Thread: What size battery?

My ( secondhand) batteries have both chosen to die lately & I am doing a trip up North soon in a mate's 4WD.
Because I have sleep apnoea I have to run my CPAP each night.
The batteries I've used were both 70 amp & would easily run the unit 1.5 to 2 nights before the low battery alarm triggered ( BTW these batteries might not have been all that good to start with).
The unit is 240 watt @ 240 volt & 1 amp
It even says on the CPAP that it is suitable for a 300 watt inverter
The inverter I have says it uses .3 amp no load, so I assume hourly consumption is 1.3 amp
Am I confusing myself by converting amps @ 240v back to amps @ 12 volt? When I do that I get 20amp usage which it can't be.
I'm thinking of getting a 50 ah Optima yellow top. My next option is to reef a 100 amp that I know is in good nick out of my Landie & see how long it can run the CPAP.
But it's easier to ask here first.

Well I found the answer after a lot of research & I have posted it up here in case it helps someone else. It turns out I had my CPAP cranked up on full heat for the humidifier, the battery I have is perfectly OK - 70 AMP & it ran the unit all night & still was on 12.5 volt after 8 hours or so with the humidifier off.
Here is what helped me...

Power Inverter for Fisher & Paykel CPAP system

I recently received a Fisher & Paykel CPAP system for testing. We did some bench testing to find out what the maximum power usage is for this CPAP system.
The machine we tested was the Fisher & Paykel Sleep Style 200 CPAP System. We tested this system here in our office on our DC power supply. All the testing was done at 12 volts. According to Fisher & Paykel, all of their CPAP’s will run on a modified sine wave power inverter. The specs call for at least a 200 watt power inverter, we used an AIMS 400 watt Modified Sine power inverter for this testing. We connected the inverter with cables, we did not use the cigarette plug. We decided not to use a cigarette plug, for one main reason. The manufacturer specs calls for a 200 watt inverter and a cigarette plug only handles up to 180 watts, if it is fused at 15 amps. If your vehicle has 10 amp cigar plugs, you can only go 120 watts maximum.
The first test was done for about 4 hours with the heater set at 10 and the CPAP set 20 cmH2O (maximum pressure). While monitoring the usage, I observed Fisher & Paykel Sleep Style 200 CPAP System with the heater set at 4 (max), consumes just over 12 DC amps per hour, maximum. We are going to use the max usage of 12.2 DC amps, this will give us an accurate amount to calculate battery capacity needed. Many people may be asking how do I convert DC amps to watts, etc. I will cover that next.
The next test I did with the Fisher & Paykel Sleep Style 200 CPAP System, was with the heater off. The usage is lower, just above 5 amps. We did not observe as much fluctuation in the usage as with the heater running. This makes sense, because it is constantly working to keep the humidity water at a certain temperature, causing the usage to cycle up and down, while maintaining the water temperature. The max DC amp usage, with out the heater on the Fisher & Paykel Sleep Style 200 CPAP System was 5.4 DC amps. This system actually runs at a very low usage, with out the heat and humidity, about 65 watts.
Next I want to review our maximum usage totals and make some recommendations on which power inverter and a battery to use in your mobile system.
We will make one small conversion while reviewing our data.
To convert from DC Amps to Watts, we need to multiply the DC Amps by the voltage of our system, in this case 12 Volts.
DC Amps X 12 = Watts
Fisher & Paykel Sleep Style 200 CPAP System maximum usage with the heater set to “on” at level 4 and pressure set 20 cmH2O (maximum pressure) was 12.2 DC Amps or 147 watts.
Fisher & Paykel Sleep Style 200 CPAP System maximum usage with the heater set to “off” at level 0 and pressure set 20 cmH2O (maximum pressure) was 5.4 DC Amps or 65 watts.
We used an AIMS 400 Watt Modified sine power inverter for all of these test. I direct connected to a battery for the test. I do not recommend that you use a cigarette plug for this equipment. Even though the watts are low enough for the plug to handle the load, most cables for cigarette plugs are not thick enough to handle the load. The cables for cigarette plugs are only rated at 80 watts per UL requirements. So to prevent damage to your vehicle, inverter or CPAP, direct connect to the battery. You will get more efficiency this way also.
If you are using a mobile battery system, for example while camping. You need to use the DC amp usage to find out what size battery is needed to run this system. Lets run over an example, if you are using the Fisher & Paykel Sleep Style 200 CPAP System, with the heater, your usage is 12.2 DC amps. A normal nights sleep is 8 hours, you will need at least 98 amp hours (12.2 dc amps X 8 hrs), to cover your daily sleep requirement. Most deep cycle batteries will give you 80 percent of their power before needing a recharge. I would recommend at least a 105 amp hour battery for this application. On our website we sell the Lifeline line of batteries. Just make sure you have provisions to recharge the battery during the day. For this application if you had the Iota 30 Amp 12 Volt Smart Battery Charger/Converter you could recharge this battery in around 3.5 hours.
For the entire Fisher & Paykel CPAP System product line, I recommend the AIMS 400 Watt Modified sine inverter, per manufacturer specs they require at least 300 watts. I chose this product because an inverter operates more efficiently with a smaller load, the closer you are to max output the less efficient the power inversion. This AIMS product will cover your power needs and give you the most efficiency out of your batteries.

It was a real eye opener on the forums as to how many people run a sleep apnoea machine..