24v battery for AirMini?

[chriscowles]

I'm researching a battery for my AirMini. A Medistrom Pilot-24 apparently works, and comes with the necessary cable.

I'm curious as to whether a 24-volt e-bike battery can be adapted for the purpose. Other than the being supplied with the connector and cable, what makes a Medistrom 24-V battery different from something like this? Medistrom's specs describe the output as 24v, 3.75A output with a 98Wh capacity. The ebike battery is 24v 12.5A output with a 300Wh capacity.

If I understand how electricity works, the ebike battery's output only gets to 12.5A if the load demands it. It wouldn't cook the AirMini because the CPAP machine wouldn't draw that much. So, if I could come up with a connector, why wouldn't this work? It would have 3 times the capacity of the Medistrom battery.

I welcome your well-intended criticism. Am I missing something?

[Hamguy515]

I'm very new to the xPAP game (about 10 days on therapy now) but given the improvement in how I feel so far, there's no way in heck I'm sleeping without it for even one night if I can help it. Since I'm pretty active and enjoy short backpacking trips and primitive camping on the weekends, I've been trying to (over)analyze how to manage therapy while out in the woods and I've done some research along the way to finally purchasing an AirMini that should arrive today. I also have a pretty deep electronics background, so I appreciate the questions you're asking. Here's my view, as well as what I personally decided to do.

First, the Medistrom (and other similar batteries like the ResMed RPSII) are intentionally limited to <100Wh to ensure they're compliant with current FAA regulations regarding primary lithium batteries in carry-on luggage aboard aircraft. Airlines are allowed to make individual exceptions, but even then only up to 160Wh per battery with a limit of two batteries per passenger in your carry-on luggage. (reference here: https://www.faa.gov/about/office_org/he ... teries.pdf)

Second, there are a couple of considerations with battery assemblies such as this, including cell manufacturer and overall system design - in other words, is the battery designed for high-current/low duration events, or for lower current draw but longer duration use? Using a battery intended for starting applications in a deep-discharge environment is a quick recipe for disappointment due to extremely shortened battery life. This is less of an issue with lithium chemistries as they're generally capable of deeper discharges and higher current ratings than more traditional cells, however, it's still something to consider as it will have an effect on system life. For an xPAP application, we're really looking for deep-discharge capability here. On the subject of cell manufacturers, the battery you linked to claims to be made with Samsung cells. Medistrom uses LG cells. Both are very good quality and should serve well, provided they are genuine cells.

Third, with increased capacity comes increased weight... regardless of how light lithium cells are, bigger = heavier. The e-bike battery in question claims a weight of 5 lbs, while the Medistrom claims a weight of 1.8 lbs. This may not be a factor if you're looking for a home backup, but may be a consideration if you're backpacking, camping, or out trekking. Just know that there is a tradeoff of weight for the increased capacity... and a tradeoff of capacity for less weight.

Finally, but very importantly, you also need to take into account overcurrent protections, whether it's designed into the product from the manufacturer, or you add current protections through proper fusing in your own design. Most manufactured battery packs will include cell balancing and some protection for charging, but not all will include protections for current delivery - that's largely left to the designer/user to incorporate.

That said, I see no reason you couldn't adapt a 24V e-bike battery over to serve your purpose, provided you are happy with the size, weight, and form-factor, and incorporate sufficient overcurrent protections in your design. You are correct that the AirMini will draw only what it needs to from the battery, but you should include at least some fusing on the output from the battery to protect yourself and the equipment in the event of a malfunction.

Now, in my case, I decided to go the Medistrom Pilot-24 route for my AirMini. I'm looking to travel internationally in the next couple of years or so, so the ability to use my machine on the airplane was a huge factor. Additionally, the lighter weight (although reduced capacity) made more sense from a backpacking use standpoint. I did also purchase the 12V DC charger for the Medistrom, and intend to use that charger along with current solar charge controller equipment I have already when I'm off-grid for a more extended period. My power budget will be close, but doable.

All that said, once I have the AirMini in-hand, I also intended to build a boost-converter to allow me to generate the required 24V from existing high-capacity LiFePO4 batteries I have available. These batteries are in the 20Ah to 40Ah range, are very lightweight, and are intended for deep-discharge use (>80% of rated capacity) and over 2000 charge/discharge cycles. I use these to support remote ham radio operations, along with foldable 120W solar panels to keep them charged, so it makes sense for me to be able to use these larger batteries when available.

Hope these thoughts help you in your decision-making.

[DavidY]

I had a Medistrom Pilot Plus 24 with the associated Co-Pilot 24 since April 2018 (Co Pilot was bought later...May or June). In the fall of 2018, the Pilot Plus stopped working (ie. not holding a charge). Sent it into Technical Support for assessment and it was replaced with Pilot Lite 24 since the Plus was discontinued. The problem was a defective cell to my understanding. Looking back, I would not buy the Medistrom setup again if I had the chance due to it's high cost, Co Pilot is useless without the base unit and my Issue. Spending $600-$700 Canadian wasn't worth it IMO. Not sure how long these batteries will now last. Thinking about the Goal Zero Yeti 400 (not the lithium ion model). I don't trust lithium ion for long term durability.

Dave

[chriscowles]

Thanks for the input. I read a thread here by another user who described his setup for a trek. My interest is similar, i.e., backpacking. I'd have to do a lot of research on discharge rates, etc, before I'd consider taking on a project like that. By my question, I was trying to clarify that I wasn't missing something obvious.

I was unaware of the FAA capacity limitations, but it makes complete sense. I am aware that they must be in carry-on baggage, only. I think the Boeing Dreamliner batteries provided a clue about what can happen when Li batteries combust spontaneously. That would be bad in a cabin but at least there somebody can react to it. If it happens in the baggage compartment, good luck. I don't doubt that some passengers unwittingly or uncaringly leave Li batteries in their checked bags. I will not be surprised if it results in disaster. Hopefully it will be only a scare, not a tragedy.

Other than the Medistrom Pilot-24, are there any commercially available batteries that the manufacturer describes as compatible with the AirMini? Does anybody other than Medistrom manufacture the connectors?

[gork]

Hello, hamguy515. I am powering an Airmini from a USB C PD power bank using a MakerHawk USB PD cable which triggers the power bank to supply 20V. I then use an XL6009 boost converter module to supply 24V at a fairly high efficiency. This same boost converter would probably be very suitable for a LFE pack in the 12-18V range, though I would suggest adding a HLVD if you are using the pack bare without a charge or load controller. I measured average consumption at about 4W at 10cm H2O.

I have a post about the USB C solution.