battery backup recommendations as10/as11

[samsam71]

I've just gone through this exercise and just bought a 120ah Lithium Iron Phosphate (LiFo4) battery as it's half the weight of a AGM battery and you can use the full 120ah capacity before it cuts out. Total power capacity is a full (12.8v x 120a) 1536W
Battery box with 12v, usb and Anderson plugs already fitted.
Total weight of battery and box is about 15kg.

250W solar panel for camp charging.

Have just tested it at home before going camping.
I can run an AS11 with humidifier(4) and heated tube(26C) at 9-12psi for 3 days at 8hrs per night without charge before it goes flat.
If you're charging each day, it takes about 3 hours in full sun to recharge back up from 1 nights use.
I plan to use in conjunction with a camp fridge, so probably 2 full days use without any charge.

[robbob2112]

Which LiFePO4 battery did you get? I've been researching and a 24volt version is pretty ideal with the $15 cable to avoid the inversion/rectification losses.

I've watched a lot of battery teardowns and reviews on youtube and am kinda spooked on a lot of the brands amazon carries because of it.

[samsam71]

The battery I got was a "Kings" which is a cheap Australian camping brand.



But all the reviews have been really good.
I'm sure there are plenty of rebranded copies available in the USA.

The battery case I'm using is like this one, it fits the 120Ah batteries perfectly with minimum space for movement.


Again there seems to be lots of rebranded versions available.

Planning to add a AC invertor onto the top of the box next.

[samsam71]

Found this youtube setup which is essentially what I'm doing, but mine is 120ah battery with 1500W output and 20amp charging.
https://youtu.be/6ftkCfbLJDg?si=5Pnp7-xpX3i7QvNp

[robbob2112]

I asked ChatGPT about the power requirements.  I was running in circles doing the conversions from watts to hours to amps.  Figured the answer might interest others.

Question -   Assume I have a cpap machines that draw 90 watts and a o2 concentrators that draw 120watts .  120vac for that.  Show me the calculations to get to the size lifepo4 battery I would need for 10 hours of run time.


Answer - 
To determine the size of a LiFePO4 battery required to run both devices for 10 hours, we need to determine the energy consumption in watt-hours (Wh) for the devices and then find a battery that can supply that energy.

Here's a step-by-step breakdown:

1. **Calculate the Energy Consumption of Each Device**

**CPAP Machine:**
Power = 90 watts
Run time = 10 hours

Energy required (Wh) = Power x Run time
= 90 watts x 10 hours
= 900 Wh

**O2 Concentrator:**
Power = 120 watts
Run time = 10 hours

Energy required (Wh) = Power x Run time
= 120 watts x 10 hours
= 1200 Wh

2. **Total Energy Requirement**

Total energy required = Energy for CPAP + Energy for O2 Concentrator
= 900 Wh + 1200 Wh
= 2100 Wh or 2.1 kWh

3. **Battery Capacity Needed**

You'd want a battery that can supply at least 2100 Wh or 2.1 kWh. However, it's important to consider a few things:

- **Depth of Discharge (DoD):** It's generally not recommended to discharge a LiFePO4 battery 100% every cycle as it may reduce its lifespan. If you assume a DoD of 80% (common for LiFePO4 batteries), the usable capacity of the battery is 80% of its total capacity.

Usable Capacity = Total Battery Capacity x DoD
2.1 kWh = Total Battery Capacity x 0.8
Total Battery Capacity = 2.1 kWh / 0.8 = 2.625 kWh or 2625 Wh

- **Efficiency & Losses:** Battery systems and inverters have some inefficiencies. If you assume an efficiency of 90% (this is just a rough average, actual efficiencies can vary), then:

Actual Capacity Needed = Total Battery Capacity / Efficiency
= 2625 Wh / 0.9
= 2917 Wh or 2.917 kWh

Based on the above calculations, you'd want a LiFePO4 battery with a capacity of around 2917 Wh or 2.917 kWh for 10 hours of run time for both devices. Consider rounding up to account for potential future capacity degradation, unforeseen power draws, and to give a buffer.



Q - How many amps do I need to be able to draw continously for this?

A - 
To determine the continuous current draw (in amperes, A) from the battery, you'll use the formula:



\[ Current (A) = \frac{Power (W)}{Voltage (V)} \]



You've mentioned that the devices run on 120 VAC, but since you're asking about a LiFePO4 battery, we're looking at the DC side of things. The inverter will convert the battery's DC output to 120 VAC for your devices. 



Assuming you have a LiFePO4 battery at a nominal voltage of 12.8V (a common nominal voltage for these batteries):



1. **Calculate Total Power Draw**



Total power draw of the CPAP and O2 concentrator:

\[ 90W (CPAP) + 120W (O2 Concentrator) = 210W \]



2. **Calculate Current Draw from the Battery**



Using the formula for Current:

\[ Current (A) = \frac{Power (W)}{Voltage (V)} \]

\[ Current (A) = \frac{210W}{12.8V} = 16.41 A \]



Thus, you'd need to be able to draw approximately 16.41 amps continuously from the LiFePO4 battery to power both devices.



However, there are a few things to consider:



1. **Inverter Efficiency:** Inverters are not 100% efficient. If we assume, for example, an inverter efficiency of 90%, you'd actually need a higher current to account for that loss:

\[ Corrected Current (A) = \frac{16.41 A}{0.9} = 18.23 A \]



2. **Safety Margin:** It's a good practice to add a safety margin, ensuring that the battery and inverter can handle occasional surges or higher draws. 


Considering the efficiency and a safety buffer, a system that can handle around 20A continuously would be appropriate.



Q  - How many amps do I need to be able to draw continously for this?

