LiFePO4 - The future for off-grid battery banks?

[steveg]

Hi John,

As in the other thread - your BMS (if any) is relative to your end application.

My LiFePO4 pack for my home has a simple BMS built into it - it protects the cells from over and under voltage with weak balancing. Other BMS'es have strong balancing as they need to miximise the power form the cells, but the BMS is more expensive.

You *will* need a solar regulator for charging the batteries from Solar PV cells - MPPT gives you more efficiency. If you want the most plain simple solution, you just need to regulate to the charge voltage/cell (3.65V in most cases for LFP)

But as I said depends on your application.

Over cell voltage will definately damage the cell ... consider this ...

you have a 200W panel the no load volatge is 45V, but if you put a 24V cell on the cell without a regulator in full sun the voltage will sag to 'charge' the cells, but the panel will continue to want to drive the cell all the way to 45V ultimately... another possible option is to size the battery pack to suit the cells, but you will get baqd efficiency of charge.

So for a 45V output cel you will put 13 LFP cells in series (to avoid over charge), but the cells will only charge very slowly - so a regulator is the best way - an MPPT one giving the best efficiency.

[john p]

Steve g that was my reply to Josko you can read my other 3 posts in this thread.. It will explain my experiences with LFP batteries..

[steveg]

Sorry - my bad.

[john p]

Steve G mostly my fault as I had made the post look like it was a question from me have redone the posts so no more confusion..

You find anything useful in my posts?

[Jonathan Cole]

Hi Steve,

LiFePo cells have a higher nominal voltage than lead acid batteries and usually the charge controller is programmed for specific batteries and their optimal charging characteristics. No charge controllers that I know of are set up for these batteries. What make of solar charge controller did you use?

By the way, an MPPT charge controller only makes economic sense if you live in the high cool lattitudes. Otherwise it is more cost effective to purchase a pulse width modulation charge controller which is less than half the price and has more than twice the warranty (5 years vs. 2 years). You are better off getting the cheaper charge controller and spending the savings on additionall PV panels which have a 20-25 year warranty.

Jonathan

[josko]

Hi everybody,
thanks for your replies.
2 solar panels, each 7,6A will be connected in 12V nominal system and 12V200Ah LiFePO4 battery block will be used.

As we are leaving in Slovenia(Europe),Steca MPPT charge controller would be the right choice. But also BMS.

Do you see any problem with standard inverter 12V/230V?

[john p]

JoscoYour solar panel output is on the low side for a 200 ah battery..
Dont know your solar isolation but doubt it be much above 3 in summer and 2 in winter.. Your panels rated output is only 7.6 amps most likely about 6.5 at best Total 13x 3 =39ahr per day..or 26ahr in winterThe most you could consume on the output side of the inverter would be about 400 whrs summer and 250 whrs taking into account battery loss charger loss inverter loss..
or put another way you could use 100watts for 4 hrs summer or 100watts for2.5 hrs not a lot for such a large expensive battery,,

[Mike90250]

Hi Steve,

LiFePo cells have a higher nominal voltage than lead acid batteries and usually the charge controller is programmed for specific batteries and their optimal charging characteristics. No charge controllers that I know of are set up for these batteries. What make of solar charge controller did you use?

By the way, an MPPT charge controller only makes economic sense if you live in the high cool lattitudes. Otherwise it is more cost effective to purchase a pulse width modulation charge controller which is less than half the price and has more than twice the warranty (5 years vs. 2 years). You are better off getting the cheaper charge controller and spending the savings on additionall PV panels which have a 20-25 year warranty.

Jonathan

a) Many of the better charge controllers are programable, I have the 60A morningstar MPPT, and can program any voltages into it, within limits. I think it can safely run up to about 62V max at the battery (which is eq voltage for a 48V battery). And my inverter should be good up to that voltage too.

b) MPPT vs PWM Well, they are different creatures. one can down convert higher voltage to battery voltage, and reduce losses in wireing. PWM is brain dead simple, but if your panels are not voltage matched to your batteries, you can have a lot of losses. PWM, in hot weather, may not deliver enough voltage for a full charge, because PV panels have lower voltage when hot. MPPT is more complicated, but also more flexible.

[john p]

PWM charge controllers are not so bad in fact I think for most uses on smaller systems there is very little difference in available output compared to a very expensive MMT controller
I have set up 3 small LFP battery systems charged with a PWM chargers and they working perfect for over 2 years now almost 3 years. my own system using Lead Acid Batteries runs using one its only 240 w of panels but puts out 78 ahr per day.. We one time tried a MPPT charger got on average 81 ahr per day..

[josko]

Did you use standard PWM charge controllers with LFP battery system?

I have set up 3 small LFP battery systems charged with a PWM chargers and they working perfect

[john p]

JOSCO yes standard adjustable output voltage PWM CHARGER..
Here is a repeat of that post explaining all..

IF anyone is interested this is how I have set up 3 x100ahr LiFePo4 batteries for charging by solar and for use with MSW inverter..
Im not claiming this is the correct way or even the best way to do it,, But dont tell me it wont work as all 3 systems been in use over 2 years now almost 3 years with NO PROBLEMS
Charger a 30a PWP 12v charger set to output 14.8v with no equalization..
Battery cell over charging 1x 3.9V 1watt zenner diode and 1x10ohm resistor across each cell
One low voltage sensor set to disconnect the inverter and anything else connected to the battery pack at 10v
Facts as found out the expensive way by me are if you discharge a cell below about 2.2 v it will be destroyed even if done just one time only.
If you over charge above about 4.4v they have a very short life.
If you look at charts from the manufacturesr you find there is little power above 3.8v and it stays fairly constant until about 2.5v then hits a brick wall.
So use between those 2 voltages and all is well with the world.
I have never found a MSW inverter that has problems with operating between 15.5v and 10.5 v and that is the usual specs given by most MSW invereter manufacturers..

[josko]

JOHN P, thank you.
As I am not good in English, I will summarize: PWM charger adjusted at 14,8V;BMS system and low voltage sensor set to disconnect the inverter at 10V.
Correct?

[john p]

Josko yes. That is correct..

Also you read my reply to your post about the 200ahr batteries and your 7.6 a solar panels .?? and why the panels are really not enough for that size battery/ies??

[josko]

John P,
Steca PR 20.20 solar regulator will be used. Controller has also a function to disconnect the load in case of battery discharge. Did you put low voltage sensor just to protect batteries once more?
Did you use the fuse too?
There will be installation of 2 solar panels, each of them 7,6V, 12V system. Isolation in Slovenia: summer average 6 hours, winter nearly 3 hours.
Now calculation looks a little bit better.

[john p]

Josko there is no way on earth you have 6 hours solar isolation in summer .. You are a long long long way north of the equator.. I be supprised even if you have 3hours..

The low voltage sensor is VERY important to protect the batteries.. Even one time you go below 2.2v per cell the battery can go DEAD