Battery lifetime/cost vs. state of charge

[transmitterdan]

The manufacturers/technicians recommending 50% DOD have nothing to gain financially by making that recommendation. If it were "marketing driven" they would recommend the 20%-80% theory and sell lots more batteries.

And the manufacturers agree with the 50% DOD and frequent 100% top-ups recommendation. The only people I hear recommending 20-80% charge/discharge profiles have never done any testing nor published any findings.

[kmacdonald]

Originally Posted by transmitterdan

The manufacturers/technicians recommending 50% DOD have nothing to gain financially by making that recommendation. If it were "marketing driven" they would recommend the 20%-80% theory and sell lots more batteries.

And the manufacturers agree with the 50% DOD and frequent 100% top-ups recommendation. The only people I hear recommending 20-80% charge/discharge profiles have never done any testing nor published any findings.

I don't think anyone would recommend the 20-80% charge/discharge profile but without wind or solar it may not be practical to get above 80% SOC. The only reason to recommend a 50% DOD max is to compensate for other abuses. It is not cost effective.
If I'm not mistaken, Maine Sail uses the 80-50% SOC as the practical capacity of a cycled LA bank. Not everyone will put a wind generator or solar panels all over their boat. They ruin the looks of a boat to some extent.
IMHO Maine Sail has the best technical advice on the internet but he may want to re-evaluate his 50% DOD max recommendation in light of the facts.

[a64pilot]

OK I'm on an IPad and don't have my glasses on, but bet that chart assumes a charge to 100% on each cycle and then discharging to 20% cuts life in half compared to a 50% discharge, but only recharging to 80% is your killer, even if you only discharge to 50%, it's the PSOC that kills at least as fast as the deep discharge, but combine them?

Oh and we have a saying in aircraft testing, "real world data trumps analysis"
The survey is actual real world data, the charts etc unless based on actual usage is analysis.

[a64pilot]

To do what you want to do is a textbook case for Lithium batteries

[kmacdonald]

Originally Posted by a64pilot

To do what you want to do is a textbook case for Lithium batteries

Yes it is. I use lithium for the weight advantage, no need to charge to 100% SOC, charge efficiency, steady voltage, cycle life. It really is a game changer for marine use but isn't for everyone yet. They have zero tolerance for abuse.

[Maine Sail]

Originally Posted by Pizzazz

Another take on your data, I think it is totally acceptable to lose 1% capacity on every cycle if not charging to 100% during offshore passages. Once the passage is complete, the batteries can be equalized and restored to most of their original capacity. How many 30-day offshore passages away from marinas does the average sailor make in 2 years time? Not that many.

That capacity loss was permanent and not recoverable. Equalizing is a too infrequently used Band-Aid for PSOC use and will only usually recover a 1-3% of capacity at best when done infrequently. Equalizing can be more beneficial if done more frequently but most boaters simply don't do this. By the time most boat owners get around to an EQ cycle, after extended PSOC use, the sulfation is hard, clustered and not recoverable thus the vast majority capacity degradation has become permanent.

We should also keep in mind that by industry standards a battery is considered failed when it can no longer deliver 80% of its original capacity. While batteries will still perform at capacities below 80% of new the risk of failure, such as an internal short, increases fairly steeply the lower we go. This is especially true in flooded batteries where weakened plates & grid structures can cause an unexpected failure. The highest rates of internal shorts/failures I see occur in flooded batteries but they can also occur AGM or GEL.

Another often over looked aspect is Peukert effect. The Peukert corrected load on a larger bank of batteries, where average discharge loads remain the same as a smaller bank, serves to make the larger bank bigger in effective usable capacity rather than smaller.

300Ah Bank - Peukert 1.27 - 8A Load = Peukert Corrected Bank Capacity - Approx 355Ah - Peukert Corrected Load 6.75A

100Ah Bank - Peukert 1.27 - 8A Load = Peukert Corrected Bank Capacity - Approx 88Ah - Peukert Corrected Load 9A

The 50% recommendation is not one created by boaters it is one recommended by the battery manufacturers themselves. In fact 50% is not really a suggested DOD. Most manufacturer's suggest the shallowest cycling possible results in the longest cycle life..

