Another view of the 50% AGM Discharge Limit

[WJones]

In my mind the exact numbers aren't important. The important point to take away from all this is that, contrary to what some might say, you are not killing your batteries within a few discharge cycles by going over 50% DOD. I've actually read that! Going to 80% DOD is a viable alternative, even if it ends up costing a bit more in the long run.
William
2000C200P

[gregmchugh]

Originally Posted by WJones

In my mind the exact numbers aren't important. The important point to take away from all this is that, contrary to what some might say, you are not killing your batteries within a few discharge cycles by going over 50% DOD. I've actually read that! Going to 80% DOD is a viable alternative, even if it ends up costing a bit more in the long run.
William
2000C200P

You've got it, the numbers in the analysis do not represent any realistic usage scenarios for typical use but they give guidance on reasonable ways to use your batteries for real world scenarios...

[gerrym51]

Greg-

does this chart take Peukerts law into account.

Isn't rate of discharge a bigger factor.

Reserve Capacity rate is probably a better way to judge a battery in todays energy gobbling devices.

[gregmchugh]

Originally Posted by gerrym51

Greg-

does this chart take Peukerts law into account.

Isn't rate of discharge a bigger factor.

Reserve Capacity rate is probably a better way to judge a battery in todays energy gobbling devices.

If you look at the Lifeline document you can see the conditions they used for the test that produced the life cycle data. Yes, you could run tests at different levels of discharge and charge to produce data at those conditions.

The purpose of the life cycle data is to give guidelines on expected battery life and the depth of discharge is likely the main factor in determining life cycles.

Since all spec data is determined with highly controlled test conditions you cannot generally use it to predict with great accuracy real world performance under the uncontrolled conditions found in actual use. You can use it for getting general guidance on expected performance which is useful.

If you want to predict actual performance at different discharge rates there is info in the manual on that but not in terms of overall life cycles at different discharge rates. Imagine the amount of testing it took to produce the life cycle chart and multiply that by the range of discharge rates (which would be constant in the tests but not be constant in real life). Lifeline seems to provide the best data of any consumer battery line, probably because of their history in the aerospace market with the Concorde battery line, but this is very time consuming and expensive to do.

[gregmchugh]

Originally Posted by gerrym51

Greg-

does this chart take Peukerts law into account.

Isn't rate of discharge a bigger factor.

Reserve Capacity rate is probably a better way to judge a battery in todays energy gobbling devices.

Not sure Reserve Capacity is more useful or not since it is just another controlled test at a higher discharge rate than you would see in the typical 20 hr amp hour test of a deep cycle battery.

Your Northstar has a 400 min reserve capacity at a 25 amp discharge rate giving about 166.66 amp hour capacity under those conditions vs 185.8 AH in the 20 hr test (9.29 amp discharge rate) vs 176.5 AH in the 10 hr test (17.65 amp discharge rate). Is the Reserve Capacity giving more insight into actual real world performance? All tests done at artificial controlled conditions so mostly useful for comparing the specs of different batteries for different uses.

If you are focusing on engine starting capability the Reserve Capacity test conditions give you better guidance when you compare batteries but it is still an artificial test.

[booster]

Getting back to the 50% rule, there were questions about the various battery types and how they behave when looking at total AH over their lives vs DOD. Getting really good, comparable data is not really very easy, so all of this is somewhat suspect because it came from the manufacturers. The actual cycles numbers are probably the most likely to be exaggerated or rated conservatively, but I would think the curve shape should be relatively good. If a chart goes to zero on the ends, it is because the data didn't cover the full range from zero to 100%. All are based on a theoretical 100AH battery of the construction listed.

First AGM which seems to be getting to be the most common type being used these days. This data comes from the Lifeline cycles vs DOD graph.



Second is a high end golf cart wet cell. The data is from Trojan's renewable energy cycles vs DOD graph.



Third is the TPPL version of AGM. This data is from the Odyssey cycles vs DOD graph.



Fourth is for lithium. This was off a site that was not a major player, and was stated to be very conservative.



As can be seen, all but the lithium are fairly consistent from 50-80%, when looking at total AH over life. The Trojan takes it the extreme, so I would guess they are plainly trying to say that their T105 doesn't suffer much, if any, by going to low DOD, which is very important to golf courses. I doubt that they are really that consistent, but with good charging, they are probably close. With aggressive charging, including a high voltage topping, good true deep cycle wet cells seem to come back from the dead very well. They have been doing that in forklifts for years.

