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Care for your rechargeable batteries High-current Battery Balancer Part 1 – by Duraid Madina Proper balancing is critical for a long battery life, especially if they are lithium-based rechargeable types. But many balancers are inefficient, as they dump excess charge for a given cell, restricting how fast you can charge the batteries and wasting power. Not this one – it redirects that extra charge into other cells, so you can charge fast with little heat or waste! M ost rechargeable batteries consist of an array of nominally identical cells, connected in series, parallel or series/parallel to meet particular voltage, current and capacity requirements. Batteries with many series-connected cells often only expose the connections at the extreme ends. For example, a typical lead-acid car battery has six cells (2V × 6 = 12V) but only two terminals. To charge such a battery, we apply a higher voltage than the total of all the cells across those two terminals, and current flows through all six cells, increasing their state of charge. But there is no guarantee that each cell starts with an identical voltage, and despite their identical construction, cell capacity can vary, especially as the battery ages. This is not a big problem with car batteries because lead-acid cells tolerate slight overcharging well. By overcharging the battery a little, cells with a lower charge get a chance to ‘catch up’ to the others, while the most highly charged cells dissipate the charging current as heat. Despite this, large lead-acid battery banks (as might be used in a renewable energy installation) will last longer if 38 they are kept balanced. In this case, you might have several batteries in series, so not only do you need to be concerned about inter-cell balancing within a given battery, you also need to consider balancing the charge between batteries. The fact that you might be using batteries with different ages and possibly even from different manufacturers makes this even more critical. Then there is the case of lithium-ion and similar rechargeable cells. There is a great variety of lithium chemistries around, but many of them do not tolerate overcharging. They also can be easily damaged by over-discharging. So keeping lithium rechargeable batteries balanced is even more crucial. This Battery Balancer can handle cell voltages as low as 3V and as high as 15V, so it is suitable for a wide range of balancing tasks, including balancing the cells within a lithium-ion battery, or balancing individual lithiumion or lead-acid batteries. Each Balancer can handle up to four cells (or groups of cells) or batteries, and you can combine multiple Balancers for larger installations. Avoiding cell damage One conservative option would be to immediately stop charging as soon as any cell reached its maximum permissible voltage, but that would leave the remainder of the cells not quite fully charged. Left unchecked, what might start as a minimal imbalance between the cells, could over repeated charge/discharge cycles develop into a much larger imbalance, with the result that as a whole, the battery has significantly less usable capacity. Worse, when the battery is fully discharged, those cells which were not fully charged could become over-discharged and damaged. So we need a way to ensure that as a battery is charged and/or discharged, the cells are kept in balance. Each is then charged to approximately the same voltage, so that the battery capacity remains good and the cells degrade equally. This way, a battery need not be discarded just because one cell has degraded more rapidly than the others (a common problem). The simplest way to do this is to shunt current around any cell that has a higher voltage than the others during charging. We have used that approach in the past, for example, in our April 2017 Battery-Pack Cell Balancer. That design could handle packs with up to six cells, but only provided about 200mA of balance current. Practical Electronics | April | 2022 The Battery Balancer is constructed on a single 4-layer PCB just over 100mm wide, so it’s small enough to slot in anywhere. Got more than four batteries? Build as many Balancers as you need! That limited it to applications with chargers up to 10A, and it got quite warm during operation, as all that power was being turned into heat. Our new Balancer, being much more efficient, produces much less heat for a given balance current and thus can handle much higher battery charge currents – to 50A or more, assuming the cells are matched to within 5% (a fairly conservative figure for a healthy battery). Operational overview This Battery Balancer helps to ensure that cells in a battery are kept in balance by periodically checking the cell voltages and moving charge from cells at a higher voltage to cells at a lower voltage. To do this, it has three main sections, as shown in Fig.1. These are: 1) A voltage sensing front-end which draws very little current from the cells. 