LiFeP04 Batteries for Solar & BMS

[tasman]

I understand the psychics involved, that's not my argument, in a perfect world you'd use 24-48v, but it's about economics and availability. Now I'm retired (70), we travel for about 7-9 months of the year and decided about 2 years ago, after a couple of expensive electrical repairs which now know could have done myself for almost nothing, decided to learn how to do electrics and simple electronics myself.

Have always fixed most 12v faults in vehicles etc, but never things like lifepo4, inverters are energy systems. So that's where I'm at, understand what you're saying, but when looking at the economics, nothing computes for a reasonable outcome. To me it's much better to have a number of cheaper replacements than spend a heap on some brand name. We've only had one problem and that was a manufacturing fault, will stick with this approach until something make me change.

Have come across lots of people who accepted the professional approach and ended up with expensive gear and when it breaks down, they either have to travel for long distances to find a technician that can handle it, or wait for an expensive replacement or import. During which time they may not have any power and its the same with off gird system in lots of Aus, because of remoteness and availability of supply.

I use easily to get components, which are cheap and seem to work just as well as the brand name expensive stuff. My BMS is one which is for an EV cost juist over $200, the same quality with less progammabilty, costs well over $500. Mine has been reprogrammed for off grid, sadly they are no longer manufactured and luckily bought 4. They perform flawlessly and never had a problem, I've altered and continue to refine the operation, if I had the expertise I'd reprogram and re-engineer it so it does everything I want. But expect that will occur with the system I'm putting together using other simple cheap cell and battery monitors, once I fully understand how to put together what I want, properly.

Paid $225 for my 3000-6000w 12v inverter, similar in 48v is more than $2000, in 24v, from $900. In the years it has been in use, nothing has happened adversely, nor come across anyone else who has problems.

Have stated a number of times, which no one seems to listen to, use 12v in our MH for most appliances and a 1000w 240v inverter.

In our home everything runs through the 3000w 12v inverter and nothing runs on 12v. We have 240v LED globes and all other things run on 240v, including vacum, two large freezers, fridge, 65cm LED tv and sometimes a PA. If they were all used together then sure there would be problems, we switch off non required things to use something else (vacum or PA). Many times checked the inverter to see if it's effected and it's fine, no hot wires or anything that looks adverse. The distance between the battery pack and inverter, is less than 30cm and use the wiring that came with it. Could have got away with a smaller inverter, but prefer to use one that is idling along, rather than peaking at times. Doubt its ever used more than about 1500w and that would be for the vacum cleaner, everything else wouldn't make 1000w.

As for the economics, try buying 24v or 48v appliances for your MH at the same price and variety you can get in 12v, almost impossible. As it is my MH is 24v and have to convert it to 12v for some vehicle things, because they don't come in 24v cheaply and it's the same with internal lights, radios, small chargers, dash and reverse camera's, try getting them in 24-48v for the same price.

Hate that title, RV makes out you are a recreational of road vehicle, when in fact the majority are bitumen conga line travellers and spend the majority of their time in van parks. Which really makes them motorised caravans and not mobile homes.

Like real MH owners, we are completely self sufficient when we travel, even for fuel as all our vehicles and machinery run on veggie oil and my MH carries 1200lt of it.

[Sunking]

Tas if you are mobile in MH aka RV, that is fine. But a wired home, 12 volt does not fit is all I am saying.

[Living Large]

Tas if you are mobile in MH aka RV, that is fine. But a wired home, 12 volt does not fit is all I am saying.

Yes, my comment was directed solely at off-grid home use 2000W or above, and I believe others were as well. Not 1000W or below. At 1000W, you are at 80A or so - which is OK IMO.

[tasman]

Tas if you are mobile in MH aka RV, that is fine. But a wired home, 12 volt does not fit is all I am saying.

