LiFePO4 - The future for off-grid battery banks?

[PNjunction]

Just saw a webinar on the new Morningstar ProStar MPPT solar charge controller; they said that it supports lithium battery packs out of the box, as long as it has a compatible BMS.

Do you mean this one:

http://www.morningstarcorp.com/products/prostar-mppt/

I REALLY like it but just from perusing the operators manual. I see they seem to have paid attention to the max-smoke model of charging, or at least a smarter/tamer way of doing it if one wants to stick to the canned presets. This is great for Pb, but I have a problem with LFP!

Unfortunately, I see NO WAY of disabling temperature-compensation for use with LiFePo4, or any lithium batteries for that matter. You don't use temp-comp with li-ion (other than monitoring for total thermal failure). Unlike my inexpensive Morningstar pwm's that will still work with the temp comp sensor ripped out for use with LFP, I'm not sure if trying to do so with one of these would outright kill it.

So as of now, NO it is NOT compatible with li-ion directly. Note that other products also claim compatability with LFP, but put the onus of responsibility on a bms that just shunts away ignorant abusive charging - which by itself can be a major point of failure.

I really like this for Pb operations. But for a guy like me who runs with NO top-balancing bleeder POF's, and sets his stuff up right, I need to disable temp comp for LFP. Maybe they could do a firmware update to enable that so I don't have to go cutting up internal traces and tricking it to do so.

[PNjunction]

Note to lukers - NOT ALL Morningstar pwm controllers will still work if you rip out the temp-comp sensor for lfp use. My Prostar 15 does. Heck, even my el cheapo EPsolar controller on my folding panel emulates it by allowing the op to set a temp comp of 0.00v.

I like Morningstar gear, but this apparent oversight is a bummer. Hopefully that will get fixed.

[Sunking]

[QUOTE=smily03;n297273]Unfortunately it is just marketing hype. Once you dig into the details, it is not compatible wiht LFP batteries. Any charger for Kithium has to bee able to communicate with a BMS and regulate charge current. This uni tis not capable of doing that.

[PNjunction]

If you know what you are doing, you can "fudge" the lfp from the canned presets, BUT the inability to disable the temp-comp is a deal breaker.

In the same manner, if you use the MSview software, you can customize the algo's, but I haven't seen the software, and still this hinges on being able to DISable temp comp for LFP. Maybe that option is in the software? I dunno'.

From a Pb perspective, I like what I see, including the mild "boost" for agm (like an EQ, but much milder), which one WILL need, since to do agm's right in a cyclic mode, that means 8 HOURS of 13.6v float daily - no matter how fast you finish absorb! - something one will never have a chance to do in a daily cyclic operation, and the higher-voltage boost is a way to deal with that.

[smily03]

Well, it was worth a hope anyhow Maybe by the time I'm ready to actually buy some batteries, one will be available If nothing else, I'll re-re-read the awesome info that you've all posted out here

Out of curiosity, would there be any way to charge cells in parallel, but discharge them in series? That way you wouldn't have to worry about them getting out of balance, right? Or would the added complexity not be worth it? (Well, that and the amperages would be so much higher too; a miniscule 200w solar array would work down to roughly 60 amps at 3v5...)

I know that most (all?) off-the-shelf charge controllers wouldn't be able to output voltages that low, but just being a curious noob

[organic farmer]

... Out of curiosity, would there be any way to charge cells in parallel, but discharge them in series? That way you wouldn't have to worry about them getting out of balance, right? Or would the added complexity not be worth it?

You guessed it. The added complexity would be horrible.

[Living Large]

(Well, that and the amperages would be so much higher too; a miniscule 200w solar array would work down to roughly 60 amps at 3v5...)

I know that most (all?) off-the-shelf charge controllers wouldn't be able to output voltages that low, but just being a curious noob

You answered your own question, both on voltage and current. Nothing will work with that low a voltage, and a moderate size system would require several hundred amps of current. And how would such a system be automatically "reconfigured"? You'd end up with something on a par with the Space Shuttle - they don't fly those anymore.

