Real world LiFePO4 lifespan data

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Real world LiFePO4 lifespan data

Post by TimF » Wed, 16 May 2012, 03:17

Hi all, longtime lurker, not much for posting.

Anyhow, I'm considering use of LiFePO4 batteries (probably Thundersky) in a battery backed solar power system. Just wondering if anyone who's actually using these cells has / is willing to share data on charge / discharge cycles, calender life, cycle life etc based on their own experiences. Also any thoughts regarding BMS, long-term maintenance etc in a stationary domestic environment (planning a metal battery box outdoors in Sydney).

Still working through the available inverter systems but I've got 4.6kWp of PV modules 18 * 280W each @ 43.78 Voc, 36.72 Vmp 7.63 Imp per module. My plan is to connect as 2S x 9P giving an array of approx 72V @ 68.3A and utilise a maximum power point tracking type regulator to maximise my energy yield (probably need to split into two arrays to minimise cabling losses). Based on tilt, orientation, shading, dirt, heat and cable loss allowances I should see average daily energy outputs from the array between 26.49kWh in Dec (best) to 11.96kWh in June (worst). Annual daily average of 20.04kWh.

Tossing up between 48Vdc or 24Vdc nominal pack voltage at this stage - there is a 96Vdc input inverter that could be used but it doesn't act as an on-grid UPS like the others I'm looking at do.

Appreciate any thoughts.
Cheers - Tim
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Real world LiFePO4 lifespan data

Post by weber » Wed, 16 May 2012, 03:57

TimF wrote: Hi all, longtime lurker, not much for posting.

Anyhow, I'm considering use of LiFePO4 batteries (probably Thundersky) in a battery backed solar power system. Just wondering if anyone who's actually using these cells has / is willing to share data ... Appreciate any thoughts.
Cheers - Tim

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Well gee Tim. I have some experience designing and installing battery-backed up grid PV systems, and I've used LiFePO4 in an EV. I was going to give you the benefit of my thoughts on using LiFePO4 for PV systems, but ...

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Post by TimF » Wed, 16 May 2012, 04:19

An unfortunate choice of words in the signature when I signed up years ago. Clearly a smile and wink at the end should have been explicit rather than implied......ah well. Image
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Post by Jodie » Wed, 16 May 2012, 17:54

Hi Tim,

You might be interested in the following ebay item No. 130573676935

For cell discharge/charge graphs go to evtv.me. There is quite a bit of information there.
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Post by weber » Wed, 16 May 2012, 18:48

That's OK Tim. I should have had a smile and a wink at the end of mine too. Image

The main reason I've balked at using LiFePO4 for PV systems so far is the difficulty of making the MPPT charge controllers, or the special battery-floating grid-connect inverters, do the right thing in regard to the LiFePO4 and its BMS, when they have been designed for lead-acid batteries.

In the electric vehicle the BMS tells the charger to deliver maximum current (in our case 5.5 A) until any one cell goes above 3.6 volts and then it tells the charger to drop back to the maximum current that the cell-top boards can bypass (in our case 0.5 A) while the rest of the cells catch up. And under load the BMS tells the load to shut off if any one cell goes below 2.5 V.

That's the crude description. In reality it ramps continuously between the various cases because as soon as you reduce the charge current a little, the cell will fall below 3.6 volts again for a while. Similarly with load above 2.5 V.

So in the EV it's all about controlling the current. The charger's voltage limit is irrelevant except as a backup in the event of BMS failure.

For typical lead-acid charge algorithms however, it's all about controlling the overall voltage, and individual cells are assumed to be able to tolerate overcharging. After all it merely electrolyses some water which either recombines later (sealed cells) or needs topping up (flooded cels). But Lithium cells suffer irreversible damage on overcharge.

For battery-backed-up grid systems I used the SunProfi SP1500E inverters until they were no longer available and then a combination of Latronics PVE1200 (grid-connect inverter with a battery-float mode) and LS1848 (standalone inverter) and Blue Sky Energy SolarBoost-3048 (MPPT charge controller).

