LiFePO4 charging - the overcharging question

[booster]

Here is a chart that is more like we are used to seeing.

[gregmchugh]

Very good Booster, where did you find that chart?

[booster]

Originally Posted by gregmchugh

Very good Booster, where did you find that chart?

Lithium Iron Phosphate (LiFePo4) - PowerTech Systems

[booster]

I think this is the style of chart and information that Avanti is referring to, which basically looks at the total amount of energy you can store and reuse over the life of the battery in relation to depth of discharge. This is for lead acid, and I haven't been able to find an equivalent chart for the lithium to this point. It does give very good real world information and kind of disputes the 50% being the best balance of cost vs utility in lead acid batteries as from 40% DOD to 100% are relatively flat. This is a flat plate battery, however. This chart shape would be what you would also expect for the Apple battery claim, i think.

[ptourin]

That's an intriguing chart (the one in #101). My usage is usually not high, so I'm looking at the C/4 curve. My draw-downs are also typically not deep - usage is usually 50-60% (90-120 Ah a day when dry camping) draw-downs with periodic deeper ones. If you couple that with the discussion about a cycle being a full discharge/charge (or 2 half discharge/charges, etc.), the picture gets very nice - it'd take many years of RV'ing to run through 5000-6000 cycles!

[gregmchugh]

Originally Posted by booster

Lithium Iron Phosphate (LiFePo4) - PowerTech Systems

Looks like they are also using 80% capacity at end of life so easy to compare to AGM specs....

[Davydd]

Originally Posted by booster

Found this one on an electric vehicle document so same chemistry. Seems to show non linearity if I am reading it right.

Is this chart telling me that if I don't discharge more than 80% I could anticipate up to 5000 cycles of use, not more than 90% up to 3000 cycles and 100% up to 1000 cycles? If so, it might explain how Advanced RV came up with their estimate of up to 5000 cycles since they make it impossible to discharge anymore than 80% without recharging. They did do some bench testing for over a year I believe. Roadtrek seems to have a set point of 90% with Voltstart enacting charging.

If the Apple example is anything right then I seldom if ever discharge more than say 33% in a day's time for easy math. So, in theory if I full timed in my RV and discharged 33% every single day or a cycle every 3 days, I could go a whopping 15,000 days or 41 years! Of course there would be more factors that would wear out a battery but it would seem impossible to wear out my batteries completely in any expected use. Also, that does not mean batteries will continue to operate at maximum expected efficiency either as witnessed by the Technomadia report of 25% loss in 3-1/2 years. What we don't know about that report is if they had DOD protection. I doubt they did as they mentioned they did not have the sophisticated BMS Elite Power Solutions offers today. Besides the heat they speculated on, they could have been nearly fully discharging with some regularity given they were powering a large Class A bus with less battery power than I have in my B.

One thing ARV and Roadtrek are doing is preventing full discharge. This is an aspect I don't think any B converter has ever done with lead-acid batteries and historically it has been very easy to fully discharge one or two lead-acid batteries via not so smart charging (like my first Pleasure-way), or leaving something on inadvertently like a 3-way absorption refrigerator on 12VDC. You could simply routinely be shooting past the recommended DOD of 50% to lower the life span of lead-acid batteries. All you can rely on is a beep sound.

Charging li-ion batteries below freezing will also shorten the life span. I have built in protection to always keep the batteries above a more suggested optimum of 41F.

Heat shortens life. This could be a concern of a southerner. As a northerner and religiously following the weather in my travels, I don't feel much concern. We spent two months (Feb/Mar) in the southwest with air temperatures as high 96F but because of day/night heat cycles the batteries never rose any higher than 88F on that worst day in Palm Springs. At home in Minnesota we had but two days last year above 90F I think. Heat is a subject that hasn't been fully explored and mostly speculated on since the manufacturers say li-ion batteries can function up to 149F.

Over charging will kill a li-ion battery. It appears my BMS is designed to prevent this. It's observable on the Silverleaf monitor. The belief of li-ion batteries existing in a 50-60% SOC is best is somewhat speculative and I am not sure in real terms what it means in the life of the battery. It definitely is not economically and functionally smart to limit the range of your battery use that much. The batteries haven't been around in real use long enough to verify theory. Anyway, instead of being plugged in or driving and having the batteries hover at 99% SOC with a use an amp, charge an amp state, ARV has reprogrammed the BMS to charge to 99% and then let the batteries discharge to 90% before charging again. I assume it helps if an optimum SOC has any bearing on life-time.

There are a lot of factors. No one can realistically believe anything can be used and last 41 years. I do feel the batteries could conceivably last the life of the B and I am confident will outlast my time with the B. I never had that hoped luxury with lead-acid.

The other major benefit is my 800ah li-ion battery pack is considerably less weight and space of 800ah of lead-acid and with the amp hour usage range is equivalent to 13 standard 12v lead-acid batteries which would be practically impossible to install in a Class B.