[Sunking]

Out of curiosity, would there be any way to charge cells in parallel, but discharge them in series? That way you wouldn't have to worry about them getting out of balance, right? Or would the added complexity not be worth it? (Well, that and the amperages would be so much higher too; a miniscule 200w solar array would work down to roughly 60 amps at 3v5...)

To complex, but not nessecary. When you recieve new Lithium batteries, .all manufactures instruct you to connect all the cells in PARALLEL. From there either Bottom Balance or Top Balance. You cannot take new cells with a BMS and Top Balance. It could take years to do that. You need something like a Powerlab 8 to do this. With a Powerlab 8 you can discharge at 40 amps, or charge at 100 amps.

As for getting depends on how you use them. If you are using a BMS with Shunt Boards or Cell Monitor you will go out of Balance. In other words if you Top Balance they do go out of Balance. If you Bottom Balance and do not connect Monitors, Shunts, or BMS they rarely ever ever go out of Balance.

[steveg]

Hi Everyone - well another post on the progress of my LiFePO4 batteries - you may have seen my initial post to this thread 5yrs ago?

Our home is off-grid and has been for approx 5.5yrs now – we have 4kw solar – we have LiFePO4 and *not* lead acid (PbA). We use no gas to cook or heat our water and our *annual average* consumption is 12kWh per day – our highest peak is 29kWh consumed in 24 hrs. In summer we easily generate 20kWh per day and in the middle of winter (on a sunny day) we generate 12kWh (my guess) per day. We get snow, but not as much as some I guess...

(late addition – I got off my arse and got the numbers - We have generated a total of 25.984MWh in 5yrs 148days (my meter tells me) so there’s the average – 25984kWh / 1974 days = 13kWh/day)

I encourage you to look into Lithium Iron cells – on paper my calculation (electronics engineer by trade) says they will have a lifetime of 15->20+ years – remember a PbA battery you can only use 1/3 of its capacity, you can’t take large prolonged loads from the cell, it doesn’t like uneven charging/discharging (i.e. clouds go in front of sun) and it doesn’t like temperature variation in order to get your 10yr lifetime. PbA also has much lower charge efficiency averaged over its lifetime so your PV panels will charge the cells less and less over time.

To illustrate - We cook on an induction cooktop, our water is heated by the excess power (along with solar hot water tubes) – we have a 320m^2 5 bedroom + study home with triple garage (3445 sq ft) – So that’s 240M^2 living space (2580sq.ft)

Our battery pack is 300Ah at 48V and weighs 160kg and fit in the back of my car (not pickup/truck) to bring it home – it is the equivalent in *usable* energy to a 1000Ah 48V PbA bank – it has a lid too of course - it cost me $7000USD in 2009 (purchased before be built house)

We have a 7kW sine inverter and a 2kw Honda petrol generator which takes 4litres to fill and can run for 3.5hrs to charge for the day when sun is hiding.

I haven't noticed any life degradation in the battery at all so far ... but I guess I cant make a strong claim on capacity claim unless I put an amp-hour meter on it and cycle it.

Steve...

[PNjunction]

Wow - pretty clean. From the looks of it, I see no physical issues with what appears to be the older Hi-Power white cells, which EV'ers used to trash pretty often.

Kind of a testament to our "sub-c" application where even the lower cost cells, if taken care of, can provide good service for at least what - 7 years now?

Have you done any preventative maintenance since then - like checking cell voltages with your own multimeter just to make sure the bms is still playing nice? Are those cell bolts snug? (careful here!!).

Thanks for the update. That's rare on most forums.

[karrak]

Hi Everyone - well another post on the progress of my LiFePO4 batteries - you may have seen my initial post to this thread 5yrs ago?

I saw the Latronics inverter in your picture and thought, here is yet another system in Australia or New Zealand.
As PNJunction said, thanks for the update. Your system is the oldest that I have seen an update on.

