Battery Type Question

System · April 2009

Hello,

If a group 31 marine deep cycle battery is cheaper (dollars per amphour) than a 6v golf cart or L16 or other commonly used off the grid battery, why are the larger, more expensive batteries still used?

From what I've read, most "marine" batteries are not true deep cycle in that "the plates may be composed of lead sponge [starting battery design], but it is coarser and heavier than that used in starting batteries."

Even if the marine deep cycle batteries have a hybrid plate design, if they are rated at an amp hour rate that makes them cheaper than other batteries (per amp hour), why not go with them?

What am I missing here? There has to be something more than just dollars per amp hour. Is it longevity of design? Rate of charge?

I'm looking for technical answers and reliable sources if possible. I'd like to be able to quote and reference.

Thanks,
Patrick

Windsun · April 2009

Re: Battery Type Question

Cheaper per amp-hour yes, but not cheaper per amp-hour x total number of cycles (battery lifetime). The main factor in total battery life is plate thickness.

In theory, you could have a 100 amp-hour battery with plates that are paper-thin, but it would only be good for a dozen cycles or so.

niel · April 2009

Re: Battery Type Question

generally you will get what you paid for, but if you are just experimenting or trying a first small system with solar then there's nothing wrong with going with the cheaper marine batteries as any mistakes will be had on those cheaper batteries. you will have to replace them in roughly 3 or 4 years if you didn't abuse them.

System2 · April 2009

Re: Battery Type Question

Hey guys,

I appreciate the responses. This question has been bugging me for a while, but it all makes sense now. I found the cycle life vs. depth of dischage charts for a few batteries and I understand.

This did bring up a new question. In a document published by East Penn (http://www.eastpenn-deka.com/assets/base/0919b.pdf) it says the cycle life for their 8L16 is 700 cycles @ 60% DOD, 1150 cycles @ 20% DOD (number of cycles based on 75 amp discharge to 1.75 vpc @ 32°C (90°F)) and the cycle life for DC27 200 cycles @ 80% DOD, 900 cycles @ 20% DOD (number of cycles based on 25 amp discharge to 1.75 vpc @ 29°C (85°F)).

When I crunch the numbers it tells me the 8L16 is more efficient/economical at a 60% DOD (as opposed to 20%) while the DC27 is more efficient/economical at the opposite end, 20% DOD.

Am I right in my calculations? Is this the nature of the thin vs. thick plates?

Patrick

niel · April 2009

Re: Battery Type Question

it doesn't appear that way to me. the dc27 only has 200 cycles of use when draining the battery 80% down leaving only 20% left while for 60% down and 40% left the l16 has 700 cycles or 3 and a half times as much cycles while having a 20% lesser discharge. at best the dc27 has 900 cycles only discharging the battery 20% while the l16 has 1150 cycles. yes, i'd say the l16 handles the deep discharges much better between the 2 of them, but 1150 is a better number than 900 every time. margins are closer at 20% dod, but that dc27 fails against the l16 both ways.

System2 · April 2009

Re: Battery Type Question

I'm with you on the L16 being a better battery than the 27.

What I was getting at is that with the L16, a shallow discharge doesnt provide you with more amp hours over the life of the battery (which seems to be the case with the 27, the point being moot in this post however).

The L16 shallow discharge provides more discharge cycles, but less overall available electricity.

A 370 Ah L16 discharged 20% = 74 Ah multiplied by 1150 cycles = 85100 Ah over the life of the battery.

Same L16 discharged 60% = 222 Ah multiplied by 700 cycles= 155400 Ah over the life of the battery.

Greater depth of discharge produces more useable electricity over the batteries life.

As far as developing the bank size I need for a particular application, is this the correct logic for shopping for the right batteries?

In other words, I if I need 222 amp hours per cycle I could get there with the above formula and pay $1024 for 4 batteries for a 24v system which would cost me $0.66 per 1000 Ah over the life of the batteries.

Or, I could use 3x the batteries for a labeled Ah of 1100, discharged at 20% = 222 Ah per cycle multiplied by 1150 cycles = 225300 Ah over the life of the batteries. The cost of 3x batteries is $3072 for a cost of $1.20 per 1000 Ah over the life of the batteries.

I realize real world applications are different from any simple equation I could post here, but I'm still learing, so let me know if my logic is off.

Patrick

BB. · April 2009

Re: Battery Type Question

You are working along the lines I would do--working up some simple formulas to see what makes sense...

I was also looking at cycle depth of discharge vs battery life... (Made up, but what I found with earlier research) Example where 4 batteries may get cycled to 50% discharge every 3 days, vs 8 batteries 75% discharge every 3 days...

The 8 batteries may last a bit longer than 2x longer vs the four batteries. Which is better for you? 4 batteries replaced every 4-5 years, or 8 batteries every 10 years? Hard to know.

Some other issues... Look at the maximum current load on the batteries--If you have lighting, computers, small fan, etc... The maximum wattage/current draw is near the continuous power rating.

If you have deep well pumps, fridge, freezer, etc., then your starting loads may be upwards of 5x greater than average load for a few seconds (or longer)... The size of the inverter and current capacity of the batteries will matter more too (larger inverter, larger physical cells and/or more strings in parallel, heavier wiring/cables/fuses/breakers).

Where you can get the batteries from (shipping, local pickup, taxes, support), Cell/Battery size (can you move a large battery vs several smaller batteries, need crane/lift gate), watering, etc. all will affect your outlook too.

-Bill

icarus · April 2009

Re: Battery Type Question

I think Bill is on the right track intuitively. If you have consistent bigger loads, (pumps, motors etc) than regardless of the math,, the bigger batteries might make better sense.

I faced a similar dilemma a couple of years ago. If found that I was using ~30-50ah out of my bank per day. (12 volt) I was running 4 L-16's. I did the math and figured that if I bought 4 t-105's the net/net cost over the life expectancy they were way cheaper than the L-16. I can't remember my exact numbers anymore,, but if the T-105's last 6 years,, and the L-16's ten,,, the t-105s would win. Time will tell if I am right.

Tony

Battery Type Question

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