2) A control section consisting of little more than a Microchip SAM-L10 32-bit microcontroller, which also draws hardly any current when idle (according to Atmel, it’s the ‘industry’s lowest power in its class’). 3)A power section for moving charge between cells. The Balancer has been designed to achieve a high level of practical efficiency in three ways. First, the amount of power consumed when not actively balancing cells is kept low, by allowing virtually everything to be switched off, and ensuring that most of what remains draws very little power. Second, the amount of power required to see if balancing required is kept low, through the use of simple but energy-efficient voltage dividers. Third, instead of using inefficient schemes for balancing such as simply dumping charge from cells that have too much charge into resistive loads, the Balancer recycles charge by taking it from cells that have too much, and adding it to those that have too little. Practical Electronics | April | 2022 We h a v e also tried to make the Balancer flexible; not only can it balance batteries of up to four cells, or sets of up to four batteries, but with a small amount of external help, it can serve as a battery charger and even a battery discharger! Transferring charge Perhaps the most critical part of the Battery Balancer is the section which transfers charge between batteries/ cells (and maybe also a charger or a load). This section is replicated four times on the board, once for each battery/cell that can be connected. SC Ó Fig.1: while highly simplified, this shows the basic configuration of the High-current Battery Balancer. Microcontroller IC2 measures the voltage across each battery/cell via resistive dividers. If one has a voltage that is significantly higher or lower than the others, it transfers power into or out of the imbalanced cells via the four power transfer blocks. These can efficiently transfer energy to or from one battery/cell to the entire ‘stack’, and by extension, between multiple batteries/cells via the stack. A simplified version of this circuit is shown in Fig.2. This section can transfer energy to or from the battery/cell shown at the left and the complete battery/‘stack’. Energy can be transferred from one battery/cell to another via the ‘stack’. Let’s suppose we notice that one cell has a voltage that is lower than the other three. We can use that cell’s power section to transfer charge from the whole battery to that cell, to bring it into balance. This happens cyclically. First, the ‘stack-side’ transistor (QX) is switched on and current begins to flow from the battery, through the stack-side transformer winding, energising the transformer’s core. Because the cell-side power transistor (QY) is off, the voltage across the cell-side transformer winding quickly rises. A moment later, the stack-side power transistor is switched off, and the cellside power transistor (QY) is switched on. This transfers the energy from the transformer’s core into the cell. When this is estimated to have completed, the cell-side power switch is turned off and the cycle repeats, with a duty cycle proportional to the desired rate of charge transfer. The inductance of the transformer can be chosen relatively freely. Transformers with higher inductance allow operation at lower frequencies, but have higher resistive losses. Transformers with lower inductance require operation at higher frequencies, but have lower resistive losses. Note, however, that transformers with particularly low winding inductances tend to have slightly reduced coupling between the windings, though only a few such transformers have coupling so poor as to be a significant factor for this Battery Balancer. The voltages at the drains of QX and QY can exhibit significant inductive ringing. If it is too severe, it might exceed the transistor ratings. We have attempted to keep the inductance of these paths low by placing these devices very close to their respective transformers. But for higher-voltage applications, it is still prudent to place series RC snubbers (ie, Csnub and Rsnub) across the transformer windings. For lower-voltage applications (eg, balancing lithium-ion cells), these snubbers can be safely omitted, and that might even result in a small efficiency gain. The micro controls MOSFETs QY and QA via an ISO7041 isolator 39 because the negative end of the battery/ cell is not connected to ground (unless it is the bottom-most in the stack). The driving scheme is a bit more complicated than shown here, as will soon become apparent. The ISO7041 is powered by its own ‘floating’ 3.3V regulator from the battery/cell, to allow for the battery/cell voltage to vary over a wide range. Note how the negative terminals of the bypass capacitors both for the individual battery/cell and for the stack are connected via N-channel MOSFETs, rather than directly to the negative terminal of the battery/cell and GND respectively. This is to provide a ‘soft-start’ function which greatly reduces the sparks generated when connecting up batteries or cells. Full circuit details The full Battery Balancer circuit is shown in Fig.3, although two of the four charge-balancing circuits have been partly omitted to save space. All four are configured identically. Now you can see the full detail of this part of the circuit, which reveals a few extra subtleties. First, the isolator outputs cannot drive the MOSFET gates and microcontroller directly as they are too weak to achieve the required switching speeds. We spent a lot of time investigating the use of integrated gate-driver ICs in that role, but most of them have a significant quiescent current draw and stop functioning at low supply voltages. While this could be resolved for the lower cell power sections by deriving their supply rails from ‘one cell up’, Features and specifications • Balances two, three or four series-connected cells or batteries • Suits Li-ion, LiPo, LiFePO4, lead-acid, AGM and other chemistries • Each cell or battery can range from 2.5V (fully discharged) up to 15V maximum • Balancing current: up to 2.5A • Charging current: up to 50A • fficienc t icall around 80% • Quiescent current: around 100µA per battery/cell • 5mm spade lug connections for high-current batteries • 2.54mm-pitch pin header for connecting smaller batteries • Switching frequency: typically 100kHz • Multiple Balancers can be combined for balancing more cells or batteries • t can also act as an efficient atter char er or dischar er • Four onboard status LEDs plus one adjustment potentiometer • Serial status/debugging interface • om act size (108 80mm ) this would leave the topmost power section needing an alternative source of power, eg, from a boost converter. Instead of using integrated gate drivers, we decided instead to use simple NMOS/PMOS transistor pairs configured as inverters. Happily, there are many dual SMD MOSFETs available which include one N-channel device and one P-channel device, so each inverter is contained within a single package. In the case of the uppermost section of the circuit, these are Q11 and Q12. In each case, the MOSFET driving the stack side of the transformer (eg, Q9) is connected source-to-ground, and is a logic-level FET. It is driven by the 0-3.3V output of the inverter pair, which are themselves driven from a microcontroller digital output pin (in this case, pin 22, labelled SSPWM3). A 10kW pull-up resistor is provided at the input of each of these SC Ó Fig.2: a stripped-down version of the circuitry in each power transfer block. Power goes between the battery/cell and the stack via MOSFETs QX and QY and the transformer. QX is ground-referenced, so it is controlled from a microcontroller output pin, while QY is referenced to the negative cell/battery terminal. Therefore, the signal from the microcontroller to control QY goes through an ISO7041 isolator, which is powered from a 3.3V rail derived from the cell/battery voltage. 40 MOSFET-driving inverters so they have a low output when the micro is not in control of that pin (eg, it is in reset or being programmed). The other transformer-connected MOSFET (for example, Q10) has its source connected to the junction of this battery/cell and the one below. So as we described above, it is driven by an isolator that runs off a 3.3V floating supply referenced to that same voltage. Therefore, the MOSFET-driving inverter is also connected across this 3.3V floating supply, to provide an appropriate swing for that MOSFET. It too has a 10kW pull-up resistor to hold the MOSFET off by default. But note that the Texas Instruments ISO7041 low-power digital isolator has variants with different default pin states. The one we have chosen provides high outputs if its inputs are not driven, or the input side of the device is not powered (as opposed to the ISO7041F, which offers low outputs). This provides us with a safe ‘resting’ state. 