A full-charge indicator for battery chargers, includes a transistor which is in a conductive state as long as charging current to the battery is not less than a level which indicates that the battery did not reach full charge. When the battery reaches full charge, a voltage drop in a resistor in the charging current path is not sufficient to maintain the transistor in a conducting state, and therefore it is switched off. When this occurs an LED is turned on, to indicate a full charge state of the battery. A photocoupler together with a photocoupler transistor are included. When the transistor is off, the photocoupler activates the photocoupler transistor to shunt out a resistor, thereby reducing the charging current to the battery to a float charging current and prevent the battery from being overcharged and damaged

Cole, Steven W.

NASA Technical Reports Server

United States Patent [191 11 4,394,613 
Cole [45] Jul. 19, 1983 
FULL-CHARGE INDICATOR FOR BATTERY 
CHARGERS 
Inventor: 
Assignee: California Institute of Technology, 
Appl. No.: 225,961 
Filed: Jan. 19,1981 
Int. Cld ....................... H02J 7/10; GOlR 19/165 
U.S. C1. ...................................... 320/48; 323/278; 
323/902; 340/636 
Field of Search ....................... 320/22, 32, 39, 43, 
320/48; 323/278, 902; 340/636 
Steven W. Cole, Covina, Calif. 
Pasadena, Calif. 
References Cited 
U.S. PATENT DOCUMENTS 
OTHER PUBLICATIONS 
Owen, “Battery State Indicator”, Practical Electronics, 
vol. 14, No. 5, Nov. 1978, p. 8, S90380037. 
Primary Examiner-A. D. Pellinen 
Attorney, Agent, or Firm-Freilich, Hornbaker, 
Wasserman, Rosen & Fernandez 
[571 ABSTRACT 
A full-charge indicator for battery chhrgers, includes a 
transistor which is in a conductive state as long as 
charging current to the battery is not less than a level 
which indicates that the battery did not reach full 
charge. When the battery reaches full charge, a voltage 
drop in a resistor in the charging current path is not 
sufficient to maintain the transistor in a conducting 
state, and therefore it is switched off. When this occurs 
an LED is turned on, to indicate a full charge state of 
the battery. A DhOtOCOUDler together with a photocou- 
320148 
0139 x 
pler transistor i re  included. WThen the transiitor is off, 
the photocoupler activates the photocoupler transistor 
nt out a resistor, thereby reducing the charging 
nt to the battery to a float charging current and 
nt the battery from being overcharged and dam- 
3,930,198 12/1975 
4,056,765 11/1977 
4,243,929 1/1981 8 Claims, 1 Drawing Figure 
https://ntrs.nasa.gov/search.jsp?R=20080005877 2019-08-30T03:06:52+00:00Z
U.S. Patent Jul. 19, 1983 4,394,613 
&= 
1 
4,394,6 13 
2 
DETAILED DESCRIPTION O F  THE 
INVENTION 
FULL-CHARGE INDICATOR FOR BATTERY 
CHARGERS 
ORIGIN OF THE INVENTION 
Attention is now directed to the single FIGURE 
5 which is a schematic diagram of a battery charger that 
incorporates the circuitry of the invention, which is 
represented by the circuitry surrounded by dashed line 
10. Except for this circuitry, the rest of the diagrammed 
circuitry represents a conventional charger with its 
cuit, represented by capacitor 14, and control circuitry 
15. The latter includes a voltage divider network repre- 
sented by RA and RE. Preferrably, to protect a battery, 
such as battery 18, from excessive charging current, a 
The invention described herein was made in the per- 
formance Of work under a NASA 'Ontract and is 
ject to the provisions of Section 305 of the National 
(72 Stat. 435, 42 USC 2457). 
Aeronautics and 'pace Act Of lg5& Law 85-568 10 power transformer 11, rectifying bridge 12, a filter cir- 
BACKGROUND O F  THE INVENTION 
1. Field of the Invention 
The present invention generally relates to battery 15 current limiter is included to limit the maximum current 
charging circuitry and, more particularly* to a full- which may be supplied to battery 18. The current lim- 
charge indicator for battery chargers. iter is designated in the FIGURE by 20. 
2. Description of the Prior Art As shown in the FIGURE, the novel circuitry in- 
In the charging of most types of batteries, and Partic- cludes a resistor R1 which is in the charging current 
ularly of the NiCd or lead acid types, it is very desirable 2o path. The emitter of a pNp silicon transistor Q1 is con- 
to be able to know when the battery under charge has nected to one end of R1 and the transistor base, through 
reached a full-charge state. Also, when battery charg- a resistor R2, is connected to the other end of R1. The 
ing is left unattended, it is important to provide some collector of Q1 is connected to ground through a third 
arrangement to prevent battery overcharged in order resistor R3. A light emitting diode or LED 25, together 
not to damage the battery. Some batteries tend to ex- 25 with a photocoupler 27, are connected in series across 
plode when being overcharged. the emitter-collector circuit of transistor Q1. For ex- 
A need exists for a simple circuit to provide an indica- plaining the first aspect of the invention, the photocou- 
tion when the battery is fully charged, and protection pler may be ignored. 
against battery overcharging while maintaining it in a 3o As should be apparent when transistor Q1 is conduct- 
fully charged state. ing, or is on, insufficient current flows through LED 25 
and therefore it does not emit light. On the other hand 
