OK, a little help here.  I have heard that using a backup battery system hooked up to an inverter that has 120v items plugged in is a very inefficient use of the battery power.

Example A- 12 v battery system >>> inverter >>> cell phone charger.

What about using a direct connect system like...

Example B- 12 v battery system >>> 12v "cigarette plug style" cell phone charger.

Not being an electrical engineer, I would guess that in "A", the inverter uses power itself, but what other losses happen in these type of systems?  

What about "B"?  Are there power losses there as well?

Is there any data to compare, say a 12v DC refrigerator vs. a 120v AC one?

Any magic formulas that are simple to understand for the layman?

Modern inverter technology makes for an inverter efficiency as much as 90 to 95 %.

Personally, I don't sweat the inefficiencies any more unless every watt-hour is critical, like in a BOB battery pack with a very small set of solar panels.

Other losses will be conductor resistive loss especially when conducting 12 vdc.

That's why I've pretty much only use 120 vac from an inverter on most systems when battery recharge isn't an issue.

[Clarification, I do use some specific 12 vdc things that I want to be independent in case of an inverter failure of some sort, especially when I'm not present, for ex, off grid heat]

Finally, your ex B would be more efficient [only one 'inverter' a DC to DC converter] vs 2 separate power supplies in ex A.

In ex B efficiency would be abt 90% and in ex A abt 90% times 90%.

The phone charger usually contains a DC-DC converter [12 vdc to 5 vdc] w/ abt 90% efficiency.

Theoretically "B" would be the most efficient.  Example A is going to take 12VDC and convert to 120VAC then convert it back to about 3-5 VDC.  While B is only going to do one conversion, 12 VDC down to 3-5VDC.

You need to think of electricity in Watts.  Watts = Volts * Amps

A 120 watt bulb is going to draw 1 amp from a 120 VAC outlet and 10 amps from a 12 VDC source but it will always draw 120 watts.

Batteries are rated in Amp Hours (AH) which means they will provide X amps for a certain amount of time.  Typically you do not want to repeatedly discharge a battery below 50% of this rating or you will shorten it's life.

You need to think of electricity in Watts.  Watts = Volts * Amps

A 120 watt bulb is going to draw 1 amp from a 120 VAC outlet and 10 amps from a 12 VDC source but it will always draw 120 watts.



ok, so if I use the above mentioned 120w bulb in a lamp and plug that into an inverter that is hooked to my battery system, it will only draw 1 amp, whereas if I were to plug that lamp directly in to a battery bank (assuming it had the cigarette lighter option) it would draw 10 amps...right?  And if so, does that mean that using the inverter will be MORE efficient since the batteries are measures in amp hours?  ...(massaging brain with another beer...)

There is NO FREE LUNCH.

A 120 volt lamp won't light up on 12 vdc.

Also, the 120 vac lamp drawing 1 amp would be a 120 watt lamp.

A 120 watt 12 volt lamp would draw 10 amps.

To me the losses would be negligible at best between the two examples.The point is that you want to charge your cell phone batteries from a 12V source. I would just go with example B. You don't have to carry around an inverter, one less thing for you to worry about, ie where did put that gosh darn inverter.

The more parts in the system, the more loss there will be.

Powering something from a battery via an inverter will not be as efficient as powering a similar item directly from the battery.

With high efficiency inverters the difference may not be enough to worry about, but it is still there.

IF you have to run power a distance from the batteries, using an inverter will likely be a much better option due to the very heavy cables and line losses of running DC power for long distances.   And by long distances, I mean more than a few feet (at high loads) up to 20-30 feet or so for lower loads.

I should have clarified-

A 120W bulb will draw 1 amp at 120 volts.  In order to get 120 volts from a 12 volt source you will need an inverter.  The inverter converts amps to voltage so 12V at 10 amps on the input side will equal 120V at 1 amp on the output side, minus about 5-10% loss.

Quoted:
The more parts in the system, the more loss there will be.

Powering something from a battery via an inverter will not be as efficient as powering a similar item directly from the battery.

With high efficiency inverters the difference may not be enough to worry about, but it is still there.

IF you have to run power a distance from the batteries, using an inverter will likely be a much better option due to the very heavy cables and line losses of running DC power for long distances.   And by long distances, I mean more than a few feet (at high loads) up to 20-30 feet or so for lower loads.

OK, so in designing a alternative power supply for a BOL, lets say I have a battery bank of 10 deep cycle 12v batteries..-pick a amp hour rating for easy math-...that is kept charged by a solar array...total off grid power.  Their total available output is "X".  Using a total off grid system I'd want to be very "power wise", so I hook up a ____ watt inverter to the battery bank.  Into the inverter I have 4 cell phone charger wall warts...(cause after TEOTWAWKI everything is fried except cell phones...scary thought is that some people probably think this way...)  Anyway, the inverter that converts the battery bank 12v DC power to 120v AC power is 90% efficient (lets say)..so there is a power loss there, correct?  Then those 4 chargers are taking 120v AC and converting it back to 12v DC..correct?   I am assuming that there is a power loss there as well...right?  So where is the trade off...  In this situation, am I better off running multiple items through the inverter that have a direct 12v charging option?  Should I try to purchase more DC powered items- fridge, etc...or...should I go the inverter route for more availability of items...what would you do?

The joy of a system is you can leave yourself options.



I have a 12 volt charger for my cell phone.  It just plugs in a cigarrette lighter or whatever they want to call em these days.