The TPPL lost a bit more due to deeper discharges, than the other lead batteries, which was a bit surprising as that is one of the things they tout, deep discharge tolerance. The only information I have seen on that was an article by Nigel Calder a while ago where he said he was investigating early capacity loss in TPPL batteries he was testing. I never did see what he found out however. His testing did beat them pretty hard, to simulate real world.

[markopolo]

Deep cycle lead acid has typically had thick plates for longevity. Thinner plates might just deteriorate faster.

TPPL can accept and deliver higher current IIRC. Basically you pick what is more suited to your needs.

Lithium lifetime amp hrs look to be around double lead acid at deep discharges according to those conservative numbers. The cost is still more than double though. It seems to range from 3X to 9X more expensive depending on what you buy.

The best bang for your buck might just be a couple of Costco or Walmart wet cells replaced as needed. IIRC lead acid batteries are something like 90% recyclable.

[booster]

Originally Posted by markopolo

Deep cycle lead acid has typically had thick plates for longevity. Thinner plates might just deteriorate faster.

TPPL can accept and deliver higher current IIRC. Basically you pick what is more suited to your needs.

Lithium lifetime amp hrs look to be around double lead acid at deep discharges according to those conservative numbers. The cost is still more than double though. It seems to range from 3X to 9X more expensive depending on what you buy.

The best bang for your buck might just be a couple of Costco or Walmart wet cells replaced as needed. IIRC lead acid batteries are something like 90% recyclable.

The bang for buck of cheap wet cells may well be a good choice, if they are GC2 style, which seem to get longer life in most brands compared their normal deep cycle batteries. Even Trojan has started showing how much difference there really is between their deep cycle wet cells.

Here is the T105 GC2 graph from above



And here is one for one of their 12 volt deep cycle batteries



Looks like about 1/2 the life and drops off more at deep discharges with the non GC battery. In the past they used to say they were same chemistry, similar life (7 years ago when I bought out first ones).

All the Trojan information came from this graph on their renewable energy site.



Really interesting how much life you get out of their top of the line wet cells.

[gerrym51]

Originally Posted by booster

Getting back to the 50% rule, there were questions about the various battery types and how they behave when looking at total AH over their lives vs DOD. Getting really good, comparable data is not really very easy, so all of this is somewhat suspect because it came from the manufacturers. The actual cycles numbers are probably the most likely to be exaggerated or rated conservatively, but I would think the curve shape should be relatively good. If a chart goes to zero on the ends, it is because the data didn't cover the full range from zero to 100%. All are based on a theoretical 100AH battery of the construction listed.

First AGM which seems to be getting to be the most common type being used these days. This data comes from the Lifeline cycles vs DOD graph.



Second is a high end golf cart wet cell. The data is from Trojan's renewable energy cycles vs DOD graph.



Third is the TPPL version of AGM. This data is from the Odyssey cycles vs DOD graph.



Fourth is for lithium. This was off a site that was not a major player, and was stated to be very conservative.




l
As can be seen, all but the lithium are fairly consistent from 50-80%, when looking at total AH over life. The Trojan takes it the extreme, so I would guess they are plainly trying to say that their T105 doesn't suffer much, if any, by going to low DOD, which is very important to golf courses. I doubt that they are really that consistent, but with good charging, they are probably close. With aggressive charging, including a high voltage topping, good true deep cycle wet cells seem to come back from the dead very well. They have been doing that in forklifts for years.

The TPPL lost a bit more due to deeper discharges, than the other lead batteries, which was a bit surprising as that is one of the things they tout, deep discharge tolerance. The only information I have seen on that was an article by Nigel Calder a while ago where he said he was investigating early capacity loss in TPPL batteries he was testing. I never did see what he found out however. His testing did beat them pretty hard, to simulate real world.

I actually mentioned this once in the past. TPPL are so close to lithium that boaters were treating them like lithium. Fast charging but not worrying about getting them to max charge like AGM. they are superior to agm but you cannot treat them totally like lithiumthey are not

[gerrym51]

continued- you can treat tppl agm with higher discharges-however you cannot ignore getting them back to max charge like you can with actual lithium.