I don't disagree with you, but been using the 3000-6000w 12v inverter for close to two years and never a problem. I expect if I was using 3000w+ continuiously, then there may be problems. But we never use more than 1500w and then only for the vacum cleaner which is supposedly 1800w. It all boiled down to economics, this was so cheap compared to higher voltage inverters, just couldntl go past it and ir works.

[Living Large]

I don't disagree with you, but been using the 3000-6000w 12v inverter for close to two years and never a problem. I expect if I was using 3000w+ continuiously, then there may be problems. But we never use more than 1500w and then only for the vacum cleaner which is supposedly 1800w. It all boiled down to economics, this was so cheap compared to higher voltage inverters, just couldntl go past it and ir works.

I don't recall anyone saying there would be "problems" or it isn't possible. I believe what the majority are saying is it would be atypical to design and build a system capable of putting out 3000W at 12V. I would never consider it, but I'm glad yours meets your needs.

I wouldn't want to *build* such a system, let alone think about what problems might pop up. For my system, I needed 33,600 Wh of battery capacity @ 48V, or 700Ah. I would need 2800Ah @ 12V.

[tasman]

I don't recall anyone saying there would be "problems" or it isn't possible. I believe what the majority are saying is it would be atypical to design and build a system capable of putting out 3000W at 12V. I would never consider it, but I'm glad yours meets your needs.

I wouldn't want to *build* such a system, let alone think about what problems might pop up. For my system, I needed 33,600 Wh of battery capacity @ 48V, or 700Ah. I would need 2800Ah @ 12V.

You use 700ah a day, what sort of equipment would use that much energy daily. The reality is, how much your system uses daily and how it will cope with spikes in use. not some fanciful figure you feel you may need. We are dealing with lifepo4, not 19th century L/A, past its use by date, junk. The differences are dramatic, especially charge times and usable capacities.

I understand my system doesn't suit the acceptable parameters, that's the case with lots of things and for me, it all boils down to economcis, reliablility and functionality. After all, innovation, development and creation of new systems, always comes about by those who challenge the accepted approach and make it work.

If we stuck with the accepted claimed reality, we'd all still believe in lead acid, fossil fuels, god and economic growth as our saviours. When the factual reality is the opposite and it's the same with all aspects of evolution. Experiment, challenge the norm, do it safely and responsibilly. Then you get real results, not perceived ideological ones. My approach works, is economical and safe with the energy reuiroments we need. The inverter seems over kill, but it means it never works beyond about 50% of it's capacity. This means it should last much longer than other inverter pushing 60-100% of its capacity constantly.

[Mike90250]

It does not matter what equipment is running, if a persons household load is 15Kwh, then a 30Kwh battery (planned usage capacity 20% - 85%) is needed, at minimum, to reduce generator runtime.

Let's compute in Watt Hours, since everyone is dealing with different voltages (12, 24, 48) just stating amp hours is confusing.

Example, I average about 7.5 Kwh usage daily in winter. To be able to skip a charging day, and NOT be totally flat after 48 hours, I need to have 3 days (72 hours) 23Kwh of USEABLE battery. 20-85% middle of the battery capacity. 35% un-useable because of damage risk to cells. So that would require I buy a 31Kwh battery so I have time to start the generator on day #3 of cloudy weather. This would be in a 48V based system, so that would need 645ah of battery.
here's a 200ah cell that looks reasonable.
batteryspace.com/lifepo4-prismatic-module-3-2v-200-ah-10c-rate-640-wh-un38-3-passed-dgr.aspx $370

I'd need 3 parallel strings, to get to 600ah, and that would be 48 cells, @ $24K With >2,000 cycles, 1 yr warranty. So I get maybe 3,000 cycles out of them, 8 years. $3K yearly battery cost.

So, I'd need to tweak the bottom and top %'s a little bit to get really accurate, but I think it's close to start with. And if I'm way off, tell me, I'd like to have some realistic numbers to plug into here.

[Sunking]

You use 700ah a day, what sort of equipment would use that much energy daily.

