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palerider wrote:
well, like I said, the converter says it's good for 12 to 24 volts dc input. like most boost converters, it's not that picky about what it eats

I think there is more to it then that. It has some smarts with regard to the battery voltage it expects. Here is a quote from some of the documentation I've read on the converter:

"The converter will shutdown automatically when the voltage drops below 10.5 volts for a 12 volt battery, or 21 volts for a 24 volt battery. This will protect the battery from damage due to being allowed to fully discharge."

So that begs the question, what happens between a maximum 12V lead acid battery charge of say 14.8V and the minimum 24V lead acid battery charge of 21V? The 4S LiPo's sit at 16.8V with a full charge.

GTSSportCoupe wrote:

palerider wrote:
well, like I said, the converter says it's good for 12 to 24 volts dc input. like most boost converters, it's not that picky about what it eats

I think there is more to it then that. It has some smarts with regard to the battery voltage it expects. Here is a quote from some of the documentation I've read on the converter:

"The converter will shutdown automatically when the voltage drops below 10.5 volts for a 12 volt battery, or 21 volts for a 24 volt battery. This will protect the battery from damage due to being allowed to fully discharge."

So that begs the question, what happens between a maximum 12V lead acid battery charge of say 14.8V and the minimum 24V lead acid battery charge of 21V? The 4S LiPo's sit at 16.8V with a full charge.

well, then, I dunno. maybe I misread the label.

Well, the ResMed DC/DC Adapter is enroute. I'll report on my testing with it once I receive it. Fingers crossed it works ok with the batteries, if not I'll use the DC/DC converter I already bought to feed 24V into it.

I tested the real world capacity of the batteries we bought. The are 270Wh each!! At minimum settings without Humidifier I think one battery may be good for 6 nights with the ResMed S9. Will report back once we try it. We have five batteries...so guess he's good for a month off grid!!

I'll post pictures of the final setup too.

Forgot to mention, the batteries weigh in at 3lb each. So 1350Wh at 15lb total weight.

Received the Resmed DC/DC Converter today!

I plugged it in to a bench top current/voltage limiting power supply so I could easily limit the input current (just in case anything went wrong).

Started at 12V and right away the adapter output 24V on two of the pins. (no CPAP connected). With no load, the adapter consumed 80mA of current at around 12V.

I slowly turned up the bench top supply voltage to 16.8V which is the maximum my LiPo battery will ever be. The good news is, the DC DC Converter didn't care!! No increase in current (in fact it decreased a bit as it should). I then turned the bench top supply down to 10V, and the adapter output shut off just as the instructions say it should (to prevent a 12VDC battery from being over discharged). I was very pleased about all this.

I then connected a load to measure power efficiency. I started with a 15 Ohm load resistor to draw about 1.6A (38W). Efficiency was excellent! An incredible 97-98% efficiency (for 12V-16.8V input). I changed the resistor out to 150 Ohm to draw about 160mA (3.8W). Efficiency here was much less at about 71%. Still not the end of the world though. This power draw corresponds with the minimum setting of the ResMed S9 with no humidifier. This will give my cousin 5 days power from one little 3lb battery on the minimalist setting. Great news.

All in all I'm quite happy. Once I receive the Pelican Case we ordered, I'll wire everything up and mount it in there. Will take pictures around that time.

Can you provide a link to the battery?

Sent from my SM-T350 using Tapatalk

CapnLoki wrote:Can you provide a link to the battery?

Sent from my SM-T350 using Tapatalk

I did higher up in the thread, but here it is again. Costs $50USD from the hobby king USA West ware house. Very good deal.

http://www.hobbyking.com/hobbyking/stor ... duct=72325

I bought this charger for it (LiPo's need good chargers!!):

http://www.hobbyking.com/hobbyking/stor ... arger.html

And I bought this low voltage alarm for it, which works great and is very load. Don't want to over discharge a LiPo!

http://www.hobbyking.com/hobbyking/stor ... ouse_.html

I also bought some miscellaneous leads and connectors to put it all together.