OK, the caveat. Despite what they say, I have a 7 year old li-ion battery iPhone I am now using as my articulating bed controller with Bluetooth operation. I do have to keep it plugged in as the charge just sitting there will barely last overnight. However, that iPhone battery has been discharged 100% more times than one knows. Bluetooth I know is a battery eater in that it is constantly searching a connection.

[booster]

Now I am getting really confused (no surprise there). I just put together a chart of the Lifetime AH for a 100AH battery vs DOD, based on the previous chart for the lithium batteries. I used the lower, blue curve from the chart. If I did the calculations right and read the logarithmic chart right, there is a huge difference in lifetime AH used based on DOD. Any comments or error finding would be appreciated, as this is exactly opposite of the Apple statements and the lead acid chart.

[gregmchugh]

Roadtrek originally stated they would discharge the Ecotreks to 90% and I have not seen any statement otherwise. With no data display of any battery status info it is difficult to confirm that the current configuration of the internal BMS and Voltstart are at 90% or not.

[booster]

It is a pretty odd coincidence that the total lifetime amp hours for 50% is just about double what you get at 80%.

With the lead acid you get double the number of cycles at 50% compared to 80%, but if you calc that the same way you only get 25% percent more lifetime amp hours.

[ptourin]

Interesting chart! It looks right - anyone who's looking at this should remember that the chart in #101 is logarithmic and this one is linear - don't get confused like I did <g>... So if this is right, you get BOTH more full cycles and more total amp-hours when you discharge more gently...

[booster]

Originally Posted by ptourin

Interesting chart! It looks right - anyone who's looking at this should remember that the chart in #101 is logarithmic and this one is linear - don't get confused like I did <g>... So if this is right, you get BOTH more full cycles and more total amp-hours when you discharge more gently...

Logarithmic charts are nice for saving space, but they sure can be deceptive when you are trying to compare life like in this case. It severely minimizes the differences.

Hopefully, we will be able to come up with some other cycles vs DOD to confirm the accuracy. I hate basing anything on one source.

[booster]

As long as I had a chart and field built, I decided to do the same lifetime AH vs DOD for a 100ah Lifeline AGM, based on their cycle life vs DOD chart. Very interesting if accurate.

[ptourin]

That does make the lith batteries look good! Interesting that you don't gain all that much from light usage, when you figure it in lifetime Ah...

[booster]

Originally Posted by ptourin

That does make the lith batteries look good! Interesting that you don't gain all that much from light usage, when you figure it in lifetime Ah...

Very interesting is that at the 70-80% depth of discharge, it looks like the lithium is only about twice the lifetime AH, and that is where they always push how much better they are, although the lithium chart was listed as conservative.

[gregmchugh]

My assumption has been that AGM batteries have a fixed amount of total energy that, to first order, was available over the life of the battery no mater what discharge scenario you used as long as you did not discharge them too deeply. That chart seems to confirm my assumption. The guideline to not discharge past 50% for a long life might be misleading, if you are going to use xxxxx amp hours of battery energy over the next yyy days it would seem the cost for your battery bank will be the about same if you pay more for a large capacity bank and discharge to 50% vs using a lower capacity bank and discharge to say 80%. The $/lifetime amp hour would be the cost to use to select your batteries taking into account the quality / reliability / performance consistency of the specific battery supplier.

[booster]

It sure makes me wonder where the "50% is best balance" rule came into effect, as this would indicate otherwise. Maybe it is related to the fact that undercharging is always the issue, in general, and deep discharges do come back better with big chargers and long charge times. It would be logical for the manufacturers to stay more in a range that folks can actually do when using their batteries, plus we would all buy more batteries

[booster]

The fixed amount of energy thing is logical, but this has to do with life, not capacity of the battery at different discharges or discharge rates. We have discussed the fact that Peukert is really a voltage drop issue and not related to battery capacity, so that all makes sense.

[ptourin]

I wonder if the 50% rule isn't just an over-simplification: "Keep your discharges to 50% or else you'll have to replace your battery frequently". The stuff we're talking over might be pretty daunting to a first-time RV'er <g>... But it doesn't seem too hard to say, "Your battery has a lifetime capacity - you can use it up slowly by discharging your battery gently or more rapidly by discharging it deeper. As Greg was saying - you can discharge 1 battery to 80% DOD or discharge 2 batteries to 40% DOD. Then you get into the pros and cons - handy to have the 2 if you ever need to discharge both to 80% DOD, but extra weight, extra cost - and so on. Even the 50% rule isn't totally simple. Before I had a battery monitor, I had no idea just where 50% was - I simply discharged until my (flooded) battery voltage under load was around 12.0 and guesstimated that I was around 50%.

[gregmchugh]

I have tried on occasion to get people to not obsess about the 50% number with the idea that the discharges will average out and you likely have a lot of shallow discharges that compensate for the times you go past 50%. I guess I wasn't pulling that completely from where the sun don't shine...