As one of the very few in the marine industry who owns the equipment to test actual battery Ah capacity, (at any given time I have two batteries under test, I have some Northstar's finishing up any time now) I can say that for certain infrequent situations, with a quality battery bank, I do recommend 80% DOD. There is only one lead acid battery that is suggest 80% DOD as the regular discharge level and that is the Firefly Carbon Foam battery.

This recommendation is usually for short duration off-shore passages, race boat passages etc., but I do not recommend that as a standard DOD for a PSOC use battery. Actually I can't say that entirely because I do have a few ultralight race boats where the deign is for regular 80% DOD but they replace the expensive AGM batteries yearly abd GEL batteries bi-yearly.. Even in just a yearly race circuit the AGM batteries are often delivering just 45-55% of original capacity by years end and are bordering on dangerous. GEL's do better but they tend to charge a bit slower. Prior to the Firefly the Odyssey or Northstar TPPL batteries were the usual choice due to speed..

If you were a day sailor who charged at a dock after each sail, sure 80% DOD could give you a decent outcome with quality batteries. The data I get to gather is based on actual real world testing of Ah capacity and being able to see and watch what different use behaviors result in out in the real world. I suspect I have collected more of this real world marine use capacity data than anyone else in the marine market.

A fair number of us in the marine industry have been talking about the misleading guidance presented in manufacturer cycle life data, with respect to real world use, and finally the manufacturers are starting to openly discuss PSOC use.

The key word here is "discuss". On a sadder & more frustrating note we are now being marketed PSOC use batteries with zero PSOC industry data, with the exception of Firefly, to back up the marketing claims. Requests for PSOC testing data has been asked for repeatedly and as of yet only Firefly has been forthcoming.

On one "PSOC marketed" battery it was found that it did no better in PSOC testing than one not marketed/sold for this use? Without manufacturer data how is one to know the truth behind PSOC marketing claims?

Will we ever see PSOC testing data from the industry? I hope so, but if I had to guess I would say this is highly unlikely because 80% DOD in a PSOC environment essentially murders most lead acid batteries in very short order and no manufacturer wants to publish that type of a $hit show.

We do have batteries that handle PSOC and deep cycles better than others do and if you want to maximize DOD then the Firefly Carbon Foam or a quality GEL battery can perform quite well for this type of discharge level. I would not suggest anyone expect much from a run of the mill flooded "deep cycle" Group 24, 27 31 etc. at 80% DOD.

Data I have from East Penn shows a flooded "deep cycle" group 27 (DC27) delivering just 200 lab cycles to 80% DOD and this was under an easy 25A reserve capacity test. The same case size battery from the same manufacturer, only in GEL, delivers 600 lab cycles to 80% DOD. While this cycle life number will be considerably lower in the real world, almost always less than half, it can still give a fairly respectable cycle number when compared to the flooded Group 27.

[Maine Sail]

Originally Posted by kmacdonald

IMHO Maine Sail has the best technical advice on the internet but he may want to re-evaluate his 50% DOD max recommendation in light of the facts.

I have real world data and I consider that data pretty factual, and extremely telling, considering it is data the manufacturers either don't have or they refuse to share, for obvious reasons. I suppose it's much easier for them to simply hide behind fairy tale cycling numbers rather than tell the truth about deep discharges and PSOC use? With the advent of lower cost capacity testing units battery manufacturers can no longer mislead people like they used to. It is my hope that within the next 5-10 years every boater will be able to afford a constant current battery Ah capacity testing device that shows them exactly what their use/charging behavior is doing to their batteries.. When this eventually happens, in large numbers, the manufactures will have no choice but to stop misleading the general public on cycling data. I suspect you will also see that some rather egregious Ah capacity claims will start dropping too.

I generally recommend that as a worst case usable cruising capacity that you want to aim for. Capacity sizing should consider cycling 50% to 80-85% of the bank or 30-35% usable capacity between full recharges. If you have solar or wind you may be able to push upwards of 40-45% of bank capacity for PSOC use. The less deep you go and the higher you get to each day means longer cycle life.

I am also pretty specific that PSOC use be as infrequent as is possible. Somewhere along the way folks have incorrectly assumed or surmised that you can do 50% to 80-85% nearly indefinitely or for multiple weeks and this is death to batteries. 100% SOC recharges as often as is humanly possible will result in the best cycle life for all lead acid batteries.

