Running a figjam swamp cooler off 12V D battery pack

[naos]

At my last playa stay I ran my figjam swamp cooler off a deep-cycle marine battery, which was great. Don't have that battery anymore, and a new one is expensive and heavy, so I'm wondering: how long could I run the cooler off an 8-pack of D alkaline batteries? I've been googling around, but I'm a total electronics noob, not sure how to calculate it.

Here's my math:
- My pump draws 500mA and the fan draws 1.6 A, wired per figjam's instructions (parallel?) for a total of 2.1A (?)
- The 8-pack of D alkaline batteries totals 12V (?)
- Eveready's D batteries are rated at about 10,000mAh at a draw of 500mA (http://data.energizer.com/PDFs/E95.pdf). Soooo: guesstimate a quarter of that mAh when drawing 2.1A = a little less than 2500mAh?
- 8 D batteries wired in series are still ~2500mAh. (?)
- So one 8-pack of D batteries will deliver ~2.5A of power for one hour, powering my 2.1A needs for a little over an hour?

Is that right? If so, better get a new marine battery.

[MrBeardy]

Can't speak on behalf of your rocket science calculations, but wanted to pass along a tip I saw posted somewhere. If you leave near a place that supplies wheel chairs, they often get people coming in needing to change out a bad battery and they are required to change out both despite if one is still okay. The poster mentioned they are often willing to part with these rather than send them to landfills. Just something to think about in case it would work for you. happy burn!

[naos]

Can't speak on behalf of your rocket science calculations, but wanted to pass along a tip I saw posted somewhere. If you leave near a place that supplies wheel chairs, they often get people coming in needing to change out a bad battery and they are required to change out both despite if one is still okay. The poster mentioned they are often willing to part with these rather than send them to landfills. Just something to think about in case it would work for you. happy burn!

Wow, great idea! Thanks so much, I'll check that out.

[SnowBlind]

My pump draws 500mA and the fan draws 1.6 A, wired per figjam's instructions (parallel?) for a total of 2.1A (?)

That's correct.

The 8-pack of D alkaline batteries totals 12V (?)

Correct.

Eveready's D batteries are rated at about 10,000mAh at a draw of 500mA (http://data.energizer.com/PDFs/E95.pdf). Soooo: guesstimate a quarter of that mAh when drawing 2.1A = a little less than 2500mAh?

The datasheet doesn't say, but that seems like you derated it too much. My gut says it wouldn't go down to a quarter. But then, I don't really know. Possible that D cells aren't too happy with such a high draw.

You could try it out, put a 2.1 amp load on a single D cell and see how long it lasts.

8 D batteries wired in series are still ~2500mAh. (?)

Correct, cause you add the Voltage, so the Capacity stays the same. (When in series. In parallel its the other way around)

So one 8-pack of D batteries will deliver ~2.5A of power for one hour, powering my 2.1A needs for a little over an hour?

As I said above, you might have derated that too much. But even if they still had 10,000mAh, it would only last for 4-5 hours.

You could put four 8 packs in parallel, that way each 8 pack only gets hit with 500mA, so you would get the full 10,000mAh from each, giving you 40,000mAh out of 32 Batteries, for 19 hours of cooling.

OTOH, at some point a lead acid might simply be easier.

[naos]

As I said above, you might have derated that too much. But even if they still had 10,000mAh, it would only last for 4-5 hours.

You could put four 8 packs in parallel, that way each 8 pack only gets hit with 500mA, so you would get the full 10,000mAh from each, giving you 40,000mAh out of 32 Batteries, for 19 hours of cooling.

OTOH, at some point a lead acid might simply be easier.

Thanks so much, SnowBlind. Nice to know my fledgling elec thoughts aren't completely off the mark. : )

4-5 hours is actually an interesting amount of time, would cover the hottest part of the day. I'm out of town now, when I get back home I'll have to try it out.

[FIGJAM]

Yes, test this idea thoroughly.

Don't get caught short on the playa.

[naos]

To follow up:

I ran a test of an 8-pack of D Duracell batteries with my swamp cooler setup. It wasn't a perfect test -- I didn't attach the ducting that would run the air into the yurt, which might put extra strain on the fan (I don't know much about fluid dynamics except that pushing air through a tube takes extra power). But close enough.

The cooler ran for 6 hours solid at what seemed like full power. Still going at 7.5 hours, but with significantly less power. So I think an estimate of 6 hours is reasonable, could theoretically get me through the hottest part of the day. For a short 3-4 day trip, might be a good solution. But for ten days on the playa, I'd need 10 x $15 (per 8-pack of batteries) = $150. And a lot of battery waste. So I'm back to thinking about getting a deep cycle marine battery for closer to $110 and adding solar panels. Comes out to more than $150 but I can reuse them next year, so cheaper in the long run. And I can get more than 6 hours a day of cooling

[naos]

...I'd need 10 x $15 (per 8-pack of batteries) = $150.

Just got back from a trip to Home Depot, and realized I got this part wrong. An 8-pack of batteries runs $10, so 10x$10 = $100 for the week. Still a lot of battery waste, but comparable to the cost of one Group 24 battery (which, at ~80aH, I think will only run my swamp cooler for 8 6-hour days).

[Captain Goddammit]

I think at this point in the game, you do whatever works!

[FIGJAM]

115 AH batteries are $90!!!

[Wrath]

adding solar panels

Screw the panels, just hook it up to your car. Don't overcharge! Halfway through the week hook it up for an hour & see what that gets you. Adjust accordingly.