WO1979001061A1

WO1979001061A1 - Method and apparatus for recharging dry batteries

Info

Publication number: WO1979001061A1
Application number: PCT/SE1979/000091
Authority: WO
Inventors: K Roestlund
Original assignee: Bele Invent Ab; K Roestlund
Priority date: 1978-05-12
Filing date: 1979-04-12
Publication date: 1979-12-13
Also published as: GB2040119A; JPS55500324A; EP0015951A1; GB2040119B; SE408610B

Abstract

A method and an apparatus for recharging dry batteries, whereby the dry batteries (4) are charged with one of the periods (7) of an intermittently pulsating alternate current, whereas the batteries (4) are allowed to discharge somewhat during the periods between the charging pulses (7). The charging current is balanced according to the charge condition of the batteries so that the batteries irrespectively of the charge condition are charged with as constant average current as possible with possible variations depending on the sensitivity of the charging components to temperature or current strength. The apparatus comprises a transformer for stepping down net current to suitable charging voltage, a diode (D1) for letting through intermittent charging pulses, a resistor (R1) connected parallelly over the diode (D1), terminals for one or several dry batteries between the diode (D1) and the resistor (R1) on one hand and a second pole terminal (2) on the other hand. Parallelly over the dry batteries (4) are connected a Zener-diode (D1) and in series therewith a PTC-resistor (R3), whereby the PTC-resistor (R3) is designed so as to balance the average charging current over the dry batteries (4) to be maintained as constant as possible.

Description

Method and appάra ύs for recharging dry batteries. The present invention relates to a method and an appa¬ ratus for recharging or^" regenerating dry batteries, espe¬ cially such types of dry batteries which hereto have not been considered rechargeable. The dry batteries are re¬ charged by one period of an intermittently pulsating alter¬ nating current, and the batteries are allowed to become discharged somewhat during the periods of time which are present between the charging pulses. Dry batteries generally have electrodes of zinc and carbon and between the said electrodes and electrolyte, and they are marketed as ordinary rod batteries for high load during a short period of time, for instance for flash lights etc., as radio batteries or transistor batteries intended for relatively low load during a long period of time. It is not quite made clear what happens during the discharge of the batteries, but until now it has generally been considered that the batteries are used up once they are discharged after the manufacture. A theory of what happens when the batteries are discharged is that a boundary layer of the electrolyte adjacent one of the electrodes or both electrodes is consumed, whereby the inner resistance of the battery successively increases and the voltage over the poles decreases. Another theory is that an oxide layer is formed on the surface of the metallic electrode, which layer reduces the ability of migration of irons between the electrodes, whereby likewise the inner resistance increases and the voltage over the pole decreases. It may also be ima¬ gined that the inner resistance of the battery increases depending on gas formation in the electrolyte during dis¬ charging of the battery.

Practically and economically and also for milieu pro¬ tection reasons it is disadvantageous that batteries are discarded as soon as they are once discharged, and therefore the basis of the present invention is the problem to provide ά method and an apparatus for recharging or regenerating dry ^r - ^{* "}-batteries of the above mentioned type. By immense attempts : £^ ^.fj_as -p_rovecj that a recharging of dry batteries is possib¬ le, and actually it has been possible to recharge one and 5 the same battery at least twenty times and still the capaci¬ ty of the battery was not substantially reduced as compared with new made batteries. It has also shown that dry batte¬ ries after having been recharged several times even have a higher capacity than new made batteriesbought on the open.

10 market.

Thus according to the invention the batteries are re¬ charged by alternative current so that the positive period of pulsating alternative current is applied to the battery as an intermittently pulsating charge current whereas the

15 battery is allowed to become slightly discharged during the intermittent periods which are present between the charge current periods, preferably by a strength of current which is between 5 and 20 % of the charging current strength and thereby it is foreseen that the average charging current

20 . through the batteries is kept as constant as possible during the entire charging period irrespectively of the load state of the batteries, however with possible minor variations depending on the sensitivity to temperature and current strength of the charging apparatus components.