1W resistors limit the power through the gate drive inverters, adding to the inverters’ intrinsic ~0.2W output resistance. This keeps the peak gate drive currents below 3A. It is not critical that the low-dropout (LDO) floating regulator (REG3 here) falls out of regulation if the cell voltage drops below 3.3V, as both the isolator and gate driver are capable of operating below this voltage. Note though that if a cell voltage is ever at less than 2.5V (a dangerously low voltage for a lithium-polymer cell, and a very low voltage for a lithium-ion cell), no attempt will be made to transfer charge to or from this cell. Instead, it is assumed that a battery with cell voltages this low is likely to have minimal charge, and so even if imbalanced, merely charging the entire battery will quickly bring the cell Practical Electronics | April | 2022 Scope1: this shows how the power switching MOSFETs are driven. For clarity, two isolated pulses are shown. The red and blue traces show stack-side and cell-side PWM signals for balancer channel 2 (gate-driving inverter inputs) as driven by the microcontroller and digital isolator, respectively. The yellow and green show the stack-side and cell-side power MOSFET gate voltages (gate-driving inverter outputs). The majority of the ringing on these traces is due to measurement error. voltages above 2.5V. Balancing can then resume long before any of the cells approach full charge. For more details on how charge transferral works, refer to scope grabs Scope1 and Scope2 and their captions. Voltage sensing To know which batteries or cells should be charged or discharged, the Battery Balancer needs to be able to take accurate voltage measurements across each battery/cell. Sensing low voltages accurately is becoming easier; high-performance analog-to-digital converters (ADCs) are readily available, and modern microcontrollers often include ADCs that would have been considered high-performance not that long ago. In our case, the SAM-L10 micro has a 12-bit ADC capable of taking one million samples per second. As we need to sense voltages up to around 60V (say, four 12V lead-acid batteries in series under charge), a kind of front-end is required to bring these voltages down into typical ADC ranges. One option would be to use operational amplifiers (op amps) that can tolerate these higher voltages, to divide (and possibly shift) the voltages as required. Suitable parts are not hard to find, but they are not cheap. Moreover, because the voltages the Battery Balancer needs to sense do not vary quickly, very little in the way of high-frequency performance is required, so offset-correcting chopperstyle op amps are applicable. However, the performance of the required op amp circuits would be dominated by the accuracy of the connected resistors. The power consumption of these op amps, while impressively Practical Electronics | April | 2022 Scope2: here, the red and blue traces are as in Scope 1, but the yellow and green traces show the drain voltages of the main switching MOSFETs (ie. the bottom ends of the power transformer) – the stack-side node is in yellow, while the cell side node is in green. Here, less of the ringing on the switching nodes is due to measurement error, particularly in the phase where both power MOSFETs are off, allowing their drains to float. low in many devices, is high enough that we couldn’t leave them powered all the time. So instead, we use a simple switchedcapacitor, switched-ground resistive voltage divider, as shown in Fig.3. To avoid the constant power consumption of an always-on voltage divider, we add low-side NMOS FETs (Q8a, Q13a, Q19a and Q24a). Even very small-signal FETs introduce only a couple of ohms of error while on. When off, however, the voltage can drift above the tolerance of the microcontroller input pins. So a second set of NMOS pass transistors (Q8b, Q13b, Q19b and Q24b) ensures the microcontroller never sees such voltages. Once again, we can take advantage of dual MOSFET packages so that each pair of transistors is just one part to be soldered to the board. To save microcontroller pins, all of the voltage dividers share a common pair of control lines. To take a set of voltage readings, first, the low-side NMOS switches are turned on, enabling the divider. Next, the pass gate NMOS switches are turned on, allowing the filter capacitors to start settling towards their respective values. Finally, the