Battery backup story!

[wlk]

This weekend I went tent camping with our Webelos 1 Scouts. I am a den leader, and I take my CPAP on all overnight excursions. I have shown the system to the boys, and explained how it worked. I put together my own battery backup system, and it has worked well for 1 1/2 years. I use a small fused inverter. I know I can power the Respironics on 12VDC, but I like the added isolation provided by the inverter. And when the battery voltage drops below around 10.5VDC the inverter sounds a shrill alarm that wakes me up. Two weeks ago I purchased a larger battery than I have been using. When tested at home, the battery lasted three nights with capacity to spare. I recharged the battery, and took it along for the weekend.
All was well until 3:30AM on night one, when the battery failed, and the inverter woke me up. There was not much I could do at that time but listen to the coyotes, and stay up and read a novel until 5:30AM rolled around. I had failed to take the old battery along!
Well....time to adapt, improvise, and overcome!! We took a battery out of an F-150 truck that one of our other leaders owns, a huge set of jumper cables, and hiked back to the campsite. I placed the battery in a garbage bag so it could not touch the tent floor, connected the jumper cable, and then connected the other end of the jumper cable to the inverter input leads. The inverter is fine, the CPAP is happy, and I had a wonderful nights sleep!
When the battery was placed back into the truck, that vehicle fired right up! A happy ending to what could have been a ruined weekend.

[GumbyCT]

I guess using 12v would have just been way to easy.

[blakepro]

Doesn't the inverter just end up sapping more energy out of your battery more quickly?

You are essentially taking 12 volts DC, inverting it up to 110 AC, then plugging your machine (or power brick) into the inverter to convert the 110 AC back down to 12 volts DC so your machine can use it. That whole process actually uses quite a bit of your battery just converting from DC to AC to DC.

If you were able to connect your DC power directly to your machine, you would get a dramatically longer battery life. If you are concerned about power fluctuations, you could put an in-line filter, fuse, or surge protector between the battery and your machine..

I've gone camping with my machine a few times and have had it die in the middle of the night on me before.. man is that a bad situation! Mine died and I just woke up gasping for air because I had the hose hooked up to my face with no air coming in. Was wasted all day because I didint get any sleep after that.

[Gerald]

WLK.....Each to his own.....but, I think you'd be happier using straight 12v. I realize that you believe it's better to use the inverter, but I can think of no reason why operating that way is beneficial. During the last hurricane, I got (2) nights out of a 35-AH wheelchair/scooter gell-cell type battery. Although I think it would have made another night, the power came on....and I didn't have to try it. My compliments to you on the F-150 improvising solution....git-r-done!

Gerald

[SuperGeeky]

Congratulations WLK on being a Den Leader and being able to enjoy Camping with your Den despite a Handicap.

I had the Black and Decker Electromate 400 all set to go camping with our Cub Scouts this past weekend. After both my Son and I became sick with the flu, I decided to bag it. Good thing, it was chilly and windy for central Georgia.

Our Pack Leader from Maryland wouldn't take his CPAP camping with him. I would offer my battery to him and he would just laugh. I guess everybody has different severity of the illness. I wouldn't be caught dead without it

Yours truly in Scouts,

SG

[timbalionguy]

No need to use an inverter between your battery and the machine. The DC out of a battery is the cleanest power there is, cleaner than even the best inverter. The only device that should be in the wiring between the battery and the machine is an appropriately sized fuse, located as close to the battery as possible (putting the fuse close to the battery minimizes the risk of a wiring fire should something short in the power cable).

The only reason to ever use an inverter is if you need the humidifier. This also triples or quadruples the required power out of the battery.

As far as the battery that failed, I have had that happen to brand new gel cells. There was a marginal cell, and one charge-discharge cycle killed it. This was an early gel cell in a Sony portable color TV.

I use a 38 AH gel cell to power my machine in the field, and so far have gotten two nights out of it with power to spare. USe the same kind of battery as a 'standby battery' at home in case of a power failure (and we had one I had to respond to last night, thankfully at work and not at home).

[kennethryan]

Doesn't the inverter just end up sapping more energy out of your battery more quickly?

When I start planning out an electronics system I'm about to design, I use a rule of thumb of 70% efficiency each time
there is a voltage conversion, 85% if I can afford a very high-quality converter (more complex, more expensive).
For DC to AC conversion only the very highest quality inverters are in the 70% range, most are worse (especially
if it''s a "stepped approximation" output rather than a "pure sine" output - you lose a *lot* of power in the
approximation converter).

The other posters are right - if you don't need a humidifier you're probably giving up 1-2 nights' worth of power
by using the useless conversion.

I know I can power the Respironics on 12VDC, but I like the added isolation provided by the inverter.

It's highly unlikely you're getting any real electrical isolation in your inverter. Unless of course you're also hauling a small
isolation transformer (probably 10-12 pounds by itself). (IIRC UL listing requires isolation on the hot side, but not the
neutral side; doesn't help against a lightning strike or miswired outlet for example)

Also, it's rare but if the inverter fails (overheats or has a latent defect) one of the possible failure modes could leave the
power transistors both turned on, effectively giving your battery a dead short. That can be quite exciting, including
starting a fire (gel cell) or "explosion" (car battery - not really an explosion, it's the water/acid violently boiling and
bursting the battery case). If you ever read about a house or business fire that got started by a computer UPS, that
is the mechanism.

For efficiency, carry weight, and safety I'd stick to the straight DC every time!