25 Since the inner resistance of the battery increases following the discharging of the battery and since the capa¬ city thereby is reduced a strongly discharged battery has a large inner resistance. If the said inner resistance is large it may be expected that the batteries can no more be

30 recharged, and it has until now been considered that it is not possible to recharge batteries, the pole voltage of which has dropped more than about 10-20 %. However, by fore- seing that the charge current is maintained constant through the batteries a strong current is applied to strongly dis-

35 charged batteries, thereupon the said current successively

drops as the charge of the batteries increase, .In t^is way it is possible to provide a quite sufficient recharging of batteries the pole voltage has dropped at least doyn to 30 of the nominal pole voltage.

In the following tables I-III is given account of an experiment of discharging and recharging dry batteries of the type R 14. Six -new made such batteries were connected in series, whereby the open circuit voltage was meassured 9,4 V. The batteries were loaded for a discharge of 39 ohm, and with the said load connected the following values were obtained for voltage, strength of current, power and inner resistance for periods of times between 0 and 45 minutes.

T A B L E

As illustrated the pole voltage for the sex dry batte¬ ries connected in series had dropped after 45 minutes from 8,4 to 6,38 W, the strength of current had dropped from 210 A to 154 mA and the power^' from 177 W to 0,98 . The said reduction of the values is depending on the fact that the

O Pl inner resistance of the batteries has increased from 1,13 ohm to 3,50 ohm.

The same batteries now were connected forrecharging with the positive period of alternative current having a voltage of 24 V. During the faded out intermittent periods some discharges of the batteries was allowed corresponding to about 20 % of the strength of the charging current. During 45 minutes of recharging the voltage of the batteries and the recharging current were measured and the values

10 thereof are stated in the following table II.

T A B L E II

It should be noted that the voltage already after 5 minutes of recharging was 8,6 V and after 45 minutes of recharging was 11,0 V. The charging current successively increased from 83 mA to 125 mA as maximum whereupon it 15 dropped slightly depending on the fact that the charging current was counteracted by the forward voltage of the bat¬ tery in the recharging apparatus which was used as will be further explained in the following.

The batteries now recharged were allowed to stand for three hours after the recharging was ended. The voltage dropped slightly during said period. After three hours the open circuit voltage had stabilized at 10,00 V and the said voltage was kept for a long following period of time.

A new discharging experiment was made for the purpose of comparing the* corresponding values of new made batteries the values of which are stated in table I. The six batteries connected in series were also in this case loaded by 39 ohm, and voltage, strength of current, power and inner resistance was periodically meassured during 45 minutes of time.

T A B L E III

It proved that the recharged batteries throughout had a higher effect and a higher capacity than the new made batteries. In order to find out the ability of the battery to recover the load of 39 ohm was disconnected and the bat¬ teries were allowed to rest for one hour, whereupon a load of 39 ohm was once again connected. After one hour of dis¬ charging the batteries .had a voltage of 7,45 V, a strength of current of 1,85 A, a power of 1,40 W and an inner resist- αnce of 1,06 ohm.

The same batteries were subjected to a large number of dischargings and rechargings, whereby the dischargings were made with different loads. Not until after ten re- 5 chargings a slight reduction of the recharge ability could be noticed. The said reduction of rechargeability, however, was of such low magnitude, that the batteries even after having been recharged 50 times could be used to run a transistor radio, a cassett tape recorder or a similar

10 apparatus. After the same batteries had been discharged and subsequently recharged 30 times and after the batteries had been allowed to rest for three days and nights the open circuit voltage over the battery poles was meassured 9,0 V which was consequently only 0,4 V less than the correspond-

15 ing open circuit voltage over new made batteries.

By different experiments it could be stated that the best recharging was obtained if the charging voltage was about 33 % higher than the nominal battery voltage and if the charging current was between 75 and 175 mA. A lower

20 charging current than 75 mA gave an unnecessary long re¬ charging time without any noticable advantages whereas a loading current higher than 175 mA involved the risk of a non-wanted-development of power in the form of heat in the batteries.

