Using a 12 volt to 24 volt DC converter from a 12 volt accessory socket.

Hello all. I am looking for some info re a DC to DC converter.

I have a part share in a narrowboat and also have sleep apnoea for which I use a CPAP machine while sleeping. In the past it hasn't been a problem as my previous machine was rated 40 watts at 23o volts for which I used a 300 watt pure sine wave inverter from a 12 volt accessory socket.

Unfortunately my new machine is 24 volt and the manufacturer doesn't recommend using it with an inverter.

They make a 12 volt to 24 volt converter which is rated at 90 watts. We have four domestic batteries of 110 amp hours each which would have very little load, other than my machine, on them overnight - The fridge is on two separate 110 amp batteries.

My questions are: What size fuse would the 12 volt accessory socket need?
Would there be any need to upgrade the cable from the battery to the socket?
Would the four batteries (fully charged) have enough charge to handle the converter for up to 8 hours?

I would be grateful for any advice you could give me.

Roy

Welcome to the forum.
You would need to know how much power your new machine takes.
The voltage converter would introduce some inefficiency and so raise the current required hopefully not by much but it could be significant so if there is an efficiency noted on the converter you would need to take this into account..
The converter being 90W just means it can supply up to 90W, but it is the machine rating that will determine the current drawn.
If the machine were say 90W then a 10A fuse should be fine.
Again with the power over time you would need to know what the machine takes.
If it were 90W then it would take about 8Ah over 8 hours and a single 110Ah battery would probably manage but would be dropping it to reasonably low charge, with four batteries there should be no problem so long as they were recharged later.
The cable size is something we do not know what you have so cannot advise but it is likely to be OK if it was running a similar machine before even though it would have to be a good quality accessory socket to take 8A.
I would expect the machine would be similar to the 40W of your previous machine.

Hello Richard, thank your for your welcome and your prompt reply.
Below is some information I have just received from another user of the same CPAP as myself:
On the ResMed Airsense 10. It sits at about 2-5W on standby (varies I think due to the cellular modem* handshaking on occasion) and runs at around 85-95W at full tilt with the humidifier cranked up and containing cold water. Once warmed up, it drops back to around 35-45W normal usage with the occasional burst to 85ish watts to keep the water warm.

The converter I am thinking of buying is made by ResMed specifically for my machine.

I only use the humidifier set to 75% so am I correct in assuming that taking into account the measurements above the maximum draw would be about 70W.

*The machine sends info on my sleep patterns back to the sleep service at the hospital.

The humidifier would very likely cycle on and off giving either max load or off.
As the converter is supplied by the company then it should be OK in use.
The load specified of 90W at 24V (3.25A) would take a current of 7.5A on the 12V side this should be OK with the batteries you have but keep them charged regularly.
I am not sure the accessory socket will like taking 7.5A continuously as the contacts are not very good, most of the standard grade plugs are fused at about 3-5A, but good quality ones should be OK. If you have the option to connect via a different type of higher current connector that would be useful.

We have a 55 hp engine with 2 x 110 watt alternators charging through a combined Sterling (I think) charger/battery management system, which automatically switches to charging from shore power if we can get a hook up. We have a 230 volt ring which is activated on external hook up.
We have 8 batteries (all 11o amp), 4 x domestics, 2 x inverter (which is very old technology and has proved to take too much of a load to run my previous equipment through the night), 1 x starter and 1 x bow thruster. We try to never let the batteries go below 50% (we have 2 x smartguage monitors for the domestics and inverter batteries). The fridge normally runs off the domestics in the day but we can switch it to the inverter batteries when we finish cruising, leaving the domestics for mainly my CPAP.
The batteries normally charge from 50% to full if we cruiser for 5 hours (pretty much our average day's cruising).
I will try and find out what fuse is in the accessory plug.
When you say it would be better to connect to a higher current connector - is there anything you could suggest.
Thanks again for you help with this.

You would probably find the domestic batteries were to about 75% after a night, taking into account some estimate of losses, but assuming a full load from the CPAP all the time, which from your report above will not be the case.
You would probably know better than I what plugs and sockets are available for caravans, etc. The Hella plugs ( a bit like accessory sockets) are rated to 10A and the so are the two pin parallel clipsal ones.

Thanks again Richard.
I will look into the Hella plugs and let you know how I go on.

Hi Richard, I've spoken to the guy who installed the socket and it is fused at 10A so hopefully it will handle my set up. I will be on the boat next week so I will let you know how I go on when I return.

Some good info there from Richard.
FWIW if the 95W maximum load is accurate, I would work it as follows:
Inverter efficiency worst case approx 85% (many DC-DCs are now achieving >90% over much of the load range)
Lowest normal battery voltage 11V if it was used heavily or didn't get charged, and then reached the end of its normal dicharge curve by the time the CPAP machine was switched off in the morning.
Typical voltage drop to accessory outlet: 0.5V
Power into converter =95/0.85=111W
Load current = 111/(11-0.5)=10.6A

As for the average consumption over a night's use, one would really need to know what fraction of the time the heater runs for, and that may depend on other factors such as ambient temp. If we said it ran at 95W for half an hour and then spent 3/4 of its time at 40W and 1/4 at 95W, it would use:
(2.125*95+4.875*40)/0.85= 400Wh approximately.
At a typical battery voltage of 12.5 the capacity required would be 400/12.5 = 32Ah.
If you aim to use your domestic batts to no more than 50% depth of discharge, i.e. 220Ah cyclic usage, then the CPAP will account for about 32/220 = 14.5% of your available capacity.

FWIW I have a similar battery configuration with 440Ah domestic / fridge / inverter, 160Ah navigation / essential services and 160Ah engine starter. The bow thruster runs on Fullers ESB and in the event of electrical failure she can take over the role of engine starter too.

Thanks for the info Lucien. I think that your estimate of CPAP/humidifier usage wouldn't be far off.
I would only be running the CPAP off the 440Ah plus the fresh water pump and lighting (even though we are all LEDs we are still in the habit of only using lights sparingly}.
The fridge is switched at night to the two inverter batteries and the inverter would be switched off.

Apologies for the long delay in replying - I couldn't find the email link to this post. Both of your estimates of usage were pretty much spot on. I used the cpap for 7 nights and the usage was between 25% and 30% (depending on how long I slept each night). Thank you again as I can now use my cpap without worrying about damaging the batteries.

Thank you for the feedback, I am glad it is all OK.