First rigorous Pb-Li comparison I have seen

[Johnnyfry]

When I embarked on my own LiFePO4 project on my van I did a lot of research before starting. One resource I used was Will Prowse's You Tube videos. While I gleaned a lot of information from these videos I don't know what qualifications Will actually had and how rigorous his testing procedures and data analysis was.
After a year of use in my 08C210P RoadTerk I consider the project a success and am happy with the results. I still have the 750 watt TrippLite Inverter/Charger and it has served me well for my usage profile.

Today I ran across a very interesting video done by an Electrical Engineer with experience in designing large battery backup systems. He did, in my opinion, the first research level analysis of LiFePO4/AGM/Flooded lead acid batteries I have run across. It is definitely worth your time:

John

 

[markopolo-ClassB]

Not useful or informative at all IMO. Pb battery low voltage cut-off is 10.5V for capacity ratings so any tests that don't take the battery to 10.5V are meaningless for accurate capacity testing. Sadly, the video maker doesn't know much about lead acid batteries it appears.

 

[booster]

Like Marko said, and that is much more common than ever among the lithium "impartial" testers who are usually not impartial. If they don't fully charge the lead acid batteries to the manufacturer's return/tail amp specs and run the capacity test to the standard 10.5v cutoff, all the testing is either biased or done by someone with very low charging knowledge.



All you have to do is look at the first screen as shown in the first post to know what is going to happen.


A Battleborn battery prominently displayed and the now normal goofy looking presenter mugging for the camera. Does anyone not believe that Battleborn doesn't have and hand in this? They are all over the web with similar "comparisons" so nothing unusual about this one in that respect.

 

[rockymtnb]

Not useful or informative at all IMO. Pb battery low voltage cut-off is 10.5V for capacity ratings so any tests that don't take the battery to 10.5V are meaningless for accurate capacity testing.

Curious about this, our AGM battery instructions always indicated that discharge should be limited to 12.2v (or sometimes 12.0) to avoid premature failure. Wouldn't that be the appropriate cutoff voltage when comparing battery performance for operational RV usage?

 

[@Michael]

Curious about this, our AGM battery instructions always indicated that discharge should be limited to 12.2v (or sometimes 12.0) to avoid premature failure. Wouldn't that be the appropriate cutoff voltage when comparing battery performance for operational RV usage?

I would not be enthused about taking an AGM down to 10.5v (100% capacity) every day, but taking a LiFePo4 to down to near 0% every day would not phase me. So I'd consider the useable capacity for an AGM to be dependent on how comfortable one is in fully discharging an AGM battery.

--Mike

 

[markopolo-ClassB]

The total lifetime kWh output is what really matters but the focus is misplaced on expected cycles.

I don't recall everyone who was involved in the original discussion on the forum re: lifetime kWh output. I wasn't, but the explanation made it clear that the penalty for deeper discharges is not as bad as presumed.

This GC2 chart was posted a while back:

The kWh math for 50% vs 80% would look something like this:

2 x GC2 210Ah 6V batteries (210Ah @ 12V)
675 cycles at 80% DOD = 12v X 168Ah (80% DOD) X 675 = 1,360 kWh total output until replacement
1100 cycles at 50% DOD = 12v X 105Ah (50% DOD) X 1100 = 1,386 kWh total output until replacement

Re: 10.5V, note that it is a common cut off point associated with inverter use. Also, the popular BD35/50F DC compressor unit used in fridges can be setup with a 9.6V cut-out. The standard cut-out for the BD35/50F is 10.4V.

There's also resting voltage vs under load voltage to consider. 10.5V resting is depleted but 10.5V under heavy load is not.

 

[booster]

I would not be enthused about taking an AGM down to 10.5v (100% capacity) every day, but taking a LiFePo4 to down to near 0% every day would not phase me. So I'd consider the useable capacity for an AGM to be dependent on how comfortable one is in fully discharging an AGM battery.

--Mike

I would not want to go to 10.5v any time except by accident, but that is the definition of capacity used in ratings. When we did all the calcs about lifetime KW hours of energy at different discharge % most of us used 80% 50% and 20% SOC cutoff points as going below 20% does steepen the loss of life vs gained KW hrs curve. IMO this is kind of irrelevant to the testing done and specs given for several reasons.


Number one is that most of the higher end, very reputable, lithium battery manufacturers tell us that leaving some "head and foot" space in you use pattern is very good for their batteries to maintain capacity and life expectancy. Just as I wouldn't use 100% of lead acid capacity, I would not use 100% of lithium capacity either. If you, or they, choose 10% on each end (the amount needed is discussed a lot and not really stabilized at this point) you come up with 80% of rated capacity, the same as with the AGM.



