Battery Charger (A3033)

© 2017-2019 Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Design
Battery Families
Charging Connections
Chargers
A3033AA3033B
Rechargeable Implants
A3028T-RA3028B-RA3028E-RA3036A
Development

Description

Warning: Using the wrong charger with your implant can damage its battery. The charger version letter is written on the modular plug boot.

[08-DEC-19] The Battery Charger (A3033) is a LWDAQ device that perform ex-vivo recharging of implantable devices equipped with lithium-polymer (LiPo) or manganese-lithium (MnLi) batteries.


Figure: Tiny IST Battery Charger (A3033A). The version letter A is written on the boot.

All versions of the A3033 consist of a cable that plugs into a LWDAQ Driver, a body that contains its recharging circuit, and two charging leads with smooth-jawed clips to hold the charging electrodes of an implantable device. But they produce different voltages and impose different current limits, so that some chargers will damage some implants, and other chargers will fail to charge other implants, or take days to charge them. Check the version letter of your charger against the version of your implant.

VersionNameCompatible Devices
A3033ATiny IST ChargerA3036A
A3033BTiny SCT ChargerA3028T-R
Table: Versions of the Battery Charger (A3033). Compatible devices implants with which we may use the charger.

The cable is terminated with a modular connector plugs. We plug this into any unused socket of a LWDAQ Driver. The LWDAQ Driver supplies power to the charger. The charger's regulator is potted in epoxy within the black-wrapped section of the charger. Two flexible leads with stainless-steel, smooth-jawed clips allow us to connect to miniature pins, bare wires, or screws. The clips deliver a charging voltage with current limiting to the implantable device through its charging leads. The positive charging lead is always red. The negative charging lead is always black.

Design

S3033A: Schematic for A3033A and A3033B.
2N3904: Bipolar transistor used as voltage source.
DMP2004VK: Dual mosfet with clamping diode used in IST (A3036) charging circuit.
BAS116LPH4: Low-leakage diode used in SCT (A3028) charging circuit.

Battery Families

Almost all the Subcutaneous Transmitters (SCT) we sell are equipped with non-rechargeable lithium primary (Li) cells, either from the BR Series or the CR Series. These batteries produce a voltage of around 2.6 V for most of their operating life, and their charge capacity per unit volume is at least double that of any other battery family. When we equip SCTs with these batteries, their operating life for continuous experiments is at least double that of a device of the same volume equipped with any other type of battery.


Figure: Example Discharge Curves for Lithium Primary Batteries. We discharge five CR1025, 30-mAhr, 3-V cells with five A3028P3 transmitters, channel numbers given in legend, each consuming ≈75 μA, expected life is 400 hr, observed life is 391-410 hr. We measure VBAT by taking the average of one second's worth of X samples. The average value of X corresponds to VCOM, which is 1.8 V. From this we deduce VBAT, which corresponds to a X = 65535.

There are two applications where we have found rechargeable batteries offer a solution where non-rechargeable batteries do not. One is when we want the smallest possible battery, for the smallest possible battery that can provide the minimum 2.3 V needed to drive an SCT is a rechargeable battery. The other is for optogenetic stimulators, for the batteries that have the highest capacity to deliver current for light-emitting diodes are rechargeable batteries.

Our rechargeable manganese-lithium (MnLi) batteries are circular disks. Their charge capacity is measured in milliamp-hours (mA-hr), so that the product of current and time is a constant, within limits. Their output voltage is close to 2.5 V during their entire discharge. The ML920 is 9.5 mm in diameter and 2.05 mm thick, with capacity 11 mA-hr and source resistance 60 Ω. The ML621 is 6.8 mm in diameter and 2.15 mm thick, with capacity 5.0 mA-hr and source resistance 80 Ω. These batteries are not suitable for implantable stimulators that drive light-emitting diodes (LEDs). When delivering 10 mA, the ML920's output voltage will drop from 2.5 V to 1.9 V, which is barely enough to illuminate a red LED, let alone a green LED. But MnLi batteries are easy to charge, because we can charge them slowly and have no fear of damaging them. We connect a charging voltage through a resistor to the charging leads of the implantable device, and let it sit for forty-eight hours. After twelve hours the battery will most likely be fully-charged, but additional charge current does no damage.


Figure: Example Discharge Curves for Manganese-Lithium Batteries. We charge the batteries through a 400-Ω resistor using various voltages 2.9-3.3 V. The ML621 is 6.8 mm dia, 2.1 mm thick, 5.0 mA-hr. The ML920 is 9.5 mm dia, 2.0 mm thick, 11 mA-hr.

