Simple Power Inverter Question

ausbran · January 2019

I'm just starting to teach myself about power and electronics, specifically for truck camping purposes.
I am going to setup an isolated dual battery in my truck engine compartment. Then I will run wires to the bed where the power inverter will be.
From here I will have a 1000w inverter for all my AC needs, but how do I power electronics that do not have an AC plug?

I am buying a small fan and probably an LED strip. Both of these are powered though two conductor power wires. So my question: Is it ok to hook up those electronics to the POS/NEG terminals on the backside of the power inverter? Will this work? Do I need to put a fuse in between the fan/lights and the inverter?

mike95490 · January 2019

Your will need a 12V fuse or breaker sub panel like they use in boats.

Your inverter DC cables are likely about 8 ga wire with a 40A fuse. Any wires on the load side of the fuse, must also be 8ga wire, so the fuse will blow before the wire melts.
With a DC fuse panel, you can use a 5A fuse for your small circuits and the 40A fuse for the inverter.

This is a bit complex to explain electrical safety over the internet. but boating sites should have some decent explanations. I prefer boating sites over RV sites, because boating is more regulated (and better safety specs) than RV's

BB. · January 2019

I suggest that you probably do not have a good match between your battery bank, loads, and things like AC inverter, and some sort of charging system.

A 1,000 Watt AC inverter on a 12 volt battery bank needs a circuit that supports:

1,000 Watt inverter * 1/0.85 AC inverter eff * 1/10.5 volts cutoff voltage = 112 Amp draw

For a "conservative design" (house/cabin/etc.), I would suggest the 1.25x NEC derating factor for fuses and wiring:

112 Amps * 1.25 NEC derating = 140 amp Branch circuit rating (wiring, fuse/breaker, connections)

In reality, as Mike says above, most RV related folks are not going to do derating step.

For the LED wiring, you probably are using less than 1 amp, so a small fuse and smaller AWG wiring... You do not want to put 20 AWG wire connected to the DC bus on the back on your 1,000 Watt inverter.

To give you an idea of what a sort of DIY solar project starts out as--Poster 2manytoyz (1/2 way down) his website has lots of photos and explanations.

http://2manytoyz.com/

Also, if you are using deep cycle batteries for more than a few minutes of 1,000 Watt loads, for a 12 volt battery bank (flooded cell lead acid deep cycle battery) will support a 250 Watt AC inverter per 100 AH of battery bank capacity. For a 1,000 Watt inverter, you should have around a 400 AH battery bank (that would be 4x 6 volt @ 200 AH "golf cart" batteries--2x in series by 2x parallel strings).

For most folks, I would be suggesting a much smaller AC inverter (something like 300 Watts)--Smaller inverters "waste" less power (smaller inverters use ~6 Watts "just turned on and no loads"... Larger inverters can be 20-40 Watts or more).

-Bill

ausbran · January 2019

BB. said:

I suggest that you probably do not have a good match between your battery bank, loads, and things like AC inverter, and some sort of charging system.

A 1,000 Watt AC inverter on a 12 volt battery bank needs a circuit that supports:
1,000 Watt inverter * 1/0.85 AC inverter eff * 1/10.5 volts cutoff voltage = 112 Amp draw
For a "conservative design" (house/cabin/etc.), I would suggest the 1.25x NEC derating factor for fuses and wiring:
112 Amps * 1.25 NEC derating = 140 amp Branch circuit rating (wiring, fuse/breaker, connections)
In reality, as Mike says above, most RV related folks are not going to do derating step.

For the LED wiring, you probably are using less than 1 amp, so a small fuse and smaller AWG wiring... You do not want to put 20 AWG wire connected to the DC bus on the back on your 1,000 Watt inverter.

To give you an idea of what a sort of DIY solar project starts out as--Poster 2manytoyz (1/2 way down) his website has lots of photos and explanations.

http://2manytoyz.com/

Also, if you are using deep cycle batteries for more than a few minutes of 1,000 Watt loads, for a 12 volt battery bank (flooded cell lead acid deep cycle battery) will support a 250 Watt AC inverter per 100 AH of battery bank capacity. For a 1,000 Watt inverter, you should have around a 400 AH battery bank (that would be 4x 6 volt @ 200 AH "golf cart" batteries--2x in series by 2x parallel strings).