microcontroller’s onboard ADC takes its samples, allowing the software to know the voltage across each battery or cell. With the 100kW 2.2kW dividers used, and the 12-bit ADC having a 1.65V reference, the nominal sensed voltage range is 0-76.65V, and the resolution is 18.7mV. That’s precise enough to detect small differences between 12V battery voltages. Refer to Scope3 for more information on how this process works. For lower voltage batteries such as li-ion, LiPo or LiFePO4 packs with cells typically ranging from 2.7-4.2V, the resistive dividers are changed to 100kW/6.8kW which gives a range of 0-25.9V and a resolution of 6.3mV, which means we can balance out inter-cell voltage differences starting at about 10mV. A virtually identical arrangement is used to sense the voltage across the whole stack using MOSFETs Q18a and Q18b (which will probably be the same as one of the cells, but not necessarily the same one, hence the separate divider) and also the rotation of potentiometer VR1 via MOSFETs Q7a and Q7b. This is used to set various parameters, which will be described later. While an independent stack voltage monitor might seem redundant, it comes in handy when using the Battery Balancer in other applications. For example, it can be used to allow charging batteries from other power sources such as solar panels, or also as a battery charger. It can even be used in conjunction with another Battery Balancer, to transfer energy between two different batteries, in either direction, while keeping both in balance. Note that to avoid error, we don’t take voltage readings while the power section is active. Soft starting/spark mitigation We found that the first prototype produced some nasty sparks when connecting batteries (as is not uncommon). This was mainly due to the inrush current to charge the capacitor banks. These sparks could possibly damage the connectors, or even weld them! We therefore decided that, since it was not difficult to mitigate this, we would do so. 41 Four-channel Battery Balancer When power is first applied, the MOSFETs in series with the negative terminals of each set of bypass capacitors are off. Those capacitors therefore slowly charge via the parallel 20W resistors. 42 After the initial battery connection is made but before any balancing takes place, the microcontroller switches these MOSFETs on, presenting the full decoupling capacitance only after the connection is made. The MOSFETs effectively increase the ESR of the capacitor banks a little. However, with on-resistances that are only a fraction of an ohm, the capacitors are still capable of doing their job of stabilising the cell and battery Practical Electronics | April | 2022 Fig.3: the full Battery Balancer circuit consists of four identical sections at left, which efficiently transfer power between the batteries/cells and the ‘stack’ connected between CON2 and CON7. The control and sensing section is at right, and is based around 32-bit microcontroller IC2. The voltage-sense resistive dividers are disconnected using MOSFETs when they are not in use to keep the quiescent current draw low. LEDs7-9 and LED11 flash to indicate when charge is being transferred to or from specific cells. voltages nicely. The MOSFET turnon time is quite slow because there are no inverters to drive them – however since they only switch on after the capacitors have charged, this doesn’t matter. Practical Electronics | April | 2022 While we still recommend taking care to positively connect batteries to the Battery Balancer and being prepared for some amount of sparking to take place, this approach does greatly reduce the sparking that typically occurs. Circuit protection No you’ve doubt noticed that all cell and battery connections are via fuses; a good idea given how much current a large battery (or in some cases, even a small one) can deliver if there is a fault. 43 Each input also has a zener diode across it (after the fuse) which provides two functions. One, if a cell or battery is connected backwards, the zener will immediately conduct and blow the fuse. Two, if the cell or battery voltage is too high for some reason (eg, you’ve connected to the wrong battery terminal), the zener will go into avalanche breakdown, and in most cases, the fuse will again blow. By the way, in the parts list we’ve specified unidirectional transient voltage suppressors (TVSs) instead of zener diodes for these parts. They are effectively zener diodes, just with very high pulse current handling capability. Also note that the actual clamping voltage will be somewhat higher than the