Monday, April 22, 2013

Lithium-Ion Batteries

The Implantable Lamp (A3024A) is powered by a BR1225 lithium primary cell. Its nominal voltage is 2.7 V and its nominal capacity is 48 mA-hr. It cannot be recharged, and we anticipate that its capacity when providing 10 mA will be closer to 24 mA-hr, because its nominal discharge current is only 30 μA. The A3024A provides 30 mA at 5.0 V for its lamp using a 90% efficient inductive boost regulator. But the battery's internal resistance is of order 40 Ω, and this resistance limits the continuous lamp current to 8.2 mA. The A3024A is equipped with a 1-mF capacitor bank in parallel with the battery, and this capacitor bank allows the boost regulator to flash the lamp for up to 10 ms. The volume of the capacitor bank is roughly the same as the volume of the battery. When we flash the lamp for 10-ms bursts every 100 ms, the battery voltage drops to 1.8 V as it provides an average current of 10 mA. We expect this current to exhaust the battery in 2.4 hrs.

The LIR1220 lithium-ion secondary cell is slightly thinner than the BR1225, has nominal output voltage 3.6 V, output resistance only 2 Ω, and can be re-charged. With its low output resistance, it can supply continuous power to the lamp without the need for a capacitor bank. Its capacity, however, is only 8 mA-hr. Consider instead the PP031012AB lithium-ion battery with capacity 19 mA-hr. Its rectangular shape occupies only 0.25 ml, compared to the 0.31 ml of the circular BR1225. When flashing the lamp 10 ms out of every 100 ms, the battery voltage will remain close to 3.6 V, and the battery will need to supply an average of only 5 mA to the boost regulator. The battery's capacity is 19 mA-hr at a discharge current of 20 mA. Here our average current is 5 mA but the peak current is 50 mA. We expect a slight decrease in capacity as a result, perhaps to 15 mA-hr. Even so, we can flash the lamp for 3.0 hrs with this battery, compared to 2.4 hrs with the BR1225. Furthermore, we can turn on the lamp continuously, although we should be aware that doing so will reduce our battery capacity.

The Implantable Sensor with Lamp (ISL), as we specified in our Technical Proposal, will combine a subcutaneous transmitter, an implantable lamp, and a microprocessor into one circuit that may be implanted in a rat. We are aiming to keep its volume less than 4 ml. Suppose we equip the ISL with a TE382030 battery pack. Its volume is 2.3 ml (30 mm × 20 mm × 3.8 mm), which leaves us 1.7 ml for the circuit, which will be sufficient because we no longer need space for a capacitor bank. The battery has capacity 150 mA-hr when delivering 150 mA continuously. It could flash the lamp with current 30 mA for 10 ms out of every 100 ms for 30 hrs, or it could turn the lamp on continuously for 3.0 hrs. Alternatively, with some lower-resistance lamp leads, we could increase the lamp current to 50 mA, thus boosting the optical power at the fiber tip, and we would still be able to turn on the lamp continuously for 1.8 hrs.

Lithium-ion batteries discharge themselves slowly. Even if we draw no current from them, they lose roughly 5% of their charge in the first month and 2% per month thereafter. This self-discharge, combined with their lower total capacity compared to the lithium primary cells, will shorten their shelf-life to a few months. If we could recharge the batteries before implantation, we could be sure of obtaining the maximum operating life after implantation. If we could recharge the battery while it is implanted, we could greatly extend the duration of our experiments.

The head fixture of the ISL offers us an opportunity to recharge a battery while implanted. Suppose we contrive to place upon the head fixture a sufficiently small, two-way, water-resistant socket. In addition to the L+ and L− leads required by the lamp, we run a B+ lead to the battery positive terminal on the ISL circuit. The B+ and L− wires connect to the two-way socket in the head fixture. When we want to recharge the battery, we connect two flexible leads with pins on the end to the head fixture. These leads in turn connect to an especially-designed battery charger, which we connect to our LWDAQ. The charger measures the resistance of the charging leads and monitors the battery voltage during a rapid charge cycle that will take roughly one hour to complete.

In both the mouse-sized Implantable Lamp and the rat-sized Implantable Sensor with Lamp, it appears that the lithium-ion battery pack provides us with a smaller, longer-lasting, and more capable power supply, even if we do not recharge them. Thus we believe we should switch to lithium-ion batteries for the ISL development regardless of whether or not we can devise a way to recharge the batteries while implanted. Furthermore, we will add a single pad to the ISL circuit that will allow connection to the battery positive terminal, so we can re-charge the battery before encapsulation, and allow for the possibility of recharging through a separate B+ lead to the head fixture.

UPDATE: [24-APR-13] We take an old cell-phone lithium-ion battery and solder wires to its terminals. This is a PBR-55D by Pantech, capacity 1000 mA-hr with nominal voltage 3.7 V. We use a 10 Ω power resistor as a load and a bench power supply as a charger. We have no difficulty charging and discharging the battery, and we find its output resistance to be 1.0 Ω. Further investigation reveals that the standard quick-charge time for lithium-ion batteries is 2.5 hrs. We begin with constant current charging at 0.5C/hr, where C is the battery capacity. When the battery voltage reaches 4.2 V, we switch to constant voltage charging until the charge rate drops to 0.05C/hr.

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