The grid-feed inverter is not adjustable in its float voltage. It is fixed at 54 V. It also allows either a boost (sealed cells) or an equalise (flooded cells) every 30 days, but the user guide doesn't tell you what the voltages are. I'd guess 58 V boost and 60 V equalise. I'm sure Latronics would tell us if we asked.

The standalone inverter has a low voltage shut-off that can be set to 40, 42, 44 or 46 volts. Once it has shut off you have to raise the voltage 8 volts above the chosen shut-off voltage for it to come back on again.

You'd have to use 16 LiFePO4 cells in series so they would get balanced once a month (using the sealed cell setting). 58/16 = 3.625 V per cell. Hopefully no cell would get so far out of balance during the month that it would spend too long above 3.625 before it's bypass circuit could bring it down.

But that's the real danger with this. It would be so easy to have 15 cells at 3.6 V and one cell at 4.0 V for some time. Or worse, 15 cells around 3.55 volts and one at 4.75 V. Then you have a fire risk from that cell venting its highly flammable electrolyte and possibly being ignited by a failing BMS board (unless it's one of ours).

You'd certainly want the highest bypass current you could get in a BMS, in this situation where the BMS can't control the charger. And you'd want to manually balance your cells on installation. And you'd want an audible and visible alarm from your BMS if things get out of hand. I guess you'd really have to have your BMS operate a contactor to disconnect all charging sources.

You'd be floating the cells at 54/16 = 3.375 V per cell which is probably quite a good choice, but unlike lead-acid the longest life for Lithiums is obtained by maintaining them at around 50% charge which is not very useful for a backup supply.

I did try it for a month or so (on a SunProfi-based system) and it seemed OK. I just diverted some cells destined for the EV. But you won't really know for some years, and it's an expensive experiment.

Lead-acid work fine in this application, and cost less. And weight and space are not an issue. So there is no incentive to use LiFePO4. I use Battery Energy SunGel.

[Edit: Sorry no info on real world LiFePO4 lifespan. Haven't had them long enough.]
Last edited by weber on Wed, 16 May 2012, 09:34, edited 1 time in total.
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Post by TimF » Wed, 16 May 2012, 19:21

Hi Weber,

thanks for your feedback. I'll certainly be looking into the lead-acid gel cells as well.

Looking back at my original post I see that I failed to define the intended usage cycle properly. What I'm really trying to do is store the day's solar energy production at time of generation and discharge as the house needs it.

In QLD (correct me if I'm wrong) I belive you still have a NET feed-in-tariff which is greater than your import tariff, i.e. 44c/kWh export vs say 25c/kWh import. This makes operation of a normal grid connect system simple - try to use as little power through the prime PV generation hours as possible to maximise your export and thus your savings. Easy for most people, turn everything off and go to work Image

In NSW however we've now got a wonderful NET feed-in-tariff which is considerably less than the import tariff rates. Typically between 4 & 8 c/kWh export vs 23.5c/kWh import on a voluntary basis. Thanks IPART. Image
Thus it makes more sense for the customer to use all their solar generated energy rather than exporting it to the grid, unfortunately this means performing energy consuming activities between say 10am and 2pm, right when most people are at work not at home......

So, the design challenge as I see it is for a suitable battery bank to be able to handle regular i.e. daily, relatively deep discharges say to 50% or 80% DOD rather than the 20% DOD generally used with lead acid batteries in a stand-alone configuration to maximise cycle life.

There is a new inverter/charger called the PowerRouter just coming out in Australia (Netherlands made by NEDCAP) which is supposed to handle lithium ion charging, and a couple of other systems like the Rich Electric SuperCombi or the Selectronics SP-Pro all of which seem to perform the function I want - supply house from PV / batteries and draw only excess from grid if needed - but I was hoping to avoid having to massively oversize the battery bank just to get a decent cycle life.

