

Most will sound an alarm at 11 volts and shut down at 10.5 volts so when your batteries are new and capable of charging up to full capacity you will be starting off at a higher resting voltage than after they have aged a bit. The batteries can be down to below 80% capacity in fairly short time especially if you regularly drain them down to shutdown voltage.

You do not get equal from both sides of the system. If you have 1500 kWh usable coming in 150 to 300 kWh of that is going to be consumed by the inverter and only 1200 to 1350 kWh can be expected from the output on the day the new batteries are put into service after which that number will steadily drop as the charge/discharge cycles accrue.

That of course is the simplistic view of things since even when you are not using the inverters output as long as its switched on its consuming power from the batteries and slowly drawing them down. This of course does not take into account using the 12 volt lights, water pump, vent fans and parasitic draws from the coaches 12 volt house systems. Anything the inverter pulls will be in addition to them unless you have a separate dedicated battery bank just for the inverter.

Anyways its a moving target that drops as the batteries age and much depends too on the quality of your converter/charger. Older coaches tended to have some pretty poor converter chargers in them that would overheat the batteries and send them to an early grave especially if they were run down to below 12 volts too often.

Bottom line is that you need to consider what output you need as your target once the batteries have reached end of life and start off at 2 to perhaps 3 times that number or you can be going through battery replacement quite often. If you start off with just barely enough to meet your goal then that goal may not be attainable for very long.

So what really is your goal here? Being able to watch the evening news for a few nights between having to run the generator or something more?

JDHarrin · 10-15-2020, 08:54 PM

I was really just curious how much power my two new house batteries hold and what it would allow me to do. Prior to this past weekend, I had no idea how long they would last for basic services or if they could support anything else (CPAP machine maybe). My wife and I went out to Boondock last weekend to figure things out. We determined that our 28-Gallon fresh water tank could basically last us two full days before we needed to dump the tanks and refill. The house batteries consumed about 20-24% each day as well, so I think we're good for two full days before having to recharge. With this in mind, I think we'll boondock for two days while traveling, and then stay at a campsite for a few days before starting all over again. Appreciate all the input.

Old Navy · 10-15-2020, 08:57 PM

Quote:

Originally Posted by JDHarrin

So based on what DavidM said, (if I interpreted him correctly), you get the same amount of power (wattage) from either side of the system - the 12-volt side and the inverted 110 volt side. Using a power calculator I found online, these two statements below are equal - it shows that the number of amps drop as the voltage increases, however, the total amount of power remains the same (minus whatever small draw the inverter uses). Does this sound right?

105A / 12V / 1260W = 11.45A / 110V / 1260W

Also, the 105Ah is a C20 rating, or at a rate that would discharge 1 battery in 20 hours. You are discharging the 2 batteries to 50% in 1 hour, or a C2 rate. At that rate, a 105Ah AGM battery only supplies about 80Ah. Add in an 85% efficient inverter and you get a little over 800W for an hour. This is why very few people have off grid solar systems.