Using the 12 volt charger for the cell phone will save the 2 conversion losses you will have going from 12 volt to 120 volt and then using the wall wart to go from 120 volt back down to 12 volt.  The more efficient I make a system the smaller a system can be or the longer a larger system will last.



Now I am not going to say I don't like options.  What if my 12 volt charger has a melt down?  Dang right I want that wall wart and the inefficiency to charge my cell phones.



So if you have a system you wire up as 12 volts you can leave yourself a way to hook up to the 12 volts.



You can also have it set up for the invertor as well.



I currently have a 120 volt chest freezer running as a fridge.  It does not really use much power and it would be worth the extra conversion loss for me to have it since the thing cost less than 200 bucks vs. me spending my money on a 12 volt chest freezer that I think costs a whole lot more.  Other than really expensive stuff I have not seen a lot of 12 volt chest freezers but if I am wrong please post a link so I can order one.



Now on some of the larger battery banks folks wire stuff up for more volts.  Getting into 24 volts or 48 volts and other numbers as well I am sure.  Some of this gets into efficiency for how long the wires are being run.



I am just starting on this road.



I plan a very small setup at 12 volts and I don't know if it will be big enough to power an invertor and my chest fridge setup.



I do know it will be perfect for charging the laptop and cell phones when the power is out and it can also charge up rechargable batteries as well.



With a small invertor it could also run some other stuff that I might find a desperate need to try to run.



Some of the stuff lehman's sells is very expensive 12 volt stuff made for off grid use.  I really like it but I have yet to feel the urge to spend the money.



When I can just go with a chest fridge at 120 volts and buy a bunch of batteries for the same cost as a really nice 12 volt fridge I kind of go for the 120 volt setup due to wanting to see if it will work.



Eventually I expect I might own some of the expensive 12 volt stuff.  



But right now I am learning and playing and being a cheapskate because I have a lot of other stuff I am messing with at once.



Anyway, if I can afford to easily skip the inefficiency I skip it.



When it comes down to a huge price difference, I am a cheapskate but I admit there might be a darn good reason to spend the money if someone had the money to spend.



For my cell phone, yeah I want my tiny battery bank to last as long as possable between generator runs.



But the invertor and wall wart are also the backup system in case the 12 volt charger explodes due to unicorn interferance.

First off, if you are building an off grid power system, you just shot yourself in the foot by using [I think you are proposing to use] 10 12 volt deep cycle batteries.

Usually when folks talk of deep cycle they are talking Walmart, Sams, etc, deep cycles that have relatively limited life vs other batteries that in the case of golf cart batteries, are longer lived and have more capacity for the $$$.

Also, I've found that not understanding care of flooded batteries results in short life -the hard way.

You seem to be concerned abt EMP if I understand, and that is likely to be the last of your problems.

You ought to consider using an inverter and common energy efficient devices with your system.

Add one more solar panel to compensate for various inefficiencies and go on down the road with the project.

If TSDHTF, then your technical knowledge of working around issues with your system that will surely arise will be FAR more important than the difference in efficiency how you charge you cell phones.

Knowledge will be the ultimate prep IMO.

This.

I see a lot of people sweating inverter ineffeciencies, not realizing that until they run #2 copper to that 12V fridge they bought, they are burning a lot of energy has heat.

Really, serious inverter system don't use 12V battery banks - Too much DC current, too hard to cable.  Look at the 48V (there is a reason it's common in telecom) inverters if you want to run a lot of loads.  It is more expensive, so that is a drawback.

For small loads, like cell phone chargers, it's a smart idea.

You can't effectivly charge 10 12 volt batteries in parallel - When the plates in one short, they have a nice habit of taking out the rest of the batteries that are in parallel, and are difficult to detect.

Much better to use series strings of batteries, i.e. 24 or 48 volts, and use higher capacity, lower voltage batteries (like 8 large 6V batteries in series for a single 48V string).  If you need to, you can make multiple parallel strings work, but they are more susceptible to unbalanced draw/charge cycles - If you do multiple strings, look at the diagonal connection wiring so that each string has the same resistance to the load.  I have 3 strings right now, but I bought the batteries used and don't need to get a lot of life out of them.  When I buy again, I'll buy larger 2V or 6V batteries in series for a single 48V string in my setup.

Add one more solar panel to compensate for various inefficiencies and go on down the road with the project.

Would this ^^^ be a better option than to add to the # of batteries in the system...assuming the $$$ equals out...?

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If you add an extra panel, you are adding extra "generating" capacity.

If you add extra batteries, you are adding to your capacity to STORE energy.

There is a balance that is determined by your power needs, sunlight thru the year, and when during the day you need access to your 'power'.

That said, except in special cases, I'd go with more panels and use all the excess energy I could during sunny parts of the day.

Now let me throw a curveball in here - what about a DC generator?

You can get milsurp 28VDC generators (one is run by a turbine) that put out decent power.  If you go with a 24V system, the configuration would be :

Generator––->Battery Bank––->Inverter––->120/240VAC system
                                            |
                                            |––>24VDC System

You'd need really short cables though - 357A is some serious current.

1989 Tiernay Turbines Mfr 51913 Mdl MEP-362A turbine engine generator set Sn HZ00131, 10 KW, 28 VDC, 357 Amps, meter indicates 190 hours, working condition unknown, **BUYER LOAD**

If you need to move the power from your battery a long distance, say 50 ft, its much more efficient to convert it to AC first. Using 12 v appliances has a slight advantage over AC in this system. I would always go 12 v if I had easy options and access. I will have 90 watts of capacity once I get my system running. It isn't much on a good day.

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