Depth of Discharge: a Contrarian View

[Wraith_Mac]

Guys,
How much of that capacity below 50% is unaffected by voltage sag, or doesn't that come in to play here?
Cheers,
Mac

[Pete7]

Rusty, why can't you get the batteries up to 100% with solar when you are away from the boat? Here is our 220 AH bank at 0900 hrs this morning at 50.N. The sun would be about 20 deg above the horizon, so not a bad figure for 125w of solar given the time of year.

Also your AH is quite high for a 28ft boat day sailing, ours is 30-40AH at anchor and more sailing at night. Does yours include an auto pilot by chance.

What I am getting at is a minor improvement in charging and you don't need 4 x 6v batteries and certainly not the extra weight. Previous owner managed 7 years from a 220 AH bank, we managed 5 years with new batteries and only destroyed one because the rotary battery switch was worn and I didn't realise it didn't disconnect. With new batteries and switches I can't remember when we last went below 12.4v.

I do think you are on the right track though for a small yacht and if you get over 5 years out of a pair of Costco batteries then you have had your monies worth and just replace them.

Pete

[Rusty123]

Originally Posted by btrayfors

1. Stu is right about sulfation. Batteries will sulfate if chronically undercharged. They will also continue to sulfate even when "fully charged" and floated at, say, 13.2-13.4VDC, albeit at a slower rate.

I think it was Third Day, not Stu, who mentioned sulfation. But no matter -- It seems to me that sulfation would affect the 2-battery and the 4-battery bank the same, since sulfation is a function of SOC, and SOC is related to charging time, vice bank acceptance or charging capacity. No?

Originally Posted by btrayfors

2. The average load is important in these calculations...Peukert comes into play here.

Ah, yes, Peukert. You are right, I did leave this out. Just for fun, I redid the comparison using various ratings from the Trojan T-105 website for different discharge rates.



As would be expected, the AH (and cost) difference between the 2-battery and 4-battery banks diminishes as discharge rate increases, and somewhere around the 5 hour rate, the 4-battery bank becomes cheaper. This is, of course, a much higher rate than will ever be seen on a boat.

Originally Posted by btrayfors

And, re: COSTCO batteries, why not install 2 more if you have the room. They only cost $70 each, and you'll spend more than $140 worrying about getting along with a smaller bank :-)

In my particular case, space is the limiting factor. To place two more 6V batteries, I would need to undertake a fairly significant construction project.

Originally Posted by btrayfors

One final thought re: small engines. There's no reason you couldn't fit a large alternator to a small engine. Even a 15HP engine can turn a 100A alternator at anchor. Think of it as a genset, not a propulsion engine.

Need it for propulsion? Just fit a good external regulator, like the Balmar MC-612 or 614, and fit the optional toggle switch. This cuts alternator output in half...at will. And, you can "de-rate" the alternator to any desired size.

Say you "de-rate" the 100A alternator by 20%. Now you have an 80A alternator which can run for extended periods. Need to power? Flip the switch and you now have a 40A alternator on your little engine.

Exactly my intention, and the focus of my winter shopping. And you are right, fitting a larger alternator will make a larger bank more attractive, not for reasons of battery economics, but for reasons of reduced charging time (which has its own set of economics, as Stu pointed out).

[Rusty123]

Originally Posted by Stu Jackson

I think you're over analyzing an issue that has been pretty much resolved over the years. There's reinventing the wheel, and there's also ctting hairs finer than necessary.

OK, now you sound like my wife.

What got me started on this analysis in the first place was the concern that occasionally, when spending the night an anchor, I might drop somewhat below 50% SOC. Based on the number of times I've read that dropping below 50% SOC is bad, bad, bad, this seemed like, well, a bad idea. But I wanted to know how bad.

Based on my analysis, dropping below 50% SOC doesn't seem to have any negative impact at all, at least not in terms of economics. And it certainly wouldn't warrant the investment and hassle necessary to make room on my boat for more batteries.

Russ

[Rusty123]

Originally Posted by Pete7

Rusty, why can't you get the batteries up to 100% with solar when you are away from the boat? Here is our 220 AH bank at 0900 hrs this morning at 50.N. The sun would be about 20 deg above the horizon, so not a bad figure for 125w of solar given the time of year.

Also your AH is quite high for a 28ft boat day sailing, ours is 30-40AH at anchor and more sailing at night. Does yours include an auto pilot by chance.

What I am getting at is a minor improvement in charging and you don't need 4 x 6v batteries and certainly not the extra weight. Previous owner managed 7 years from a 220 AH bank, we managed 5 years with new batteries and only destroyed one because the rotary battery switch was worn and I didn't realise it didn't disconnect. With new batteries and switches I can't remember when we last went below 12.4v.

I do think you are on the right track though for a small yacht and if you get over 5 years out of a pair of Costco batteries then you have had your monies worth and just replace them.

