Battery voltage measurement system

A measurement system for measuring discrete voltages of serially connected battery modules making up a battery power supply wherein there would be connected to each junction between cells one terminal of a dual-level voltage responsive switch and wherein the other terminal of each switch is connected to a common terminal. The switches are identical, and each would close at a first voltage across it and open at a second, slight higher, voltage. A ramp voltage is generated and connected from the common terminal and the high voltage terminal of the supply. The ramp voltage and current through this circuit are concurrently measured and displayed wherein the difference in voltage between the appearance of two current pulses is illustrative of the battery voltage of the battery module connected between switches giving rise to the current pulses.

FIELD OF THE INVENTION 
This invention relates generally to measurements of battery power supplies 
and particularly to a system for measuring the voltage of individual 
battery modules making up a power supply. 
BACKGROUND OF THE INVENTION 
There are many instances in which battery cells, or battery modules, are 
serially connected in a bank or banks to provide a higher voltage than 
otherwise available. Unfortunately, the condition of the bank of cells as 
a whole is dependent upon the condition of individual cells or modules, 
and it is well known that in order to detect and locate a deteriorating 
one of these, individual measurements must be made at the cell or module 
level. As an example, where there is a bank voltage of 126 volts, it might 
be in the form of ten 12.6-volt lead acid batteries. In such case, 
condition measurements would typically require that each individual 
12-volt battery be subject to measurement, which would necessitate the 
wiring harness extending from each battery back to the point of 
instrumentation making the measurement, and wherein there would be a wire 
for each terminal, or, in this case, a total of 11 wires. This has been 
regarded as quite cumbersome. 
It is an object of this invention to eliminate all but two of the wires and 
at the same time eliminate the necessity for switching between battery 
modules in order to make individual measurements. 
SUMMARY OF THE INVENTION 
In accordance with this invention, one lead of a single voltage responsive 
switch is connected to each of the interconnecting battery terminals and a 
common terminal. In addition, such a switch is connected between this 
common terminal and one of the end terminals of the bank of batteries. The 
ramp voltage is then applied between the other end terminal and the common 
terminal, this ramp voltage being such as to extend from zero voltage to a 
voltage just in excess of the total voltage of the bank of batteries. As 
the voltage is ramped up, the switches are opened and closed, 
respectively, giving rise to current pulses. Both the ramp voltage and 
instantaneous current through this arrangement are coordinately provided 
as outputs, wherein the voltage between discrete pairs of pulses are an 
indication of the battery voltage, the switches giving rise to these 
pulses.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a battery assembly 10 consisting of a 
plurality of battery modules designated B1-B16, only seven of which are 
shown. The negative terminal of battery module B1 is connected to terminal 
12, and the positive terminal of battery module B16 is connected to 
terminal 14, the intermediate batteries being connected by connecting 
terminals designated T1-T15, only T1-T2 and T13-T15 being shown. 
Two-threshold, or bi-level, switches S1-S17 are employed wherein one 
terminal of each is connected to common terminal 16, and the opposite lead 
of each is connected to one of terminals 12, T1-T15, or 14. Ramp generator 
18 and current sensing shunt 20 are connected in series between negative 
terminal 12 and common terminal 16. 
For each scan, ramp voltage generator 18 generates a ramp voltage waveform 
22 which commences at zero and rises to approximately 4 volts above the 
nominal voltage of 96 volts of battery assembly 10. As the ramp voltage 
rises on terminal 16, it sequentially rises above the voltage appearing at 
each terminal T1-T15 and finally just above that on terminal 14. The ramp 
voltage is also applied as an X axis or horizontal sweep voltage to 
oscilloscope 28, and the voltage output of current measuring shunt 20 is 
connected to the Y or vertical axis input of oscilloscope 28. 
A bi-level response switch, one of switches SW1-SW17, is connected between 
common lead 16 and each of terminals 12, 14, and T1-T15. These switches 
are identically set to close at a like discrete voltage at their terminals 
and to open at a slightly higher potential. 
FIG. 2 shows a schematic circuit diagram of one of the switches which is 
generally labelled a bi-level switch (FIG. 1). Switch SW is voltage 
responsive across its terminals to close and pass current when its right 
terminal is more positive than its left by about 2 volts and to open and 
block current flow when this voltage difference rises to approximately 2.5 
volts. These differences are reflected by the voltage current waveforms 
appearing on oscilloscope 28 and shown in calibrated form in FIG. 3 
wherein, for example, the difference between the leading edge and trailing 
edge of pulse P1 marks this difference. 