Another point is if you look at how the ratings are done, especially for lead acid. They are done at a fixed amount of time, normally like 5,20,100 hours to use the full capacity. That means the current is much lower at 100 hrs than 20 and even more less than at 5 hrs. Lead acid batteries are prone to have voltage drop based on output, so the ratings at the different hours are significantly different. Very few of us use our whole capacity at the 20 hour rate, and we for instance are closer to the 100 hour rate so we have more usable amp hours. Lithium has less voltage drop from discharge amps so the capacity doesn't change as much by the hour rating compared to lead acid. Many, many, of the lithium comparison tests use a high discharge rate which will inherently make them look much better than lead acid. This is a bias, IMO, as they are the ones that always are gung ho on the lithiums and are often showing specific brand placing like in the original post video.


Lithium certainly has some advantages, but it also has some disadvantages which you rarely hear about in the comparisons. Needing heat, no charging if under 32* (or 40* depending on who is asked). No discharge or storing under -4*, bottom and top space, full cutoff charging, abrupt shutoff that require special methods to restart and/or activate charging sources. A good place for us all to watch will be to watch and see how Avanti handles all the "difficult" parts of building a good lithium system as he appears to be on his way to doing it well.


I think the big issue with me is that there are so many of what I consider bought and paid for reviewers/comparers out there doing videos that a lot of non technical and inexperienced people are going into the lithiums with a severe shortage of knowledge and of what to expect.

 

[folivier]

Of course you can draw down any battery to 10.5 volts. But do you want to? Not only for the health of the battery but 12 volt devices especially motors are designed to work on 12 volts and not 10.5 volts. The lower voltage could lead to overheating or reduced life. This is where the LiFePO4's really shine with the much reduced voltage sag.

 

[RTRanger]

I accept the depth of discharge comparison to total amp hours delivered as valid. A test I would like to see would be a rate of discharge and lifespan.

I would suspect that if a small load of 20A is used in one test and then compare it to drawing a 100A load. We might see a different story unfold when looking a battery types. With more AC loads in modern all electric RVs we see higher prolonged current draws than just a few years ago when I started, we had a couple lights and a furnace fan as our main load on the batteries.

But just the amount of time it takes to recharge a lithium battery is a big selling point for me. It used to take forever to stuff those last couple amp hours in a lead acid battery to fully charge them (which lead acid batteries should be kept at).

 

[booster]

I accept the depth of discharge comparison to total amp hours delivered as valid. A test I would like to see would be a rate of discharge and lifespan.

I would suspect that if a small load of 20A is used in one test and then compare it to drawing a 100A load. We might see a different story unfold when looking a battery types. With more AC loads in modern all electric RVs we see higher prolonged current draws than just a few years ago when I started, we had a couple lights and a furnace fan as our main load on the batteries.

But just the amount of time it takes to recharge a lithium battery is a big selling point for me. It used to take forever to stuff those last couple amp hours in a lead acid battery to fully charge them (which lead acid batteries should be kept at).

Yep, exactly my point, and add to it, as you say, you have to apply it to the real world of RVing.


The load thing is so highly variable depending on the RV and the user, that it all needs to be tested at more representative test points the would more closely come to real world use.


In many cases, the higher discharge rates and being able to hold voltage become non issues because there is not enough capacity to maintain it decently in battery bank. That is why a single lithium 100 amp hour battery is not as beneficial as it would be in a 400ah bank for instance because you run out of AH too quickly if you draw a big load.



Most of the systems that are being done in smaller bank also seem to increasing bank size and that in itself is a bit deal in use. The savings of weight and space with lithium can help in that kind of install, except they should be inside the van which can hurt space. Lithium also often takes more charging and control equipment which east s up space.



On the small, more drop in type, installs that are being done it will of interest to see how the life of the lithium batteries is. Lead acid can last a long time with very good charging, which few systems have, and we routinely see considerably shorter life based on that. It may also be true for lithium.



It will be interesting to see where the final charge rate limits settle in at. In the beginning of all the lithium stuff we saw them touting 3C rates for instance, but now we see Battleborn, who also used to be very high rate charging, at .5C for instance. That will lengthen the charge time but still not as long a time as an AGM would take, if you go to full charge based on return amps. Of interest, I think, and at least to me, is that with Battleborn at .5C the charging from 20% SOC to about 80% SOC would be only 1.25 times faster than AGMs charged at .4C which is about the max we can do continuously. This means putting a day or two use back in a bank would take nearly the same amount of time. You do need to take the AGMs to full charge every 7-10 cycles though to maintain life, where with the lithium you would want to leave the under full charge for longer life, from what we now hear.


The place were lithium stands out is in big bank, high draw amps, big daily use systems. The voltage holding properties and higher initial voltages help there, for sure. Running AC off of batteries for instance.


For more moderate daily use, but with need for lots of time off grid, an AGM large bank can work well as you don't need the high discharge rate and voltage holding as much. Put in a bit of solar to top off when in the final 20% SOC to maintain life.

 

[GallenH]

Related question

Somewhere I read/heard that over time a AGM battery should be "re-rated." IOW: After several years of use a 100AH battery when fully charged will no longer be 100AH but, say, 96AH.

Is this true? And if so, is there a general rule of thumb about how to guess safely about the expected level of, say, a 5-year-old battery.

thx.glenn

 

[Davydd]

Having wet cell lead-acid, to AGMs to two very different lithium ion battery systems in 17 years I can say I would never go back to lead acid. I’ve had lithium ion now for 7 of those year.