That is completely meaningless information. Amp hours do not mean squat unless a voltage is specified. Watt hours is the measure of electric power consumed, not Amp Hours. You have not made that connection or understand that. 700 AH at 12 volts = 8400 watt hours. 175 Amp Hours at 48 volt = the same 8400 watt hours.

Maybe this will help. Let's say you go out and buy 4 12 volt 100 AH batteries. You have 3 possible ways you can configure them of 12, 24, and 48 volts

4 in parallel is 12 volts @ 400 AH = 4800 watt hours. We connect a 1000 watt load and it will draw 84 amps
2 in series in parallel with 2 in series (2 x 2) gives us 24 volts @ 200 AH = 4800 watt hours. We connect a 1000 watt load and it will draw 42 amps
All in series is 48 volts @ 100 AH = 4800 watt hours. We connect a 1000 watt load and it will draw 21 amps.

The 48 volt configuration is much less expensive to install, and far safer than 12 volts. There is no way you can win the argument using lower voltage is less expensive or safer. Sorry but if you think otherwise you are delusional.

[Living Large]

You use 700ah a day, what sort of equipment would use that much energy daily. {snip}

No, I didn't mean that, if that is how it was interpreted. A 48 volt 700 Ah battery bank, with 2 to 3 days of autonomy and 50% to 60 DOD. So, 700 Ah * 0.6/3, Or about 140 Ah a day (at 48V).

This was for an FLA bank, but we have strayed significantly from LFP and BMS and for the sake of comparing 12V, 24V and 48V systems, I don't believe it matters too much.

[northerner]

Getting back to the topic of this thread, I came across a really informative write up on an LiFePo4 based system. This is by a fellow that uses his system in a marine environment, but there are many similarities to using LFP as a house bank. He gives really good info about precautions, the bms, cell balancing, etc... Really great writeup!!! There are 3 pages.

http://www.pbase.com/mainecruising/lifepo4_on_boats&page=1

[Living Large]

Getting back to the topic of this thread, I came across a really informative write up on an LiFePo4 based system. This is by a fellow that uses his system in a marine environment, but there are many similarities to using LFP as a house bank. He gives really good info about precautions, the bms, cell balancing, etc... Really great writeup!!! There are 3 pages.

http://www.pbase.com/mainecruising/l...n_boats&page=1

I quickly browsed that article, and there is a lot there. Thanks for the link.

True confessions - I have to admit I have to go back to before this thread left the LFP and BMS discussion - Dereck was helping me to understand how to integrate a BMS into my proposed system, and I didn't read his last few contributions several days ago. This article and the previous discussion here should keep me busy...

[codo1982]

Alliance Renewable Energy, Inc

Please help, I'm looking to buy some LifeP04 Batteries and I found this place. Is it any good? Is any of you buy LifeP04 Batteries from here? Any suggestion where to buy them. Thank You for your help.

[PNjunction]

What I have been hearing and seeing in videos of users is it tends to stay in balance for extended periods of time (months).

If you are running <1C, preferably <.5C, and are not going into the extreme ends of charge / discharge, they tend to stay in the *state* of whatever balance you last left them in. So it is important to get them close, but if you aren't going to extremes in either current or SOC, you may never see a problem.

I think we forget why we (or at least for me) chose lifepo4 in the first place. Cycle life wasn't the main criteria.

Unlike EV or RC, where one is usually concerned with total miles / flight per pack, and the associated issues of weight, with solar, we have the luxury of running in a sliding-window of SOC.

Just like Pb, I recommend designing capacity for no more than 50% discharge, but for different reasons.

With lifepo4, you are not penalized for partial-state-of-charge operations, and are actually rewarded for it. Makes it a 'natch for solar. Since we are not dealing with Peukert to any extent, it also makes calculating your capacity needs much easier. I have found that simply de-rating the stated capacity to only 80%, and with no Peukert to deal with, I can get very close mentally with static loads.

