Next up a battery. - Page 2 - Fiberglass RV


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Old 08-25-2013, 07:56 AM   #15
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Originally Posted by Jared J View Post
I probably should have explained better, I was wondering what people had luck with. I won't buy exide or optima.
Forgive my ignorance, but what's the scoop on Optima, at least in your opinion? Too expensive for what you actually get? Or is there some other issue?
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Old 08-25-2013, 03:33 PM   #16
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I used to run optima red tops in all my vehicles. They ran for years. When I started replacing them, the new red tops failed repeatedly.
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Old 08-25-2013, 03:43 PM   #17
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Like all batteries, the Optima has a set of trade-offs. For starters, their spiral-wound, cylindrical design, which hails from the very first commercial AGM battery design, makes them highly vibration and drop resistant as well as very slightly more efficient when accepting a charge.

The downside of the cylindrical design for us RV owners, on the other hand, are many. Their cylindrical design leaves lots of unused space between the cylinders that square cell battery designs use to for additional capacity. A spiral-cell AGM battery's capacity is about a third, 50Ah (Amp-hours) capacity, compared to a box-shape design that packs 75-80Ah into the same footprint.

Flipping back to advantages for a moment, AGM batteries weigh slightly less per Ah than traditional flooded cells do, and the the amount of weight per Ah is pretty comparable between Optima and other AGM batteries. So, if you are trying to save weight and all you need is a 50Ah battery, the Optima might be your best bet.

Another difference is Optima brand doesn't make a pure deep-cycle design. All their "RV/Marine" designs are hybrids that can be used as a marine starter battery and deep cycle.

Finally, there's the name. The Optima name tag seems to boost prices by a third or so per Ah.

Another difference -- I won't call it an advantage or disadvantage -- is that Optima has been using the same basic battery design for 25 years, and a few of those Optimas are still in serviceable condition. With a desulphination float charger you can expect to get 10 or more years out of a quality AGM battery, Optima or otherwise, as long as you don't bust its charge down to flat nothing every year or overcharge it frequently.

Overcharging is, however, a great way to kill an AGM battery. Unlike other designs, there's not a lot of electrolyte (aka water) in an AGM battery, so it's easy to "boil" the water off into oxygen and hydrogen gas by overcharging them. To compensate for this, AGMs have a reaction chamber that re-combines any hydrogen and oxygen that boil off back into water, but it's a slow process that can't keep up if you overcharge the battery often.

I have an Optima in my Scamp 5er. It's been a good performer, but I've learned that the Scamp's converter has a tendency to overcharge batteries and is not good for my battery life. So, most of the time, I don't even turn it on and depend on either on our tow vehicle or solar cells to charge the battery when we're out on the road. When the trailer is in storage I use a small, smart desulphination trickle charger (a BatteryMinder) to maintain my battery.

Longer term, I'm thinking of removing the converter all together and replacing it with a smart 6 or 10 amp charger with a desulphination feature, and will do exactly that with the Surfside project trailer we're working on. (Our Surfside is currently hollowed out and sitting in our garage, devoid of windows, doors, cabinetry, or appliances and detached from its frame. It does have a new floor . . . )
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Old 08-25-2013, 04:01 PM   #18
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Originally Posted by peterh View Post
Their cylindrical design leaves lots of unused space between the cylinders that square cell battery designs use to for additional capacity. A spiral-cell AGM battery's capacity is about a third, 50Ah (Amp-hours) capacity, compared to a box-shape design that packs 75-80Ah into the same footprint.
I assume that was supposed to be one-third less capacity, not one third of the capacity.

The area of a circle is pi/4 or 79% of the area of a square, and fitting industry-standard package sizes further compromises spiral cell packing efficiency.
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Old 08-26-2013, 08:47 PM   #19
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Thank you, Brian. Yes, the Optima has about a third less capacity that a square-cell battery with the same footprint.
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Old 08-27-2013, 07:31 AM   #20
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Why would any one buy a battery with 1/3rd less capacity?
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Old 08-27-2013, 07:44 AM   #21
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The area of a circle is pi/4 or 79% of the area of a square
When I was at school (last century) the area of a circle was pi r squared.
My how thing have changed.
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Old 08-27-2013, 08:01 AM   #22
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When I was at school (last century) the area of a circle was pi r squared.
My how thing have changed.
Just a typo - it is pi * D squared / 4
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Old 08-27-2013, 11:46 AM   #23
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Quote:
Originally Posted by widgetwizard View Post
When I was at school (last century) the area of a circle was pi r squared.
My how thing have changed.
Quote:
Originally Posted by Thomas G. View Post
Just a typo - it is pi * D squared / 4
Things have not changed, and it is not a typo. I wasn't posting the area of a circle, I was referring to the ratio of the area of a circle to the area of a square which with sides equal to the circle diameter.

area of circle: Ac = pi * r = pi * D / 4
area of square: As = D
Ac/As = (pi * D / 4) / D = pi/4 = 79%

... just as I said before:
Quote:
Originally Posted by Brian B-P View Post
The area of a circle is pi/4 or 79% of the area of a square.
The big difference is the first time I spent about six seconds to work it out in my head, and typing this took much longer.

I suppose it would have been more clear if I said:
The area of a circle is 79% (the value of pi/4) of the area of a square.
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Old 08-27-2013, 12:21 PM   #24
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Why would any one buy a battery with 1/3rd less capacity?
... compared to what?
A spiral-cell battery has about one-third less capacity than a flat-plate battery of the same outer package size; there is no meaningful difference in capacity for the same weight. I think in most applications, the package size is not critical.
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Old 08-27-2013, 07:06 PM   #25
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Nerd alert

Area of six circles = 6(pi(d^2))/4 = 4.71d^2

Area of a rectangle scribed around six circles (hexagonally packed) is 3.71d x 1.71d = 6.34d^2

(4.71d^2)/(6.34d^2) = .74 or 74%

Irrelevant, just an interesting puzzle.

Edit: if you connect the center of the circles you get an equilateral triangle with of course 60* angles. I used 45*. The corrected value is 72%. Of course this occurred to me in the middle of the night
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Old 08-27-2013, 07:14 PM   #26
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Brian,

Are you saying a spiral battery has a higher energy density per unit volume compared to a flooded plate battery?
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Old 08-27-2013, 07:54 PM   #27
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Originally Posted by P. Raz View Post
Area of six cylinders = 6(pi(d^2))/4 = 4.71d^2

Area of a rectangle scribed around six cylinders (hexagonally packed) is 3.71d x 1.71d = 6.34d^2

(4.71d^2)/(6.34d^2) = .74 or 74%

Irrelevant, just an interesting puzzle.


Actually relevant, as one of the Optima models does (or did) stagger the cells (maybe not all the way to hexagonally close packed) to fit one of the BCI formats (sorry, don't remember which one, but the length would have been more than 1.5 times the width).

The pi/4 (79%) factor is the ideal rectangularly arrayed case. Cylindrical (spiral-wound) cells in hexagonal boxes would hit 91%, and real battery case proportions are all going to be worse than that.

The point is just that cylindrical cells don't fit nicely in battery boxes, so the spiral-wound batteries are little bulkier (but not heavier) for the same capacity, or they are lower capacity (and lighter to match) for the same standard box size.
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Old 08-27-2013, 07:57 PM   #28
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Are you saying a spiral battery has a higher energy density per unit volume compared to a flooded plate battery?
No, about the same energy density per unit volume of working parts and - since both are made of lead and electrolyte - about the same energy density per unit mass. Since the box isn't filled with working parts, the spiral-cell battery has a lower energy density per unit of case volume, but still about the same energy density per unit mass.
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