Small Battery Pack Configuration

[KWeirich]

Hello,

I am building a small battery pack and would like to hear the opinions and ideas of the forum. It will provide back-up power for a CPAP along with the usual phone/laptop charging if needed. The CPAP requires 12 volts at a max of 3.14 amps, well within the capabilities of a 18650 pack.

The pack has to be air transportable so that limits the max WH of the pack. I could use a pre-built battery pack with a USB trigger board and (if necessary) a buck-boost converter to provide the steady 12 or 20 volts but:

1. Packs that actually will provide the amperage I need at 12 or 20 volts are still relatively expensive, and
2. I want this to be an opportunity to learn how to build a batter pack.
3. Almost none provide any sort of non-USB outlet

From the research I have done opinion seems to be evenly split on how to configure the pack, either with a 4s2p or a 7s configuration. Both keep the battery within carry-on limits.

Thoughts? Recommendations? Warnings?

Thanks,
Kurt

 

[sunshine_eggo]

Lotsa posts about CPAPs

Search results for query: cpap

diysolarforum.com

When you say 18650, LFP is less common in that form factor than NCA or NMC chemistry. I would encourage you to stick with LFP, and don't lock yourself into a cell type without exploring options.

Recommend you use a kill-a-watt or similar to determine your actual CPAP energy requirements. Mine was somewhere between 0.2 and 0.3kWh with heater.

 

[KWeirich]

Hi! It's been a busy couple of weeks.

My Cpap can run off of 12 volt DC and I have a very good idea of the energy requirements. Since I don't use the heater it is pretty low.
You are correct about the battery chemistry and I would prefer LFP - I was thinking about size more than chemistry. No matter the chemistry I am still have to decide between configurations.

-Kurt

 

[sunshine_eggo]

4S LFP is a great match for lead-acid 12V operating range allowing use of off-the-shelf 12V chargers. CPAP instantaneous power requirements are low, so you don't need to deal with a lot of current.

There is no good way to get 12V with 3.7V Li-Ion chemistry.

A 4S LFP battery with a wide input 12V regulator (8-40V input, 12V output, 10A) to output a constant 12V would be my choice.

 

[Rednecktek]

What are the limitations on the air travel side of things? I've never seen a Lith sticker that didn't say No Airplanes! on it.

 

[DIYrich]

What are the limitations on the air travel side of things?

Amp hour capacity.

 

[Rednecktek]

You may have to make multiple small packs you can combine on site to get the power density you need.

 

[DIYrich]

Thoughts? Recommendations? Warnings?

Thanks,
Kurt

Zopec battery. You can build cheaper yourself. You can get cheaper battery packs. But, if you want some assurance it will work reliably ...

Also has power label in case they really inspect it. Home made could be denied if you get close to the limit and no easy way to prove it is under the limit.

 

[DIYrich]

You may have to make multiple small packs you can combine on site to get the power density you need.

FAA has single passenger limits.

 

[KWeirich]

Zopec battery. You can build cheaper yourself. You can get cheaper battery packs. But, if you want some assurance it will work reliably ...

Also has power label in case they really inspect it. Home made could be denied if you get close to the limit and no easy way to prove it is under the limit.

Thanks for the recommendation. This would be a good option if they had a direct DC outlet jack - I am trying to stay away from a inverter. But it is an option and you are correct about the possibility of an airline objection.

 

[KWeirich]

FAA has single passenger limits.

The FAA limit is 100wh per device. Interestingly from what I have researched the FAA has no limit on the number of devices, but some airlines do have limits.

 

[KWeirich]

You may have to make multiple small packs you can combine on site to get the power density you need.

On of my thoughts was to make the batteries removable and then I could bring additional batteries and a separate charger; rotating the batteries if necessary.

 

[DIYrich]

Thanks for the recommendation. This would be a good option if they had a direct DC outlet jack - I am trying to stay away from a inverter. But it is an option and you are correct about the possibility of an airline objection.

Zopec has dc versions.