What continuous wattage can I get out of a 12v lantern battery?

[Algr]

TL;DR Summary

I'm trying to power a device, but can't find info about the battery.

I have a C-Pap machine, and I'd like to put together something to power it for a few hours if we have a blackout. Wiring two of the above lantern batteries together should produce 24 volts. But I can't find any info on how much wattage you can draw from these before the voltage will sag. It says 7200 mAh capacity. Do I multiply that by 24 to get 172 watt/hours? This would power my (90w) C-Pap for about an hour and a half, which seems low, given the size of the batteries. What continuous wattage can I get out of a lantern battery?

I don't need rechargeable batteries, we have a blackout here less then once a year. The devices that are made for this cost $400 to $1200 dollars.

 

[Baluncore]

Lantern battery = 7.2 A·h, 12V
Two in series = 7.2 A·h, 24V
W·h = 7.2 * 24 = 172.8 W·h
90 W will last for 172.8 / 90 = 1.92 hours .

 

[jack action]

It might be noteworthy to specify this:

One battery with 7.2 A.h & 12 V will give you 86.4 W.h. This could be 86.4 W for 1 hour or 1 W for 86.4 hours as previously shown.

Each additional battery will add 86.4 W.h, no matter whether they are connected in series or in parallel.

 

[phinds]

90 W will last for 172.8 / 90 = 1.92 hours .

Without any drop in the voltage? @Algr, how low can the voltage go before your device stops working?

 

[jack action]

Without any drop in the voltage? @Algr, how low can the voltage go before your device stops working?

Not significantly for the most part. I know that for a car battery the voltage is at 12.6 V when the charge is 100%. Then it goes:

• 12.4 V @ 75%
• 12.2 V @ 50%
• 12.0 V @ 25%

These are the numbers I use to charge my car batteries without overcharging them. The rating used is RC (for reserve capacity) which is actually equal to 1 A.h. So if your battery is 100 RC and has a voltage of 12.1 V (37.5%), you need to "fill" 62.5 RC. So you can either set your charger at 10 A for 6.25 h or at 40 A for 1.5 h.

 

[Algr]

I already tried a different power supply with a quite large battery, and I found that if you try to draw 90 watts from it, it just shuts down and beeps at you. It could last for days connected to the router it was designed for, but did not work at all for a higher wattage device. W = AV only works up to a point. If I draw too much power from a regular battery it might start a fire or leak acid.

So I guess "Peak output" might be the proper term of what I am looking for?

 

[hutchphd]

W = AV

This is always true but as one draws more amps from a battery the actual terminal voltage diminishes and finally plummets. That condition is very hard on the battery and available power also plummets. Peak output is an independent spec, usually to be used for very infrequent conditions.

 

[phinds]

So I guess "Peak output" might be the proper term of what I am looking for?

I don't think so. "Peak output" when it IS possible that it would start a fire or leak acid. What you want is a battery that has a peak power output well ABOVE your required instantaneous power so that you are not stressing it.

If your device requires 90Watts, you should go for a battery that has a peak power output of at least 125 watts.

That is, a battery with 90 Watts peak power output WILL drive your device but you are stressing the battery to its limit and that's not a great idea.

 

[DaveE]

It can be difficult to get a good datasheet for consumer batteries. No one but you and me want or understand that data. 1st step: a manufacturer (RayOVac?) and a part number. 2nd step google. We don't even know the chemistry they've used. This is why EEs look for the datasheets before they buy the parts, not after.

 

[DaveE]

Maybe this one: https://www.rayovacindustrial.com/wp-content/uploads/2021/08/Lantern-Barricade-926.pdf

 

[Algr]

Ah, that looks useful! Thanks Dave!

Their example has an 18 ohm load. E=IR means that is 8 watts, right? Way less power than I need. If that is a typical load, then I doubt 45 watts per battery is going to happen. It doesn't exactly say that, but it isn't looking good. Ugg. It looks like next time we have a blackout I'm just going to have to sleep in my car.

 

[Baluncore]

Google; cpap power consumption
Most CPAP machines use between 30 and 60 watt, but need about 100 watt to start.
https://www.newtechpub.com/sleep/magazines/psnews/power03.html