For most lead acid batteries I prefer to design a cruising bank that does not cycle to 50%. This gives significantly better voltage support to equipment and results in considerably better cycle life. I also prefer to design in solar, wind, hydro etc. so that we can get to a higher SOC daily than we can from generator or alternator charging.

50% is not a magic bullet and if on occasion you need to dip to 80% go for it, but do follow it up with a 100% SOC recharge and an hour or so of EQ as soon as possible..

Where it is appropriate for 80% DOD I do recommend it, but based on the actual 20 hour capacity testing I do, on a daily basis, 80% DOD mixed with PSOC use is simply murderous for most batteries. Those are the facts as I measure & test them based on real world usage testing of batteries under various charging & discharging scenarios.

In a perfect world I would love to take more lead acid marine batteries to 80% DOD. There are some major benefits to 80% DOD not even being discussed here, but for most batteries, this makes for far too big a dent in cycle life to make it worthwhile ...

[Pizzazz]

Agree than lithium batteries are best for deep discharge. In fact, I was considering replacing my battery bank with lithium, then started to do the lifecycle cost calculations (based on 30% useable capacity for wet cells vs. 90% for lithium), then looked at the additional investment required in chargers, etc. then came up with the idea of running my golf cart batteries to the ground and replacing every two years as the most cost effective solution. Based on all the comments so far I think discharging to 80% will be fine in my case. I spend more than 2/3 of the time plugged in, my passages are 1-3 days mostly and I will start equalizing monthly (thank you MainSail).

Another example, I just looked at my group 27 starter battery. It is 10 years old and still going strong. Need to do a load test to see where it is. Some people would be proud of taking such good care of a battery. I feel dumb for not abusing it more and replacing it every three years at $100. Different strokes for different people.

[Maine Sail]

Originally Posted by Pizzazz

Based on all the comments so far I think discharging to 80% will be fine in my case. I spend more than 2/3 of the time plugged in, my passages are 1-3 days mostly and I will start equalizing monthly (thank you MainSail).

As long as you are happy with that model then it works fine. Many boat owners are horrified to have to replace batteries at 7+ years let alone two.

I replaced a failed bank in August where the owner was bemoaning the cost and "frequency" of battery change outs. His charging and use is pretty decent compared to most. His off-brand golf cart batteries were date coded December of 2005 and they had been installed in April of 2006. Ten years is fairly decent life..

I would suggest having a solid reserve/start/get you home battery as back up though..

[S/V Alchemy]

Originally Posted by Maine Sail

I have real world data and I consider that data pretty factual, and extremely telling, considering it is data the manufacturers either don't have or they refuse to share, for obvious reasons.

Indeed. Because I was in a position to onboard a large bank of six L-16s (steel boat, batteries low and under the mast, 1575 Ah size), my goal is to never, save for an emergency, to dip below 75-80% SOC. I just consider that 20% to be my "working reserve" and run the boat accordingly.

I have yet to do my first equalization (the bank's has yet to drop low enough on mostly shore power) but thorough monitoring and SG tests, and the installation of various remewable charge sources, I feel confident I will keep these batteries healthy for many years, simply because I sized them to my energy budget and have treated them according to precepts I learned from Maine Sail's considerable reporting on how to get the best from your batteries.

If you're off a dock, you have to consider other and often multiple ways to keep the batteries at full capacity, meaning the cost of batteries alone is only a partial consideration. There's the cost of an efficient, perhaps large-case alternator, the cost of decent external regulation, and a fairly strict regime of usage and charging monitoring. By the way, the burning of diesel to put amps at correct voltages into the bank is the least-preferred of our options, and is always done in the context of having to motor for other reasons.

[kmacdonald]

Maine Sail, do you think cycling batteries to 80% DOD is OK if they are charged to 100% SOC routinely? In other words, is it the 80% DOD or the PSOC that is the worst for batteries. I have always assumed it was the PSOC that is worse for them and that is not the use profile of boats plugged in to the dock. In Maine most boats a kept on moorings and therefore are not plugged in to run a charger so you may experience more of the worse case scenario.