Our rechargeable lithium-polymer (LiPo) batteries come from various sources. They are all rectangular, so they have length, width, and depth measured in millimeters. When we buy a new batch of them, we can usually get something almost identical to what we purchased before, but rarely will it be exactly the same part number from the same manufacturer. Their capacity is also measured in mA-hr. Their voltage when freshly charged will be around 4.2 V, but quickly drops to 3.8 V and declines slowly to 3.5 V when the battery is nearly exhaustsed.


Figure: Example Discharge Curves for Lithium-Polymer Batteries. Each chart is C/I where C is nominal battery capacity in mA-hr and I is current drain through a transmitter in μA. The fraction thus expressed is the expected operating life in thousands of hours. We charge with a 4.2-V source current-limited to 20 mA for 19-mAhr and 150 mA for 190 mA-hr batteries.

We use LiPo batteries in our implantable stimulators because they are the only miniature batteries capable of delivering tens of milliamps to an LED. Their output resistance is an order of magnitude lower than that of MnLi or Li cells. Our 19 mA-hr LiPo battery has output resistance 5 Ω, while our 190 mA-hr LiPo battery has output resistance 1 Ω. These batteries can deliver 30 mA to an LED easily. Their higher output voltage means they store more energy, which is not useful when powering an SCT that requires only 2.3 V, but is useful when driving a blue LED that requires 3.0 V.

But LiPo batteries are more difficult to use. If we allow them to discharge completely, they lose half their capacity, as we describe in Discharge Rate and Capacity. So we must be sure to monitor their voltage during use, and turn them off before their output voltage drops below 3.4 V. When we charge them up, we must limit the charging current and the charging voltage or else they will over-heat and lose some portion of their capacity, or start to swell and crack the epox encapsulation of the implant.

The ideal way to rechrage a LiPo battery is to connect it to a bench-top power supply set to 4.2 V output with current limit set to the C, where C is the numerical value of its capacity in mA-hr. So a 19 mA-hr battery we charge with a 19 mA-hr current limit. At first, the power supply will apply something like 3.5 V to the battery and 19 mA will flow in. After half an hour, the battery will have charged to 3.7 V and 10 mA will be flowing in. After two hours, the current will be 1 mA or so, and the voltage will be 4.2 V. Now we disconnect the charger.

Neither terminal of the battery of an implantable devices is accessible to us directly. We must charge the battery through a pair of leads, each of which present a resistance of 10-100 Ω, depending upon the device. Once the charging current reaches the base of the leads, it must pass through one or two diodes before reaching the battery. In SCTs, we use the sensor leads, and the current must pass through two diodes. In ISTs, we use the stimulator leads, and the current must pass through one diode. The diodes are what makes it possible to use the leads for charging as well as their sensors or stimulators. The diodes and lead resistance make recharging the battery more difficult.

Charging Connections

The rechargeable versions of our Subcutaneous Transmitter (SCT, A3028) are made with the A3028PV1 circuit board, circuit schematic S3028P. This circuit provides only one amplifier, so all rechargeable versions of the A3028 are currently single-channel devices. Their input leads are X, a red or yellow lead, and C, a blue lead. Recharging takes place through diodes D1 and D2, which are BAS116LPH4. The plot below gives the forward current versus forward voltage relation for these diodes at various temperatures. Our chargers assume that the ambient temperature will be close to 25°C.


Figure: Current versus Voltage for the BAS116LPH4 Diode. Charging current must pass through two such diodes on its way to the battery in all rechargeable versions of the Subcutaneous Transmitter (A3028).

The Implantable Stimulator-Transponder (IST, A3036) input leads are L+, a red or orange lead, and L−, a blue or purple lead. From the base of the L+ lead, charging current passes directly into the battery's positive terminal. This same current emerges from the battery's negative terminal and passes through one diode before reaching the base of the L− lead. The plot below gives the characteristics of this diode as measured in an IST circuit.


Figure: Current versus Voltage for Charging Diode in the IST A3036A. This diodes it the clamping diode of the ITs' stimulator switch, a DMP2004VK mosfet.

At 0.45 forward voltage, the current is 30 μA through the diode, which is small enough that it will take man days to damage a fully-charged battery. If we want to charge to 4.25 V, a charging voltage of 4.70 V with a series resistance of order 100 Ω will provide safe charging over about eight hours to a 19 mA-hr battery.

Chargers

All versions of the A3033 are encapsulated devices with charging clips at one end and a network connector on the other end. Plug the network connector into an open socket of a LWDAQ Driver (the balck box). Fasten the charging clips to the two charging electrodes of a compatible rechargeable implant.

A3033A

The A3033A is a charger for small lithium polymber (LiPo) batteries. It provides a 4.7-V charging voltage with source resistance roughly 60 Ω. The source resistance combined with the resistance of the leads provided by the rechargeable device serve to limit the current flowing into the device's LiPo battery. The A3033A is compatible with the IST A3036A, which it will charge in 8 hr.