when transistor Q1 is off i.e. is non-conducting, suffi- 
In accordance with the present invention, a circuit, cient current flows through L E P  25 thereby activating 
for inclusion in a battery charger, comprises a resistor 35 it, to emit light. To switch transistor Q1 on a known 
which is designed to sense the charging current. As emitter to base voltage difference must be present. It is 
long as the charging current through the resistor is typically 0.6 volt. 
greater than a known level, thereby indicating that the In Practice the &stance of R1 is chosen SO that as 
battery is not yet fully charged, the voltage across the long as the charging current is not less than a given 
resistor exceeds a chosen voltage. As a result, a transis- 40 level, which indicates that the battery did not reach a 
LED, is turned on. Thus, the LED is not energized. less than 0.6 volt and therefore transistor Q1 conducts. 
However, once the battery reaches full charge and, as a For 
level, the transistor is cut off and the LED is illumi- 45 resistance of R1 is chosen to be 12 ohms. Thus, as long 
as the current is 50 ma or above the voltage across R1 nated, thereby indicating a full-charge state of the bat- 
I _ _ _ _  is not less than 0.6 v and therefore transistor Q1 con- 
SUMMARY O F  THE INVENTION 
tor, connected in parallel with an indicator e.g. an full charge state as yet, the voltage drop across R1 is not 
assuming that when battery l8 is 
result the charging current drops below the known charged, the charging current drops below 50 ma* the 
ducts. The function of R2 is to limit the base current. As wry. To protect the battery from becoming Overcharged, a shown, two diodes CR1 and CR2 are connected across 
photocoupler, a resistor and a photocoupler transistor 5o R1 to limit the voltage drop across it not to exceed 1.5 
are added. The photocoupler is connected in series with 
the LED. Thus it is turned on or illuminated whenever When transistor Q1 conducts LED 25 is not ener- 
the LED is illuminated which occurs when the battery as the 
reaches charge. Once the photocou- battery reaches a full charge state, the charging current 
pler activates the photocoupler transistor which causes 55 drops below 50 ma. Therefore the voltage drop across 
the charger's output voltage to decrease to a float level R1 is less than 0.6 v. Consequently, Q1 is cut off. With 
and thereby Protect the battery from becoming over- Q1 in a non-conducting state, sufficient current flows 
charged. through LED 25 and therefore it emits light, which 
The novel features that are considered characteristic indicates that the battery is fully charged. 
of this invention are set forth with particularity in the 60 Quite often chargers are left unattended, in which 
appended claims. The invention will best be understood event a danger exists that the battery may be over- 
from the following description when read in connection charged, which may greatly damage or destroy the 
with the accompanying drawing. battery. To prevent overcharging from occuring, the 
v, by providing a current path around R1. 
gized sufficiently to emit light. H ~ ~ ~ ~ ~ ~ ,  as 
photocoupler 27 together with additional circuitry are 
65 included. The additional circuitry consists of a DESCRIPT1oN OF THE 
The FIGURE is a schematic diagram of a battery 
charger incorporating the ciqcuitry of the present in- 
vention. 
photocoupler transistor 30, which turns on when the 
photocoupler 27 emits light, and a resistor R4. The 
transistor 30 and the resistor R4 are connected in series 
4,394,613 
3 4 
across RA which is in the charger’s voltage dividing 
branch. energized, 
In operation, as the battery becomes fully charged 
and the charging current drops below 50 ma, Q1 cuts 
off. Therefore, in addition LED 25, photocoupler 27 5 plied, 
emits light. The light from photocoupler turns on 
photocoupler transistor 30 and thus R4 is effectively 
across RA, thereby reducing the effective resistance of 
RA in the voltage divider network. Consequently, the 
voltage across the battery decreases. R4 is chosen so 
that the voltage drops to a safe floating voltage level, 
and only a float charging current is applied to the bat- 
tery. The floating voltage is chosen so that the battery 
is protected from becoming overcharged. The floating 
voltage level is generally specified by the battery manu- 15 
facturer. 
As seen from the FIGURE, a capacitor 35 is con- 
nected across LED 25 and the photocoupler 27. The 
function of capacitor 35 may be described as follows: 
When the charger is first turned on to charge a battery, 
which is other than fully charged, a finite time elapses 2o 
before the charge current builds up to the required, 
charging level. Without the capacitor 35, as the charger 
is turned on, the charging current may be less than 50 
ma, thus turning on photocoupler 27 which would in 
turn, turn on transistor 30, thereby locking the charging 25 
voltage at the float voltage level, unless the battery is 
discharged so that the charge current exceeds 50 ma 
even at the floating voltage level. However, with the 
capacitor 35, it delays the activation of photocoupler by 
a short period which is sufficient for the charger’s 30 
charging current to build up to 50 ma or more, thereby 
preventing the charger from becoming locked at the 
float voltage, except when the battery is fully charged. 
In summary, in accordance with the present inven- 
