Car battery capacity question

[czgunner]

I have a CPAP and I’m heading out on a 4 day deer hunt. My machine’s power supply is rated at 80 watts.

ETA: I have the 12 volt DC cable for it and it draws 6.7 amps.

I’m looking for some guidance on getting a battery (probably a deep cycle) to run this for 8 hours a night without recharging.
We are driving into camp, so I won’t be packing it around.
No generator available.

 

[Bretny]

Is the power supply 120v or 12? Whats the efficiency of your inverter if the power supply is 120v? Most are 80*.

Not factoring in inverter loss you need 53ah per 8hr night. Lead acid batterys really only like being 20-30* used so....this is a job for a generator, a solar system or some other way to charge the batterys every morning.

 

[MattT]

If it's straight DC I figured 53 AH per night so you need to be north of 100 AH for one night to keep the battery above 50%. That's do-able with a large deep cycle but you'd have to recharge it every day with the vehicle.

And like Bret said if you need to use an inverter you'll have to figure the additional AH required to cover it's losses.

 

[theoldwizard1]

The only TRUE deep cycle batteries readily available ar 6V golf cart batteries. Those marine dual purpose batteries are basically regular car batteries with a different label and a higher price tag.

If my life depended on it, I would spend the money for 2 golf cart batteries. Over 200Ah of energy. Some of the ResMed machines with built in humidifiers require at least a 60Ah battery.

(Running any lead acid battery down to 50% will significantly shorten it life.)

 

[tym]

AGM batteries can be deep discharged more than flooded batteries, so might be worth considering by the OP. Might need 2+ of them.

 

[Dozerhand]

Sounds like an excellent time to buy that Honda EU2200i generator you've always wanted. 3 gallons of gas and an extension cord and you're set. Very quiet. Plus you can run a few other luxurys . You know you need it.

 

[mike93lx]

Sounds like an excellent time to buy that Honda EU2200i generator you've always wanted. 3 gallons of gas and an extension cord and you're set. Very quiet. Plus you can run a few other luxurys . You know you need it.

Maybe a local Honda power dealer has an eu1000 for rent. That would be more than enough

 

[kythri]

How long would something like this:

https://www.homedepot.com/p/RYOBI-1...erator-for-18-Volt-Battery-RYi150BG/308460871

Last on a 9.0aH battery, with the stated 80W draw?

 

[mcbane]

If you get a deep cycle battery and want to recharge by idling the car every day, you may need extra fuel to cover several hours of idling each day. Cars these days all have 80-100 amp alternators but they still regulate to 14 volts. That will provide 10-15 amps of charge to a battery at the end of jumper cables.


Sent from my iPhone using Tapatalk

 

[mike93lx]

How long would something like this:

https://www.homedepot.com/p/RYOBI-1...erator-for-18-Volt-Battery-RYi150BG/308460871

Last on a 9.0aH battery, with the stated 80W draw?

The Math is simple. 9 ah x 18 volts = 162 watt hours / 80 Watts = 2 hours, minus losses in power conversion.

This setup necessitates a generator, or a big *** battery bank

 

[kythri]

Didn't know it was that simple - the 80 watt draw is going to be 110V/120V for the CPAP machine, so I didn't know how that played into the equation.

Much appreciated!

 

[mike93lx]

Didn't know it was that simple - the 80 watt draw is going to be 110V/120V for the CPAP machine, so I didn't know how that played into the equation.

Much appreciated!

No sweat. You'll lose a bunch to the inverter... Probably 20ish percent. Assume that 80 Watts is now 100. Way less than 2 hours.

 

[mike93lx]

To the OP, if you are dead set on batteries, you really need to know how much power it actually uses. A kill-a-watt or clamp on multimeter could tell you. Might be a lot less than 80 watts

 

[czgunner]

I just checked the CPAP again and it is actually 12 volts DC at 6.7 amps.
AC adapter is what I was looking at. I have the 12 volt cable for it that I would be using.
I’m sorry for the confusion!

 

[Downwindtracker 2]

Philips Restonic sells a battery they claim is good for three nights. It looks like a AGM. I use a deep cycle Blue Top Optima 24, and charge as needed. The CPAC is 12 volt and I have the cigarette plug adapter for it. A multimeter and the chart from "12 volt life" is your friend. I use a Yami 2400 and a smart charger. Good Hunting

 

[mike93lx]

I just checked the CPAP again and it is actually 12 volts DC at 6.7 amps.
AC adapter is what I was looking at. I have the 12 volt cable for it that I would be using.
I’m sorry for the confusion!

Is that the power supply's rated output or the actual power draw?

Those numbers don't change any of the advice above. Still too much for 4 days of use if that is the actual power draw

 

[Citation]

Is that the power supply's rated output or the actual power draw?

Those numbers don't change any of the advice above. Still too much for 4 days of use if that is the actual power draw

I was thinking the same thing. Presumably the power supply is sized for the peak load, not average. The battery will have no trouble with the peak load requirement.

I would guess... and this is a SWAG... that the continuous/average draw is more like 3A. So assuming 4 nights of 8hr each you would need 96Ah of 12V battery capacity. So a 200Ahr battery would probably be just fine. I would drive it right off the battery, not via a battery -> inverter -> power supply. That might waste 50% of your power.

 

[mike93lx]

I was thinking the same thing. Presumably the power supply is sized for the peak load, not average. The battery will have no trouble with the peak load requirement.

I would guess... and this is a SWAG... that the continuous/average draw is more like 3A. So assuming 4 nights of 8hr each you would need 96Ah of 12V battery capacity. So a 200Ahr battery would probably be just fine. I would drive it right off the battery, not via a battery -> inverter -> power supply. That might waste 50% of your power.

I would be willing to bet the power supply was selected because it met a price point. As you guessed, I wouldn't be surprised if it was 2-3x the capacity actually required.