[S/V Alchemy]

Originally Posted by Maine Sail

I would suggest having a solid reserve/start/get you home battery as back up though..

I have both an ACR for the start battery and an Echo Charger for the windlass battery for these purposes. Ganged, they can make an emergency house bank, should that be required. The requirement on me is to make sure the monitoring is accurate.

[Cadence]

Originally Posted by StuM

20-80% SOC is not "abusing" lithium batteries. But it is abusing FLA.

Charging above 80% is a not "costly, inefficient, not practical" if you use solar, wind or motor a lot. Only if you rely on a genset or running an engine at anchor.

Apart from the reduced life, you will very quickly get reduced capacity and charging efficiency as the 80% charged batteries start to sulphate.

I also doubt your 2 versus 4 year figure. Properly looked after you should get a lot more than 4 years out of a good set of LA batteries.

It is not the in thing to use plan old LA batteries.

[Adelie]

Originally Posted by Pizzazz

It is often said on this forum that it is advisable to keep wet cell batteries above 50% state of charge to ensure maximum life. I have been analyzing my battery setup and came to the conclusion that it is cheaper to abuse the batteries than to follow the advice above.

The logic is that if we discharge the batteries to 20%, they will last half as long (650 cycles vs. 1200 cycles) but will cost half as much for the same capacity (from 80% to 20% SOC vs. from 80% to 50%; I think we all agree that charging above 80% is costly, inefficient and not practical at sea).

So why do people feel so religious about the 50% SOC cut-off? This logic also defies the case for lithium batteries. I can see many benefits to "abusing' the batteries down to 20%: you carry half the weight, bulk charging from 20% to 80% is nearly 100% efficient, i.e. the batteries take in all the current from the alternator and so on. The downside is that the batteries need to be replaced every two years vs. four years (heavy), probably more maintenance (checking the water level, etc.) and slightly higher risk that one of the cells will short circuit in the most inopportune moment (we have the starter battery for that). Any thoughts?

Regards,
SV Pizzazz

I've thought about this before and ultimately saw several problems with plan.

A] There are installation and opportunity costs as well as purchase costs. Let's assume the cycle life really was as advertised. In terms of power extracted x number of cycles vs cost over time then cost is a wash.

However, how many and how big are the batteries? Would you swap it out yourself, or pay to have it done?

If you have a couple of 80lb or smaller batteries, you can probably pull them yourself, move them to your car, get them to the store, swap them, then back to the boat and reattach the wiring in 4-6hr.

If you have more batteries add 1 hour per battery.

If you have larger batteries, that's a different ball game. 8D batteries weigh in the vicinity of 165lb, 4Ds are about 125lb. Getting them in and out of their location on the boat probably means an extra person. Gettting them out of the cabin onto the dock probably means a block and tackle . Into the car is the extra person again. Hand trucks to get them up and down the dock. Let's say that 3-8D batteries are going to be 8-10 hours to replace.

So you looking at spending 2-3 days every 4yr replacing batteries instead of 1-1.5d days.

Or you pay somebody to do it for you.



2] The available life-cylce graphs are predicated on fully charging every cycle. If you do a lot a Partial-state of Charge (POC) operation (discharge after a partial recharge) as envisioned above then the curves aren't valid, the POC operation will dramatically shorten life, so you will wind up replacing batteries every year instead of every other year. I don't know if anyone has produced POC/DoD vs life cycle to know if the 2:1 life cycle ratio remains constant.

If you don't do POC operation but recharge fully every time or almost every time then you are no longer even breaking even on purchase cost. Theres a 50:40 advantage to the 50% DoD operation. Plus double the time and effort costs.




For specific purposes like reducing battery weight for a long distance race discharging to 80% DoD would make sense. For general use I don't see it.

[tanglewood]

Originally Posted by kmacdonald

Maine Sail, do you think cycling batteries to 80% DOD is OK if they are charged to 100% SOC routinely? In other words, is it the 80% DOD or the PSOC that is the worst for batteries. I have always assumed it was the PSOC that is worse for them and that is not the use profile of boats plugged in to the dock. In Maine most boats a kept on moorings and therefore are not plugged in to run a charger so you may experience more of the worse case scenario.

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.