specified voltage, depending on the current being delivered from the source. We have taken that into account when selecting the parts, so that the protected parts of the circuit will not be exposed to damaging voltages at any reasonable current level. As the micro monitors all the various voltages, it will shut down if any of them are out of range. For example, if a cell voltage is too low for the circuit to function. Control section The microcontroller section is quite straightforward due to the high level of integration on the SAM-L10 micro (IC2). Its internal oscillator is more than adequate as an instruction clock source in this application. Currentlimiting resistors on digital outputs 15, 16, 23 and 24 are provided for it to drive four status LEDs directly (more on these later). ESD clamps are connected across the programming and UART interfaces to protect them from static discharge as these pins could be externally accessible. The microcontroller derives its power from linear regulator REG1, another NJW4184U3-33B. This was chosen to minimise quiescent current and operate over a relatively wide input voltage range (up to 35V). Its output passes through ferrite beads before reaching the microcontroller supply pins. It also provides power to the ‘near side’ of the various low-power digital isolators and the stack-side gate drivers. As these consume only a few milliamps while active, the power dissipated in the linear regulator is only a few tens of milliwatts in the worst case, when it is powered by a fully-charged 12V battery. While the gate drivers consume small amounts of current on average, they do so in an extremely bursty fashion, so they each have a local bypass capacitor. 44 Parts list – High Current Battery Balancer (suitable for 12V battery balancing – see below for other options) 1 four-layer plated through PCB coded 14102211, 108 x 80mm, available from the PE PCB Service 4 4.7µH 1:1 transformers (T1-T4) [eg, Coilcraft MSD1278**] 5 3A fast-acting SMD fuses, M6125/2410-size (F1-F5) [eg, Bourns SF-2410FP300W-2] 1 0.75A fast-acting SMD fuse, M6125/2410-size (F7) [eg, Bourns SF2410FP075W-2] 2 SMD ferrite beads, 470W <at> 100MHz, M2012/0805-size (FB1,FB2) [eg, Taiyo Yuden BK2125HM471, Murata BLM21AG471SZ1D or Kemet Z0805C471BSMST] 1 100kW vertical multi-turn trimpot (VR1) 1 momentary SPST tactile pushbutton switch (S1) 11 5.08mm pitch PCB-mount vertical spade lugs (CON2-CON12) [eg, Altronics H2094/H2095] 1 5-pin straight or right-angle header (CON13; optional – for smaller battery packs) 1 4-pin header (CON14) 1 8-pin header (CON15; optional, for ICSP) 1 2x4-pin header (JP1) 1 jumper/shorting block (JP1) Semiconductors 1 ATSAML10E16A-AUT 32-bit microcontroller programmed with 1410221A.hex, TQFP-32 (IC2) 4 ISO7041 4-channel digital isolators, QSOP-16 (IC4,IC6,IC8,IC10) [Note: not ISO7041F] 5 NJW4184U3-33B# 3.3V LDO regulators (REG1,REG3,REG5,REG7,REG9) 4 BUK9Y4R8-60E* NMOS FETs, LFPAK-56 (Q1-Q4) 1 BUK9Y8R5-80E* NMOS FET, LFPAK-56 (Q5) 1 UM6K34N dual NMOS FET, SOT-363 (Q7) 5 UM6K31N dual NMOS FETs, SOT-363 (Q8,Q13,Q18,Q19,Q24) 8 BUK9Y14-80E* NMOS FET, LFPAK-56 (Q9,Q10,Q14,Q15,Q20,Q21,Q25,Q26) 8 QS6M4 dual NMOS+PMOS FETs, SOT-457T (Q11,Q12,Q16,Q17,Q22,Q23,Q27,Q28) 4 3mm or 5mm through-hole LEDs (LED7-LED9,LED11) 4 SMD 24V* TVS diodes, SMB size (M3226/1210) size (ZD1-ZD4) [eg, SMBJ24A] 1 SMD 64V* TVS diode, SMB size (M3226/1210) size (ZD5) [eg, SMBJ64A] 2 5V ESD clamp diode arrays (D6,D10) [Littlefuse SP0503BAHTG] Capacitors (all SMD M2012/0805 size X7R ceramic unless otherwise stated) 4 100µF* 35V radial organic polymer electrolytic (eg, Kemet A759KS107M1VAAE031) 2 47µF* 80V radial organic polymer electrolytic (eg, Kemet A759KS476M1KAAE045) 4 4.7µF 100V or 10µF 75V M3226/1210 11 10µF 50V ** for lower-current applications, Coilcraft 8 4.7µF 6V MSD1278-562 is a suitable alternative 6 1µF 50V # AP7370-33Y-13 is a suitable alternative 8 470nF 6V 3 100nF 50V Note: Csnub and Rsnub components 5 1nF 50V C0G are not fitted for 4V/cell version 8 470pF* 250V C0G (Csnub) Resistors (all SMD M2012/0805 size 1% metal film unless otherwise stated) 5 100kW 0.1% 8 10kW 5 2.2kW* 0.1% 4 680W 5 330W 5 100W 5 20W 8 30W* (Rsnub) 8 1W M1608/0603-size Parts for ~4V cell balancing (eg, li-ion) – substitute for asterisked (*) items above 5 BUK9Y1R3-40H NMOS FETs, LFPAK-56 (Q1-Q5) 8 BUK9Y12-40E NMOS FET, LFPAK-56 (Q9,Q10,Q14,Q15,Q20,Q21,Q25,Q26) 4 SMD 10V TVS diodes, SMB size (M3226/1210) size (ZD1-ZD4) [eg, SMBJ10A] 1 SMD 24V TVS diode, SMB size (M3226/1210) size (ZD5) [eg, SMBJ24A] 4 100µF 16V radial electrolytic polymer capacitors 2 33µF 