For most folks, I would be suggesting a much smaller AC inverter (something like 300 Watts)--Smaller inverters "waste" less power (smaller inverters use ~6 Watts "just turned on and no loads"... Larger inverters can be 20-40 Watts or more).

-Bill

Thanks this is great information. I didn't consider the draw of the larger inverter. I just assumed it would be best to get an inverter that would have more than enough wattage. Here is my plan:

I'll most likely be getting this battery after doing some research:
https://www.amazon.com/Vmaxtanks-Vmaxslr125-rechargeable-Solar-Inverters/dp/B00ACNO2AO/ref=as_li_ss_tl?ie=UTF8&qid=1507673045&sr=8-3&keywords=vmax+battery&linkCode=sl1&tag=gnomad0f-20&linkId=f0ae2ff05a84de9ce636b767ce35feb6

Also, now I will probably be getting a smaller inverter. Something like this 400w:
https://whistlergroup.com/products/xp400i-power-inverter-1#features

And lastly, I'll be getting a very low power draw fan. This one:
https://www.defender.com/product.jsp?id=46808

So with a 400w inverter being powered by a 125AH battery will last for how long? I want to be able to charge my phone and laptop, and also my girlfriend will want to do the same, before going to sleep. Then it would be ideal to have the fan going while asleep at night. And the lights.

I'm breaking it down like this:
(2) Cell Phone = 25 watts each = 50 watts total
(2) Laptop = 100 watts each = 200 watts total
(1) Fan = 5-6 watts total
(2) LED Light bars = 5 watts each = 10 watts total
Overall wattage = ~270 watts

With this in mind, I feel that a 400w inverter would be ideal. Its also safe to assume I won't be drawing all of that power simultaneously at all times. Very rarely actually.

BB. · January 2019

Just to be clear... There are two major types of measurements here...

There is your "power usage" which is in Watts (or Amps @ volts)... This is like miles per hour or gallons per hour -- A "rate" of energy usage.

And there is your "energy usage" which is in Watt*Hours (or Amp*Hours @ xx volts). Like total miles driven (in a day) or gallons pumped in a day. An "amount" of energy usage.

So, most cell phone chargers are ~10 Watt maximum units, and typically you charge in 1/2 to 1.5 hours time frame per day. So an example may look like:

Cell phone charger 10 Watts
Cell phone usage 10 Watts * 1.5 hour per day = 15 Watt*Hours per day

Your computer may use 60 Watts maximum (running computer + charging), but average 30 Watts * 8 hours per day:

Laptop uses 60 Watts (peak)
Laptop usage 30 Watts average * 8 hours per day = 240 WH per day

So, your above numbers need to be clarified. It may look something like this. Inverter sizing:

2x cells * 10 Watts (peak) each = 20 Watts
2x laptops * 60 Watts (peak) each = 120 Watts
1x DC fan * 6 Watts (not used with inverter--) = 0 inverter watts
2x 12 VDC Led Lights * 5 Watts each (no inverter) = 0 inverter watts
=====================================
Total inverter load = 140 Watts

Note that AC power is "complicated"... Assuming PF~ 0.8, then the VA rating of the inverter (typically VA=Watt max inverter rating for non-commercial units):

140 Watts * 1/0.8 PF = 175 VA (aka "Watt") rated inverter

To measure your energy usage, a Kill-a-Watt type meter for AC loads, and a DC AH/WH meter for DC loads is pretty much needed (links below are just starting points for your research--There are lots of devices out there).

https://www.solar-electric.com/kiacpomome.html
https://www.rc-electronics-usa.com/ammeters/rv-battery-monitor.html

A DC Current Clamp DMM (digital multimeter) is very handy for measuring current and debugging too:

https://www.amazon.com/gp/product/B019CY4FB4 (midrange unit)
https://www.amazon.com/gp/product/B07546L9RT (cheap/good enough for our needs unit)

