Battery lifetime/cost vs. state of charge

[conachair]

Originally Posted by kmacdonald

I guess the best answer to the OP's question is it's fine to cycle to 80% DOD IF you recharge to 100% SOC.

And if you don't care that your fridge keeps tripping up because the voltage is so low.

[thinwater]

One more variable. A smaller battery bank will be easier to drive to a higher state of charge with solar etc. So the smaller bank that is discharged more completely is more likely to be returned to full charge while away from the dock, unless the over use is truly chronic... in which case a large bank would also be gradually dragged down.

The danger is that a really small bank will lose more ground in the typical 2-3 days of cloudy weather. That is the one I don't think you can easily get around. One solution, of course is austerity; if you are not going to charge for a few days, drop the load. We've all got stuff we can turn off.

[sailorboy1]

You know this battery cost "issue" has more to do with whether you use expensive or "cheap" batteries. I just replaced my 6 year 460AH bank of 6V golf cart batteries and it cost me $539 for 4 Trojan batteries. The old ones had some life still in them but were probably down to 50% life after being out on the mooring for 5 years with the refrigerator on all time time and 290W solar to hopefully maintain the batteries. The old bank cost me less and it worked out to $83/yr.

If everything on my boat was as cheap to use as batteries boating life would be even better!

[ranger58sb]

Originally Posted by sailorboy1

You know this battery cost "issue" has more to do with whether you use expensive or "cheap" batteries. I just replaced my 6 year 460AH bank of 6V golf cart batteries and it cost me $539 for 4 Trojan batteries. The old ones had some life still in them but were probably down to 50% life after being out on the mooring for 5 years with the refrigerator on all time time and 290W solar to hopefully maintain the batteries. The old bank cost me less and it worked out to $83/yr.

If everything on my boat was as cheap to use as batteries boating life would be even better!

FWIW, our oldest bank of 3x G31 AGMs (Odyssey PC2150s) cost $318.95/ea including shipping... $956.85 total, for that bank... and they've now finished 11 seasons... so $86.99/year total, for that bank.

Time to replace, next season, though.

-Chris

[kmacdonald]

I've had good luck with batteries also. I used to own a 28' Bertram with two Sams Club golf car batteries. They were 8 years old when I sold the boat and still in good shape. With a small sailboat now I am more interested in weight savings than battery life.

[Cadence]

Originally Posted by kmacdonald

I've had good luck with batteries also. I used to own a 28' Bertram with two Sams Club golf car batteries. They were 8 years old when I sold the boat and still in good shape. With a small sailboat now I am more interested in weight savings than battery life.

Maybe when buying batteries people should consider the weight, not at weight savings but plate thickness thus life expectancy. If light weight works for your application great. If you want long life, the more lead on the plates the better and weight. All solutions don't fit all needs.

[a64pilot]

Originally Posted by Cadence

Maybe when buying batteries people should consider the weight, not at weight savings but plate thickness thus life expectancy. If light weight works for your application great. If you want long life, the more lead on the plates the better and weight. All solutions don't fit all needs.

Yes, often you can literally weigh two brands of the same size battery and one will weigh considerably more, meaning of course more lead.

[S/V Alchemy]

Originally Posted by tanglewood

Bingo. I think you guys are both right, but are talking about different things.

1) How frequently you come back to full charge, and how most people don't and it seriously cuts cycle life (MaineSail's point)

2) Does discharging to 80% DOD half the cycle life, or does it cut cut by some exponentially large amount. (kmacdonald's point).

Now if you regularly operate to 80% DOD, you are necessarily spending more time in a partial state of charge compared to 50% DOD. But if you regularly return to 100% charge, how much does it matter? According to the manufacturer's data sheets, the loss is pretty much linear.

Another way to look at this, and a lot of battery people do look at it this way, is that any given battery has a fixed number of life-time Ahs of power that can be stored and retrieved. The cycle life vs DOD charts plot this. You can utilize those life-time Ahs in lots of smaller chunks (50% DOD), or fewer large chunks (80% DOD). If you take the cycle life charts and convert it into a chart plotting life-time Ahs (or kwh) against DOD, it's a remarkably flat curve, save for the extreme ends.