35V radial electrolytic capacitors 5 6.8k 0.1% M2012/0805 size metal film resistors Software The Battery Balancer software is fairly simple, but it took some development to get it right, and there were a few choices to be made along the way. Perhaps the most critical task the CPU has to perform is producing the eight PWM signals required for balancing. There are many larger microcontrollers, frequently aimed at Practical Electronics | April | 2022 to configure both the peak balancing current and the cell mismatch threshold above which balancing takes place. Scope3: this shows the voltage sensing circuit in operation. The yellow trace shows the voltage to be measured (~12V) and the green trace shows the divided voltage present on the micro input pin (~240mV). The red trace is the voltage divider enable line, which has a duty cycle of less than 1%, minimising the power consumption of the voltage dividers. The blue trace is the divided voltage pass control line, which ensures that only stable divided voltages reach the micro input pin. motor control applications, that feature large numbers of advanced PWM generators. The SAM L10 is small, inexpensive, and sips power, but has a more limited set of peripherals. The Battery Balancer needs to produce short pulses of variable length at variable frequencies; if a pulse is too long, substantial currents can flow through the Battery Balancer, leading to a blown fuse and possibly damage to other components, particularly the power MOSFETs. Moreover, the Battery Balancer needs to produce two PWM signals per cell. To achieve this, we use a software-driven approach. When a cell is to be charged or discharged, we define a ‘blip’ routine as a series of instructions that are either NOP (no-operation), or single-I/O set/clear instructions. With a 16MHz CPU frequency, this allows us to control pulse trains with roughly 60ns precision. We then compute the desired number of ‘blips’ up to a safe maximum (currently set to 10,000), disable interrupts, and call the blip routine in a loop. Once the blip routine has run the desired number of times, the software stops all power train activity and determines the next course of action. Voltage sensing When not in the middle of charging or discharging cells to bring a battery into balance, the Battery Balancer periodically checks the cell/battery voltages to determine which should provide charge, and which need to be given charge. We set the ADC voltage reference to Vdd/2 (ie, around 1.65V), noting that as the power train is inactive, the power consumption and consequently noise on the Vdd LDO output will be relatively small. Therefore, this voltage should be nice and steady. To measure a set of cell voltages, we first enable the resistor dividers by connecting their bottom ends to ground via the small-signal NFETs, and then enable the passtransistor NFETs. We then pause for about 1ms while the capacitors on each of the sense lines settles towards their final value. Finally, we use the ADC to sample each of the settled lines before disabling the pass transistors and voltage dividers. The rotation of potentiometer VR1 is sensed at the same time that the other voltages are measured. It can be used Practical Electronics | April | 2022 Serial/USB interface The microcontroller features a UART, which is connected (via slew-limiting resistors and ESD clamping diodes) to pin header CON14. This can be easily converted to USB through the use of third-party ICs or cables such as FTDI’s ‘TTL-234X-3V3’, though note that these cables cannot be plugged directly into this header; some jumper leads will be required. If electrical isolation is required (or at least desired), our Mini Isolated Serial Link project in lat month’s issue, could be connected between the Battery Balancer board and the USB/serial adaptor. This board can be programmed by plugging a PICkit 4 into the ICSP header (CON15). For safety, this should only be done with no batteries or cells connected to the Battery Balancer. The board features four LEDs, one for each battery/cell. These are off by default but blink slowly if a battery/cell is being charged, or rapidly if a battery/cell is being discharged. The power consumed by the Battery Balancer’s control logic is small compared to that consumed by the LEDs while switched on! For this reason, the LED duty cycles have been kept low. Next month In part two of this feature next month, we will cover building the Battery Balancer, testing it, configuring it and using it, as well as some safety tips. 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