GTSSportCoupe wrote:I slowly turned up the bench top supply voltage to 16.8V which is the maximum my LiPo battery will ever be. The good news is, the DC DC Converter didn't care!!

maybe I didn't misread the label

iirc, 16.8 isn't unheard of at times on an automotive charging cycle, so nominal 12v things that are designed to be run from a 'cigarette lighter' need to be able to not blow up with a fairly wide range of voltages.

palerider wrote: iirc, 16.8 isn't unheard of at times on an automotive charging cycle, so nominal 12v things that are designed to be run from a 'cigarette lighter' need to be able to not blow up with a fairly wide range of voltages.

I agree that anything that plugs into a cigarette socket has to be tolerant of a wide range of voltage, but 16.8 is well into the over voltage range for a lead acid battery. The voltage for battery equalization (a deliberate overcharge that rejuvenates flooded batteries) is often 16.2, so 16.8 would probably start killing a battery after a hour or two, Maybe I want a dc/dc converter to generate 12 V for my Respironics. This is why I'm drawn to LiFePO4, which can use the 12V voltages directly.

So how does this compare to other batteries? At 270 Watt hours for $49 they are much cheaper than the packs we've seen. However, they special charger at $45 is required so the real advantage is being able to use multiple batteries to extend time. Compare to the U1 AGM battery (forgetting weight) - the U1 is maybe 300 Watt hours useful, so it about the same, although charging is more straight forward with AGM. Still, the weight difference is huge.

palerider wrote:

GTSSportCoupe wrote:I slowly turned up the bench top supply voltage to 16.8V which is the maximum my LiPo battery will ever be. The good news is, the DC DC Converter didn't care!!

maybe I didn't misread the label

iirc, 16.8 isn't unheard of at times on an automotive charging cycle, so nominal 12v things that are designed to be run from a 'cigarette lighter' need to be able to not blow up with a fairly wide range of voltages.

A 12 volt car lead acid battery's design charge rate is 14.8 volt dc for a max, not 16.8, most voltage requlators correct cars are set at 14.4 volts d/c. This voltage will change with temp by design. Over 14,8, water usage, plate warpage, and shorter lifespans can be expected.

That's why using a stand alone battery charger is faster, than using a cars charging system, for recharging lead acid batteries, they use a higher voltage to recharge the battery, there is a price for high power recharging, it length of battery lifetime. Jim

Goofproof wrote:A 12 volt car lead acid battery's design charge rate is 14.8 volt dc for a max, not 16.8, most voltage requlators correct cars are set at 14.4 volts d/c.

I never said anything about design rated charge, or even proper voltages, you might notice.

palerider wrote:

Goofproof wrote:A 12 volt car lead acid battery's design charge rate is 14.8 volt dc for a max, not 16.8, most voltage requlators correct cars are set at 14.4 volts d/c.

I never said anything about design rated charge, or even proper voltages, you might notice.

Maybe that's why I posted the true facts of how normal auto batteries work.Jim

"Equipment intended to be powered by the receptacle needs to account for intermittent contact, and voltages outside the nominal 12 V DC, such as maximum voltage 9–16 V continuously, or maximum voltage of 20 V lasting 1 hour, 24 V lasting 1 minute, and 40 V lasting 400 ms.[12] An example of protection component ratings tolerance is +50 to −60 V DC.[13] Robust equipment must tolerate temperatures varying between −40 to +85 °C (−40 to 185 °F), plus possible high humidity and condensation of water.[12]"
https://en.wikipedia.org/wiki/Cigarette ... receptacle

The question is do the pumps live up to these standards?

Well, we've had great success! My cousin got five days out of ONE battery. And we bought FIVE batteries! He took some excellent photos. Here they are. Please let me know if you have any questions.

Components

Charging Setup

In Case

Very nice project - thanks for the pictures! The problem that I have always had with Hobby King is their excessive shipping and handling charges - how much did they ding you for the parts that you ordered from them?