tion, the state of charge of the battery, being charged, is 35 
monitored by sensing the charging current, which flows 
to the battery, rather than the voltage across the bat- 
tery. Only when the charging current falls below a 
certain level, thereby indicating that the battery is fully 
charged is an indicator e.g. an LED energized to indi- 40 
cate full charge battery state. To prevent a battery from 
being overcharged, means are included, so that when 
the battery reaches full charge the charger is controlled 
to reduce the voltage across the battery to a float level 
and apply to the battery a current, which can be applied 45 
to the fully charged battery without any danger thereto. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art and consequently, it is 
intended that the claims be interpreted to cover such 50 
modifications and equivalents. 
sistor is in a nonconducting state, said indicator is 
unidirectional conducting means in series with said 
battery through which charging current is sup- 
a voltage dividing network comprising at least first 
and second series resistors connected between a 
conductor along which current is conducted into 
said battery and a source of reference potential for 
said battery, 
switching means for reducing the effective resistance 
of one of said voltage dividing resistors when the 
voltage drop across said control means is less than 
said preselected voltage to reduce the charging 
voltage applied to said battery, said switching 
means comprising a photocoupler and a photocou- 
pler transistor, the former being connected in series 
with said indicator between the emitter and collec- 
tor of said transistor, and the later being connected 
across one of said voltage dividing resistors. 
2. A circuit as defined in claim 1 further including a 
capacitor connected across said photocoupler and light 
indicator. 
3. A circuit as defined in claim 2 wherein said indica- 
tor is a light source. 
4. A circuit as defined in claim 2 wherein said light 
source is a light emitting diode. 
5. In a battery charger, a circuit for protecting the 
battery from being overcharged, comprising 
control means in the charging current path for pro- 
viding less than a preselected voltage drop there- 
across when the charging current level is less than 
a known level which is related to full-charge bat- 
tery state, 
a resistive network connected between a conductor 
along which current is conducted into said battery 
and a source of reference potential for said battery, 
a transistor switch having its base-emitter junction 
connected across said control means to switch 
from one conduction state to a second state when 
the voltage drop across said control means is less 
than said preselected voltage drop, and 
switching means for reducing the resistance provided 
by said network when the voltage across said con- 
trol means is less than said preselected voltage 
drop, said switching means comprising a photocou- 
pler responsive to said transistor switch to emit 
light when said transistor switch is switched to said 
second state and a photocoupler transistor con- 
nected in parallel with a portion of said resistive 
network, 
whereby light from said photocoupler activates said 
photocoupler transistor to reduce the resistance 
What is claimed is: provided by said network. 
1. A circuit for use in a battery charger to indicate 6.  A circuit as defined in claim 5 further including a 
when the battery to be charged reaches a full charge light indicator connected in series with said photocou- 
state and for protecting said battery from being over- 55  pler, whereby when said transistor switch is in its sec- 
charged, comprising ond state to activate said photocoupler to emit light, 
a control means in the charging current path for pro- said light indicator is energized to indicate said full 
viding less than a preselected voltage drop there- battery charge state. 
across when the charging current is less than a 7. A circuit as recited in claim 6 wherein said light 
known level, which is indicative of a full-charge 60 indicator is a light emitting diode. 
battery state, 8. A circuit as recited in claim 5 wherein said resis- 
a transistor connected with its base-emitter junction tance network comprises a plurality of resistors con- 
across said control means, said transistor being nected in series, and said control means includes a sepa- 
switchable to a nonconducting state only when the rate resistor connected in series with said photocoupler 
voltage across said control means is less than said 65 transistor, and said photocoupler transistor and separate 
preselected voltage, resistor are connected across at least one of the resistors 
of said network. an indicator connected between the collector and 
emitter of said transistor, whereby, when said tran- * * * * *  
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
PATENT NO. : 4,394,613 
D A T E D  : July 19, 1983 
INVENTOR(S) : Steven W. C o l e  
are hereby corrected as shown below: 
It is certified that error appears in the above-identified patent and that said Letters Patent 
Col. 4, line 25, delete "2" and substitute -- 3 -- 
4igned and 5ealed  this 
Tenth Day of January 1984 
(SEAL,] 
Aftest: 
Atfesting OfJicer 
GERALD J .  MOSSINGHOFF 
Commiuiontr of Portnrs ond Tmdcmorks 


Full-charge indicator for battery chargers

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