25 It should be observed that the charging current of course should be adapted to the type of battery so that a battery having a higher capacity than the above discussed batteries of the type R 14 are given a ^"higher charging . ^• current than the above mentioned one, whereas a type of

30 battery having a less capacity should be given a less charg¬ ing current than the above mentioned one. Irrespectively of

; the type of battery the best result is obtained if the charging voltage is adjusted so that each celi of -1,5 V nominal value is charged by 1,8 - 2,0 V.

35 In the following the invention will be further ex- plained with reference to an apparatus for executing the method, which apparatus is diagrammatically illustrated in the accompanying drawings.

In figure 1 of the drawings is shown a circuit diagram of a simple apparatus for executing the method according to the invention.

Figure 2 shows a curve illustrating the charge voltage and drawn from an oscillograph.

Figure 3 is a diagram which schematically illustrates the voltage variations of a battery cell when being charged, in fully charged but non-loaded state and during load res¬ pectively.

Figure 4 is a diagram illustrating the function of a Zener-diode included in the apparatus according to figure 1, and figure 5 is a circuit diagram of a more developed and automatically acting battery charger.

It is to be understood, that the described method and the embodiments of the invention described in the following and illustrated in the drawings are only illuminating . ■_.-^' examples and that the invention is only restricted by the appended claims.

The apparatus illustrated in figure 1 is the very apparatus which was used in the above mentioned experiments. The charging apparatus includes a transformer which is not shown in the drawings and which gives 24 V alternate current voltage over the pole terminals 1 and 2. The charging mainly is made by the positive period of the alternate voltage, and the apparatus is designed for the best possible charging of dry batteries of the type R 14. From the pole terminal a diode Dl, for instance a silicium diode of 0,5 A is connec¬ ted, and paralleUy over the said diode a large resistance R 1 of 470 ohm and 1 W is connected. The diode D 1 and the resistance R 1 are both connected to a conduit 3 which is connected to the positive pole of six batteries 4 each of 1,5 V which are connected in series. In the conduit 3 a small resistance R 2 of 21 ohm and 9 W is connected, and paralleUy over the batteries 4 connected in series is both a Zener-diode D2 having a breake through voltage of 12 V and in series therewith a so called PTC-resistance R 3 (positive temperature coefficient) of between 0' and 100 kilohm connected. The PCT-resistance R3 acts so as to form a very high resistance when loaded with high current, where¬ as the resistance decreases when the current decreases. This means that the PCT-resistance gives a high resistance when the batteries 4 have a high internal resistance, i.e. when the batteries are strongly discharged, whereupon the resistance of the PTC-resister -drops following the increased charge of the batteries. From the negative pole of the batteries extends a conduit 5 which closes the circuit at the pole terminal 2.

The dry batteries 4 are intended to be charged with the positive periods of an alternate current and during the said periods a current flows from the pole terminal 1, over the diode Dl, through the conduit 3 and the little resist¬ ance R2 and through the batteries 4 and the current is closed over the conduit 5 at t-he pole terminal 2. Since only the positive period is used the maximum-voltage during the period is half the transformer voltage of 24 V, conse- quently 12 V, which is 33 % over the nominal voltage of the batteries. Since the resistance R1 is large practically all current flows through the diode Dl. The Zener-diode D2 has a brake through voltage of 12D, but still a small amount of current passes through the diode, and the higher resistan- ce of the batteries 4 is the higher current should pass through the Zener-diode D2 if it was not connected in series with the PCT-resistance R3. As mentioned above the resistance of the PTC-resistor R3 is very high if the batteries are strongly discharged and therefore practically no current will pass through the unit of the PTC-resistor R3 and the Zener- diode D2. In the discharged state the batteries 4 therefore are loaded with a strong current what is necessary to over¬ come the inner resistance of the batteries so as to force any charging current through the batteries. When the charge of the batteries increases the inner resistance thereof drops,, and as a consequence the resistance of the PTC-resis¬ tor R3 drops. The resistor R3 thereby balances the charge current so that the charge current is maintained substantial¬ ly constant through the batteries irrespectively of the charge condition of the batteries. By designing the appa¬ ratus, accordingly it is possible to recharge batteries which are very strongly discharged, and in practise a charging has successfullybeen carried through of six batteries connected in series the combined pole voltage of which was as low as 3 V.