Nigel found the issue was boaters were treating the tppl just like lithium which they sort of duplicate with the fast charging andeeper discharge.

However agm even tppl agm need to be charged back to max asap unlike lithius that don'td

you still have to take care of your ltppl agm

[markopolo]

TPPL appears to be expensive to own if deep discharging when you look at the lifetime amp hours. Take a look 80% DOD on the posted charts.

Lithium ......... 100,000+ lifetime AH
Trojan T105 .... 60,000 lifetime AH
Lifeline .......... 42,000 lifetime AH
TPPL ............. 24,000 lifetime AH

I'm assuming the data reflects manufacturer recommended care of the batteries.

It certainly is an interesting way to look at the data. It seems like we've focused on number of cycles for so long now.

[gregmchugh]

The TPPL batteries seem more useful for the typical marine deep cycle/engine start application rather than as deep cycle house batteries, at least based on the data for the Odyssey ones...

Northstar doesn't seem to have detailed info on life cycle performance for theirs that I could find...

[markopolo]

Originally Posted by booster

The bang for buck of cheap wet cells may well be a good choice, if they are GC2 style, which seem to get longer life in most brands compared their normal deep cycle batteries. Even Trojan has started showing how much difference there really is between their deep cycle wet cells............................................

Great point. Deep Cycle is the key.

[booster]

Originally Posted by markopolo

TPPL appears to be expensive to own if deep discharging when you look at the lifetime amp hours. Take a look 80% DOD on the posted charts.

Lithium ......... 100,000+ lifetime AH
Trojan T105 .... 60,000 lifetime AH
Lifeline .......... 42,000 lifetime AH
TPPL ............. 24,000 lifetime AH

I'm assuming the data reflects manufacturer recommended care of the batteries.

It certainly is an interesting way to look at the data. It seems like we've focused on number of cycles for so long now.

It is hard to figure how it all settles out, I think, especially the Odyssey information. Over the years, almost all the reading I have done about Odyssey and Lifeline mostly called them both among the best batteries (for boats in particular), but both also have some problems in some applications. To see that much difference in the life specs is very surprising.

I think you have it right that the specs would reflect very good charging per manufacturer spec, which very few boaters or RV folks have. Lifeline has been refining their charging specs lately, increasing voltage allowed, increasing the charge rate minimum for deep discharges, emphasizing getting the charge to the .5% of capacity amps while in absorption (the big one IMO), etc. Odyssey is a lot squishier in their recommendations, and a lot of their charging hype about speed has small print explaining the you would only get to 90% full that way and still had to spend lots of hours to get the rest of the way full. I think Gerry is right in saying boaters abuse them by partial cycling, but I think they do that to all brands, mostly out of necessity, but Odyssey also seems to kind of encourage it in their literature.

[booster]

Originally Posted by markopolo

Great point. Deep Cycle is the key.

It would sure be fun to see a cycle life vs DOD curve for Costco, Sam's, and Walmart GC2 batteries to see where they fall in relation to the two versions of the Trojan deep cycles.

[booster]

Originally Posted by markopolo

It certainly is an interesting way to look at the data. It seems like we've focused on number of cycles for so long now.

I completely agree. It is hard to imagine how the 50% rule got so ingrained into the battery knowledge base, unless it is just the easy excuse for early battery failures (which more likely were from bad charging). You constantly are having folks saying you should NEVER go under 50% SOC or you will kill the batteries very quickly, and they size their banks at double what their maximum, ever, use would be before charging.

I would rate this right near the top of the RV/marine urban legend list, I think, as there are really very few folks who don't believe it, including most of the manufacturers in their literature. Of course it could also be because knowing the actual SOC has been very inaccurate historically, so maybe they are trying to keep us from going completely dead due to lack of accuracy.

[Davydd]

Originally Posted by booster

I completely agree. It is hard to imagine how the 50% rule got so ingrained into the battery knowledge base, unless it is just the easy excuse for early battery failures (which more likely were from bad charging). You constantly are having folks saying you should NEVER go under 50% SOC or you will kill the batteries very quickly, and they size their banks at double what their maximum, ever, use would be before charging.

I would rate this right near the top of the RV/marine urban legend list, I think, as there are really very few folks who don't believe it, including most of the manufacturers in their literature. Of course it could also be because knowing the actual SOC has been very inaccurate historically, so maybe they are trying to keep us from going completely dead due to lack of accuracy.