So, figuring how much energy per day (night) you use... An example:

2x cells * 10 Watts (average) each * 1.5 hours per day = 30 Watt*Hours per day
2x laptops * 60 Watts (peak) each * 8 hours per day = 960 WH per day
1x DC fan * 6 Watts (not used with inverter--) * 8 hours per day = 48 WH per day
2x 12 VDC Led Lights * 5 Watts each (no inverter) * 5 hours per day = 50 WH per day
==================================================================
1,088 WH per day

To make things easy, I am going to do one calculation that assumes everything above is running on an AC inverter. You have little DC loads, so it will not make much difference (DC loads will be 15% bigger than they really are):

1,088 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank = 107 AH per day

For a 125 AH battery bank @ 12 volts, that would pretty much drain the battery in 1 day. In reality, for longer battery life, highly suggest that you use only 25% to 50% of battery capacity per day. For an off grid home (full time off grid RV), suggest that you only use 25% of battery capacity per day.

From my calculations, obviously the computers are a huge amount of your energy usage (based on my guesses)... Using computers less, using "ECO mode" on laptops, using smaller/more efficient laptops (Chromebooks, tablets, etc.) all help a lot. My cheap 14" chromebook uses closer to 5-10 Watts vs ~30 Watts for a mid-range typical laptop.

Before I type too much more: Questions/corrections regarding above?

-Bill

ScoobyMike · January 2019

A couple of options you may want to consider:

1 - Check out portable battery power stations on Amazon. They don't have a lot of capacity to run a fan all night, but they come with 150-200 watt inverters for AC, USP ports for cell phones and some have 12V out for lights and fans. I would suggest getting D chargers for the laptops and maybe never even use the inverter. This option would be good if you only need power for an hour or two although you can get solar chargers for these units as well.

2 - For more continuous power Harbor freight has a 700 watt 2-stroke generator called Tailgator that is a little less than $100. It gets good reviews, is easy on fuel and not very noisy.

ausbran · January 2019

BB. said:

Just to be clear... There are two major types of measurements here...

There is your "power usage" which is in Watts (or Amps @ volts)... This is like miles per hour or gallons per hour -- A "rate" of energy usage.

And there is your "energy usage" which is in Watt*Hours (or Amp*Hours @ xx volts). Like total miles driven (in a day) or gallons pumped in a day. An "amount" of energy usage.

So, most cell phone chargers are ~10 Watt maximum units, and typically you charge in 1/2 to 1.5 hours time frame per day. So an example may look like:

Cell phone charger 10 Watts
Cell phone usage 10 Watts * 1.5 hour per day = 15 Watt*Hours per day

Your computer may use 60 Watts maximum (running computer + charging), but average 30 Watts * 8 hours per day:

Laptop uses 60 Watts (peak)
Laptop usage 30 Watts average * 8 hours per day = 240 WH per day

So, your above numbers need to be clarified. It may look something like this. Inverter sizing:

2x cells * 10 Watts (peak) each = 20 Watts
2x laptops * 60 Watts (peak) each = 120 Watts
1x DC fan * 6 Watts (not used with inverter--) = 0 inverter watts
2x 12 VDC Led Lights * 5 Watts each (no inverter) = 0 inverter watts
=====================================
Total inverter load = 140 Watts

Note that AC power is "complicated"... Assuming PF~ 0.8, then the VA rating of the inverter (typically VA=Watt max inverter rating for non-commercial units):

140 Watts * 1/0.8 PF = 175 VA (aka "Watt") rated inverter

To measure your energy usage, a Kill-a-Watt type meter for AC loads, and a DC AH/WH meter for DC loads is pretty much needed (links below are just starting points for your research--There are lots of devices out there).

https://www.solar-electric.com/kiacpomome.html
https://www.rc-electronics-usa.com/ammeters/rv-battery-monitor.html

A DC Current Clamp DMM (digital multimeter) is very handy for measuring current and debugging too:

https://www.amazon.com/gp/product/B019CY4FB4 (midrange unit)
https://www.amazon.com/gp/product/B07546L9RT (cheap/good enough for our needs unit)

