Battery charger for charging a plurality of batteries

A battery charging apparatus that charges a plurality of batteries to equal voltage levels. The apparatus has a transformer that acts as a flyback circuit to charge the plurality of batteries. The transformer has a primary winding and a plurality of secondary winding circuits, each secondary winding circuit having a secondary winding and a diode. A different secondary winding circuit is coupled across each battery. The diodes cause only the battery having the lowest voltage among the plurality of batteries to receive energy on a transformer pulse. This occurs until that battery is charged to the level of the next lowest voltage battery, at which time both batteries will receive energy during the pulse. The sequence is followed until all the batteries are charged equally and to a predetermined threshold level.

FIELD OF THE INVENTION 
The present invention relates to the field of battery chargers, and more 
particularly to a battery charging apparatus which simultaneously charges 
a plurality of batteries. 
BACKGROUND OF THE INVENTION 
In order to provide protection against power outages, modern computer 
systems typically use battery backups. When the AC voltage supplied to the 
computer drops below a certain voltage level, a trigger circuit will cause 
a battery voltage to be discharged to the computer. This allows the 
computer to save its memory during the outage, and if necessary, to 
perform a graceful shutdown. 
Typically, the battery voltage is supplied by a plurality of battery packs, 
each containing a plurality of battery cells. These batteries are coupled 
in series to provide the input voltage when the normal input source fails. 
The battery packs must be kept charged to assure that the battery will be 
able to supply the proper level of voltage to the computer, when called 
upon to do so. This function is performed by a battery charger. 
In the prior art, battery chargers are known which sense the voltages of 
the series-coupled battery packs, and then provide a charge to 
simultaneously raise the voltage levels of each battery pack. The charge 
will be applied until the voltage of the lowest voltage pack is raised 
beyond a predetermined threshold voltage level. The problem with this 
approach is that the battery packs do not often have the same voltage 
levels before charging. Therefore, when the lowest voltage battery pack is 
raised to the predetermined threshold level, the other battery packs are 
potentially charged to much higher voltage levels. In other words, in 
order to assure that the lowest voltage battery has a minimum voltage 
level, the prior art battery chargers allow the possibility of the 
remaining battery packs being charged to too high a level. 
There is therefore a need to provide a battery charger that would keep a 
plurality of battery packs charged to a minimum voltage level, but at the 
same time assure that these 3-battery packs are each charged to the same 
voltage levels. 
SUMMARY OF THE INVENTION 
The present invention satisfies the need for a battery charger which will 
maintain a plurality of battery packs charged to equal voltage levels. The 
present invention provides a battery charging apparatus that charges a 
plurality of batteries to equal voltage levels. The apparatus has a 
charging voltage supply and a transformer coupled to this voltage supply. 
The transformer has a primary winding and a plurality of secondary winding 
circuits. Each secondary winding circuit is coupled across a different one 
of the batteries and includes a secondary winding and a semiconductor 
switching element. 
The secondary winding circuits act to charge the battery having the lowest 
voltage first, until its voltage reaches the voltage level of the next 
lowest voltage battery. At that point, both of these batteries have equal 
voltages and will be charged. This sequence is continued until all the 
batteries have equal voltages. The batteries are charged until each one 
attains a predetermined threshold voltage level. By this invention, equal 
charging of the batteries is assumed. 
The present invention also provides in a computer system, a power supply 
comprising a primary voltage source, and a battery backup. A trigger 
circuit senses when the primary voltage drops below a certain level, and 
triggers an SCR to allow the battery backup to supply power to the 
computer. The battery backup is kept charged by a battery charger that 
includes a transformer with a plurality of secondary winding circuits 
coupled in parallel across individual series-coupled batteries. The 
secondary winding circuits each include a secondary winding and a diode. 
The windings and the diodes act to ensure that the battery or batteries 
having the lowest voltage will receive energy from the transformer on any 
particular pulse. After the batteries have been charged to equal levels, 
they will continue being charged until they all reach a threshold charged 
voltage, as sensed by a voltage sensor and charge control. This charger 
control will then reduce the charging rate to a trickle.

