Lithium batteries, what is the current thinking.

@solorone said, For my info , has Lithium reached  practice  use ? 

Amidst a sea of negative opinions, perhaps based on misinformation and hearsay,  I made the decision to switch to LFP  and will never look back to iead acid. A vast majority of negative comments are based on the single word Lithium, which encompasses a range if differing chemistries, some of which can be extremely dangerous if not treated with extreme caution, while some Lifepo4 for example are much safer.

No battery is absolutely safe, including the stalwarts of the industry, however with Lifepo4  care is needed  to ensure operating within safe parameters this is mostly done with electronic protection wether by voltage limitations on charging/dischargeing or through the use of a battery monitoring system or BMS to take care of the control.

Having a strong electrical  background  is needed,  particularly with a DIY bank, less so perhaps with drop in replacement types which have an on board BMS to protect itself though they have their limitations, they cannot be connected in series for example to achieve higher voltage. No personal experience with this type.

The direct advantages in my experience are as follows. They are able to accept higher charge current without the need for long absorption resulting in better efficiency especially with generator charging . High discharge current without excessive volt drop,.  Low temperature gain, when charging and ability to operate at higher temperatures, within reason. The  ability to discharge to a much lower state of charge, no need to fully charge as needed bu LA to prevent sulfation, no watering as with FLA  (not applicable to AGM )  higher cycle potential especially when lightly used.

Other possible advantages,  higher energy density, resulting low weight and physical size, particularly useful in mobile applications.

Disadvantages, cannot be used in temperatures below 0°C. More  electronics adds to complexity in most cases. Higher initial cost, which is subjective if the  life expectancy is longer and the need for a lower overall Ah capacity is possible.

These are my views and opinions, others may differ..........There are others using LFP, please share your experiences.

what caused the failure within 3 years?  As we all know the first batt. sets are  murdered... or was it too much discharging or too little PV to properly recharge the bank or too high a charging voltage, AGMs are particularly sensitive to too hi a charge V setting , ......

Li starts to have performance degradation below 40F, both discharge and recharge.  Recharge has to cease (for all practical purposes, a c100 rate is pretty pointless) at frost (32F / 0C)

https://batteryuniversity.com/index.php/learn/article/charging_at_high_and_low_temperatures

So, your 2 golf cart batteries have a daily 30% usage for  longest life or 50% usage for a shorter life

That takes your 12V 200Ah battery (2400wh) to 720wh usable or 1200wh usable

Li batteries generally avoid near full and near empty conditions.   so the middle 90% ( 5% off the top, 5% off the bottom of a well controlled battery) is the MAX you can get.   So to replace the 1200wh of the 50% FLA battery, you need a 1320 wh LFP battery which would be 110ah running right to the razor edge .   A more conservative approach to battery management  would be 10% from top and bottom, needing a 1440wh battery or 120ah

A bit fuzzy this am, I hope I got the math right

I replaced my 2 GC2 golf cart batteries (rated about 220AH @ 12V for the pair) with a single LiFePO4 (100AH), and am happy with the change.

As noted above, the Lead vs. Lithum advertised ratings aren't computed the same way.  In fact, most reputable Lithium battery makers are giving ratings of true usable capacity - 100AH advertised is a true 100AH delivered.  

So you don't even need to shave off the 5% at top & bottom like Mike did above.

Another benefit of Lithium is the voltage is higher and stays higher under load.   My old system would sag to 12V under heavy load, whereas the Lithium stays closer to 13V.  

dmsims said:

Some LiFePO4 batteries can be charged in the range "-20°C – 55°C"

This is a somewhat misleading statement, there are some drop in replacement types which are self heating which may allow charging at below freezing, however this is not applicable to the vast majority of passive cells or batteries.

That's why words have to be chosen carefully, there has been evolution of the chemistry since 1996 when LiFePo4 was discovered, the linked Winston cells do indeed claim extremely low/high  temperature limits but as can be seen from the discharge curve at -45°C it would be in the LBCO regon of most well designed systems.

Strangely there's no charging/temperature  curve on the link or website, that I could find, my assumptions would be it would be better not to venture into the extreme limitations, both high or low, much like anything for that matter. When designing a system it would be prudent to provide extra capacity for supplemental heating or cooling as required to circumvent such extremes, neither of which I personally have needed to be concerned with.

At cold temp, besided reduced llithium ion mobility, the critical limitation is ability of graphite anode (neg side) to accept Li-ion charge.  Graphite will accept lithium ions storage during low temp charging at a lower charge rate.  Critical is when graphite cannot accept the rate of charging , the result is lithium plating at the anode side.  This results in non recoverable lithium which results in loss of capacity,  There is also lithum plating that can also create lithium metal dendrites which can grow severe enough to puncture into separator.

Nextgen, for example, recommends charging at 0 to -10 degsC with a reduced rate of 10%C maximum.  At -10 degsC to -20 degsC they recommend a charging rate at 5%C rate maximum.  Since most chargers, and inverter/chargers do not have any temp based automatic charge current control this would have to be adjusted manually in advance of anticipated cold temp.

Other factor relavent to charge or discharging, is battery impedance goes up as temp decreases. 
At    0 degsC it is about   85% increase in Rs over room temp.
At -10 degsC it is about 145% increase in Rs. 
At -20 degsC it is about 215% increase in Rs. 
At -30 degsC it is about 300% increase in Rs. 

If you really have to deal with a cold environment, with a little insulation, thermostatically controlled heating pads consuming about the same power as an inverter idle current you can keep them warm enough.

Probably number one cause of LFP battery 'murder' is overcharging of a cell.  There is only a 40 mV rise in single cell no load voltage from 50% state of charge to 85% SOC.  Folks get lulled into complacency at the slow voltage rise during charging.  At about 90-95% SOC the voltage rise rockets up quickly. The result is an overcharged cell that is bloated up with electrolyte decomposition and a murdered cell.  Manually watching cell voltage of a series group of cells that are not SOC matched is where this gets you into trouble.  Must maintain reasonable cell SOC balance.  Must use a BMS with balancer with per cell monitoring for high and low voltage cutout protection.

Their big advantage particularly for solar off grid is they are pretty happy not being fully charged.  Only caveat is many BMS use a simple resistor dump to balance a cell that does not activate until above about 3.4v per cell so you have to hold cell above this voltage occasionally to achieve any cell balancing.   The resistor dump only does 100 mA to 200 mA so it only compensates for cell self discharge leakage variance.  There are active balancers that operate all the time and transfer charge efficiently between cells to balance. 

When you buy a group of cells they may not be all at the same state of charge so fully charging cells individually to 3.65v is best way to start with all cells SOC matched at full state of charge.  This can be done with multiple cells in parallel with a voltage limit of 3.65v and current limited power supply.  It can take a long time for a low current power supply and multiple parallel cells. Most new cells arrive at about 3.300v which is about 50% SOC.  If you have 16 50% SOC 280AH cells in parallel you need to put in 16 x 140 AH = 2240AH's  which is over a week on a 10 amp power supply.