Ah well - back to the spreadsheet

[EDIT] As for the over-voltage risk I absolutely see the problem. There's no way I'd consider an lithium chemistry pack that didn't have a BMS that was able to isolate the incoming charge from any source.

[EDIT2] Hmm, rather than a full blown inverter/charger setup, maybe something like the Latronics PVE1200 / 48V lead acid battery combo as mentioned above but with a suitably rated contactor or solid state relay between the inverter and the grid. Sense the load on the house via current transformer, use an off the shelf micro-PLC or smart relay for control and only turn on the inverter once the load goes above a preset limit (e.g. nominal AC output of inverter) in the afternoon then turn it off once the day's solar charge has been exhausted rather than running the battery right down. Basically just a peak-lopping application. Thoughts?
Last edited by TimF on Wed, 16 May 2012, 09:39, edited 1 time in total.
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Post by weber » Wed, 16 May 2012, 20:56

TimF wrote:supply house from PV / batteries and draw only excess from grid if needed - but I was hoping to avoid having to massively oversize the battery bank just to get a decent cycle life.
Ah. That seriously changes the equation in favour of LiFePO4.

You're correct about the Qld feed-in tarriff. But it discriminates against retirees and parents at home with babies/small children and people working from home. So Coulomb and I designed and built a load shifting system that does the exact opposite of what you want to do. It maximises PV energy fed to the grid by running the house off the batteries during the sunny parts of the day and recharges fully from off-peak power at night. Only if the grid goes down does the pv array get switched over to charge the battery.

This would never pay for the batteries, but since the customer already had the batteries for other reasons we thought it worthwile to cycle the top 20% every day for this purpose. It was all done automaticaly using a battery SoC monitor, a 24 hour timer, and some relay/contactor logic.

I'm not familiar with those other inverters you mention, but I have read up on the Selectronics SP-Pro models in the dim past, but haven't ever used one as they were usually too high powered and expensive for the job. But you have 4.6 kW of PV so they might make more sense than the Latronics PVEs.
Hmm, rather than a full blown inverter/charger setup, maybe something like the Latronics PVE1200 / 48V lead acid battery combo as mentioned above but with a suitably rated contactor or solid state relay between the inverter and the grid. Sense the load on the house via current transformer, use an off the shelf micro-PLC or smart relay for control and only turn on the inverter once the load goes above a preset limit (e.g. nominal AC output of inverter) in the afternoon then turn it off once the day's solar charge has been exhausted rather than running the battery right down. Basically just a peak-lopping application. Thoughts?


What occurs to me is, why bother having a grid-feed inverter at all? The only time you would want to export is when you can't use the PV power as it is being generated and the battery is already full. What's the average daily consumption of the household?

You would have to figure out whether the amount exported would pay for the grid-feed (or dual purpose) inverter over its say 10 year life.

Otherwise you could just have a standalone system with only one day of storage (using LiFePO4) instead of the usual 5 days, and switch the household loads over to the grid if the battery SoC falls below 30%.

Having separate standalone and grid-feed inverters does make for a more flexible system than the all-in-one inverters like the SP-Pro and Sun-Profi. So you can reconfigure it when you get a sensible government again.
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Post by TimF » Thu, 17 May 2012, 01:16

weber wrote:
Ah. That seriously changes the equation in favour of LiFePO4.
Thought I was on the right track   Image
weber wrote:
You're correct about the Qld feed-in tarriff. But it discriminates against retirees and parents at home with babies/small children and people working from home.
Ironically enough these are not only the people who arguably need cheap energy the most but are the only people in NSW that a grid connect system with no storage makes any sense for (other than businesses wanting to cut their air-con loads etc) .... and in many cases the ones with the least available disposable income to purchase one.
weber wrote: So Coulomb and I designed and built a load shifting system that does the exact opposite of what you want to do. It maximises PV energy fed to the grid by running the house off the batteries during the sunny parts of the day and recharges fully from off-peak power at night. Only if the grid goes down does the pv array get switched over to charge the battery.

