OK, so I accidentally managed to get some LTO or Lithium Titanate pouch cells(li4 Ti5O12) . I thought they were NMC cells, but no – Lithium Titanate cells they are, and to be honest, quite happy I am to have them. Of all the Lithium cell technologies, LTO is among the safest as they do not have carbon in the anode. It is the carbon in the anode which causes most of the overheating issues. LiFePo4 batteries are very safe, but still have carbon anodes.
A lithium–titanate battery is a modified lithium-ion battery that uses lithium-titanate nanocrystals on the surface of its anode instead of carbon. This gives the anode a surface area of about 100 square meters per gram, compared with 3 square meters per gram for carbon, allowing electrons to enter and leave the anode quickly. This makes fast recharging possible and provides high currents when needed.
A disadvantage of lithium-titanate batteries is that they have a lower inherent voltage (2.4 V), which leads to a lower specific energy of about 30–110 Wh/kg than conventional lithium-ion battery technologies (which have an inherent voltage of 3.7 V). Lithium-titanate batteries are reported to have an energy density of up to 177 Wh/L.
Among the advantages of LTO technology is the high discharge rate capability, very internal resistance so low heating under load or charge together with exceptional charge rates and life cycle times. Recharge life capacities in excess of 100000 cycles with less than 10% capacity loss have been quoted – far in excess of anything else out there. Add that you can charge LTO batteries to around 90% capacity at -30 degrees C in around 30 minutes, and things start to look really interesting. Indeed fast recharge
The discharge voltage limit of a traditional Li-ion battery must be tightly controlled as copper used as the negative electrode current collector will dissolve in the electrolyte below about 2V. This can lead to shorting and thermal runaway. A traditional Li-ion battery (Figure 2) must be highly controlled in terms of its temperature and voltage states and has a limited window of safety operation with only six regions of temperature and voltage safety stages. An LTO-based battery demonstrates a greater range of safety ” anywhere from -40°C to +260°C.
The down side – nominal cell voltage of only 2.4v means serial strings of 5x to reach only 12v nominal, that’s a little on the low side meaning higher currents that we would expect for LifePo4. It also means a different charge profile as we have a target max volts of only 2.8v per cell (14v absolute).
Constant current constant volt charge profiles are somewhat different to the lead/acid brigade.
With lead acid, you charge predominantly within the fixed voltage, variable current regime. Whereas with Lithium based technology you need to operate a very strict constant current constant voltage type regime, not paticualrly friendly to alternators or similar charging sources.
one of the big issues with lithium technology is the ability of the battery to swallow comparatively huge charge currents. Typicalil lead acid batteries automatically decrease charge currents by increasing internal resistance quite quickly with charge level. This means that lead acid are progressively harder to charge, the more charge they receive.
If you thought that LiFePo4 batteries were fast to charge, then you will really like LTO. Lithium technologies are different. they can accept far higher charge rates than Lead acid, often in the 1C range. LTO cells take this to a whole new level and the newer cells can potentially accept charge at up to 10C! That’s a full charge in 6 minutes!!!!
OK, so my cells aren’t quite that fast – they recommend only 5C, but that still means empty to full in only 12 minutes IF you can source the amps. It also means frightening maximum discharge rates. My cheapy cells have a maximum continuous rate of 6C and a peak rate of 12C. That means a 180A/hr nominal stack can potentially put out a whopping 2160Amps, and can handle 1080amps continuous drain quite happily.
Let’s put this into context. Imagine you are running a 2Kw inverter on your 12v house bank. Lets imagine you have a 325A/hr lead acid bank – that’s the 20 hr rate. Your 2Kw inverter is going to be pulling something in the order of 160-180 Amps – let’s say 0.5C. Assuming a fairly normal Peukerts constant for a good AGM battery, you can expect around 32 minutes of run time at that rate. On the other hand, a 180 LTO is more than happy to trundle along at only 1C discharge for the full rated capacity. Net result half the battery capacity but twice the run time – what is not to like. When you add that the LTO battery will swallow charge at up to 900 amps (5C) without complaint, and weighs in at around only 30Kg things really take shape.
Lets look at a typical AGM setup:
Mastervolt 12/160 AGM battery (Group 4D)
Nominal capacity at C/20 = 160 A/hrs
Weight = 42.3Kg eack (x2 = 84.6Kg for 320 A/hr)
Max charge rate = 45A (charging takes a Loooong time)
CCA (max discharge) = 870A (to SAE spec)
Self discharge rate = approx 1-2% per month when new up to 2% per WEEK when near end of life.
Life expectancy at 50% DoD = approx 700 cycles assuming 100% recharge each time
It is worth noting that 100% charge takes a considerable time.
Our Lithium Titanate Battery pack
nominal capacity 180A/hr – at any rate
Weight = Approx 30Kg
Max charge rate = 900A ( you can go from 10 to 100% in under 11 minutes – if you can supply the juice!)
CCA (Equiv) = approx 2700A
Self discharge rate = approx 1-2% per month (still under investigation)
Life expectancy at 90% DoD = approx 7-10000 (over 10 – 14 times as long)
Also worth pointing out that 100% recharge can be as fast as you can charge it, and battery life is not affected by less that 100% recharge. In fact they prefer to be less than 100%.
So, with all that said, I currently have only 36A/hr worth of cells and I am trying to obtain a further 54 cells to give me a total of 70 cells. These will be made up into a pair of 7P5S battery packs which will supply a total of 168A/hr nominal. This is intended to be the mainstay of the Catamaran power system. The reason for going with 2x separate battery packs is to provide a degree of additional resilience in the event of a pack fault.