The function of the apparatus is best evident from figure 2 which shows three positive charging pulses 7 over the zero-line and Between the discharging pulses 8 which re¬ present the discharging which is allowed from the batteries during the periods between the charging pulses.

In this case it is intended to charge six batteries connected in series with the positive periods of an alterna¬ te current which periods have a maximum at 12 V. At a charging pulse as mentioned above to the main portion passes through the diode Dl which can be a silicium diode, and through the little resister R 2 and practically all current enters the battery cells 4 as a charging pulse 7. After a charging pulse 7 the previous negative pulse of the alter¬ nate current fades out, and during the gap of current there¬ by formed the battery cells 4 have an opportunity to dis¬ charge, whereby a pulse of current flows from the battery cells 4 back over the little resistor R2 and.the large re¬ sistor Rl since the diode Dl does not allow a flow of current in the said direction. The current circuit is closed over the pole terminal 1 and the transformer, over the pole ter¬ minal 2 and the conduit 5. The discharging pulse 8 which is oppositely directed to the charging pulse 7 extends as a negative pulse 8 on an oscillograph. Since the discharging pulse is substantially less than the charging pulse 7 the main part of the charge is remained in the battery cells.

As the battery cells 4 are charged the voltage thereof increases and during the said increase of voltage and in¬ creasing amount of the current of the charing pulse passes the PCT-resistor R3 and the Zener-diode D2. Not until the voltage over the battery cells 4 reaches the value of the break through current of the diode D2, which in this case is supposed to be 12 V, the diode D2 opens fully, and since the battery cells thereby do not accept further charing the resistance of the resistor R3 increases. The output of cur¬ rent drops, and since the Zener-diode D2 is open any possib¬ le current passes the diode D2 and the resistor R3. The battery cells 4 consequently are disconnected from the charging circuit. The Zener-diode D2 and the resistor R3 thereby prevent any overload of the batteries.If the battery voltage by time should drop the Zener-diode closes and the resistance of the PTC-resistor- increases and charging pulses are once again fed into the batteries until the voltage of the batteries once again reach a value corresponding to the brake through voltage of the Zener-diode D2.

In figure 2 is illustrated by dotted lines how the peaks of the charging pulses are" somewhat cut off as the voltage over the battery shells increases, and figure 2 correspondingly illustrates how the voltage of the dis- charging pulses increases.

In figure 3.is diagrammatically illustrated the vol¬ tage function of a battery cell of 1,5 V during charging, a subsequent rest period and a further subsequent discharging period. In the diagram it is presupposed that the recharging starts when the battery cell has an .orbit circuit voltage of

O slightly over 1 V and that the recharging is stopped when the voltage over the battery cell has reached the intended charge level which is supposed to be between 1,8 and 1,9 V. When the recharging is stopped the open circuit voltage of the battery drops relatively quick for a short while to about 1,65 V, and for another one or two hours the voltage drops slightly until it stabilizes on a level which is maintained practically unchanged for a long time. When the discharging starts the voltage likewise drops relatively strongly for a short while, whereupon the drop of voltage follows rather slowly until a certain limit 9 at which the voltage starts to drop somewhat quicker. It may be advanta¬ geous to recharge the batteries from the "point of limit 9, but a recharging can also be made of substantially more discharged batteries.

The large resistor Rl has for its main purpose to restrict the discharge current from the batteries and the little resistor R2 is intended for adapting the charge current for different types of batteries. For batteries having a large capacity only a little resistance is used or the resistor R2 is simply excluded and for batteries having a little capacity a relatively large resistance of R2 is used. In the above described case, where six battery . cells of type R14 connected in series are recharged and in which the recharging and the discharging respectively is illustrated in the above tables I-III. The resistor R had a value of 470 ohm whereas the little resistor R2 had a value of 27 ohm. This gave a recharging current of about 150 mA. The PTC-resistor R3 had a working area from 0 to 100 kilohm,, and the Zener-diode D2 was of the type IN 5927. In order to make it possible to recharge batteries of diffe¬ rent types the resistor R2 can be formed as a variable resistor, and in such case preferably an ampere meter is mounted in direct connection to the resistor to make it possible to check that the charging current is within the optimum value of 75-175 mA.