This would never pay for the batteries, but since the customer already had the batteries for other reasons we thought it worthwile to cycle the top 20% every day for this purpose. It was all done automaticaly using a battery SoC monitor, a 24 hour timer, and some relay/contactor logic.
Neatly done. Cheeky bit of work with the off-peak battery charging must see if our DNSP's will allow that. Some areas of Sydney that might make sense even without PV now as Time Of Use tariffs roll out - 10c/kWh off peak, 19c/kWh shoulder, 40+c/kWh peak - Suspect I'll wind up with something along those lines for the first system but with slightly modified logic. As you say, the cost of the fully integrated inverter/charger units is my main reason for looking at alternatives using "dumb" standard grid connect inverters and a bit of clever logic.

Average household usage is around 21kWh/day peak and 30kWh/day off peak HWS (all cooking etc is electric as no gas available in the street). 5 adults so it could be worse. Lighting is being modified to LED as time/budget/SWMBO permits....
weber wrote: What occurs to me is, why bother having a grid-feed inverter at all? The only time you would want to export is when you can't use the PV power as it is being generated and the battery is already full.
1) with a grid-feed inverter and PV modules the system would be eligible for REC's / STC's, as opposed to the capped REC's / STC's allocation and additional requirements (distance/cost) for stand-alone systems.

2) yep, figured it was better to be able to export the power to the grid (even at the pittance they offer) when the battery is full rather than overcharge the batteries, switch in a dump load or just disconnect the array

3) must confess I didn't examine closely the price differences between grid-connect and standalone inverters. More rigorous analysis is required, clearly.

4) Figured I could make the system more or less modular - normal ELV configured PV array + ELV grid-connect inverter as initial install. Add battery pack / load-shifting control later if desired.
weber wrote: Otherwise you could just have a standalone system with only one day of storage (using LiFePO4) instead of the usual 5 days, and switch the household loads over to the grid if the battery SoC falls below 30%.
Our NSW DNSP's get stroppy about people using the grid for backup. In fact I'm fairly sure there's something about that in the connection terms and conditions. Must read up.
weber wrote: So you can reconfigure it when you get a sensible government again.


Sensible government? Is that like military intelligence?

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Post by weber » Thu, 17 May 2012, 02:38

TimF wrote:Cheeky bit of work with the off-peak battery charging must see if our DNSP's will allow that. Some areas of Sydney that might make sense even without PV now as Time Of Use tariffs roll out - 10c/kWh off peak, 19c/kWh shoulder, 40+c/kWh peak
The only requirements in SE Qld are that the charger must be permanetly wired to the controlled tariff, not plug-in, and there must not be any alternative charger on the general tariff. Yours may be the same.
Suspect I'll wind up with something along those lines for the first system but with slightly modified logic. As you say, the cost of the fully integrated inverter/charger units is my main reason for looking at alternatives using "dumb" standard grid connect inverters and a bit of clever logic.
If you have a standard grid connect inverter, or even a Latronics PVE in battery floating mode, there is no way to use the energy stored in the batteries, whether to feed to the grid or to power the house. I'm not sure that even an SP-Pro will draw down the batteries to feed the grid. They certainly aren't allowed to do so in Qld as you could then charge your batteries off the grid and get paid the premium FiT to re-export dirty power.

The only way I can see to use the energy in the batteries is to have a standalone inverter and run the house off it, either permanently or switched.
1) .. 4)
All good reasons for having a grid feed inverter. I'm convinced.
Our NSW DNSP's get stroppy about people using the grid for backup. In fact I'm fairly sure there's something about that in the connection terms and conditions. Must read up.
Hmm. But that's effectively what you will be doing, whether or not you have batteries. That's effectively what anyone is doing, who has a large amount of grid-connect PV. They should be happy if your load is typically off peak.

