Usable capacity of batteries

Zephod · July 16, 2017, 02:12:25 PM

Thus far I have been mightily disappointed by the gel batteries I've bought.

My 5ah battery was fine for ventilation when I was running two CPU fans as ventilating units. I upped it to 10AH with my 2.5A fan. Between a full charge of 13v and 11.9v I get maybe 6 minutes of ventilation. That just doesn't sound like I'm getting too much out of these batteries. My calculation is that a full (13v) battery should get me at 50% usage, 2 hours of fan usage before the thing needs recharging!

I might upgrade to a pair of 7ah batteries but even 9 minutes ventilation is poor.

All I want to do is run my extraction fan for 10 to 15 minutes, power my door lock and possibly power two CPU fans used to keep air circulating and hence cut down on heat.

Somebody did suggest dumping these crappy gel cells and getting a $20 lawnmower engine battery instead.

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muldoonman · July 16, 2017, 02:52:20 PM

Get you a AGM Type 31 and forget it. Ought to run those little fans quite a while. Those 7AH are too small for anything on your bus. Maybe good for starting your atv.

Zephod · July 16, 2017, 03:13:43 PM

Quote from: muldoonman on July 16, 2017, 02:52:20 PM
Get you a AGM Type 31 and forget it. Ought to run those little fans quite a while. Those 7AH are too small for anything on your bus. Maybe good for starting your atv.

I don't want something excessive. All I need to power are two CPU fans, my 2.5a fan and my door lock. A type 31 is way bigger than I need.

Lighting on my bus is powered by lanterns running off D cells. At $2 for several weeks of lighting, it's just not worth the cost to install permanent powered lighting.

Even the door lock will run happily for a couple of months on AA alkaline.

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muldoonman · July 16, 2017, 03:37:26 PM

You have a solar charger, get that larger battery and set sail. Heck you can even run your tv and boombox.

Zephod · July 16, 2017, 05:30:30 PM

Quote from: muldoonman on July 16, 2017, 03:37:26 PMYou have a solar charger, get that larger battery and set sail. Heck you can even run your tv and boombox.

I'm going to modify my system slightly. I'll put in some thermal switches so that below 25C, the fans don't do anything. That should optimize power usage.

I have 35w of solar panels producing probably about a third to half that.

My total system usage is about 3A at maximum load. That's 36W.

In 10 minutes my fans can expel all the air from the bus. All I need is to be able to run my fans for ten stinking minutes. Then the panels can recharge the batteries.

I can't believe a 10ah battery is too decrepit to provide 0.5AH of usable power. I must have been sold a duffer!

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TomC · July 16, 2017, 06:00:56 PM

I currently have 2-8D AGM batteries totalling 510amp/hrs. On my truck, I'm going with 4-L16 AGM that are 6V, 400amp/hr each to give me 800amp/hrs at 12v. Enough to run the truck for 48hrs, except for A/C. Good Luck TomC

Geoff · July 17, 2017, 09:52:01 AM

Good old Harbor Freight sells 35ah gel batteries for $66 less a coupon. I use them a lot and they are the same as an Interstate battery.

--Geoff

Zephod · July 17, 2017, 10:51:27 AM

Quote from: Geoff on July 17, 2017, 09:52:01 AM
Good old Harbor Freight sells 35ah gel batteries for $66 less a coupon. I use them a lot and they are the same as an Interstate battery.

--Geoff

People get very snooty at Harbor Freight but I've had almost all my tools from them plus one of my solar panels.

I'm sure you'll agree, that panel looks just like a destination board on my stealth motorhome.

I'm going to try dropping the cutoff voltage on my charge controller a bit to see what happens. I have several gel batteries that I've bought at different times for the project.

I'll have to see whether the 35ah battery will fit in Harbor Freights 50cal ammo box.

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Zephod · July 17, 2017, 11:31:31 AM

Well, I've just changed the settings on my charge controller. Fan starts at 13v, fan goes off at 11.1v. So far I've had 17 minutes out of the fan. That's far better than before. The inside of the bus doesn't feel any cooler though, which is odd. On the other hand the thermometer still says 84 which is what it said when I came in here, 20 minutes ago.

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Zephod · July 17, 2017, 11:31:43 AM

Well, I've just changed the settings on my charge controller. Fan starts at 13v, fan goes off at 11.1v. So far I've had 17 minutes out of the fan. That's far better than before. The inside of the bus doesn't feel any cooler though, which is odd. On the other hand the thermometer still says 84 which is what it said when I came in here, 20 minutes ago.

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Tony LEE · July 17, 2017, 12:29:39 PM

If your gel cells are the same as typical 100Ah GEL, then they have a charging sequence totally unlike normal lead acid or AGM batteries. Voltages are different and timing of each stage is also vastly different. Quite likely your batteries are either way undercharged or substantially stuffed by serious overcharging.

As for cooling the place down using just an exhaust fan. The bus is a large mass of metal and other materials that is all at 85F and sucking a little bit (in terms of thermal mass) of air into all that high mass material isn't going to make any difference especially given the air you are sucking in is probably not much cooler than the inside air.

Zephod · July 17, 2017, 01:16:31 PM

That battery is new. It seems to max out at 13v. It won't go higher. In the end, after 20 minutes the battery was on 11.7v so I adjusted the cut off to 11.7v. It should now cycle on for about 20 mins every time the charge reaches 13V. I'll put a thermal on switch in later so that it switches on only in temperatures above 77F.

The internal temperature rose to 86 which is the same as the outside temperature. I'll have to see how it does on hot days. Ideally I want to maintain parity with outside temperatures.

I'll add some permanently mounted cooling fans in the autumn.

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Zephod · July 17, 2017, 02:32:40 PM

Quote from: Tony LEE on July 17, 2017, 12:29:39 PM
As for cooling the place down using just an exhaust fan. The bus is a large mass of metal and other materials that is all at 85F and sucking a little bit (in terms of thermal mass) of air into all that high mass material isn't going to make any difference especially given the air you are sucking in is probably not much cooler than the inside air.

I'm hoping to match external temperatures. I think you're underestimating my fan. It is a 3" bilge fan that blows 130 cubic feet of air a minute. My bus is nose to tail 32.5 feet. The cabin is about 24 feet by about 6 feet by about 7.5 feet. That's approx 1080 cubic feet or 8 minutes for my fan to blow all of the cabin air out the back of the bus.

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sledhead · July 18, 2017, 10:03:30 AM

a exhaust fan will bring in fresh air when on but unless the out side air is a lot cooler then the inside air it will never change the inside temp. as there is the the radiant temp. of the walls , floor , the counter top and every thing else in side .

a fan works great if it is blowing at your face even if it is blowing hot air

dave

Zephod · July 18, 2017, 01:35:13 PM

Quote from: sledhead on July 18, 2017, 10:03:30 AM
a exhaust fan will bring in fresh air when on but unless the out side air is a lot cooler then the inside air it will never change the inside temp. as there is the the radiant temp. of the walls , floor , the counter top and every thing else in side .

a fan works great if it is blowing at your face even if it is blowing hot air

dave

But with the hot air removed, cooler air will cool the interior.

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