The A3033A's quiescent power consumption is 300 mW, confined within its epoxy-encapsulated body. Over the first ten minutes, the circuit heats up, and this results in its output voltage rising by roughly 0.1 V. After that, the output voltage is stable. We adjust the A3033A's output voltage so that it will be 4.7±0.05 V when warmed up.


Figure: Output Voltage versus Time after Power Connected for A3033A Charger with No Battery.

The output current of the A3033A is limited by its own output resistance, so it is not possible to damage a LiPo battery with the charger, nor to damage the charger with a short circuit.


Figure: Output Voltage versus Output Current for A3033A Charger.

The plot above shows the decline in output voltage with increasing current for four A3033A chargers. The initial slope is ≈ −60 Ω. The final slope is ≈ −200 Ω.

A3033B

The A3033B is a charger for small manganese-lithium (MnLi) batteries. It provides a 4.2-V charging voltage with 1-kΩ source resistance. The source resistance limits the current that flows into the battery to one or two milliamps. The A3033B is designed to charge MnLi batteries through the EEG leads of a rechargeable SCT. The charging current must pass through two charging diodes in the SCT before reaching the battery. The A3033B is compatible with the SCT A3028T-R, which it charges in 48 hr.

Rechargeable Implants

A3028T-R

The A3028T-R, with displacement volume 0.5 ml, is the smallest SCT we make. It turns out that the smallest batteries we can buy are manganese-lithium (MnLi) rechargeable batteries, so the A3028T is equipped with an ML621 MnLi battery with capacity 5.0 mA-hr. To charge the battery, we use a 1-kΩ resistor to deliver a 4.2-V charging voltage. The battery will be fully charged after forty-eight hours. Our Battery Charger (A3033B) is designed for use with the A3038T-R, and provides a full charge in 48 hr. Connect the charger's red lead to the red stimulator lead, and the charger's black lead to the blue stimulator lead.

A3028B-R

The A3028B-R, displacement volume 1.4 ml, is equipped with a 19 mA-hr lithium-polymer (LiPo) battery. We use a 100-Ω resistor and a 5-V charging voltage. We monitor the battery voltage by disconnecting it from its charger every half-hour, turn on the transmitter, and use the average value of its X input to deduce the battery voltage (Vbattery = 65535 × 1.8 V ÷ Xaverage). When the battery voltage reaches 4.2 V, we stop charging. If we continue charging, we will damage the battery.

A3028E-R

The A3028E-R, displacement volume 4.0 ml, is equipped with a 190 mA-hr lithium-polymer (LiPo) battery. We use a 100-Ω resistor and a 5-V charging voltage. We monitor the battery voltage by disconnecting it from its charger every half-hour, turn on the transmitter, and use the average value of its X input to deduce the battery voltage (Vbattery = 65535 × 1.8 V ÷ Xaverage). When the battery voltage reaches 4.2 V, we stop charging. If we continue charging, we will damage the battery.

A3036A

The A3028A, with displacement volume 0.9 ml, is our smallest implantable stimulator. It is equipped with a 19 mA-hr LiPo battery. To charge the battery, we use 4.7-V with source impedance 100 Ω connected to the red (positive) and blue (negative) leads. The charging current flowing into an exhausted battery will no more than 10 mA. The battery will be fully charged to 4.3 V in eight hours. At this point, the current flowing into the battery will be of order 10 μA and we can leave the charger connected indefinitely. Our Battery Charger (A3033A) is designed for use with the A3036A, and provides a full charge in 8 hr. Connect the charger's red lead to the red stimulator lead, and the charger's black lead to the blue stimulator lead.

Development

[10-DEC-19] We construct our first A3033A, using a commercial ethernet jumber cable with a sliding boot. We adjust the output voltage to 4.70 V and pot in black epoxy.


Figure: Construction of the A3033A, Potting the Circuit in Epoxy. The protruding component is the 470-Ω potentiometer we use to set the charging voltage.

[11-DEC-19] We plug our A3033A prototype into a LWDAQ Driver and measure open-circuit output voltage 4.65 V. We load with 1 kΩ and wait ten minutes. The open-circuit output voltage rises to 4.69 V and when loade with 1 kΩ it is 4.38 V. We expect the output voltage to increase as Q1, 2N3904. warms up. From these observations, we deduce output resistance 71 Ω. Short-circuit current is 31 mA, in which state Q1 is saturated and we have 0.5 W being dissipated in R2 = 470 Ω, a 0.25-W resistor encapsulated in epoxy. We connect to an A3036A and see output voltage 4.61 V, which implies 1.1 mA flowing into the device.