Below:
(1) Safety advice.
(2) Charge rate
Safety circuit: A "safety circuit" is a small PCB with electronics that is integrated as part of each cell and which protects the battery, the equipment and you from the more excessive behaviours of LiIon batteries. Without a safety circuit a LiIon cell is a small bonfire waiting to happen.
LiIOn 18650 cells really really really should have a safety circuit per cell. Genuine Panasonic cells are usually NOT supplied with safety circuits - as it is assumed a battery integrator will add them.
Non-Panasonic cells often will not have a safety circuit.
Pack Integration:
Charging individual LiIon cells is a reasonably straightforward task.
Integrating and then charging multiple Lithium Ion cells in series or parallel (let alone series parallel) is a challenging task which is far easier to do wrong than right.
Done wrong it can shorten cell life and can lead to spectacular expensive and dangerous destruction of the battery.
There are ICs available which can be used to reasonably safely charge LiIon battery packs. Failure to properly manage charging of multiple cells is about the battery equivalent of driving drunk, or blindfolded.
Brand: If you buy shoes, clothes, handbags or jewellery, brand may matter but often branding and high price benefit the manufacturer far more than you.
With LiIon cells, it is possible to but low price unknown brand cells that are of good quality and whose datasheets and claimed specifications are legitimate.
However, many LiIon cells that not made by major manufacturers are of low quality, datasheets may be copied from other makers or not available and actual performance is often inferior to what is claimed.
Genuine Panasonic cells are usually of good quality and datasheets are available and reliable and specifications are meaningful. Pansonic are not the only trustable brand, but are about as good as any. Even major manufacturers have had problems with LiIon batteries. This tells you something about what is liable to be experienced with products from unknown manufacturers and whose spec sheets are unavailable or untrustable.
The cells you provided a link to claim to be Panasonic NCR18650As.
If they don't say Panasonic on them then they almost certainly aren't.
The picture in the ad did not show a Panasonic logo and the cells do not look like cells in Panasonic literature. (That said, here are a number for sale on ebay that look similar, that do claim to be genuine and that do not appear to have Panasonic labelling. For example, the person selling these in a group buy very specifically claims them to be genuine Panasonic cells.)
If they DO say Panasonic on them they MAY not be.
They do say that they are made in Japan. Generally, Japanese manufacture is a sign of reasonable quality. In many cases low quality cells which are not made in japan claim to be Japanese made.
Panasonic NCR18650 data here
Panasonic NCR18650 data sheet here
Panasonic LiIon safety precautions
So, finally -
Charging rate
IF these are genuine Panasonic NCR18650A cells
then the recommended charge rate is 0.7C or 2 amps per cell or per series string.
If these are not genuine Panasonic cells
then the maximum charge rate is in the data sheet
and the data sheet may or may not be available
and the datasheet if available may be believable.
Or not.
In such cases 0.7C or 2A max per series string is probably "safe enough" but this is not certain.
Clone cell capacity may be as low as 2000 mAh.
0.7C x 2000 Mah = 1.4A.
2A for a 2000 mAh cell is 1C.
This is often"safe enough" for a clone cell.
YMMV *
"*" Your mileage may vary. ~~~= Don't try this at home.
Comparative results from different brands
The table below is reported on a Mountainbike Forums pag by a user whose competence is unknown, but who seems capable enough. All such results need to be treated with due care but also can be useful. mAh capacity results for cells tested showed a genuine Panasonic protected cell having highest capacity and an unprotected Panasonic cell as having slightly less (by about 2%). This result is somewha surprising and needs to be considered with care - but may be due to different cell batches or statistical differences between cells.
From this useful discussion page
Q&A:
So its meen that I can charge my 3S3P battery pack with 24v max 6AH Charger?
A cell requires up to 2A x 4.2V = 8.4W.
Your charger is rated at 24 x 6 A = 288 Watt.
So using a boost or buck converter that will charge about 288/8.4 =~ 288/9 = 30+ cells. BUT you will need to use a converter to do this.
If you connect to battery with a linear requlator you have 6A max or up to 3 strings in parallel and not more than 5S so 5 x 3 = 15 cells max.
.
3S3P = 9 cells.
You say 21 cells.
Do you means 3S7P = 21 cells?
BUT
3S = 14.6V max, 9V min.
4S = 16.8V - 12V
5S = 21V - 15V
6S = 25.2V - 18V
7S = 29.4V - 21V
A 24 VDC charger will charge up to 5S without a boost converter.
If you want 24V nominal you need 6S or 7S or maybe 5S.
How do you intend to do speed control?
What IC or controller do you intend to use?
Just connecting 24 V to cells will not work - as I think and hope you know. .
You need a specialist LiIon charger IC. Digikey sells a number that would be suitable.
I meen 7S3P - 7X3.7=25.9V
For Lithium Ion = LiIon cells
Vmax = 4.1V or 4.2V fully charged
Vmin = ~= 3.0C. Slightly lower is possible but there is almost no energy left and you risk cell damage.
7 x 3.0V = 21V min.
7 x 4.2 = 29.4V max.
So IF you charge 7 cells in series you need about 30V DC.
IF you want to charge 3P strings at full capacity you need 3 x 2A = 6A.
You can charge slower.
Your 24V supply is not high enough to charge all 7S cells directly.
You need a boost converte or a higher voltage supply.
21 cells requires about 21 x 2A x 4.2V max ~= 180 Watts - although less can be OK.
Your 24C, 6A supply can provide 24 x 6 = 144 Watts.
If you use a boost converter to 30V if overall efficiency is 80% then you need 180/80% =
225 Watts OR your 14 Watt supply at 24V will deliver 144 x 80% = 115 Watts at 30V.
115W into 7S3P at 4.2V cell = 115/21/4.2 = 1.3A = 65% of maximum allowed. This is acceptable - charging will take less than 50% more time than best allowable.
But you still require a charge controller IC.
.
I also have some tiny 50mAh LTO cells. Perhaps the same ones?
I got mine at Battery Space . . .
http://www.batteryspace.com/LTO-1020-Rechargeable-Cell-2.4V-50-mAh-2.0A-rated-0.12Wh.aspx
They have larger LTO cells as well. Some are offered pre-wired into 5 cell packs
(nominal 12V) and the charger they offer for these is a basic flood lead acid
car battery charger, with instructions to unplug as soon as the LED indicates
full charge (ie: don't trickle charge your lithium batteries.) No BMS in these packs either. It's a kind of crude solution to the problem, but LTOs are, apparently, rather good at self leveling in these 5 cell configurations and
a lead acid charger happens to be constant voltage/current, proper voltage,
and only lacking automatic cutoff.
But, yes, what about just one cell and at a smaller amperage rating?
This is an ideal solution for that . . .
http://www.prodctodc.com/5a-constant-current-led-driver-lithiumion-battery-charger-digital-ampvolt-mete-p-428.html#.VUzYTdNVhBc
I actually found the two I own from a Chinese eBay vendor (he might be the original manufacturer,) but it's nice to find them generally available on
a US-based web site.
Two tiny set screws allow you to dial in the specific voltage and amperage settings and the multi-colored LED on the bottom board indicate when
charging is complete (although I'm not 100% sure if the charger automatically
cuts off at this point.) In any event, this is the answer to your (and my) problem. It will not only charger LTOs, but anything else you can think of in regards to single cell lithium . . . even ones that haven't been invented yet (which will, invariably, come in different voltages than what we have now.)
Have fun!
Best Answer
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.