How to charge lithium-titanate battery with lower voltage

Look closely at the top plot and you can see it is just applying a simple maximum current OR voltage scheme. Up to about .9 hour the maximum current (looks like about 700 mA) causes lower than the maximum voltage, so the battery is charged with that current. When the battery reaches the 4.2 V limit, the charger holds the voltage there and the current then decreases. Eventually the battery gets so full that the charging current tapers off to nearly 0.

The charger puts out either 700 mA or 4.2 V, whichever results in lower voltage (or current). The battery then decides the other of the two.

Short: You very likely need a balancer.
This page quotes a user who says his SCIB LTO batteries work well without one. Other people sell LTO balancers and other brands are generally 'less reputable' than Toshiba's SCIB.

Longer: There is no reason to expect a string of more than a few LTO calls to not need a balancer. Balancing is required when small differences in cell characteristics, due to initial and/or lifetime characteristic differences, cause cells to age differently, to have different capacities and so to charge and or discharge in slightly different manners. The result is that some cells will reach either charge or discharge endpoints before others. Continuing to charge or discharge the series string in those conditions will drive the cells into disallowed conditions and may result in cell or whole battery pack destruction.

The "good news" is that LTO balancing should be no harder than for standard LiIon cells - except that, if charging is carried out at the fast charge rate then or at the maximum permissible charge and discharge rates the switches used will need higher current ratings for a given cell Ah capacity. It would be possible to voltage monitor all cells in a string and enter balancing mode at lower charge rates when any one cell reaches its permissible limit.

LTO cells typically allow fast charge at 10C and maximum charge at 20C so a 40 Ah cell will typically fast charge at 400 A and have a max allowable charge rate of 800A.

Notes:

LTO (Lithium Titanium Oxide) batteries have been available commercially for 'some years'. The Toshiba "SCIB" LTO battery was introduced in 2008.
Suzuki use the Toshiba "SCIB" LTO battery in part of the battery in a number of their electric vehicles.

LTO differences compared to LiIon include:

• Substantially higher charge and discharge rates, due to the use of high surface area Lithium Titanate 'nano crystals' rather than Carbon on the Anode. An area increase of about 25 x compared to carbon yields the increase in charge/discharge rate.

• Somewhat lower and higher charge and discharge temperatures.

• Substantially higher claimed cycle life at 100% DOD. Claimed lifetimes vary quite widely with manufacturer and "Caveat Emptor" applies with respect to lifetimes and utility of cells from little known manufacturers.

Toshiba continue to announce 'improvements' to their SCIB cells. If Toshiba are still learning, other manufacturers and/or their users also are.

In the Suzuki EVs the LTO battery is usually used as a "front end" to the main LiIon battery, allowing fast charge and discharge for energy regeneration and hard acceleration purposes. LTO chemistry has lower energy density than standard LiIon technology making it unattractive in mobile applications where its significant improvements over LiIon are not highly valued.

Superb LTO discussion page - recommended!

Good LTO EV bike article.
He uses balancing boards intended for use with super-capacitors. Obtained on ebay here

Analog devices active cell balancing during discharge.

Information page
and PDF

LTO dedicated balancer

A "Sort of datasheet" for Yinlong 40 Ah LTO cells

EEtimes LTO introduction

Specific energy: approximately 30-110Wh/kg
Energy density: as high as 177 Wh/L
Specific power: 3,000-5,100 W/kg (peak load)
Discharge efficiency: approximately 85%; charge efficiency over 95% (low-rate)
Energy/consumer-price: 0.5 Wh/dollar
Service or shelf life: >10 years (some to 20 years)
Self-discharge: 2-5 %/month
Cycle durability: 6,000 cycles to 90% capacity (some models >10,000 cycles)
Nominal cell voltage: 1.9 to 2.4V (Toshiba SCiB cells 1.5-2.7V operating; 2.3V
nominal)
Cut-off voltage: 1.5V typical (some at 1.7V)
Temperature: -40 to +55°C (extended models)
Charging technique is using standard constant current, followed by constant voltage until the amps threshold is reached.

_____________________________

Caveat Emptor "specifications" for Yinlong 40 Ah cell.

Toshiba SCIB LTO home page

SCIB news December 2018

and here

Demand for SCiB™ is growing fast, and Toshiba is expanding production capacity through capital investment and alliances. In Japan, the company will construct a new production facility in Yokohama, Kanagawa prefecture, and reinforce the current manufacturing facility, Kashiwazaki Operations in Niigata prefecture. Separately, in 2017, Toshiba, Suzuki Motor Corporation and Denso Corporation agreed to establish a joint venture company to produce automotive lithium-ion battery packs in India, and Toshiba will also collaborate with Johnson Controls Power Solutions in the U.S.