LifePO4 batteries for use with Midnite Classic 150?

[createthis]

I own a midnite classic 150 and I'm interested in replacing the small test agm battery in my diy portable solar generator with a more useful 100ah lifepo4 battery.

I exchanged emails with the folks at midnite solar and they recommend using a LifePO4 battery with a BMS so the midnite classic can be shut off via AUX2 when the battery is fully charged.

This seems to mean I can't/shouldn't use batteries like the StarkPower UltraEnergy series: http://store.starkpower.com/12V-100A...tery_p_24.html because their BMS is internal and doesn't have any outputs.

I see a few other options, but I thought I'd ask around here. Is anyone here running LifePO4 with a Midnite Classic? Which battery are you using and how do you have it set up?

Thanks!

[inetdog]

If the BMS is designed to cut off charging at a reasonable maximum voltage, then you should not need to cut off the Classic.
More importantly, for maximum life of a typical Lithium battery you should not routinely charge and hold it at above about 80% SOC.
If you set the output voltages of the Classic to that voltage level AND turn off temperature compensation, you should be OK and not get near the overcharge point.
Sunking and others can tell you why most internal BMS systems are NOT appropriate for batteries in PV service and you are better off without them. Search the old posts for very good discussions. (Google search adding site:solarpaneltalk.com to your search string.

[Sunking]

Unless you have a lot of experience with LFP batteries, do not even try LFP. In addition to a good BMS you also need other equipment you do not have just to balance the batteries and maintain them properly. It takes a lot of know how and just one mistake will destroy LFP.

[createthis]

Unless you have a lot of experience with LFP batteries, do not even try LFP. In addition to a good BMS you also need other equipment you do not have just to balance the batteries and maintain them properly. It takes a lot of know how and just one mistake will destroy LFP.

Thanks for the warning. I know it's a risk, but I'm not aware of any way to gain experience with LFP solar without actually doing LFP solar.

[inetdog]

...
I'm not aware of any way to gain experience with LFP solar without actually doing LFP solar.

Absolutely right, but please read everything first and do not cut corners on monitoring!
You can accumulate knowledge without doing, but you will not get all the benefits that come from experience.

You avoid mistakes through experience and you acquire experience by making mistakes. Just try to limit the consequences of those mistakes.

[createthis]

If the BMS is designed to cut off charging at a reasonable maximum voltage, then you should not need to cut off the Classic.
More importantly, for maximum life of a typical Lithium battery you should not routinely charge and hold it at above about 80% SOC.
If you set the output voltages of the Classic to that voltage level AND turn off temperature compensation, you should be OK and not get near the overcharge point.
Sunking and others can tell you why most internal BMS systems are NOT appropriate for batteries in PV service and you are better off without them. Search the old posts for very good discussions. (Google search adding site:solarpaneltalk.com to your search string.

It's strange that Midnite would recommend top balancing with a BMS while this forum recommends bottom balancing with no BMS. I understand the logic, I'm just not sure why Midnite is misinformed. It makes me think that not many people are running LFP. I know LFP is big in the boat scene though.

[createthis]

Absolutely right, but please read everything first and do not cut corners on monitoring!
You can accumulate knowledge without doing, but you will not get all the benefits that come from experience.

You avoid mistakes through experience and you acquire experience by making mistakes. Just try to limit the consequences of those mistakes.er

Sigh. I'm here, aren't I? Research is my middle name.

[inetdog]

It's strange that Midnite would recommend top balancing with a BMS while this forum recommends bottom balancing with no BMS. I understand the logic, I'm just not sure why Midnite is misinformed. It makes me think that not many people are running LFP. I know LFP is big in the boat scene though.

I am not sure what is behind Midnite's recommendations, so I can't really comment on that. But part of the problem may be that the most common DIY use of Li batteries is the electric vehicle market and the second most common is for mobile power for boats, etc. Both of these have different use characteristics and different criteria for evaluation than the stationary PV market. As a result the charging and battery management needs will be different too.

For EVs, top balancing and using a BMS to cut off current for the whole string when one battery reaches its low voltage limit gives you the maximum total energy per charge for a single use, and therefore the best performance and range. For RE you should never need to discharge the battery below 20%, nor will you design to to charge above 80%.
And you are (hopefully) going to avoid multiple C discharges.

