Cable size to minimise voltage drop?

Agree with RMD, I've been doing it this way for 20 plus years. CPAP works fine, I can also run/charge phones etc with the 240volt into my tent.

From memory, the CPAP has a 240VAC and 12v input (typing this up at work).

If I remember my electronics theory from way too long ago, it uses a full wave rectifier and regulates the output to 12.x volts DC out which then plugs into the CPAP.

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Siringo, if you are right about that, then taking the path of bumping your 12v battery up to 230vac via an inverter, then putting the 230v into the CPAP 230vac-to-12vdc converter will incur two lots of losses. Then you have the cost of the inverter.....
Clearly, the CPAP is designed to run on 12vdc..... why not use that?

My comment above about inputs is totally incorrect.

I found a tech guide. The CPAP converts 240VAC to 24VDC at 3.75 amps via an inline adapter.

We purchased an adapter so we could run it off a car battery, the adaptor must have a step up transformer in it.

The CPAP has a single DC input.

So am I better off placing the adaptor near the battery or the CPAP?

I'd think near the battery, with the thought that voltage drop my be less pushing 24v through a length of cable rather than 12v?

Any thoughts?

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I would still use 8B&S cable and place the adaptor near the battery.

Hi Allan,
How do you work out the voltage drop?

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Peter (below) has provided a formula but here is a online simple calculator which is adequate for most practical purposes. With this one, you nominate the cable size you are considering and it will provide the voltage-drop. There are others that require nomination of an acceptable volt-drop and the calculator provides the required cable size.

The value of "acceptable volt-drop" can vary by user tolerance and appliance requirement but 0.2v would be OK for most situations. It is 1.6% of 12v.

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I would not consider using an inverter to be a useful solution if you already have a CPAP that can operate from 12vdc. It is a lossy and expensive solution.

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Peter, it's also "pretty worthless" if you cannot nominate how much voltage-drop is acceptable.

What voltage drop is acceptable varies with the appliance being powered.
You or I can not specify that. The product specs or manufacturer should.
Cheers,
Peter
OKA196 motorhome

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Your'e right Peter, on both counts.

This thread illustrates the futility of trying to respond to a question with false or inadequate information!
You just get led up the garden path. Think I'll go back to my solitaire app. lol

You also need to know the voltage starting point, which can very significantly depending on the batteries and other loads.
Cheers,
Peter
OKA196 motorhome

I expect Siringo already has the cpap he intends to use, but for those reading this in the future if you know you will be travelling take a look at the Devilbiss Blue. Runs natively on 12v . The ‘natively’ is the important part - no internal voltage conversions & no need for an inverter. Different technology humidification to any other make. Result is a machine comparable to the market leaders which uses far less power. Using humidification I average around 20 to 25Ah per night.

Siringo
If you had the fridge set LOW it will draw more out of the battery than if set higher. So what is "hardly ticking over"? Whenever the fridge starts it will be drawing the same current each time, ie, same tick over, and ALSO at startup the initial loading will do a sudden voltage drop for an instant. The CPAP will detect that lower voltage as the fridge starts and if at critical voltage it will drop out when the battery dips to a lower voltage. One item or the other will possibly be OK, but both is asking a fair bit when considering the total load. Allowing your deep cycle to discharge to below 12 is going to shorten it's life. If done a few times, although you think it is 105ah, it may now be operating at say 80ah max, ie, possess less total capacity, and that will definitely exacerbate your problem. If the CPAP is inverter based power from 12v source, as the battery voltage lowers, for whatever reason, the CPAP WILL draw more and more 12v current to attempt to continue making it's set working voltage to operate. That increase will hasten the discharge. You haven't got enough battery reserve to do both items it seems. A105 ah battery is, if your lucky, only a 105 ah if in top condition AND you know for sure it was charged to the max before nightfall. If the fridge dragged off it for a while beforehand, pre Cpap. then 105 goes out the window.

There you go Siringo. You haven't got enough battery. :)
Dave.

Thanks RMD, that has cleared up a few things. I appreciate your input.

The Danfoss compressors will continue to run down to 10.5V.
Cheers,
Peter
OKA196 motorhome

Theory sounds good, but in reality you should have more battery capacity. After one night I'd like to have only 20% of battery capacity used, maximum of 50% after 2 nights.

Lithium can be discharged lower, but you still have to recharge with something, and lithiums can accept higher recharge currents, so you can have more solar panel sizes, which means more money for more solar because you used the lithium for longer than an AGM....vicious circle sometimes. Nothing that money can't fix...if you have it !!

Thanks Gonk. I wanted a 2nd pair of eyes on my workings. Now I can work out which way to go.

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Siringo,

Gronk is right. Your assessment is based on an ideal battery, fresh out of the box, fully charged with no other discharge than the CPAP. And the certain ability to recharge it fully before the next CPAP demand.
The reality is never as good as our aspirations.
Try it out, But be prepared to upgrade!

Yep, you're right Allan, but we can only work out these figures based on 'advertised' known figures. I'm also using worst case consumption figures for the CPAP, so it's kind of swings & roundabouts.

Lithium recharges faster than other battery types so I'm expecting to be able to recharge daily either via solar or driving or both.

Thanks again.

Seeing the CPAP is basically to keep you breathing which is more healthy than not, I would be making sure some insurance/over capacity in essential items/components is present, so next day Breakfast is achieved. Borderline figures with all this stuff is probably not a good idea, as Gronk mentioned, it is only money to make it right.

Thanks Andrew. I'll see if I can find some more tech specs for our CPAP, I only found it's typical consumption figures. Thanks for the humidification tip.