I have been asked to start a lithium batteries thread. Unlike lead acid my lithium experience is limited to the last month which hardly qualifies me as someone to start something like this. I however do like information catalogued so that I can refer back to it easily so on that basis I plough ahead and hope this thread develops into a resource with many contributors.
I will use lead acid batteries (LA) as a reference point and assume that folk are familiar with them.
What is Lithium? It is the 3rd lightest element and those of you who were the chemistry teacher nemesis at school will know it is highly reactive in both water and air and is usually stored in a hydrocarbon solvent. It readily sheds its 3rd electron and therefore its highly reactive nature.
Lithium ion batteries are all similar in that their cathodes have a matrix of some lithium containing compounds and the lithium ion since it is so small moves in and out of this matrix allowing charge and discharge of the cell. There are numerous flavours of lithium ion but I think in the solar industry lithium iron phosphate (LiFePO4) has the biggest potential. Lithium iron phosphate (often called lithium ferrous phosphate to prevent the lithium ion/lithium iron confusion) is more stable and therefore safer than the other lithium ion batteries, has a greater cycle life and greater heat tolerance. Since it is charged to 3.65V rather than the more common 4.2V it has a lower specific energy not a huge problem with our stationary modules. Thermal runaway with LiFePO4 only happens >250°C whereas other lithium ion variants this temperature is a lot lower.
There is a LA versus Lithium debate and this post is not about continuing the debate but to understand the beast one must make comparison to what is familiar.
LiFePO4 batteries are like an AGM or Gel battery in that they are not vented – there is no giving off of gas.
LiFePo4 batteries have a high charge and discharge rate and do not require a long tapered absorb and float phase. The sudden drop in charging from 30A to <2A is a clear indicator that the batteries are fully charged. My batteries in fact discharge slightly to balance the cells.
Like LA batteries in theory LiFePO4 batteries should last forever but in our imperfect world LiFePO4 suffer from a process akin to sulphation. The electrolyte of LiFePO4 batteries contains LiFePO4 dissolved in an organic solvent. This precipitates out as a Li2CO3 layer on the cathode, reducing the availability of the active LiFePO4. This process is irreversible and speed up at elevated temperatures (>50°C). There is a fair amount of R&D to slow down this process. Unlike sulphation this is a process triggered by charging and LiFePO4 do not suffer from storage partially discharged like LAs do from sulphation.
LiFePO4 batteries achieve nearly all their charging at about 3.3V/cell and the jump in SOC is from roughly 30% at 3.2V/cell to 90% and over at 3.3V. Since LiFePO4 batteries do not suffer from sulphation and overcharging has a negative influence on battery life some manufacturers cheat and do not fully charge the batteries thereby extending battery life for a small loss of capacity, a sensible trade off in my opinion.
The biggest advantage to me besides the longevity of LiFePO4 batteries is the ability to “grow” your battery bank as the need arises. With LAs there is a problem with combining batteries of differing ages into a single bank. With LiFePO4 due to the Battery Management System (BMS) modules of differing vintages can successfully accommodated within a single battery bank. Thus one can grow one’s bank as the need arises or finances allow.
I hope @plonkster, @Coulomb, @weber, @PurePower and others will add to this and correct any errors in my post making it a useful resource.