Converter confusion

[Ian-Vicki]

I suspect that our converter is not performing properly and I'd like to know how to check it. 12-volt wiring is not my best thing!
Thanks in advance for any help you can give me.
cheers
Ian

[Thomas G.]

I'd check its output voltage and the current that it is delivering to the battery. Also measure the battery voltage with the converter unplugged.

THE BEST Multimeter tutorial (HD) - YouTube

Post your results

[Alex C]

Originally Posted by Ian-Vicki

I suspect that our converter is not performing properly and I'd like to know how to check it. 12-volt wiring is not my best thing!
Thanks in advance for any help you can give me.
cheers
Ian

I have disconnected my battery using the disconnect switch which you should have near or on your battery. Plug in a 12V Meter and see what you have. If you have a multi-voltage converter like I do in my Trillium it should read 14.2 volts or greater. When your battery is in the circuit the voltage will drop in 3 stages down to a 13.2V. You can buy a 12v meter at WalMart. The picture is what the voltage is in my trailer without the battery in the circuit. Good Luck

[Alex C]

Originally Posted by Ian-Vicki

I suspect that our converter is not performing properly and I'd like to know how to check it. 12-volt wiring is not my best thing!
Thanks in advance for any help you can give me.
cheers
Ian

If you check my post on "Trillium Wiring" you can see a general wiring diagram of a RV trailer. It's the last diagram. (I made changes to the other ones)

[Paul west]

The converter is much like an alternator on your car. The voltage output of the converter has to be greater than the battery voltage. With the battery hooked up you should read 13 to 14.5 volts give or take. Meters will read differently. Unhooked the battery should read 12v. If you have a dead cell in your battery it will read considerably less. Your lights may still work at lower voltages but not at full potential. Bad converter, bad battery both will aggravate equally until problem is resolved. Like the previous post a meter is essential. Learning to read one can be fun!
Paul w

[Ian-Vicki]

So far, using a multi meter, the battery shows 12.7 volts plugged in, 12.6 not plugged in. Dunno how paul got 14+.
Also wonder how many amps a cpap machines draws and how many nights it id good for on a battery.
cheers
Ian

[Paul west]

The voltage will vary somewhat. I was giving ball park figures. Some newer converters are " smart " varying the voltage according to the charge.
Your
c-Pap amperage can be figured by dividing the listed wattage by the voltage.
Here we are talking 110-120 volts depending on where you live. For example if you have a machine listed at 1200 watts and divide that by 120,
that gives your amperage at 10 amps, 110 would give you10.9 amps. This is just an example. The wattage should be listed on the machine somewhere.

Paul w

[AlanKilian]

There's some information about camping and cpap this site

Their example has a 1.5 Amp at 12 Volts DC power use.

They seem to imply that modern cpap machines all have a DC power input. That would be more efficient than going 12 Volts DC to 120 Volts AC and then having the machine convert it back to DC internally.

Does your cpap machine have a DC input?

[Ian-Vicki]

thanks for the responses. CPAP has 100-115V-1.20A (1.30A max) printed on its label, and I do have a dc connector for it. How do I determine how many hours a fully charged battery will run the machine?
Thanks
Ian

[AlanKilian]

I would contact the manufacturer and ask them how much current it will draw at the 12 Volts DC input.

The numbers you gave are really high. Maybe it has a heater and humidifier that take a lot of power?

Other "Camping with cpap" sites say they draw more like 0.25 Amps at 120 Volts AC.

[Ian-Vicki]

Here's what the manufacturer's website has for electrical info. Even though they are Australian, apparently they write in Greek. If i try to run the CPAP using the inverters I have, they overload. This model CPAP has no direct DC link.
Bigger inverter?
cheers
Ian


ELECTRICAL RATINGS:
Supply Frequency: Supply Voltage and Current:
50-60 Hz 1.2 A, 1.3 A ~ (100-115 V max), 0.8 A, 1.8 A ~ (220-240 V max)
NOTE: These values represent average current.
DC to AC Inverter Requirements: 115 V - 200 W/300 W surge 230 V - 300 W/500

[RogerDat]

Originally Posted by Ian-Vicki

So far, using a multi meter......Also wonder how many amps a cpap machines draws and how many nights it id good for on a battery.
Ian

Your deep cycle battery has an AH or Amp Hours rating, probably on the label. You only want to draw deep cycle down to 50% to avoid premature failure of battery, they don't take well to being drawn below 50% charge so...

If for example your batter is rated at 120 AH you have 60 AH of usable power. Usable AH divided by amp draw give you how long you can run. In this example 60 hrs @ 1 amp draw, 30 hrs @ 2 amp draw, 20 hrs @ 3 amp draw.

You might find it easiest to hook up the CPAP and with everything else off and CPAP running measure the amp draw at the battery with you meter.

That approach will give you a real world measurement of your amp draw, and will take into account any "extra" draw due to inverter conversion inefficiencies or cooling fan, you might need to run CPAP for awhile to trip the inverter cooling fan so you capture that when you measure amp draw at battery.


Then just take the amp draw from your meter reading and divide 50% of your battery amp hour rating by the draw to get hours you can run.


(battery AH divided by 2) divided by amp draw = hours you can run.

Batteries draw down differently depending on size of draw, so this is not an exact calculation. You might be able to support 60 hrs @ 1 amp but not 6 hrs @ 10 amp or vice versa. There are some here who know a whole lot more about those details than I do.


Margin of error would be a good thing, you may not have 100% charged battery, or there could be things such as dehumidifier on CPAP that will have periodic high draw to take into account.

[AlanKilian]

It looks like this CPAP machine draws a lot of power.

Using the numbers you got from the manufacturer:

1.2 Amps at 115 Volts AC = (1.2 * 115) 140 Watts.
DC-to-AC converters are probably like 75% efficient, so let's account for that.
140 Watts / 0.75 = 184 Watts needed input to the converter.
184 Watts at 12 Volts dc = 184/12 = 15 Amps. (This is large!)
If you have a 150 Amp-hour battery and want to discharge it only half-way ...
150 Amp-hour / 2 = 75 Amp-hour
75 Amp-hour / 15 Amps = 5 hours.

After reading other camping with cpap threads in other forums, I think you should look for a different cpap machine if possible.

Look for one with a 12-Volt DC input.
Look for one that draws less than 2 Amps when running on 12 Volts.

If you use the same 150 Amp-hour batter now you get:
150 / 2 = 75 Amp-hours
75 Amp-hours / 2 Amps = 35 Hours of use.

That seems better.

[Carol H]

Originally Posted by Ian-Vicki

So far, using a multi meter, the battery shows 12.7 volts plugged in, 12.6 not plugged in. Dunno how paul got 14+.
Also wonder how many amps a cpap machines draws and how many nights it id good for on a battery.
cheers
Ian

Ian I was taught that once you have fully charged your battery let it sit for 3 hours before taking a reading - the one taken right after charging is likely to be higher and inaccurate. As far as I know the max reading your going to see on a 12v fully charged battery in good condition after resting is 12.7. Readings taken before the battery has rested or taken while the battery is being charged will be higher and don't mean a whole lot in regards to the actual state of the battery. As said thats just what I was taught but its an approach that has worked well for me in determining if & when its time to change out at battery due to its inability to hold a charge &/or fully charge.