[createthis]

I am not sure what is behind Midnite's recommendations, so I can't really comment on that. But part of the problem may be that the most common DIY use of Li batteries is the electric vehicle market and the second most common is for mobile power for boats, etc. Both of these have different use characteristics and different criteria for evaluation than the stationary PV market. As a result the charging and battery management needs will be different too.

For EVs, top balancing and using a BMS to cut off current for the whole string when one battery reaches its low voltage limit gives you the maximum total energy per charge for a single use, and therefore the best performance and range. For RE you should never need to discharge the battery below 20%, nor will you design to to charge above 80%.
And you are (hopefully) going to avoid multiple C discharges.

RE = Residential?

You're saying EV designs for max power output and residential designs for max longevity? That's weird though. If you cut off the bottom 20% and the top 20%, you're cutting off 40%. That's only 10% better than AGM's 50% DOD. I thought LFP usually could handle 80% DOD. Since this is a portable system, I have to wonder if I should design it more like an EV system. Maybe the additional cost of the BMS makes sense from a weight savings/power capacity perspective.

[inetdog]

RE = Residential?

You're saying EV designs for max power output and residential designs for max longevity? That's weird though. If you cut off the bottom 20% and the top 20%, you're cutting off 40%. That's only 10% better than AGM's 50% DOD. I thought LFP usually could handle 80% DOD. Since this is a portable system, I have to wonder if I should design it more like an EV system. Maybe the additional cost of the BMS makes sense from a weight savings/power capacity perspective.

RE = Renewable Energy. Daily cycling to far less than the full energy capability of the batter.
Do not believe the 50% figure for AGM. I do not think anybody here would recommend that for a long term off grid installation.
The discharge extremes that the battery can handle without immediate damage and the low and high end cutoff points that give you the most watt-hours over the lifetime of the battery are two very different things.
If you are trying to size the battery as small as possible to minimize weight or initial cost, then maybe you do want to go closer to EV design rules of thumb. Sunking?

[createthis]

RE = Renewable Energy. Daily cycling to far less than the full energy capability of the batter.
Do not believe the 50% figure for AGM. I do not think anybody here would recommend that for a long term off grid installation.
The discharge extremes that the battery can handle without immediate damage and the low and high end cutoff points that give you the most watt-hours over the lifetime of the battery are two very different things.
If you are trying to size the battery as small as possible to minimize weight or initial cost, then maybe you do want to go closer to EV design rules of thumb. Sunking?

Yeah, I know 30% is better than 50%. Less discharge equals more cycles.

So, if I want to maximize the capacity of the battery at the expense of cost and cycle life, I want to top balance with a BMS, right? I run the risk of the BMS failing and destroying the battery, but I get the most capacity per cycle, though not most capacity over the life of the battery.

In that case, which BMS/Battery combo would you recommend? Is the GBS stuff any good? It seems to be the most visible on the internet at the moment.

[inetdog]

It is outside my area of expertise to make a concrete recommendation. Try PNJunction instead.

[PNjunction]

In that case, which BMS/Battery combo would you recommend? Is the GBS stuff any good? It seems to be the most visible on the internet at the moment.

GBS is fine for our purposes. I run them in a simple 4S / 12v bank. GBS are more an "energy cell" than a "power cell", as far as capabilities go, and that is actually way more capable than we need *if you treat them in a sub-c application*. Like a solar housebank for instance, that doesn't usually see more than about 0.2C charge / discharge. 3C is available in short bursts, but we don't do that!

And along with good inter-cell connectors, they come with a 2-cent purple cover for each cell! Amazingly safe - since with other brands YOU have to build your own cell covers, (dropped wrench syndrome), and in your haste to get up to speed, these little covers might save your *ss right from the start.

Everyone has their own idea of how to run LFP. Much of it is spot-on depending on your application and your skill level.

If you are willing to take on some responsibility, YOU can become the BMS, which is made far easier for a low voltage - low current application.