But what if you over-estimate your bank capacity? While nobody promotes "growing into" a battery capacity, if you do make a mistake and make it larger than needed, the hit in your wallet will be compensated somewhat by running narrow PSOC with extended cycle life beyond the norm. Yes, nobody I know has taken these things beyond 10 years in this mode, but at least that's what the manufacturer's say. Take that with a grain of salt, but chemically that is supposed to be beneficial by staying away from the top and bottom edges of capacity.

Example: if you design for no more than 50% discharge, you can easily run 100/50, 90/40, 80/30, or 70/20 SOC windows and are never penalized for doing so. In addition, NO corrective topping-up or in the case of flooded, EQ is necessary. You may have a genny on hand, but if you do your homework right, even with sliding windows of SOC, as long as you have ample capacity to run your loads, the fuel in your genny might go stale before ever having to use it. Of course voltage is a poor indicator of capacity in the flat part of the curve, so one will want to count / calculate coulombs.

In the EV / RC world, nobody in their right mind would over-size their bank to take advantage of this PSOC - incurring an unnecessary weight and size penalty to stay away from the edges. We have that luxury be it intended or a goof.

What seems to get overlooked are the *practical* aspect advantages of running lifepo4, and how simplifying operations by choosing decent capacity with sliding windows of psoc may alleviate the daily worry of pin-point precision balancing needed with EV / RC. If you design it right, you'll never reach top or bottom regularly and increase cell life further.

Other than the hit on your wallet, over-purchasing lifepo4 won't take a toll on the cells, unlike Pb where you MUST reach 100% SOC on at least a somewhat regular basis. In addition, leaving your lifepo4 in a partially discharged state, provided you don't have any parasitic loads, won't hurt you either.

So balancing / bms discussions are fine - but don't forget some of the practical reasons that differ a LOT from EV/RC. Before spending a dime on a huge permanent setup, get a small bank (like a simple 12v 4S battery of 40ah or so) and witness it yourself. This is no different than getting a "learner" battery for first-time Pb solar operations.

[Living Large]

If you are running <1C, preferably <.5C, and are not going into the extreme ends of charge / discharge, they tend to stay in the *state* of whatever balance you last left them in. So it is important to get them close, but if you aren't going to extremes in either current or SOC, you may never see a problem.

I think we forget why we (or at least for me) chose lifepo4 in the first place. Cycle life wasn't the main criteria.

Unlike EV or RC, where one is usually concerned with total miles / flight per pack, and the associated issues of weight, with solar, we have the luxury of running in a sliding-window of SOC.

Just like Pb, I recommend designing capacity for no more than 50% discharge, but for different reasons.

With lifepo4, you are not penalized for partial-state-of-charge operations, and are actually rewarded for it. Makes it a 'natch for solar. Since we are not dealing with Peukert to any extent, it also makes calculating your capacity needs much easier. I have found that simply de-rating the stated capacity to only 80%, and with no Peukert to deal with, I can get very close mentally with static loads.

But what if you over-estimate your bank capacity? While nobody promotes "growing into" a battery capacity, if you do make a mistake and make it larger than needed, the hit in your wallet will be compensated somewhat by running narrow PSOC with extended cycle life beyond the norm. Yes, nobody I know has taken these things beyond 10 years in this mode, but at least that's what the manufacturer's say. Take that with a grain of salt, but chemically that is supposed to be beneficial by staying away from the top and bottom edges of capacity.

Example: if you design for no more than 50% discharge, you can easily run 100/50, 90/40, 80/30, or 70/20 SOC windows and are never penalized for doing so. In addition, NO corrective topping-up or in the case of flooded, EQ is necessary. You may have a genny on hand, but if you do your homework right, even with sliding windows of SOC, as long as you have ample capacity to run your loads, the fuel in your genny might go stale before ever having to use it. Of course voltage is a poor indicator of capacity in the flat part of the curve, so one will want to count / calculate coulombs.

