WARNING to CPAP Users: PREPARE for the Unexpected - When the power goes out

[GWild]

Gratz! Sounds like you are in business.

[archangle]

I just got back from the Marine store. I bought a west-marine--15-amp-automatic-battery-charger-with-engine-start-timer--14982045
and a west-marine--gel-deep-cycle-marine-gel-battery, with 97 amp hours at 12v--P015020365

Archangle, did I get the right stuff? That is the charger the very experienced salesman told me I needed for my battery, and we even went on to a cpap site, and I showed him exactly what I was using it for.

Note - I see you took it back, but the info might be useful to someone else.

Yes and no.

The specs on that charger are good, however, I STRONGLY recommend you never hook the charger and the CPAP machine to the battery at the same time. This applies EVEN IF you unplug the charger. It mentions "high frequency technology," which is probably a "desulfation" circuit. This sends high frequency high current pulses to the battery, which might do strange things to your CPAP.

Some of these desulfator circuits run off of the DC power and will run off of the battery, even if you unplug the charger

The charger might do the right thing, however, it has to do the right thing 24/7 for a couple of years. I'm more comfortable with the Schumacher SE-1-12S because I've got 20 or so "battery years" experience. Also the SE-1-12S only puts out 1.5 amps, so even if it "goes nuts," it won't kill the battery as fast.

In my opinion, the Schumacher SE-1-12S is the only choice for a 24/7 maintainer. I can't say others are bad, but I know this one is good. I also know some other "trickle" and "maintainer" chargers do eat batteries over a period of months or years.

Supersleeper, I took the 15 amp charger back to the marine store, and I bought a 1.5 amp, 12 volt battery trickle charger/tender, at the auto parts store. I also purchased the proper cable for my machine from a DME shop. Might buy second battery and tender soon. The things are as heavy as f*ck, but it is comforting to have one/them.

What model did you get? Some are good, some are bad.

[archangle]

As far as the common lead-acid batteries (either sealed or the type where you can add distilled water), you CANNOT discharge them much below a 70% charge numerous times without completely damaging the battery. Again, we're talking about the kind of batteries that are like car, boat and UPS batteries (commonly lead-acid).

I'm beginning to believe this is a lot less important than we think it is for CPAP backup batteries. I'm reading some sources that say that a lead acid "starter" battery will be good for 30 or more deep discharge cycles.

If this is really true, us CPAP users are probably unlikely to do much damage to a starter type battery if we're using it for power failures. All lead acid batteries die eventually, even if they're never over discharged. You'd need to use the backup battery 30 times in this time period, and you'd also need to use it long enough off AC power to get into the "deep discharge" range.

I need to see if I can dig up better data on the number of deep discharge cycles it takes to damage various lead acid battery types.

Yes, deep discharge is important if you're doing something like running a forklift, electric wheelchair, or trolling motor and your use it every day or even every week. I'm not sure it matters that much for power backup usage for us pappers.

If the starting battery is a lot cheaper, it might not be a bad idea.

[GWild]

Not a chemist so can't explain the reasons behind why some LA batts last 30 cycles and some 5000, but the average car battery can survive a few 100% cycles. Ever had a bettery go dead, push start or jump the car and the battery snaps back? The battery loses some capacity, but rarely fails abruptly. Most car batts die from overcharging due to constant 14.5 volts applied rather than an intelligent float level nearer 14 volts.

The reasons against wet cells, especially car batteries, is the self discharge rate, and safety. AGM's are better, and gels are best, for their LA type. But having to maintain a wet battery (keep the electrolyte channels full), and that they generally don't have gas recovery caps, means a lot of work. A few bucks more buys an AGM... and off-loads a lot of work keeping the battery healthy... not to mention they are generally sealed and keep the gas inside - some even using gas recombination technique (oxygen and hydrogen are very flammable, so a non-sealed LA car battery inside a home on a charger is a bad thing).

Suffice it to say a 20 or 30 AH AGM is a fair choice for emergency use... at least enough time to get to a motel. And 50AH is a bit better and likely allows for a several days. A 100AH gel, well, it's the Cadillac (or Aston Martin if you are of that persuasion).

Arch is right about use one case: using any charger while connected to the machine is a clear violation of logic, and a bad idea.

The method of constant desulfation - the theory anyway - is to use a pulsed DC circuit for controlling charge current during float. Well, most chargers were once analog brute force with little or no control of voltage or current. Then came 'smart chargers' with switching supplies. High current filter capacitors and inductors are expensive, and since a battery has a pretty solid ultra low ESR, these parts are almost pointless unless the exact battery load is known. So why not market the ripple current as a feature?

I recall some tests where it helped, some where it didn't. All depends on the battery construction. In the end, controlling the charge voltage versus temperature has a major impact on battery life, and that's where you want to spend money to get the most out of a set of batteries. That's why you see three stage chargers.

So what is a good three stage? One that allows you to set the absorption and float conditions. Absorption V, C/rate Switchpoint, and Float V are the main parameters. I haven't seen many charges that allow for setting the critical switchpoint; most just use their own peak charge as a reference. It means small batts get overcharged and large batts get undercharged. And few have any type of intelligent temperature control of charge current. How hot the battery gets during charge is crucial to keeping it healthy.

Anyway --- batteries are not simple devices.

[GWild]

Thought a few curves would be handy... they should be self explanatory.

A midrange but good deep cycle AGM (5yr warranty):
[attachment=1526]

A high end deep cycle wet cell (7yr warranty):
[attachment=1528]

A very high end deep cycle wet cell (10yr warranty):
[attachment=1527]

And for grins, a common 1500mAH Lithium Ion battery pack for a cell phone:
[attachment=1529]

ps: cheap wet cell deep cycles don't come close to these specs... and car batts from xyx auto are even worse.

[GWild]

And, per an un-common knowledge rule, you want a minimum charge of C/10 (temp corrected, of course)... so your 105AH battery wants at least 10 amps during bulk charge phase.

By the way - the above is written incorrectly -- optimal charge is supposed to be 10% to 25% of the C/20 rate. For a 100 AH battery the C/20 rate is 5 amps (20 hours at 5A = 100 AH) -- so charge with 0.5 amps up to 1.25 amps for optimal capacity without boiling too much electrolyte away.

...

[GWild]

And, in another thread someone pointed out the newest tech LiFePO4 battery type...

This is a 12V 26AH version:
!! claimed 1,700+ cycles at 100%, or 8,000+ at 30% !!
[attachment=1534]
... now a drum roll please: ... ... it costs $473.00, ouch!

[Sleepster]

We get power outages that lasts about 10 days. One in 1983 (Alicia) and another in 2008 (Ike). Those are rare enough that if we have another I will rely on my car battery. I'll take it out of the car for the night, then put it back in and charge it up the next day. I'll also buy a new battery for the car to have as a back up.

I have boarded up the windows in preparation for a storm maybe once every ten years. The next time I do it I will include a new car battery in my shopping preparations.

[archangle]

Thought a few curves would be handy... they should be self explanatory.

Good info. Got any similar info for cheap car batteries?

[GWild]

I've looked - but found nothing recently for car battery charge-discharge cycle life.