I’d like to purchase some lithium ion batteries from Core Electronics and perhaps a dc to dc converter to step down to 12 volts so that I can power my Devilbiss CPAP machines for camping.

I will need to power 2 x Devilbiss CPAP machines which each run on 12 volts and consume 2 amps for 3 nights. Does anyone able to help me work out how many 18650 batteries to buy and what dc to dc converter would step down the voltage down to 12 volts please? I already have one of those kits that enables me to join 18650 batteries together without soldering.

I’m thinking I need the dc to dc converter as 4 x 18650 at 3.7 volts = 14.8 volts. I don’t know how many batteries are required in series so as to power the 2 CPAP devices.

Also I’d like to add to the battery a cell logger to show the voltage on each battery in series which has an audible alarm when the voltage drops too low. Does Core Electronics sell these or something similar?

Many thanks.

Hi Rick,

To power those machines for three nights it will take a 96Ah battery. The 18650 battery has a capacity of 2600mAh at 3.7 volts, if that is converted to 12v (not taking into account the ~80% efficiency of a regulator) then you are left with 800mAh. My back of the napkin calculations would indicate that you would need 120 batteries if the system was perfectly efficient and more likely about 144 batteries. I don’t need to tell you that this isn’t a cost-effective solution.

What you need is a deep cycle battery with a capacity of 100Ah or more. You can get these at your local auto parts store. If these devices are made to run off 12v, and are intended to work in a car/caravan then they are probably tolerant of the range of voltages that a 12v battery will have. From about 13.5V-11V. So you shouldn’t need a regulator.

You could monitor voltage with one of these:

and use a simple Arduino to indicate when the battery charge has gotten too low.

Good luck with your project!

Thanks Stephen. I have a Yeti 400 which is basically a deep cycle battery which works well but I was trying to build a battery that was heaps lighter.

Do the calcs change if I build a battery using 4 x 18650 @ 2600mAh in series 12volts at 80% efficiency = 8.3Wh. If so does that mean my 2 x CPAP (4amps) will run for 2 hours so I will need 24 sets in parallel = 96 batteries? I’m sure my maths is wrong!

Rick Polito

Axsapt - 1300 AXSAPT

www.axsapt.com.au

Hi Rick,

Lets work through it, if each machine runs on 12V and consumes 2A per hour, and you need them to run for three nights (lets say 8 hours a night) then that will be:

12V*2A = 24Watts

2 machines so 48 Watts

48W * 24 hours = 1152 Watt Hours needed.

The 18650 are 3.7V batteries with 2600mah of capacity. Capacity is relative to the voltage output. If you combine four 18650 batteries together you will end up with a 14.8V 2600mAh battery.

14.8V * 2.6Ah = 38.48 Watt Hours

We will ignore converting the voltage for now and focus just on the Watt-Hours that the batteries can produce, since this isn’t dependent on the voltage you are using.

1152 Watt hours are needed to for the three nights.

1152/38.48 = 29.9 sets of four (lets say 30)

30*4 batteries in a set = 120 batteries total needed.

I have been ignoring losses due to the efficiency of the power conversion in my calculations. Thats probably why our results are different.