Yes. My L-16s show this in terms of expected cycles (and I realize this is "lab theory", but we have to start somewhere) given 20-25% discharge versus the apparently more typical 50%, which I've always tried to avoid. At 30% DOD, I can expect 3000 cycles, assuming I keep 'em clean, cool and watered:

[transmitterdan]

If you subscribe to the theory that a given battery will produced a fixed number of lifetime amp-hours then automatically a larger bank will be more cost effective. It costs less per AH and it will produce more AH for a given load.

[Cadence]

Look at how long automotive batteries last. It's seems forever, kept charged other than if you park and leave the lights on, which today they turn off by themselves. When they don't start the car it is replacement time. The point being they are not run down and are charged all the time once the car is stated. Taking more out than putting in is the problem. Maybe a simple analogy.

[Camille1]

You guys must watch your instruments every ten minutes! Many of us now have a fairly heavy power drain with al the kit now available. I have tried most battery types and have a lot of sympathy with Pizzazz. My worst experience was with Odyssey whose batteries I bought to be sure of reliable power crossing the pond. What a waste of money! They lasted no more than eighteen months and Odyssey told me, in the most courteous way to 'get lost'.
Maybe Golf cart batteries are the way to go for us non techies. Or an I wrong?

[thinwater]

Originally Posted by Cadence

Maybe when buying batteries people should consider the weight, not at weight savings but plate thickness thus life expectancy. If light weight works for your application great. If you want long life, the more lead on the plates the better and weight. All solutions don't fit all needs.

That is an interesting point.

What is a pound of weight savings worth to you? Some folks will put a high value on it; carbon fiber speedsters sell for $60-100/pound, where an old lead mine might sell for a few $/pound. How much more would you pay for a boat that was 1000 pounds lighter but with the same durability and quality? Since I sail a medium perforce cat, I reason $10-25/pound is reasonable, with more at the bow and masthead, and less in the center where the batteries are. But if I save 50 pounds, that's $500. IF the life cycle cost is even close, I'm ahead to cut the bank just a little thin.

And then there is the trade off of adding more solar. While it may be in the way an ugly on some boats, on a cat with a big hard top it's easy to install and nearly invisible. A panel adds more power variability than an extra battery, though the trade off is not one-to-one. The cost is probably similar.

The up-shoot for me is that I think I will add another panel. I've got the space on the roof and capacity in the controller. Cheaper and lighter. It does not help the rainy stretch problem, but it helps with sustained draw, short winter days, and sulfation.

It also helps to turn stuff off. Given that my first boat had practically none of the electrical gear I have now, it stands to reason I can turn it all off most of the time. For example, my fridge load is now zero. Temperatures have dropped to where I don't need it. I just use a cooler to moderate the swings.

[StuM]

Originally Posted by transmitterdan

If you subscribe to the theory that a given battery will produced a fixed number of lifetime amp-hours then automatically a larger bank will be more cost effective. It costs less per AH and it will produce more AH for a given load.

And the bigger the battery bank, the lower the draw rate and thus Peukert gives us considerably more Ah for a given DOD. (or conversely a higher SOC after drawing the same number of Ah).

[tanglewood]

Originally Posted by transmitterdan

If you subscribe to the theory that a given battery will produced a fixed number of lifetime amp-hours then automatically a larger bank will be more cost effective. It costs less per AH and it will produce more AH for a given load.

How will it cost less per Ah?

Two batteries delivers twice the live-time Ah, and costs twice as much. So cost per life-time Ah is the same.

[mitiempo]

Originally Posted by tanglewood

How will it cost less per Ah?

Two batteries delivers twice the live-time Ah, and costs twice as much. So cost per life-time Ah is the same.

Not really. With a given load doubling the battery bank in size increases the AH by more than double, thanks to Peukert.