On a typical day your array will be charging your battery all day. If the battery gets to full, the excess will be fed to the grid via the grid-feed inverter. Then you will run the battery down in the evening, by running your house off it via the standalone inverter. If the battery gets down to 20% you will switch the house over to the grid. No need for a 240 Vac battery charger.
Sensible government? Is that like military intelligence? Image

You are so right. I should have written "a less stupid government". Image
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Post by TimF » Thu, 17 May 2012, 04:50

weber wrote:
On a typical day your array will be charging your battery all day. If the battery gets to full, the excess will be fed to the grid via the grid-feed inverter. Then you will run the battery down in the evening, by running your house off it via the standalone inverter. If the battery gets down to 20% you will switch the house over to the grid. No need for a 240 Vac battery charger.


Pretty much how I envisaged it (i.e. as per the Latronics et. al. app notes), but without the standalone inverter and ATS. Naturally this also means there's no backup function in the event of a grid failure, its purely acting as a load-shifting system. Contactor in the active between the grid-connect inverter and the solar supply main switch. Current transformer pickup on the active from the house main switch to the house loads (normal tariff). No 240Vac charger (might have to have the grid-connect inverter's battery equalisation / float functionality disabled where export tariffs are greater than import tariffs but shouldn't be a problem if it's the other way around like NSW)

I.E. for maximum self use of PV generation
Daytime - state 1 (sunlight present, battery <100%SOC, 5min ave house load < rated output of grid-connect inverter)
Charge batteries from PV. Grid inverter isolated via contactor.

Daytime - state 2 (sunlight present, battery = 100%SOC, 5min ave house load < rated output of grid-connect inverter)
Grid inverter turns on to drain off excess PV generation.

Daytime - state 3 (sunlight present, battery >20%SOC, 5min ave house load (someone's home today) > rated output of grid-connect inverter)
Grid inverter turns on to "support" the load of the house, thus minimising current drawn from grid through bi-directional NET meter. Fed from PV array and/or PV energy stored in batteries.

Evening - state 1 (no or little sunlight, battery >20%SOC, 5min ave house load < rated output of grid-connect inverter)
Grid inverter isolated via contactor

Evening - state 2 (no or little sunlight, battery >20%SOC, 5min ave house load > rated output of grid-connect inverter)
Grid inverter turns on to "support" the load of the house, thus minimising current drawn from grid through bi-directional NET meter.
Fed from PV energy stored in batteries.

Evening - state 3 (no or little sunlight, battery <=20%SOC, house load don't care)
Grid inverter isolated via contactor, wait for batteries to recharge next day.

Pros:
Reduced system cost (CT/smart relay/contactor vs stand-alone inverter + ATS)
Inverter usually only turns on when house load exceeds inverter rated output or battery is full, thus minimal energy exported, maximises self-use of energy where FiT < import energy cost.
Reduces peak load on grid by discharging batteries at required time of use.

Cons:
No UPS function if grid goes down - you're in the same boat as a normal grid-connected system in that you can't use the PV output or your stored energy.
Contactor should probably be a solid state relay to minimise contact wear over lifetime.
Reduces energy company revenues so likely to be regulated against. Image

[EDIT]
All of which leads back to the original issue of working out the most economically viable battery bank for this sort of usage in terms of usable kWh/cycle vs lifespan.
[/EDIT]
Last edited by TimF on Wed, 16 May 2012, 18:56, edited 1 time in total.
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Post by weber » Thu, 17 May 2012, 06:15

Tim, I don't see how your scheme can possibly work without a standalone inverter (or a grid inverter that can also work standalone, like the SP-Pro). The Latronics PVE1200 will not operate except when the nominally 48 V battery is full (54 V), i.e. it will not take any energy from the battery under any circumstances.*

And for the same reason, I don't see why you need a contactor between the inverter and the grid.

