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I would like to charge a lithium-titanate battery rated at 2.4V (capacity 50mah, max charge voltage 2.75V), which is lower than the typical li-ion battery (3.7/4.2V).

Most charger ics seem to be fixed at 4.2V, so what is the correct way to do this? If I need a battery charger ic, I would prefer to use a standalone charger ic, instead of a mcu controlled one. Power source will be USB.

*edit: I found this article on how to change the termination voltage of a charger ic http://www.maximintegrated.com/en/app-notes/index.mvp/id/4520 using an opamp to change the volage to the sensing pins. Does this approach work with other charger ICs as well (do they all function in the same way)? Or what would be the best way to get a suitable charger?

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  • \$\begingroup\$ You need a charger tailored to the battery chemistry. \$\endgroup\$ – WhatRoughBeast Mar 10 '15 at 17:26
  • \$\begingroup\$ That's the problem, there are lot's of chargers for Li-Po, LiFePo4 and NiMH available, but I couldn't find any simple/cheap solution for charging Li-Titanate yet. \$\endgroup\$ – Ruu Mar 10 '15 at 17:51
  • \$\begingroup\$ If you're asking for a charge algorithm, one data sheet b2i.cc/Document/546/50Ah_Datasheet-012209.pdf implies a 1C charge rate with a cut-off voltage of 2.8 to 2.9 volts. \$\endgroup\$ – WhatRoughBeast Mar 10 '15 at 18:03
  • \$\begingroup\$ According to the spec, charging should be cc/cv to 2.75v. I found this article on how to change the termination voltage of a charger ic maximintegrated.com/en/app-notes/index.mvp/id/4520 using an opamp to change the volage to the sensing pins. Does this approach work with other charger ICs as well (do they all function in the same way)? Or what would be the best way to get a suitable charger? \$\endgroup\$ – Ruu Mar 11 '15 at 15:43
  • \$\begingroup\$ Frankly, I have no idea where to find appropriate chargers for LiTi. And I simply cannot make a general evaluation of charge control ICs. But good work finding the app note. \$\endgroup\$ – WhatRoughBeast Mar 11 '15 at 23:00
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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!

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I just encountered a similar problem, where I needed a regulated voltage below that available from a zener, and I didn't want to go to the expense of a LDO regulator.

So stack up 4 regular silicon diodes (IN4001-5) in series, forward biased with a shunt resistor selected to match your supply voltage and the charge current you desire. And since USB ports can be found just about anywhere it makes sense to use that as your supply, and the whole mess is simple enough to incorporate into a spare USB cable, with a bit of heatshrink.

For a 2hr charge time, you'll need 25mA, and this works out to a 100 ohm shunt resistor. (5.0V-2.4V)/.025A = 104 ohm. Near enough for government work.

One caveat, the forward breakdown voltage of silicon diodes is somewhat dependent on current flowing through the diode, so you may do better playing with low current signal diodes or a larger number of schottky diodes in series instead. With luck the circuit might even be self-limiting, since the less juice the cell draws, the better the diodes will conduct.

Suck it and see if theory stands up to practice. <50c worth of components and a cable from the spares bin certainly won't break the bank.

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