To determine the continuous current draw (in amperes, A) from the battery, you'll use the formula:



\[ Current (A) = \frac{Power (W)}{Voltage (V)} \]



You've mentioned that the devices run on 120 VAC, but since you're asking about a LiFePO4 battery, we're looking at the DC side of things. The inverter will convert the battery's DC output to 120 VAC for your devices. 



Assuming you have a LiFePO4 battery at a nominal voltage of 12.8V (a common nominal voltage for these batteries):



1. **Calculate Total Power Draw**



Total power draw of the CPAP and O2 concentrator:

\[ 90W (CPAP) + 120W (O2 Concentrator) = 210W \]



2. **Calculate Current Draw from the Battery**



Using the formula for Current:

\[ Current (A) = \frac{Power (W)}{Voltage (V)} \]

\[ Current (A) = \frac{210W}{12.8V} = 16.41 A \]



Thus, you'd need to be able to draw approximately 16.41 amps continuously from the LiFePO4 battery to power both devices.



However, there are a few things to consider:



1. **Inverter Efficiency:** Inverters are not 100% efficient. If we assume, for example, an inverter efficiency of 90%, you'd actually need a higher current to account for that loss:

\[ Corrected Current (A) = \frac{16.41 A}{0.9} = 18.23 A \]



2. **Safety Margin:** It's a good practice to add a safety margin, ensuring that the battery and inverter can handle occasional surges or higher draws. 


Considering the efficiency and a safety buffer, a system that can handle around 20A continuously would be appropriate.



Q - Assume I can use a DC-to-DC converter for each device


A - If you're using a DC-to-DC converter for each device, you can skip the losses associated with an inverter. However, DC-to-DC converters also have efficiency losses, albeit generally smaller than those of inverters. Let's re-calculate:



1. **Calculate Total Power Draw**



As before:

\[ 90W (CPAP) + 120W (O2 Concentrator) = 210W \]



2. **Calculate Current Draw from the Battery with a DC-to-DC Converter**



Assuming the DC-to-DC converter is converting from the 12.8V of the LiFePO4 battery to the required DC voltage for each device:



Using the formula for Current:

\[ Current (A) = \frac{Power (W)}{Voltage (V)} \]

\[ Current (A) = \frac{210W}{12.8V} = 16.41 A \]



3. **Account for DC-to-DC Converter Efficiency**



Let's assume a DC-to-DC converter efficiency of 95% (a typical efficiency for a good quality converter):



\[ Corrected Current (A) = \frac{16.41 A}{0.95} = 17.27 A \]



With the DC-to-DC converter, you'd need to draw approximately 17.27 amps continuously from the LiFePO4 battery.



4. **Safety Margin:**



Again, it's a good idea to add a safety margin to account for any unforeseen higher draws or inefficiencies. If you round up, a system that can handle around 20A continuously would still be appropriate.


Remember, when selecting a DC-to-DC converter, ensure it's capable of handling the required current and can convert to the specific voltage your devices need. If your devices indeed run on DC, ensure that the DC voltage requirements match the output of your converter. If they run on AC, then you can't use a DC-to-DC converter and would need an inverter instead.

[robbob2112]

Calculation for a 24v 100ah LifePO4 battery - running the aircurve 10 asv with heater and heated tubing on - 



Let's calculate the runtime for only the CPAP machine, which runs at 24V and will be powered directly from the 24V 100Ah battery without any converters:

1. **Determine the Total Energy Capacity of the Battery**

\[ Energy (Wh) = Voltage (V) \times Capacity (Ah) \]
\[ Energy (Wh) = 24V \times 100Ah = 2400Wh \] or 2.4kWh

2. **Power Consumption of the CPAP Machine**

The CPAP machine uses 90W.

3. **Determine Run Time**

\[ Run Time (hours) = \frac{Total Energy (Wh)}{Power (W)} \]
\[ Run Time (hours) = \frac{2400Wh}{90W} \approx 26.67 hours \]

So, if you power the CPAP machine directly from the 24V 100Ah battery, you can run it for approximately 26.67 hours. 

Again, for battery longevity, it's recommended not to fully deplete the battery on each use. Always good to leave a buffer.

[Crimson Nape]

Have you consulted the Resmed Battery Guide? The 90 watts is the maximum load of the power supply.

Here is the link: https://document.resmed.com/documents/ar...lo_eng.pdf

- Red

[robbob2112]

Good information - I do not see the aircurve asv listed in there anyplace. I figured I'd plug through the kill-o-watt meter tonight and see what my actual draw is and refine the calculations some.

I asked the AI about solar generator/power stations and without heaters assuming 35 watts draw and it came up with a size of 466Whr. I've read a bunch of threads and the watts listed varies from 13watt ~ 40 watts in general.



Of course that lead me down the garden path to a 24kWhr kit with rooftop solar, inverter, transfer switches, etc.... expensive hobby to research.

[HappySailor]

From my measurements on the Resmed S10 and Resmed S11, they use a lot less than what you have asked ChatGPT. The bare minimum is about 90 WattHours per night. Comfortable is 100 WattHours per night.

Your battery 24 v, 100Ah equals 2400 Wh. Which would get you through 24 nights without the need to recharge.

As CrimsonNape already mentioned, the 90 W that you used is the maximum the power supply can supply continuously. It is absolutely not what the Resmed draws.

Lucky for us, we don't need a big heavy battery to get us through the night. Maybe try asking ChatGPT again, but this time with realistic numbers?

Cheers,

Charan

[Nightynite]

My nightly test with a 300 WH lifepo4 battery pack. Dc To DC converter cord. A10 with humidifier Off, no heated hose .


Night / time unit used / percent battery used.


1 - 8:17 - 21 %
2 - 8:13 - 22 %
3 - 9:01 - 23 %
4 - 8:51 - 23 %
11 % battery left.