In figure 4 is diagram atically illustrated the func¬ tion of the Zener-diode D2 as far as to the breake through voltage of 12 V and without reference to the resistor R 3 connected in series with the said diode. As evident a re- latively weak current is capable of passing in the break through direction up to about 7 V, whereupon a slightly increasing current passes as far as to 12 V, whereby the diode opens completely and the current can pass through the diode practically without any resistance. In figure 5 is shown an alternative embodiment of a recharging apparatus for executing the method according to the invention. The said apparatus is intended to be con¬ nected to 220 V alternate 'current and it comprises a trans¬ former 10 which transforms the current to a suitable charg- ing current, for instance 24 V alternate current, for re¬ charging six dry batteries 4 each of 1,5 V connected in series. To one terminal 1 of the transformer a large resist¬ or R 1 is connected, and in series therewith a weak resistor R 2. ParalleUy over the resistor R 1 a diode Dl for instan- ce a silicium diode is connected, and paralleUy over the weak resistor R 2 a capacitor-C is connected. The diode Dl and the capacitor C are connected in common to a point 11 and the output of the capacitor Cl is connected to the out¬ put of the little resistor R2 at a point 12. To the same point 12 are connected the battery cells 4 to be recharged, a two-pole connection relay Rel and a one-pole disconnection relay Re2. The opposite ends of the battery cells 4, the connection relay Rel and the disconnection relay Re2 are interconnected over a conduit 16, and paralleUy over the battery cells are connected a Zener-diod D2 and a PTC- resistor R3 from point 12 to a point 13. From point 13 three conduits are branched, viz. a first conduit 14 which is connected to the pole terminal 2 of the transformal 10 and which contains a fuse 15, a second conduit 16 which contains a check lamp LI indicating that charging is on and a third conduit 17 containing a check lamp L2 normally indicating that the batteries 4 are fully charged. The lamps LI and L2 are commonly connected to the pole terminal 1 of the transformer 10 over a conduit 18. The conduit 18 contains a diode D3 which with the anode portion is connect¬ ed to the pole terminal 1 of the transformer. ParalleUy over the connection relay Rel a capacitor Cl is connected and paralleUy over the disconnection relay Re2 a capacitor C2 is connected. With reference to the transformer the con¬ nection relay Rel is connected after the battery cells 4 and the disconnection relay Re2 following the connection relay Rel. In the conduit 19 to which the two relays are connected a diode D4 is connected between the battery cells 4 and the connection relay Rel, and between the connection relay Rel and the disconnection relay Re2 a variable resist¬ or R4 is connected for controlling the disconnection voltage of the disconnection relay Re2. The two-pole connection i - relay Rel contains two switches 19' and 20 which are dis- connected when the relay Rel is non-actuated. The switch

19' is connected by the conduit 16 to the check lamp LI and the switch 20 is connected by" the conduit 14 to the pole terminal 2 of the transformer. The one-pole disconnection relay Re2 contains a switch 21 which is connected by the conduit 17 to the check lamp L2 and which is disconnected when the relay Re2 is disconnected.

The diodes Dl, D3 and D4 may be of any type, for inst¬ ance silicium diodes, whereas the diode D2 is of the Zener type and has a breake through voltage which is about one third higher than the nominal voltage over the battery cells 4. When recharging for instance six battery cells each of 1,5 V connected in series, whereby the nominal battery vol¬ tage is 9 V the Zener-diode preferably should have a breake through voltage of 12 V. Both relays Rel and Re2 have a permitted operation voltage corresponding to the maximum charging voltage, in the above mentioned case 12 V, but at least the connection relay Rel should have a connection voltage which is the same as or less than the lowest voltage at which it may be possible to start the charging. Since as shown possible to recharge six 1,5 V batteries connected in series from a pole voltage which is as low as 3 V the con¬ nection relay Rel consequently should have a connection vol¬ tage of 3 V. If the battery cells have a lower pole voltage than the said 3 V the relay Rel does not switch in, and the switches 19 and 20 remain disconnected and there is no charging.