40% weight of LiFe to AGM is a major factor and how I achieved a design of a short Sprinter with a 2500 chassis and still got a 576ah battery bank with 4 group 27 Valence batteries. My previous van had 800ah and the batteries were under the van where an Onan generator usually goes. ARV has been installing 6 Valence batteries inside short and long 3500 vans for over 800ah but for space and weight considerations I settled for 4 batteries.

The cutoff point of having a van operate electrically transparently whether boon docking or on shore power is probably 280ah commonly provided in Li-ion batteries for overnight use but I wanted 2-3 days. 400ah would be OK but I could still get more in my under bed design and still achieve weight goals.

You can draw power from below freezing Li-ion batteries but not charge them. You shouldn’t let them go below -4F. Consequently in travel or storage you need a heat source. In traveling I have an external heat source under the batteries to maintain them at a minimum of 41F. Obviously, that heat source will have to come from the Van heater or the batteries themselves. While driving a second alternator is going to supply amps more than taken out. I think Winnebago Revel says you can heat your van from the engine to bring them up above freezing. I question that approach. So external heat or some internal battery heat is necessary coming off the batteries is the safe way. My BLM monitors the battery heat which in turn calls for the external heat to come on. On long term storage you have to have a shore power hook up to provide external heat, garage in a heated space or remove your batteries to a heated space. The discharge rate of disconnected Li-ion batteries can span a winter. If you have the potential to go below -4F then it is a no go without external heat.

Here is my battery bank under the bed.

 

[markopolo-ClassB]

My first comment in this topic wasn't about the advantages and disadvantages of using LiFePO4 batteries. I just happen to prefer accurate information and found the linked video to be lacking.

Like several here, I have both lead acid batteries and LiFePO4 batteries. Some of the advantages are near impossible to beat. Some of the disadvantages are trip interrupting.

If you're going to test a lead acid battery the first learn how to charge it. Lead acid RV type batteries typically have a 20hr rating and sometimes a Reserve Capacity rating.

The 100Ah rating on a lead acid RV type battery is usually the 20 hour rating which is the number of Amperes of current the battery can deliver for 20 Hours at 80F until the voltage drops to 1.75 V / Cell which is 10.5 Volts for a 12V battery.

RC or Reserve Capacity is the time in minutes that a battery can deliver 25 Amperes at 80F until the voltage drops to 1.75 V / Cell which is 10.5 Volts for a 12V battery.

Randomly inventing a test other than those then complaining that the battery doesn't achieve those well defined ratings shows a lack of knowledge of lead acid batteries at best.

 

[booster]

Somewhere I read/heard that over time a AGM battery should be "re-rated." IOW: After several years of use a 100AH battery when fully charged will no longer be 100AH but, say, 96AH.

Is this true? And if so, is there a general rule of thumb about how to guess safely about the expected level of, say, a 5-year-old battery.

thx.glenn

Yep, pretty much all batteries of all types lose capacity over time based on use, conditions, charging, etc. so pretty much all batteries should be capacity checked once in a while, especially if you are using a good battery monitor SOC as it is calculated by using the entered actual capacity if done well.


Well taken care of batteries, including lead acid cells of all kinds, can last multiple times longer than ones that don't get great care or charging, so very had to tell how much they have deteriorated over time with testing.


I always suggest doing a capacity test down to what you would normally use for maximum discharge amount when you get new batteries so you have a baseline for the future and then check again periodically or if you start to question the capacity in use. When I did our first check I did not take them to 105v full capacity rared point. I put in the rated capacity and discharged 80% to 20% SOC and let the batteries rest there without load for a day and then compared voltage to Lifeline voltage specs for that SOC to see if it was the same as they said it should be. My voltage was actually higher than they said so we probably had a bit over rated capacity which is not surprising. I didn't use the tested capacity, and just left it at rated, and it later tests over 5 years time we still check with higher voltage than the Lifeline chart says so they are holding up very well. We do take good care of them though.

 

[GallenH]

Thanks, booster. I'll try the test out when RV is out of storage next. And I'm guessing that if discharging to 80% revealed a lower voltage than LifeLine specs, that would mean that your battery was not 100AH but less, correct? And from that voltage reading you could approximate/calculate the true AH in the battery (at least fairly close). Then you'd reset the monitor AH rating of the battery.

 

[booster]

Thanks, booster. I'll try the test out when RV is out of storage next. And I'm guessing that if discharging to 80% revealed a lower voltage than LifeLine specs, that would mean that your battery was not 100AH but less, correct? And from that voltage reading you could approximate/calculate the true AH in the battery (at least fairly close). Then you'd reset the monitor AH rating of the battery.

In our case it isn't 100ah as I tested the whole 440ah bank together but yes the voltage below what they saw would indicate under 440ah of capacity. I would probably just take 20ah or so off the capacity setting and run it again to see what I got. We have a lot of extra capacity as exact accuracy has not much to do with running out of power issues, but it would be good to know when they are starting to head downhill.