DC Only Power Bank

Snowblind · April 2020

Need help designing 24 volt power bank for extended use CPAP machine. The only use for the bank will be to run a cpap machine and charge a cell phone or two. I began researching and found some battery cabinets and think using series 31 vrla batteries might be a good choice. I can keep a charge in them via house current until the trees come down and we experience total power grid collapse like we do that can go on for days to weeks. The reason I thought a 24 volt system would be the way to go is the cpap machine will run on 24 volts @ 3.75 amps. I have a choice of filling the cabinet with either six series 31 batteries or eight batteries. At a glance this looks affordable. Need advice. Anybody knowledgeable. TIA

Estragon · April 2020

Do you have a generator or other means to charge in a weeks long outage?

It's generally best to keep the number of parallel strings to one or two by using larger lower voltage batteries. Eg 8 x 12v batteries at say 100ah = 4 strings of 2 for 400ah@24v. Doable but attention to wiring layout and careful monitoring needed to avoid imbalances. An alternative might be 2 strings of 4 x 220ah 6v batteries.

Also, if the group 31s are an automotive starting type, they're designed for fast charge and discharge (thin plates for more surface area) and may not work out well if deep cycled.

Snowblind · April 2020

The batteries are 100ah 12v sealed lead acid deep cycle for Solar, RV, Ups, Off grid. I have interest in these batteries because of the cost. I almost lost consciousness when I found out what you can pay for one single battery. My plan for mid crises recharging would be a small Honda generator. It's only purpose would be for off grid recharging of this bank. I'm also lost as to what charger to apply to the batteries.

Marc Kurth · April 2020

What size is your generator?

BB. · April 2020

Have you measured the power requirements for your machine with a Kill-a-Watt type power meter (120 VAC) or other DC Watt/AmpHour meter?

Have you looked at "travel" CPAP machines (know nothing about machine or website, just getting some information):

https://www.cpap.com/productpage/z1-travel-cpap-machine#specs-tab

This one uses an average of 20 Watts (not a cheap machine, and an optional battery sleeve is not cheap either).

Say, just for back of the envelope math:

30 Watts * 8 hours per night = 240 Watt*Hours of 120 VAC power
240 Watt*Hours per night * 1/0.85 AC inverter eff * 2 nights storage * 1/0.50 max discharge * 1/12 volt battery = 94 AH @ 12 volt battery bank

Use a 300 Watt or smaller AC inverter (this one has remote on/off, and "search mode" to save energy when AC power is not needed):
https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

If you do not need the humidifier/"boiler", CPAPs should use much less energy.

I know (believe) you already have a 24 VDC TSW AC inverter(?). How large is it (xxx Watts?)?

Note that even sealed lead acid batteries (AGM, GEL) can/do vent when "abused" (overcharged) as a failure mode as they get old and die... So venting is still a good idea (and installing the bank where fumes will not get into family belongings, etc.).

How many days do you want to run on the batteries between recharging? 8x 12 volt @ 110 AH AGM type batteries seems overkill (assuming you recharge them every two days with a genset).

-Bill

Snowblind · April 2020

The generator on the whishlist would be a compact 2200 watt. We just went four nights no power. Where we live there are more deer than poeple. So when the power goes out we're last on the list!

mike95490 · April 2020

4 golf cart batteries should run a basic CPAP @ 8hours for at least 3 days without charging. (four 6V 200ah batteries in series)
Be aware that when charging the batteries, the battery voltage can exceed 26V easily. In the daytime, the machine is not likely to be running, but it may be expecting a well regulated supply voltage.

Recharge could be daily solar with about 400w of panels, but winter/bad weather you will need a generator to power a 30A/24v golf cart charger for an hour every other day

Marc Kurth · April 2020

Based on your 3.75 amp number x 8 hours, you will use around 30 amp-hours per 8 hour night. You stated that you want four nights of operation between charges, so 4 x 30 = 120 amp hours from the battery.

The next question is how often will you do this? You said "long term" so is this continuous for every day of the year, or is this intended for emergency periods of no power? Battery selection is dependant upon the answer to this.

I suspect that you find that it will be better to run the generator more often, as opposed to selecting a larger battery bank to achieve shallow cycling. Tell us more about what you need.

Snowblind · April 2020

First, Thanks! This is all great information. I'm not opposed to six volt golf cart batteries. I have no inverter, I thought a 24v system would allow me to skip the 110v to 24v transformer marked as 24v-3.75 amp output used with the cpap and just plug the machine directly to the raw 24v from the battery bank. This is probably not possible so the the voltage system could be whatever is most efficient. I do have to opportunity to wire in one of those orange emergency power outlets if I use an inverter. I would like to get at least four nights and perhaps up to seven before replenishing the batteries. I could use the bank as the default power source for the cpap if the batteries need to excercise to stay healthy. Power outages might only be once or twice a year, but it's just so important the machine functions when needed. I had a friend actually die from sleep apnea years ago. I found a cabinet in the store that looks like it would make a good home for the batteries. I have a spot it could live on an exterior wall for venting. So all the other components are up for grabs as far selection. The good news it's from scratch and there's a chance to do it right. After 4 nights without the machine I got the death stare and suddenly realized what my new hobby was. So new questions would be charger for golf cart batteries , bank voltage, inverter, any other stuff I need?