Pete

If/when I get to the point where I'm spending lots of time at anchor, solar will definitely be on the short list, for the reasons you mention.

Regarding consumption: out of my 75 AH daily use, 26 AH is due to a refrigerator. Does your consumption figure include a refrigerator?

Disclosure: I've only had a battery monitor for a few weeks, so my consumption figures are based on short term readings at pier-side, rather than long term measurements at anchor.

[sailinglegend]

Originally Posted by Rusty123

...In fact, if the charging source is the limiting factor (such as solar might be), wouldn't a smaller battery bank actually reach a higher SOC in a given time (and thus be less subject to sulfation?).

Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.

It takes a bit of very over-simplified maths to prove the point, but a 100 Ah battery that is discharged to 50% may accept 20Ah in the first hour during the boosts stage, maybe 10Ah in the second hour during the start of the less efficient absorption phase, and the remaining 20Ah in another 5 hour. Doubling the battery size to 200Ah, with the same charging source of 20 amps, will accept 10Ah into each battery in 1 hour, that’s 20Ah into the bank. In the second hours it will store another 20Ah. That’s 40Ah replaced in two hours, as compared to 30Ah with a single bank. In the 3rd hour it may still accept 20 amps into the bank because a single battery in the start of the absorption phase could accept 10 amps. That’s 60Ah in three hours.The key point is that for two hours it is still in the more efficient boost stage where the battery is taking all the current the charge source can give it. Note that the initial boost charging stage has captured 40Ah in two hours, and 60 Ah in three hours. With the smaller bank it could only capture 20Ah in the first hour during boost and 30Ah after the second hour during the start of absorption. The third hour may add another 5 amps. That’s 35Ah with one batteries and 60Ah with two batteries. So a bigger bank will be more efficient and accept more Ah more quickly from all charging sources.

Since a lot of the time we are only charging up to the absorption stage which is about 80-85% then this increase in stored Ah is significant.

Another safety feature is that if you have a larger bank - or many smaller batteries in one large bank, it is easy when they start failing to just disconnect the bad ones and run on the others as long as you can until you can replace the whole bank. This may also allow skippers to search around and find the batteries they really want - not just be forced to buy the local "rubbish" because they are desperate.

[Pete7]

Originally Posted by Rusty123

Regarding consumption: out of my 75 AH daily use, 26 AH is due to a refrigerator. Does your consumption figure include a refrigerator?

Disclosure: I've only had a battery monitor for a few weeks, so my consumption figures are based on short term readings at pier-side, rather than long term measurements at anchor.

Yes includes the fridge. Runs about 3AH when running but only 2 minutes in every 15 during the day when we are in and out of the fridge, less at night time. Also includes laptop and phones lights etc. Diesel heating takes a lot more and we can use 20AH during a night but not often needed during the UK summer.

Pete

[sailorboy1]

Seems a lot of thought for nothing really. We are talking $140 for a battery bank here. If you don't have the ability to regularly fully charge and equalize the batteries then just figure on 1-2 year life. If you are only going be able charge to 80% whole underway but return to the dock every few weeks and can fully charge and equalize your 1 bank will probably last. 5-6

The problem here isn't really a battery bank size, it is a charging problem. I figure I can use 50% of of bank rating (more really for small normal ) because I know my solar will recharge to 100%

[Rusty123]

Originally Posted by sailorboy1

Seems a lot of thought for nothing really. We are talking $140 for a battery bank here.

Two points:

1. In my case, we're not talking about $140 for a battery bank. We're talking about a construction project to accommodate them (perhaps $500), and a periodic replacement cost of $300 (vice $150) every 5 years or so. And, the investment in an upgraded alternator and regulator ($800), in order to take advantage of the higher battery acceptance (since that appears to be the primary advantage of a larger bank).

2. The accurate calculation of battery life/cost/value doesn't just apply to my narrow circumstance. It also applies to those considering going from a 4 battery bank to a 6 or 8 battery bank. Or those contemplating a switch from FLA to AGM in order to increase acceptance rates. Or even those contemplating a switch to LiFePO4. I'll bet you a box of donuts that the vast majority of people evaluating these alternatives have built into their calculations an assumption that they should never drop below 50% SOC (for FLA). As best I can tell, even taking the comments of this thread into account, that assumption has no basis in science or economics, and should be disregarded.

[CharlieJ]

Unless all of the manufacturer's specifications are followed to the letter; T105s or any FLA battery will never achieve the life cycles advertised. These specifications include:
> Temperature compensation on all charging sources
> Correct absorption voltage on all charging sources
> Correct float voltage on all charging sources
> A cool environment
"When operating batteries at temperatures above 80°F (27°C) they will deliver more than the rated capacity but the battery life will be reduced."