DETAILED DESCRIPTION 
The battery charger apparatus of the present invention is illustrated in 
FIG. 1. An AC input source provides AC voltage to a bridge 11 connected to 
an output 12. The output 12 is connectable to a computer 13 to provide it 
with operating power. The bridge 11 can be a one-phase or three-phase 
bridge. 
Should the AC source 10 fail, power will be supplied to the computer by a 
plurality of battery packs. In the embodiment of FIG. 1, there are four 
battery packs 25, 26, 27, and 28. In the illustrated embodiment, each of 
these battery packs 25-28 are 60 volt battery packs, each containing 
thirty two-volt battery cells. The battery packs 25-28 are coupled in 
series to provide 240 volts across capacitor C1 at the output 12. 
The battery packs 25-28 are kept charged by a transformer 14 that is 
connected to a source of DC voltage 30 at the transformer input. The 
transformer 14 has a single primary winding 16 and four secondary windings 
21, 22, 23 and 24. The transformer 14 also has a core 18. Each of the 
secondary windings 21-24 is connected across a different battery pack, and 
each will have the same number of turns. 
Each secondary winding 21-24 is connected through a diode D1, D2, D3 or D4 
to one of the battery packs 25-28. For example, secondary winding 21 is 
coupled across battery pack 25 through diode D1, and secondary winding 22 
is coupled across battery pack 26 by diode D2, etc. A transistor 34 is 
coupled to the primary winding 16 and is controlled by a voltage sensor 
and charger control 32. The transformer 14 is used as a flyback circuit, 
with the transistor 34 being switched at 25 KHz. When transistor 34 is on, 
the diodes D1-D4 are reverse biased and the secondary windings 21-24 do 
not conduct. When the transistor 34 is turned off, the secondary windings 
21-24 all reverse their polarity and the energy stored in the transformer 
14 is transferred to the battery packs 25-28. 
As stated before, the secondary windings 21-24 all have equal turns. If the 
battery pack 25-28 all had equal voltages, the current in all the 
secondary windings 21-24 would be equal. However, it is more probable that 
the battery packs 25-28 all have different voltages. When the secondary 
windings 21-24 try to conduct (transistor 34 is off) the voltage across 
each secondary winding 25-28 will be the voltage of the lowest voltage 
battery 25-28. This is because the lowest secondary winding voltage will 
be reflected on all of the other secondary windings. 
Only one diode will be forward biased, this diode being the one that is 
connected to the battery pack 25-28 that has the lowest voltage. For 
illustrative purposes, assume that battery pack 26 has the lowest voltage, 
battery pack 25 the next lowest voltage, battery pack 27 the next lowest 
voltage, and battery pack 28 the highest voltage of the four battery 
packs. Since battery pack 26 has the lowest voltage of the battery pack 
25-28, its associated diode D2 will be forward biased so that it will 
conduct and the battery pack 26 will receive all the energy on that 
particular pulse. The remaining diodes D1, D3, D4 will be reverse biased 
since their respective secondary windings 21, 23, 24 will all have 
voltages that are less than the voltage of their battery packs 25, 27, 28. 
The voltage on battery pack 26 will increase as it receives energy, until 
the voltage of battery pack 26 increases to a voltage equal to the next 
lowest battery pack, battery pack 25 in this example. At this point, when 
battery packs 26 and 25 have equal voltages, both diodes D1, D2 will be 
forward biased and will conduct. The two secondary windings 21, 22 will 
now share the energy in a pulse. The sequence continues until all the 
battery packs 25-28 are charged to equal voltages and all secondary 
windings 21-24 are conducting. All the battery packs will then be charged 
to a predetermined threshold level. This threshold level is detected by a 
voltage sensor and charger control 32 which is coupled across one of the 
secondary windings, such as secondary winding 24. Since the voltage across 
any one of the secondary windings 21-24 will always be equal to the 
voltage of the lowest battery pack, the voltage sensor and charger control 
32 can determine when all of the battery packs have been charged to a 
required voltage level. When the voltage sensor and charger control 32 
detects that each of the battery packs 25- 28 has reached a threshold 
voltage level (for example, 78 volts per battery pack), the control 
circuit 32 will change the charging rate to a trickle. It does this by 
controlling the transistor 34. In this manner, the charging scheme acts as 
a two rate constant current charge. 
The discharge of the battery packs 21-24 occurs when an SCR 36 receives a 
trigger signal from a trigger circuit 38. The trigger circuit 38 will 
activate the SCR if the AC input voltage source fails. When this occurs, 
the battery charger is inhibited and the SCR turns on, to cause the 240 
volt battery to be dumped into the load at output 12. At a predetermined 
voltage (for example 57 volts per pack) the load would be shut off and 
the SCR 36 could recover. When the AC power source returns, the charge 
cycle would begin all over. 
The voltage sensor and charger control 32 and the trigger circuit 38 are 
conventional circuits, known to those of ordinary skill in the art and are 
not further illustrated so as not to obscure the invention. Also, 
different types of batteries can be charged other than four 60 volt 
battery packs. For example, eight 30 volt batteries could be charged if 
eight secondary windings and eight diodes are used. The present invention 
is to be limited only by the terms of the appendant claims.