S2T_Bama · April 2020

My $0.02

My wife uses a CPAP when we camp 1/year. She purchased a cigarette lighter adapter from the mfg. that bypasses the inline transformer "brick," because its output is 12VDC. She disconnects the humidifier, and is able to run the CPAP all night with a 12v group 29 deep cycle marine SLA battery. In the morning the voltage is usually 12.2 - 12.3, so pretty lightly discharged. We charge with a 50w solar panel, manually tracking / resetting 2-3x per day. At bedtime the battery reads 12.5v or better. Rinse and repeat for a week.

I hope that helps.

mike95490 · April 2020

Lead acid batteries do best when recharged within 48 hours, or they begin to sulfate, so having a couple hundred watts of PV to recharge them on a daily basis would be good if the power is out. but, if you have power 360 days a year, a 24v trickle charger that plugs in the wall, and a 20A charger for a small generator is likely more affordable.

Snowblind · April 2020

I think this covers all the information I need to move forward. But one last question of coarse, we also have a "brick" inline transformer like S2T_Bama speaks of. Only ours reads 24v@3.75 amp output. Can this also be eliminated and run directly off a 24v bank? If the banks output is 24v@20 amps will the machine only draw 3.75 amps? Thanks to everybody for all the help.

mcgivor · April 2020

Personally I would have reservations about putting my life on on the line of a cigarette lighter connection, but that's just me, wouldn't want to wake up dead. They are not the most reliable of means to connect the least critical loads, just saying, not that I need a CPAP.

wellbuilt · April 2020

I’ve used the morning star 300 watt inverter for 5 years with 2 golf cart battery’s and 400 watts of solar .

It’s plenty of power if you don’t use the heater .

I ran my camper on it , radio , tv lights , charging cordless makita battery’s .
It worked real good 8 months out of the year .
The place was closed up for 4 months , to cold and snowy to stay in a camper .

We got 4” last nite .
You can run for 2 nights and then yo need to charge any way 6 battery’s is a lot

BB. · April 2020

A 24 volt lead acid battery bank voltage runs from ~21.0 volts to 31+ volts (time, temperature, charging/discharging state, etc.)... That is usually too wide of voltage range to run a system that is not designed for it.

Usually much better to have something like a wide range input DC to DC converter, or a DC to AC inverter take the "wide/varying voltage, and safely regulate to the +/- 5% (or up to 10%) voltage range expected by the load.

-Bill

Snowblind · April 2020

I like that! dc to dc converter. Checked some out.. on the list. Girls from 30 miles away came to rake out the dog pen. They just got power back, 8 days without! Suddenly clobbered with 18" of wet heavy snow takes a toll on the grid. Power company still milling around today. But I'll never complain about the area, spent most of my life living in Boston, moved here to the Maine highlands because I like the winter. Home is just "inaccessible" once the snow flys in the winter. Gotta take sleds up a steep hill just to bring grocerys in. -20F for a solid month got my attention. Had to set an alarm for every three hours to make sure someone got up to feed the wood stove. Waking up and finding out it's 16 degrees in the house is something you'll remember for a bit. I guess the only thing that becomes a problem is the cpap. So glad I found this site. I'll start putting this thing together while We waite for the new Bombastic Cyclones or the springtime 100 mile an hour microbursts that flattened 15 - 100 foot pines that lay in place in the back as reminders that Mother Nature is the boss. Power dudes rule!

BB. · April 2020

Get a Kill-a-Watt type energy meter (for 120 VAC @ 15 amp plug-in loads).... When you have the "real numbers", it is much easier to design a system that works right the first time:

https://www.amazon.com/s?k=kill+a+watt&ref=nb_sb_noss_2

And it just a good way to monitor overall energy usage (is the old fridge starting to consume too much energy/$$$, is the condensor covered in dust and fur, etc.).

There are DC versions too:

https://www.amazon.com/s?k=dc+wh+ah+meter&ref=nb_sb_noss

And you can use a DC current clamp meter (really AC+DC Current Clamp DMM) to see what is happening/debugging/understanding your systems & Loads:

https://www.amazon.com/UNI-T-Digital-Current-Capacitance-Multimeter/dp/B0772FYF5M (inexpensive)
https://www.amazon.com/gp/product/B019CY4FB4 (mid-price)

The above are just suggested starting points. Do your own research and ask questions (there are AC only current clamp meters, they are fine meters, but don't do DC current measurements).

-Bill

DC Only Power Bank

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