The operative current path of switch SW is through diode 30 and N-channel 
field effect transistor (FET) 32, the anode of diode 30 being connected to 
terminal 18, its cathode to the drain lead D of FET 32, and the source 
lead S of FET 32 being connected to the left-hand terminal of switch SW. 
FET 32 is controlled by the voltage divider consisting of resistor 34, for 
example, 1 megohm, and the collector-emitter circuit of transistor 36, 
these being connected through diode 30 across the switch. The control 
voltage for FET 32 appears across the collector-emitter circuit of 
transistor 36, it being connected between the gate lead G and source lead 
S of FET 32. Transistor 36 is in turn controlled by a voltage divider 
consisting of fixed resistor 38 and variable resistor 40, the former being 
on the order of 10 megohms, and the latter extending to 3 megohms. The 
voltage divider is connected across switch SW through diode 30, with 
variable resistor 40 being connected between the base and emitter of 
transistor 36. 
Initially, the operation of switch SW1 (FIG. 1) will be considered, with 
ramp generator 18 just starting its rise from zero voltage. When the ramp 
voltage rises just above approximately 0.6 volt, a tiny current commences 
flowing through diode 30, resistor 38, and resistor 40 of switch SW1. This 
small current is not discernable in FIG. 3. There will be no current flow 
through switches SW2-SW17 as diode 30 of these switches will be blocked by 
virtue of the battery voltages on terminals T1-T15 and terminal 14 being 
higher than the voltage on common terminal or lead 16. 
As the voltage on terminal 16 increases with respect to voltage on 
terminals 12, the largest percentage of this voltage will appear across 
the high impedance of the collector-emitter circuit of transistor 36, 
which at this point is turned off; and when this voltage rises to 
approximately 1.96 volts, its application between the gate and source 
leads of FET 32 causes the drain source circuit of FET 32 to conduct, 
effectively closing the circuit across the switch. There then follows a 
rising current as indicated by the leading edge of pulse P1 shown in FIG. 
3. 
Next, when the ramp voltage rises to approximately 2.48 volts, the voltage 
across variable resistor 40 rises to about 0.45 volt; and the latter 
voltage, being connected across the base-emitter circuit of transistor 36, 
causes it to conduct, lowering the impedance of its collector-emitter 
circuit and thereby the voltage across the gate-source terminals of FET 
32, causing the impedance between the source and drain leads of the FET to 
change to essentially an open circuit. As a result, the current flow 
through the switch drops to essentially zero, this being shown by the 
trailing edge of pulse P1 occurring at approximately 2.5 volts, as shown. 
Next, as the voltage of ramp generator 18 rises and reaches a point at its 
right terminal 19 which exceeds the voltage on terminal T1 (FIG. 1), 
taking into account the voltage of battery module B1, there occurs current 
conduction through the switch, this being illustrated by pulse P2 of FIG. 
3. Thereafter, as the ramp voltage continues to rise, the switching action 
just described will repeat in the balance of the switches, that is, 
switches SW3-SW17 giving rise to the succeeding current pulses shown in 
FIG. 3. Significantly, if one reads the voltage from FIG. 3 from 
succeeding on otherwise identical points on the current pulse waveforms, 
the voltage difference will be indicative of the voltage of the particular 
battery module between which succeeding switches have been operated to 
create the current pulses. 
It is to be noted that the base point for the current pulses gradually 
rises on the display of FIG. 3, this occurring by virtue of there being 
increased current flow with increasing ramp voltage as more of diodes 30 
of the switches conduct through resistors B8 and 40. Where module voltages 
are simply determined by visual examination of the display, such as shown 
in FIG. 3, this poses no point of error. Whereas, where battery module 
voltage is determined from an amplitude points on the trailing or leading 
edges of the pulses, appropriate compensation would be provided to 
comparators making such an examination either by hardware or via software. 
From the foregoing, it is to be appreciated that applicant's system 
provides a simple, in both hardware and operation, means of battery module 
analysis. No switching systems are needed, and wire connections from the 
environs of the batteries to any remotely located measurement system need 
only total two wires.