I think you hit it. How many RVs have any accurate way to measure SOC? Not many I would venture to guess. Then there is usually but one or two house batteries. It is just way too easy to totally discharge with that amount from stupid things like walking away and ignoring during storage, forgetting to switch an absorption refrigerator off 12VDC when parked, etc. It doesn't take too much to blast through a batteries capacity given those numbers. So it is safe to say 50% as a rule of thumb which makes those idiot lights a little easier to understand and time to catch your mistakes.

[booster]

Originally Posted by gregmchugh

The TPPL batteries seem more useful for the typical marine deep cycle/engine start application rather than as deep cycle house batteries, at least based on the data for the Odyssey ones...

Northstar doesn't seem to have detailed info on life cycle performance for theirs that I could find...

I haven't found any graphs on the Northstar either. What they do say is 900 cycles at 50% and 400 cycles at 80% down, so that would be 45,000 and 32,000 amp hours in the theoretical 100 ah battery. This is quite a bit better than Odyssey claims at 33,000 and 24,000 ah, but still quite a bit short of the Lifeline and Trojan. It also shows more of a loss at the deeper discharges (ratio) than the others do.

The only other TPPL batteries I know off offhand are Optima and Vmax Tanks.

[papab]

I think what's missing is lifecycle data with extended time at partial state of charge. For the manufactures tests, they discharge, then charge up right away. What about the real world for dry camping RV's or sailboats? I didn't know much about batteries when I jumped into this. I sized mine to go down to 50% or less over 3 days, then I find out they (lifeline) want you to charge them up fully every day (tech support phone call). What's the life of a battery if its between 50-95% for days, with weekly or biweekly full charges?

[booster]

Originally Posted by papab

I think what's missing is lifecycle data with extended time at partial state of charge. For the manufactures tests, they discharge, then charge up right away. What about the real world for dry camping RV's or sailboats? I didn't know much about batteries when I jumped into this. I sized mine to go down to 50% or less over 3 days, then I find out they (lifeline) want you to charge them up fully every day (tech support phone call). What's the life of a battery if its between 50-95% for days, with weekly or biweekly full charges?

This has been the big question for a lot of people. As best that I have been able to find out, from multiple calls to manufacturers and specs that are available:

* The number one killer of marine batteries is the very common practice of cycling them between 30-50% and 80%, with never getting full, or at least very rarely full. This can be because of the practicality of the way they are used, or because of less than great charging systems. The everyday full recharge that you have been told, I think, is not really accurate based on lots of other things I have seen.

* The consensus seems to be if you get a totally full charge (based on the amps the battery takes), at suggested absorption voltage, or even better the highest suggested voltage, every 5-7 charge cycles, the damage to the batteries will be minimal. This assumes, however, that the recharge to full is timely. You don't want the batteries to sit discharged for weeks at a time.

* If you do have the deeper discharges of more than 50%, having a charge system that will do at least 20% of the battery bank capacity, in amps, will increase battery life.

* Some manufacturers are recommending a short, current controlled, higher voltage stage at the end of the charging cycle for some of the AGM batteries, especially if they are discharged deeply. This is basically a mini "conditioning" cycle similar, but shorter and more often, to what Lifeline has recommended to get back lost capacity in their batteries. It is pretty impractical to do on every cycle in an RV due to the equipment and voltages required. I would also add that if the charging system is good, and the other recharge best practices are followed, it probably would rarely be needed.

All of this said, almost none of the RVs around have a charging system that will reliably get the batteries 100% full at every full recharge cycle, and many would also have trouble being at 20% of bank capacity. As I mentioned earlier, I think the 50% discharge rule came in mainly because of early battery failures due to other problems, like inadequate chargers or knowing what SOC you are actually at. I only know of a couple of shore chargers that will charge to the amp based full criteria. Solar chargers have more selections that will do that. The smart regulators for engine charging don't have any that will do the amp based charging, AFAIK.

For reference, I sized our battery bank to be at 80% down in our worst case scenario. That would be off grid, no sun, hot weather so frig and fans using more power, and 5-7 days without moving. Two of our 3 charging sources will do the 20% of capacity (solar won't). At the end of the 5-7 days, we would want to have a very long drive and/or shore power to get back to totally full. I don't think it is practical, or necessary, to go much beyond this for us.