Experience shows that floating FLA batteries for extended periods of time will cause electrolyte stratification and sulfation, both detrimental to life cycles. Batteries must be used, not abused to maximize their cycle life and your capital investment.

[Stu Jackson]

Originally Posted by Rusty123

OK, now you sound like my wife.

Russ,

Sorry about that!

I think you've answered your question since the $140 for the second two batteries will cost you a lot more.

You also mentioned you're new to your battery monitor and are working on loads.

In my acceptance link Page 1, Reply #10, I mentioned that "what goes OUT is EASY, it's what goes back in that makes a battery monitor so invaluable."

There have been so many energy budget posts and answers done over the past 20 years that if someone can't come up with what goes OUT they are really in deep doo-doo. The loads have been known for decades and the only variable is time. That part really is easy and doesn't require a battery monitor. Here's my energy budget, confirmed by my Link 2000, 100 ah a day with a fridge.

The All-Important Energy Budget:
Energy Budget

Record of Daily Energy Use of 100 ah per day:

"Breaking In" New Wet Cell Batteries


So, my advice? Give it a try with your two new 6V batteries for your house bank and spend the $140 (PLUS !) on a solar panel and good controller for it. I think you'll be just fine.

Good luck, but I don't think luck will be a part of it, you seem to know what you're doing.

[sailinglegend]

Originally Posted by Stu Jackson

Record of Daily Energy Use of 100 ah per day:

"Breaking In" New Wet Cell Batteries

You keep posting links to sites I can't access????

"You have reached a page that is either non-existant or in error!"

This your c34.org site with this error message - with a spelling mistake!!!!

I can't register, even if I manage to get to the home page - it doesn't work - but I did manage to register a few months ago - maybe I was using a VPN.

Don't you want non American locations to see your precious information?

I am in Turkey at the moment.

Please, please can you fix it?

[Rusty123]

Originally Posted by Stu Jackson

Russ, Sorry about that! I think you've answered your question since the $140 for the second two batteries will cost you a lot more. You also mentioned you're new to your battery monitor and are working on loads. In my acceptance link, I mentioned that "what goes OUT is EASY, it's what goes back in that makes a battery monitor so invaluable." There have been so many energy budget posts and answers done over the past 20 years that if someone can't come up with what goes OUT they are really in deep doo-doo. That part really is easy and doesn't require a battery monitor. Here's my energy budget, confirmed by my Link 2000, 100 ah a day with a fridge. The All-Important Energy Budget: Energy Budget Record of Daily Energy Use of 100 ah per day: "Breaking In" New Wet Cell Batteries So, my advice? Give it a try with your two new 6V batteries for yoru house bank and spend the $140 (PLUS) on a solar panel and good controller for it. I think you'll be just fine. Good luck, but I don't think luck will be a part of it, you seem to know what you're doing.

Thanks Stu.

My energy budget calculation was done with essentially the same methodology as yours. My comment about pier side calculations vs. real world observations is based on the fact that the current draw of some loads (autopilots, for example), is difficult to measure accurately unless underway. Also, unless actually measured, assumptions about operating time for fans, lights, radios, etc. can be off.

All that said, I'm fairly confident in my 76 ah daily budget.

Good advice about solar vs. more batts. I've begun investigating that angle.

Best,

Russ

[Cheechako]

Originally Posted by Rusty123

Yes, that is another way of stating the point. To me, the big eye opener was that the often cited "don't exceed 50% maximum depth of discharge" guidance appears to be baseless for true deep cycle batteries (such as golf cart batteries), and that bank sizing should be based on other things (economics, risk, weight, etc.).

Of course, even deep cycle batteries have a practical DOD limit. Here is a quote from Trojan's website: "Do not discharge your battery more than 80%. This safety factor will eliminate the chance of over-discharging and damaging your battery".

I gave up worrying about % of discharge, the reality was the voltage would get too low to be useful.... and that's when (or before that) I had to bulk re-charge!

[Stu Jackson]

I am truly sorry that you can't access those pages. I am not the webmaster, so I have no way to fix it. I didn't even realize it was broken! For you at least. As you can imagine, most of our respondents are in the USA, Canada or Mexico, with a few in Europe, Australia and New Zealand, who haven't mentioned any issues. Perhaps it's at your end. And our site doesn't post the error message, somebody at "the internet" did that, not our website. I simply don't know, and again I'm sorry that you can't access the information. Please, don't be mad at me.

Originally Posted by sailinglegend

You keep posting links to sites I can't access????

"You have reached a page that is either non-existant or in error!"

This your c34.org site with this error message - with a spelling mistake!!!!

I can't register, even if I manage to get to the home page - it doesn't work - but I did manage to register a few months ago - maybe I was using a VPN.

Don't you want non American locations to see your precious information?

I am in Turkey at the moment.

Please, please can you fix it?