* However, in a strange coincidence, I happened to run into an old friend tonight, who now works for Latronics, and so I discussed load shifting with him. He mentioned a trick where you use a 60 V battery with a PVE1200 so that when the inverter shuts off at 54 V it actually _has_ discharged the battery somewhat. Apparently there are a few MPPT charge controllers that can be setup to work with a 60 V battery. He mentioned Midnight and AERL. So you could have maybe a 3.2 kW array with a PVE1200 and the PVE would not be able to export all that the array is generating and so the battery would be recharged by the excess. Then the PVE would keep running for maybe 4 hours after the sun goes down, until it has discharged the 60 volt battery down to 54 volts. So you get to use a half-sized inverter and spread the generation over double the time, provided your battery has sufficient capacity.

You could of course double those figures by using a PVE2500 with a 120 V battery but MPPT charge controllers for that may be hard to find, and you may have to settle for a simple on-off charge controller.

But a better solution might be to parallel two PVE1200s on a 60 V battery.

This system doesn't quite do what you want, which is to only export power when the battery is full. However with the addition of your contactor or SSR between inverters and grid, and your logic, I think we're there!

However I don't think you need the complication of measuring power and thresholding it to feed into the logic.
All of which leads back to the original issue of working out the most economically viable battery bank for this sort of usage in terms of usable kWh/cycle vs lifespan.

I'm pretty sure LiFePO4 will work out better than lead-acid for this, as you guessed.

If you use 20 cells, 54 V/20 = 2.7 V and it would be best to measure SoC and disconnect the inverters at 20% or 30%, before the voltage gets that low.
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Post by Damnthematrix » Tue, 05 Feb 2013, 15:28

Hi Dave....... I ran into this forum (which I note is 9 months old, and 9 months is a long time in this kind of technology...) while looking for LiFeSO4 battery info myself... I'm going to have to do something about replacing my SP1500E... it flattened the batteries (which seem to have recovered very well) over that nasty bit of weather we just had. It's the second time this has happened (destroyed two batteries in the old bank you put in) mysteriously. i swear it happened overnight, because one day it was fine, and it was flat the next. In the afternoon of the first good day of sunshine, we actually had a 6 hour blackout, and the only partially recharged batteries coped very well with that, running the fridges and freezer no problem..... Beats me.

Anyhow, back to Lithium batteries (which I'd like to use in Tasmania when we get there)..... the other day I visited the Nedap Power Router dealers in Nambour (Eclipse Solar) and saw one actually running. Very nice units, but pricey at $4600! (3.7kW - which he told me not to repeat Image was exactly the same as the 5kW except for software...!)

They have a really smart charging menu built into them where you can select what sort of batteries you have (flooded, gel, AGM and Li ion - but NOT LiFeSO4)

Do you know if the charging regime between the two different kinds of Li batteries is different?

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Post by Richo » Tue, 05 Feb 2013, 21:13

Li-ion and LiFEPO4 both use CC-CV charging.
Only the terminal voltage and Charge currents may vary.
The low voltage cut-out may be different too.
So the short answer is NO but the long answer is YES.
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Post by Damnthematrix » Wed, 06 Feb 2013, 17:21

Excuse the ignorance, but.......what's CC-CV charging?

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Post by coulomb » Wed, 06 Feb 2013, 19:41

Damnthematrix wrote: Excuse the ignorance, but.......what's CC-CV charging?

That would be Constant Current followed by Constant Voltage.

Basically, the charger has a voltage limit and a current limit. Initially, the pack voltage will be less than the voltage limit, so the current limit is in force. In effect, the charger runs "flat out", supplying all the current its design can safely supply. When the pack voltage rises to the set point, the charger cuts back the current to ensure that the pack voltage doesn't exceed the voltage limit.

You can do the exact same thing with a laboratory power supply.

That's all you really need for lithium chemistries, although some chargers will provide fancy thee, four, and more stages. Also of course the Battery Management System (BMS) may cause the current to throttle back even further than the current and voltage limits would allow, because one or more cells is full.
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