The resistors Rl and R2 are adapted to the batteries to be recharged, and when recharging battery cells having a rated voltage of 9_. V the resistor Rl is preferably chosen 470 ohm and the resistor R2 27 ohm. The resistor R4 is a- variable resistor which can be controlled for instance between 10 and 2200 ohm, and the resistor R3, which is a PTC-resistor, can be designed for an acting field of between 0 and 100 kilohm. In order to make it possible to get information from the apparatus whether or not the batteries are fully charged a resistor R5 can be connected paralleUy over the battery cells-4 via a switch 22 as will be further explained in the following. The battery cells are connected between the conduits 19 and 16 with the anode facing the conduit 19. If the pole voltage over the batteries is less than the connection voltage of the connection relay Rel the contact 20 thereof intftfe .main conduit 14 remains disconnected and no current can flow into the apparatus. Both check lamps LI and L2 thereby remain turned off. If, however, the pole voltage over the batteries 4 exceeds the connection voltage of the connection relay Rel, which in the above mentioned case is 3 V the relay Rel turns over to connected state and the switches 19' and 20 are connected.

OMPI

A AAλs,-,_. ^~ i^po Charging:

A current hereby flows from the pole terminal 1 of the transformer 10 through the diode Dl, through the little resistor R2, through the battery cells and back to the pble terminal 2 over the main conduit 14. ParalleUy therewith current flows through the conduit 18, the diode D3, the check lamp LI and back through the main conduit 14. The check lamp LI thereby indicates that charging is on. The said current passes during the intermittent charging pulses.

Discharging: During the periods between the intermittent charging pulses a discharge current is allowed to pass from the batte¬ ry cells 4 to the little resistor R2, the large resistor Rl, over the transformer 10 and through the main conduit 14 back to the battery cells 4. ParalleUy herewith a current flows through the diode D4, through the connection relay Rel and back to the battery cells 4, which current keeps the main switch 20 connected.

The PTC-resistor R3 and the Zener-diode D2 connected in series therewith prevent current from passing therethrough when the battery cells 4 are strongly discharged. As the charge of the battery cells increase the inner resistance thereof decreases and thereby also the resistance of the PTC-resistor R3 decreases and by time some charging current can pass through the conduit 23 containing the said two components.

Fully charged condition:

When the battery cells 4 are charged to a predetermined level the counter electromotive force thereof is so strong that the voltage over the battery poles reach the level of the connection voltage for the relay Re2. The connection voltage thereof can be adjusted by the variable resistor R4. When the said voltage is reached the relay Re2 connects and the switch 21 in the conduit 17 is closed whereby the check lamp L2 lights and thereby indicates that the batteries 4 are fully charged. At τhis stage also the resistance of the PTC-resistor R3 has reached its maximum level, and also the breake through voltage for the Zener-diode is reached.

Any further current will pass both through the conduit 23 containing the resistor R3 and the Zener-diode D2, . through the connection relay Rel and through the full- .. - charge-relay Re2, Consequently the batteries 4 cannot be overcharged and therefore the apparatus Gάn also be used for constant maintenance charging of dry batteries.

If the pole voltage of the battery cells 4 by time should drop the resistance of the resistor R3 increases and a charging current is brought to pass through the battery cells until they are once again fully charged. At the same time the relay Re2 disconnects and thereby lights out the check lamp L2 indicating the fully charged condition of the batteries.

It is obvious that the charging apparatus according to the invention also can be used for charging of.ordinary rechargable batteries and accumulators like rechargeable nickle-cadmium accumulators or silver-zinc accumulators. It is to be understood that the above described method and the apparatus described above and shown in the drawings is not restricting the invention and that all kinds of modifications may be presented within the scope of the appended claims.

