Maximizing the Cycle Life of Rechargeable Battery Packs with Multicell Battery Monitor ICs

Rechargeable battery packs can prematurely degrade if any one or more cells are allowed to over-discharge. When the pack reaches a fully discharged state, the ILOAD•RINTERNAL voltage drop of the weakest cell or cells will exceed the internal VCELL chemical potential, and the battery terminal voltage will become negative (relative to the nominal voltage). At this point, irreversible chemical processes begin that alter the internal material properties that originally provided the battery's charge storage capability, so that subsequent charge cycles of that cell will not retain the original energy content. In addition, once a cell is damaged, it is more likely to suffer a polarity reversal during subsequent use, exacerbating the problem and rapidly shortening the pack's useful cycle life.
When using nickel-based battery chemistries, over-discharging a group of series-connected cells does not necessarily pose a safety hazard, but it is common for one or more cells to suffer a polarity reversal long before the user notices any significant degradation in performance. By then, it is too late to repair the battery pack. When adopting lithium battery chemistry with higher energy storage density, polarity reversal must be prevented as a safety measure against overheating or fire. Therefore, monitoring the voltage of each cell is absolutely necessary to ensure long battery pack life (and safety when using lithium batteries).
Consider using the LTC6801, an integrated solution developed specifically to address this specific problem. The LTC6801 can detect overvoltage (OV) and undervoltage (UV) conditions of up to 12 series-connected battery cells and use cascadable interconnects to handle extended device chains, all without the need for any microprocessor support.

Features of the LTC6801
Operation modes and programmable threshold levels are set by pin strapping. Nine UV settings (from 0.77V to 2.88V) and nine OV settings (from 3.7V to 4.5V) are available. The number of cells monitored can be set between 4 and 12, and the sampling rate can be set to one of three different speeds to optimize the relationship between power consumption and detection time. Three different hysteresis settings are also provided to accommodate the operating state of the alarm recovery function circuit.
To support extended configurations of series-connected batteries, fault signaling is sent through bidirectional transmission of galvanically isolated differential clock signals in a "stacked" chain of devices, providing excellent immunity to load noise applied to the battery pack. Any device in the chain that detects a fault will interrupt its output clock signal, so any fault indication in the entire chain will be propagated to the "end" device in the stack. The clock signal is generated at the end of the stack by a dedicated IC (such as the LTC6906) or a master microprocessor (if one is needed) and circulates completely through the device chain when conditions are normal.
In many applications, the LTC6801 is used as a backup monitor for more sophisticated acquisition systems such as the LTC6802 (for example, in hybrid vehicles). However, it is also well suited as a stand-alone solution for lower-cost products such as portable tools and backup power supplies. Because the LTC6801 draws its operating power directly from the battery it monitors, the usable battery voltage range of each device varies depending on the battery chemistry to provide the voltage required to operate the device - from about 10V to as high as 50V or more. This voltage range supports the use of stacked groups of 4 to 12 lithium-ion batteries or 8 to 12 nickel batteries. As shown in Figure 1, it is very simple to use the LTC6801 to monitor a nickel battery stack (containing 8 nickel batteries) and protect it from improper use such as over-discharge. Note that although there is only an undervoltage alarm associated with the Nickel battery chemistry, a pack continuity fault will still be detected during a charging operation due to the presence of an OV condition.

Figure 1

Avoiding Reverse Battery
Reverse battery is a major damage mechanism in conventional nickel-based multi-cell battery stacks and can actually occur long before other obvious symptoms of charge depletion appear.

Take the following scenario as an example. An 8-cell NiCd battery pack is powering a hand tool such as a drill. The average user will use the drill until it slows to about 50% of its initial speed, which means that the nominal 9.6V battery pack drops to about 5V after the load operation. Assuming that the cells are perfectly matched (as shown in the left sketch in Figure 2), this means that the voltage of each cell has run down to about 0.6V, which is acceptable for each cell. However, if there is a mismatch in the cells (so that perhaps 5 of the cells are still above 1.0V), the voltage of the other 3 cells will be below 0V and experience a reverse stress, as shown in the center sketch in Figure 2.

Figure 2

Even assuming only one weak cell in the stack (a realistic scenario), as shown in the right-hand sketch in Figure 2, the first cell reversal will most likely occur while the stack voltage is still 8V or higher, with only a slight drop in the stack's ability to deliver power being sensed. Because of the practically unavoidable cell mismatch, users will periodically reverse cells without realizing it, thereby reducing the capacity and life of their battery packs. Therefore, a circuit that can detect a dead cell early on can provide significant value to the user.

Solution with LTC6801
The lowest available UV setting of the LTC6801 (0.77V) is well suited for detecting depletion of nickel battery packs. Figure 1 shows a MOSFET switch used as a load disconnect device, which is controlled by the output state of the LTC6801. When a cell is depleted and its potential drops below the threshold, the load is removed, thus avoiding the effects of battery reversal and its resulting performance degradation. It also allows maximum energy to be safely extracted from the battery pack because no assumptions are made about the relative matching of the cells, which may be required when using an overly conservative single-cell stack potential threshold function.
A 10kHz clock is generated by the LTC6906 silicon oscillator, and the LTC6801 output status signal is sensed and used to control the load disconnect action. Since this example does not involve stacking of devices, the cascadeable clock signal is simply looped back instead of passing to another LTC6801. An LED is used to provide a visual indication of "power available to the load." When the switch opens, the weak cell’s voltage tends to recover slightly and the LTC6801 restarts the load switch (no hysteresis with a 0.77V undervoltage setting). The cycling rate of this digital load limit action depends on the configuration of the DC pin; in the fastest response mode (DC = VREG), the duty cycle of the delivered load power decreases and tapers to zero, while pulsation becomes noticeable and slower as the weakest cell safely reaches a fully discharged state.

In some applications, it is unacceptable to automatically disconnect the load when the weakest battery is close to a fully discharged state (as shown in Figure 1). For these cases, the circuit shown in Figure 3 may be a good alternative. Rather than forcing some kind of load intervention, this circuit simply provides an audible alarm indication of a nearly depleted battery. Here, the LED provides an indication that the alarm circuit is operational and that no battery is depleted.

Figure 3

When the source clock is not present, an LTC6801 idle mode is invoked and power consumption is reduced to a miniscule 30μA, well below the typical self-discharge current of a battery pack. In both Figures 1 and 3, the circuits shown have a switch that disables the oscillator (and other peripheral circuitry) to place the circuit in idle mode when not in use, minimizing battery drain.

ConclusionThe
LTC6801 can simultaneously monitor up to 12 individual cells in a multi-cell battery stack, enabling the capacity and life of the battery stack to be maximized. Multiple LTC6801s can also be cascaded to support larger battery stacks. The device is highly integrated, configurable, and has well-thought-out features, including an idle mode to minimize battery stack drain during inactivity. This makes the LTC6801 a compact solution for improving the performance and reliability of battery-powered products.