Battery thoughts; interesting but useless

[Geoff K]

So far everything I've read about batteries says the longevity is severely compromised by deep discharge. This is based on number of cycles. However, total amphours extracted from the battery over it's lifetime would be a more meaningful measure of its useful life. I haven't been able to find this data so, using the data available on Deka's tech page, I calculated it. My house batteries are a pair of Deka AGM 4Ds.

Unfortunately I could not copy the data to this note and preserve the format. The first column is the percent discharge, 2nd is life expectancy of the battery based on number of cycles assuming 50% discharge is "normal," 3d same based on total amphours extracted from the battery. The bottom line is the life of the battery is shortened by deeper discharge but not nearly as severely as counting cycles would suggest.

25% 250% 125%
50% 100% 100%
80% 54% 86%
100% 41% 81%

Here's 2 links to my calculations; first link numbers, second excel. You can plug the data from your battery into the spreadsheet if you're interested.

http://dl.dropbox.com/u/15870341/Battery longevity data.numbers http://dl.dropbox.com/u/15870341/Battery longevity data.xls

Geoff

 

[Stu Jackson]

Yup, pretty much very good information. Which simply translates to what most folks have been saying for many, many years, and as you so aptly put it:

deep discharges, especially beyond 50% of C is not good

battery life, no matter what kind of battery is affected by the number of cycles, but not so much...

The "real world" issue is this: they'd last forever if you didn't use them (!), but when you use them, take care of them.

One also has to add the "Battery Basics 101" Rule that is so very important: don't overcharge and don't leave them discharged or chronically undercharged, too.

Nice report, thanks.

 

[Maine Sail]

I am a firm believer in sizing your bank conservatively and then adding just one more battery, if you can. We only regularly cycle to about 70% SOC (using just 30% of the available capacity) and as a result our very in-expensive Wal*Mart batteries are on year five and a gnats hair away from new in Ah capacity. They have also been charged with a, "gasp" dumb regulated alternator, and still perform perfectly at year five. This bank can still start our 44 HP 4 Cylinder diesel and barely drop below 12 volts, even with the in-rush.

The secrets I've found are that shallower discharges lead to significantly longer bank life and topping them up as often as possible limits sulfation. Knowing their actual state of charge also greatly helps and this is why a good calibrated battery monitor can be soooo useful. We top our batts up to full every week via solar so they rarely if ever have a chance to sulfate badly. We often get back to 80-85% SOC with the alt and then the solar takes care of the rest. I have seen much better quality batteries, in a similar system to ours, last only a year or two at best due to overly deep discharges. The deeper the discharge the more rapid the sulfation.

I am in the middle of a personal study on bank life and trying to see what I can get out of this VERY, VERY inexpensive bank. When I did a 20 hour Ah load test on them this spring they came in at about 98% of the stated new capacity..

Here are the parameters I am using and am trying to keep it as "real world" as possible with two exceptions the solar and shallow discharges.:

-Cheap "deep cycle" batteries
-Oversized bank for shallow discharges (no greater than 30% depth of discharge on a regular basis)
-Regular maintenance and topping up with water when necessary
-Dumb regulated stock alternator
-Solar to get the bats back to 100% SOC often (not at a dock)
-No equalizing charges over the life of the testing (very few boaters do this)
-Batteries tested regularly via multiple methods
-Batteries stored on-board all winter and only topped up in the fall then left to sit.
-No temperature regulation on the alt or solar controller

So far the shallow discharges and the solar keeping them topped up have served us tremendously well for an obscenely inexpensive bank.... At this point I fully expect to get 6-7 years out of them.

 

[Stu Jackson]

I agree completely. Too many boaters simply don't have enough house bank capacity for their intended use. A fridge uses 60 ah per day, so a daily load is usually about 100 AH. See: http://c34.org/bbs/index.php/topic,6353.msg41471.html#msg41471

We have a 360 ah house bank. Works great for a 24 hour period, and then we use the engine or get back to shorepower to charge. For a longer cruise, I usually turn the fridge off when we hit the sack and back on again when we wake up, last longer and with the fridge closed and not being used at night, it all works out.

Do an energy budget for your boat and size the house bank accordingly, and, as Maine Sail says, the very best thing you can do for your electrical system (after you buy a battery monitor!!) is to increase your house bank as much as you can.

 

[Geoff K]

Well, I've done one thing right; I have 400 ah and have never used 100 ah in 24 hrs. Thanks for the input. You've confirmed some of what I had thought. I put in the electrical system during a 6 year on-the-hard refit. I had little experience with sailing and less with fixing sailboats. Thus everything is by the book. i.e. expensive. I'm sure Maine Sail will get far more amphours per dollar than I will and I'm a bit envious. However, I'm gaining my experience in my retirement and am willing to pay to avoid hassles. So far I've only day sailed but I'm hoping to cruise Lake Superior, starting with a trip to the Apostles in two weeks. (50km) Next year further.

So I'm glad to have the large capacity and not unhappy about the relatively trouble free but expensive AGMs. Probably wasted some money on the Xantrex charger and Ballmar alternator. OTOH, peace of mind, especially my aging mind, carries some value too.

Geoff

 

[Stu Jackson]

Geoff, no, I don't think you wasted $$ on the charger or the alternator. You may want to read many of Maine Sail's topics on his Musings page on the Featured Contributors portion of this website. Lots of handy things to know for when you go off the grid. For instance, there's a superb discussion of the care that AGMs need to stay in working shape.

 

[Geoff K]

Thanks

I will check out Maine Sail's treatise on the care of AGMs. I appreciate the input.

Geoff