In the EV / RC world, nobody in their right mind would over-size their bank to take advantage of this PSOC - incurring an unnecessary weight and size penalty to stay away from the edges. We have that luxury be it intended or a goof.

What seems to get overlooked are the *practical* aspect advantages of running lifepo4, and how simplifying operations by choosing decent capacity with sliding windows of psoc may alleviate the daily worry of pin-point precision balancing needed with EV / RC. If you design it right, you'll never reach top or bottom regularly and increase cell life further.

Other than the hit on your wallet, over-purchasing lifepo4 won't take a toll on the cells, unlike Pb where you MUST reach 100% SOC on at least a somewhat regular basis. In addition, leaving your lifepo4 in a partially discharged state, provided you don't have any parasitic loads, won't hurt you either.

So balancing / bms discussions are fine - but don't forget some of the practical reasons that differ a LOT from EV/RC. Before spending a dime on a huge permanent setup, get a small bank (like a simple 12v 4S battery of 40ah or so) and witness it yourself. This is no different than getting a "learner" battery for first-time Pb solar operations.

I am a little confused as to your main point. I'll explain my thinking for my application, and perhaps that will allow you to tailor a followup.

I chose LFP because it is relatively lightweight, relatively small, is sealed, charges relatively quickly. The cycle life is good in comparison to others, but honestly cost is not my main concern so I really haven't put a lot of thought into lifetime cost.

I was going to undersize my bank in the sense I was targeting using 10/90 over three days of autonomy. Why 80%? I'll admit I didn't want to spend more than I had to up front on capacity, but am I taking more of a hit on cycles than you think I should? Are you saying that your belief is a 50% window in the long run will be more economical due to the increase in cycles the increase in minimum SOC results in?

[Sunking]

LL what PN is saying is the less usable range you use, the less important a BMS becomes.

Example say you have a 100 AH cell and only want to operate on 50% of the capacity or 50 AH. So you only charge to 75% SOC and discharge to 25% SOC. At that narrow level you don't really need to worry about individual cell voltages. You can operate like FLA working only with total pack voltages So for a 16S aka 48 volt battery you only charge the pack up to 53.6 volts vs 3.65 volts x 16 = 58.4 volts at 100%SOC. Even if the cells are out of balance you are never going to push any cell to 3.65 volts with 3.35 volt average at 75% SOC. Likewise you are never going to discharge a cell down to 2.5 volts if your low voltage cutoff is set to 49 volts 25% SOC or 3.1 volts per cell. Using this method you would want to BOTTOM BALANCE initially and never have to worry about anything. All you gotta do is set the charger to 53.6 volts and LVD to 49 volts.

However the same can be said is you want to charge to 90%, and LVD @ 10% as long as you keep charge current to 1C or less and Bottom Balance initially. Set the charger to 56.8 volts, and LVD to 46.4 volts. This is the method 60% of the DIY EV guys operate. They BOTTOM BALANCE and use no BMS or any battery monitoring other than setting the charger and letting the motor controller operate the LVD. The other 40% use a lot of expensive BMS and have the most reported failures.

As for cycle life if you charge to 100% and discharge to 20% you get around 2000 cycles in a lab. If you limit to 90% or less at charge, and do not go below 10% you get around 3000 cycles. So nothing gained operating 10/90 or 25/75.

One more note here if you operate 10/90 the LFP charge curve is nearly linear under 1C load and less of 3.4 volts @ 90%, and 2.9 volts @ 10%. Makes it very easy to estimate SOC based on voltage.

Bottom line or take away here is if you want to go to or near 100% SOC you have to use a BMS to TOP BALANCE and make sacrifices.

• It is going to hit your wallet because BMS cost money and can get expensive. At a minimum for a passive BMS is $16/cell that does not work very good using Vampire Bleeder Boards, nor gives you any useful iNFORMATION other than an idiot light.
• You will give up to 33% cycle life.
• Run a much higher risk of destroying very expensive LFP cell or cells