Here is what I do for a solar application. YMMV. Generally I'm running from a max of about 10-80% DOD. That's about worst case, since with LFP, you want to stop thinking this way since there is no need to fully charge all the time. So, don't skimp on your battery and force it into a high-current application by getting too little capacity.

1) Charge each cell to full upon receipt. You can do this with a "nominal" 3.2 (three point TWO) volt single-cell charger, or your own variable bench supply. Set it to 3.6v, at no more than .5C (.3C or so preferable) and let the current fall to .05C, which for a 100ah bank would be 5A. Stop and move on to the next cell. You can do this to each cell individually while they are strapped together even.

2) Discharge normally with your regular loads, but don't let any cell fall below 2.7v while under charge. So that means some form of monitoring and of course an LVD. My personal LVD is set for 12.7v at the pack level. BUT, I try not to go there. That is a dead-man's catch - which is actually not deep enough to hurt the cells, but will start to get my attention.

3) When recharging, don't use the single cell charger - that's only for initial charging and PM maintenance down the road. Set your solar CC or ac charger to no more than 3.45v per cell. In my case, I have boosted that a teeny bit to 3.425v. This way, you won't exceed about 90% SOC, AND you'll never actually fully charge the battery if you mistakenly forget about the charger. Note that I'm not advising that floating the battery forever is ideal - this is just a "I forgot" kind of thing for a day or week or so.

Note, when dealing with the small range of voltages for LFP, one had better TRUST their multimeter for accuracy! Flukes of course do nicely - plenty of multimeter threads around.

In essence, my charger is the HVC. I run a small Powerwerx LVD at 12.7v. I have fully charged my batteries upon receipt, and am trusting in the GBS retailer to be providing me cells that are reasonably close in both capacity and internal resistance. Actually I have measured each, but that's the gist.

But that's me. Some may want to go in a totally different direction. Bottom balance, full-blown individual cell monitoring, automatic balancing - the list goes on and on. For me, if I have to treat LFP like if I sneeze it will die, I don't want to have anything to do with it. Still, I'm safe and I think what I'm doing is sane for a "sub-c" application like ours.

I have a bevy of lithium charging setups, balance chargers and so forth that I could use on my GBS cells. And I have! One can take it to that point if they want to like they would with an RC modeling setup, or racing EV setup. BUT, in this case, I stick to a simpler KISS method, with just a tad of sanity, and find it quite easy to handle.

Probably the best way to start out is to get a bank already assembled for you - that means already linked, and the pack strapped. Some guys do it themselves, but forget the strapping - and if you put some external force on your bank from a loose object, and the terminals twist or bend - bad news. Of course, get your cells from a reputable distributor, NOT some used junk or deal too good to be true. Lot's of LFP sharks out there waiting for the next sucker to buy junk.

You might be ready for LFP too - but maybe not the guy next door! I think a simple 4S / 100ah GBS bank is entirely manageable for someone who actually takes an interest in their batteries, and doesn't just ignore them.

[Sunking]

It's strange that Midnite would recommend top balancing with a BMS while this forum recommends bottom balancing with no BMS. I understand the logic, I'm just not sure why Midnite is misinformed. It makes me think that not many people are running LFP. I know LFP is big in the boat scene though.

You are misinformed. Any commercial application demands top balancing and using a BMS. Reason is you do not need to know anything about LFP batteries, and consumers don't know chit from shinola. Problem with Top Balance is it shortens battery life, and prone to failures which destroys the batteries.To Bottom Balance takes a lot of knowledge and know how which you do not have. Many DIY EV builders and a few Solar users use Bottom Balance to extend battery life, eliminate over discharge, and a lot less expensive to implement.

[Sunking]

Well I must say GBS is the worse of the Chi-Com batteries. Three points.

1.They are the most expensive of the Chi-Coms.
2. They have the shortest cycle life of the Chi-Coms
3. They have the longest Short Circuit Discharge Time of any LFP manufacture. (Highest Internal Resistance per AH)

1 + 2 = Real Expensive Deal Breaker.

They are just not worth it, I speak from my own loss and experience. FWIW to learn how to work with with LFP, you use a couple a $5 cells to destroy, and not a $2000 set.