Our home is off-grid and has been for approx 5.5yrs now – we have 4kw solar – we have LiFePO4 and *not* lead acid (PbA). We use no gas to cook or heat our water and our *annual average* consumption is 12kWh per day – our highest peak is 29kWh consumed in 24 hrs. In summer we easily generate 20kWh per day and in the middle of winter (on a sunny day) we generate 12kWh (my guess) per day. We get snow, but not as much as some I guess...

(late addition – I got off my arse and got the numbers - We have generated a total of 25.984MWh in 5yrs 148days (my meter tells me) so there’s the average – 25984kWh / 1974 days = 13kWh/day)

Looks like a perfectly sized LFP battery if you are running a generator with just over one days storage capacity (~15kWh). I would be interested to know how often you have to run your generator.

We have a smaller system and requirements, we have 1140W of solar panels, 360Ah@24volts LFP battery(~9kWh around three days storage capacity) installed in April 2013, 4kW inverter and no generator. Our winter requirements are ~2.5-3kWh and summer ~5-6kWh. Like you, cooking is done on induction hotplate and electric oven in summer and wood stove in winter.

If one didn't want to run a generator, now that solar panels are so cheap I would look at oversizing the solar array and decreasing the battery to maybe two days storage capacity.

If the battery lasts ten years I have calculated that the cost of storing power in our battery will be ~$0.40-$0.50/kWh, for your system I calculate around $0.30/kWh assuming you are storing around half you daily consumption.

Simon

[PNjunction]

If the battery lasts ten years I have calculated that the cost of storing power in our battery will be ~$0.40-$0.50/kWh, for your system I calculate around $0.30/kWh assuming you are storing around half you daily consumption.

Kind of a side note about costs vs Pb ... maybe it's been hammered to death, but most cycle-life comparisons and so forth don't take into account that most people murder their Pb batteries long before the lab-rated performance data is reached! Ie, sulfation, no eq for flooded, undercharging agm, etc in a typical solar setup.

For LFP, once you get it under control, be it a commercial system, or a diy version for a battery geek, the chances of murdering cells in OUR application can be remote. Not impossible surely, but the chances of meeting lab specs, even half-way, is probably greater in probability than lead acid in the real world. That real-world maintenance issue (with QUALITY lfp cells, not counterfeits or used hacks) vs lab-specs can make it less costly than marketing charts make it out to be.

[steveg]

Hi All - Well, we're almost at 6 years for install from my original message - batteries seem as fresh as the day we put them in - LiFePO4 so far seems quite superior to PbA. We still use an induction cooktop and dishwasher and oven when we have power for them... Generator runs are in winter months or more than 2 days rain/dark days. Approx 90hours generator total run time each winter season ~ approx 135Ltrs gasoline. haven't had to replace an LED lightbulb.

[solar pete]

Howdy steveg, thanks for popping in with that info, very good to see 6 years on the same set of batteries and all is well, good stuff, well done sir

[karrak]

Hi All - Well, we're almost at 6 years for install from my original message - batteries seem as fresh as the day we put them in - LiFePO4 so far seems quite superior to PbA. We still use an induction cooktop and dishwasher and oven when we have power for them... Generator runs are in winter months or more than 2 days rain/dark days. Approx 90hours generator total run time each winter season ~ approx 135Ltrs gasoline. haven't had to replace an LED lightbulb.

We went from aging NiCd batteries (ex railway locomotive batteries that I got for free) that you never knew how much energy they had stored in them with woeful efficiency to the LFP battery. The change in performance is hard to believe!

Thanks for the information on generator run time.

I would be interested to know what voltage and cut off current you charge your battery to and if you float the battery.

I used to think that floating LFP batteries could be bad for them, but am now of the view that charging to 3.45 V/cell and floating at around 3.35 V/cell will not decrease the lifespan of them and will give you a battery that is close to 100% full at the end of the day if there has been enough sun.

Thanks
Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger
Latronics 4kW Inverter, homemade MPPT controller