So, figuring how much energy per day (night) you use... An example:

2x cells * 10 Watts (average) each * 1.5 hours per day = 30 Watt*Hours per day
2x laptops * 60 Watts (peak) each * 8 hours per day = 960 WH per day
1x DC fan * 6 Watts (not used with inverter--) * 8 hours per day = 48 WH per day
2x 12 VDC Led Lights * 5 Watts each (no inverter) * 5 hours per day = 50 WH per day
==================================================================
1,088 WH per day

To make things easy, I am going to do one calculation that assumes everything above is running on an AC inverter. You have little DC loads, so it will not make much difference (DC loads will be 15% bigger than they really are):

1,088 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank = 107 AH per day

For a 125 AH battery bank @ 12 volts, that would pretty much drain the battery in 1 day. In reality, for longer battery life, highly suggest that you use only 25% to 50% of battery capacity per day. For an off grid home (full time off grid RV), suggest that you only use 25% of battery capacity per day.

From my calculations, obviously the computers are a huge amount of your energy usage (based on my guesses)... Using computers less, using "ECO mode" on laptops, using smaller/more efficient laptops (Chromebooks, tablets, etc.) all help a lot. My cheap 14" chromebook uses closer to 5-10 Watts vs ~30 Watts for a mid-range typical laptop.

Before I type too much more: Questions/corrections regarding above?

-Bill

Ok, this is all great, thank you!! Ideally, we are only charging laptops and cell phones for 1-2 hours at the very max each night. The only things that would get extended use would be the lights and the fan. The lights will probably run about 3-4 hours average each night.

I'm still unsure of how you would connect a red and black wire connection from the fan to the inverter. Should the fan run to a small fuse box and then the fuse box runs to the ports on the inverter?

Estragon · January 2019

You wouldn't connect the fan to the inverter at all. The inverter is for AC loads - it changes lower voltage DC (eg 12vdc) from batteries to higher voltage AC ( eg 120vac).

The fan is a DC device. Assuming it's the same voltage (12v) as the house bank, it would connect to the batteries (via a properly sized fuse or DC breaker on/near the positive/red battery terminal).

Some other small loads (eg phone/laptop chargers) may be able to run directly off DC using relatively cheap automotive adapters.

littleharbor2 · January 2019

These are great 12 volt fans. I bought a few at Wal mart for under $20.00each. Run on D batteries or a 12 vdc power cord from 12 volt tap or wall wart.

Image: https://us.v-cdn.net/6024911/uploads/editor/qb/0ggff6dw9upa.jpg

BB. · January 2019

If the fan is 120 VAC and less than a few hundred Watts, any AC inverter rated at ~1,500 Watts or less would not need a fuse/breaker in the AC output (USA wiring is rated for 15 Amps--So is anything plugged into an AC outlet).

For a small RV system, a 300 Watt AC inverter (about 5 amps output), it usually more than enough for most folks.

https://forum.solar-electric.com/discussion/10865/most-efficient-12v-dc-fans

Or go straight 12 VDC fans... The O2 fan is a brushed motor and has limited life. Brushed motors can also make a bunch of radio frequency noise that can interfere with HAM and AM radio.

Get computer fans (ideally ball bearing, and you might find magnetic bearings)--Available in 12/24 and sometimes 48 VDC. There are high volume fans (turn fast, kind of noisy), and other fans that turn at a slower RPM and you can barely hear them.

You can also run many fans directly from a small(er) solar panel for daytime ventilation and avoid the battery/AC issue all together.

-Bill

BB. · January 2019

I should add, that for 12 VDC devices, even a single 12 volt lead acid battery can output 100's of amps into a short circuit... Always use a fuse or circuit breaker from + battery terminal/+ bus on the wire going to the DC device.

Circuit breakers can be worth the extra costs over fuses--If you can use them as an On/Off switch too.

-Bill

ausbran · January 2019

Thanks Bill, tons of great info!!

Simple Power Inverter Question

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