Claims

C L A I M S:

1. Method of recharging dry batteries, whereby the dry batteries are recharged by one_, of the periods of an intermittently pulsating alternate current, and whereby the batteries are allowed to discharge somewhat during the periods between the charging pulses, c h r a c t e r- i z e d in that the charging current is balanced according to the charge condition of the batteries, so that the batte¬ ries irrespectively of the said charge condition are charged with an optimum constant average current and with possible variations depending on the sensitivity of the charging component to temperature and current strength.

2. Method according to claim 1, c h a r act e r¬ i z e d in that the maximum voltage of the charging pulses is 25-35 % higher than that of the rated voltage of the batteries.

3. Method according to claim 1 or 2, c h a r a c¬ e r i z e in that the dry batteries during the discharge periods are allowed to discharge by a current strength which is between 5 and 20 % of the charging current strength.

4. Method according to claim 1, 2 or 3, c h a r a c¬ t e r i z e d in that the charging current strength is adjusted to 75-175 mA and in that the said charging current strength is maintained substantially unchanged through the batteries during the entire charging thereof.

5. Method according to any of the preceding claims, c h a r a c t e r i z e d in that the charging is restricted by means of a Zener-diode and a PTC-resistor connected in series therewith, which -components are connected paralleUy over the battery or batteries to be recharged and with the cathode facing the positive current pole, whereby the Zener- diode is designed for a breakethrough voltage which is about 30 % higher than the rated voltage of the dry batteries.

6. Apparatus for executing the method according to any of the preceding claims comprising a transformer for stepping down alternate current from the net to a suitable charging voltage, a diode (Dl ) for letting through inter¬ mittent pulses of one of the periods of alternate current, a resistor (Rl) connected paralleUy over the diode (Dl), 5 terminals for one or several dry batteries (4) between the diode (Dl ) and (Rl) on one hand and a second pole terminal on the other hand,^" c h a r a c t e r i z e d in that the apparatus as connected over the dry batteries (4) a Zener- diode (D2) and connected in series therewith a PTC-resistor

'0 (R3) designed so as to balance the average charging current over the dry batteries (4) to be maintained as constant as possible, whereby charging current pulses are intermittently supplied to the dry batteries (4) over the diode (Dl), where¬ as a counterdirected discharge current appears during the 5 periods between the charging current pulses, whereby the discharging follows through the resistor (Rl) connected paralleUy to the diode (Dl ).

7. Apparatus according to claims 6, c h a r a c¬ t e r i z e d in that Zener-diode (D2) has a breake through 0 voltage which is about 30 % higher than the rated voltage of the dry batteries (4).

8. Apparatus according-* to claim 6 och 7, c h a r¬ a c t e r i z e d in that. he resistor (Rl) is variable for adapting the discharge current strength to be 5-20 % of 5 the charging current strength during the charging periods (7).

9. Apparatus according to claim 6, 7 or 8, c h a r¬ a c t e r i z e d in that a charging ^"resistor (R2) is con¬ nected in series with the* discharging resistor (Rl ) and the 0 dry batteries (4), which charging resistor (R2) is variable for controlling the charging current strength according to different types of dry batteries.

10. Apparatus according to claim 9, c h a r a c t e r- i z e d in that a capacitor (C) is connected paralleUy over the charging resistor (R2).

11. According to any of claims 6-9, c h a r a c¬ t e r i z e d in that it comprises a connection relay (Rel), which turns on the charging current only when the pole voltage over the batteries (4) exceed a predetermined minimum voltage.

12. Apparatus according to claim 11, c h a r a c¬ t e r i z e d in that it comprises a check lamp (LI ) which is lit by the connection relay (Rel) when said relay is turned on.

13. Apparatus according to claims 11. or 12, c h a r¬ a c t e r i z e d in that it comprises a full-charge-relay (Re2) which turns on when the battery is fully charged and which thereby lights another check lamp (L2).

14. Apparatus according to any of claims 6-13, c h a r¬ a c t e r i z e d in that the PTC-resistor (R3) has a work¬ ing field between nearly 0 and about 100 kilohm.