is this battery backup with charging capability ok

[nikhilele]

The ckt which i have shown is for giving battery backup to a system which should work continuosly, system req 9V and i have used 12 v battery , now when the light goes, the battery is connected to ckt via diode and when the battery is near to danger level of discharge it should cuttoff itself from the load.
and when the ac power is regain then the battery should charge but little faster,

100 ohm resistor should be changed to 10ohm

 

[Davood Amerion]

yes it is good idea but you write:

and when the battery is near to danger level of discharge it should cuttoff itself from the load

i dont see that circuit.
i think 120 ohm resistor is not correct value.
you can remove 2K2 120ohm and transistor (right most) and connect terminal + of battery to anode of right hand diode.

 

[nikhilele]

2K2 120ohm and transistor is the ckt which will cutoff the battery from the system.

2K2 and 120ohm forms voltage divider which makes transistor ON when battery voltage is above 11 and make it off when it is below

 

[Davood Amerion]

I dont think this circuit is correct!
if you change right transistor with PNP one and swap resistors 120 and 2K2 it may corrected.
but it is not a good circuit and has some problems.

 

[nikhilele]

u r right, i got ur point, but even though the ckt has one problem for making on and off transistor i have to used other things rather than resistor, a comparator will do which will switch on and off transistor rather playing near the baising point.

 

[nikhilele]

i have modified the ckt according to your suggestions. and i am posting that also. if it is all correct than others can also use it

i think i have connected last med pwr pnp transistor in wrong way plz invert it

 

[nikhilele]

is the above ckt ok

 

[Davood Amerion]

hi nikhilele
I draw one circuit which its output disconnect when battery voltage drop-out to about 6 volts.

Regards,
Davood Amerion.

 

[artem]

Davood, may be i am wrong, i will appreciate if you can point to my errors :
- better to connect R3 to Battery's positive , than Q2's collector. And then eliminate R5.
- The battery will be under continous charge - that will harm it . So overcharging protection is to be drawn as well
- Measuring voltage based on bjt's be juction could be cheap and reliable only within narrow temperature range - you have a 1/11 divider . be junction temp variation could be about 0.4 - 0.7 in wide temperature range .multiplied by 11 can give detection 4.4 - 7.7 V range . better to put termocompensation . Yes for room constant temperature it will be OK . Or put bandgap at Q1 emitter and use less divider for out voltage monitoring .

Best Regards

 

[Davood Amerion]

Hi Artem,

- better to connect R3 to Battery's positive , than Q2's collector.
And then eliminate R5.

I designd circuit so that when battery disconnected from circuit,
it consum current as low as possible to protect battrey (from over dicharged).

- The battery will be under continous charge - that will harm it .
So overcharging protection is to be drawn as well

yes, but not for all type of battery, it is a simple transistor circuit!

- Measuring voltage based on bjt's be juction could be cheap and reliable only within narrow temperature range -
you have a 1/11 divider . be junction temp variation could be about 0.4 - 0.7 in wide
temperature range .multiplied by 11 can give detection 4.4 - 7.7 V range .

In BJT for PN diode temperature coeficient are 2mV/°C.
In the other hand shut down voltage isnt needed to be precise.

Regards,
Davood Amerion.

 

[nikhilele]

The battery will be under continous charge - that will harm it .
So overcharging protection is to be drawn as well

yes, but not for all type of battery, it is a simple transistor circuit

diode will take care of above, when battery is overcharge then battery voltage will increase when it is greater then Vo - diode voltagedrop the diode will stop conducting.

thank you for your responses
please point out erorr in my ckt also

 

[artem]

The battery will be under continous charge - that will harm it .
So overcharging protection is to be drawn as well

yes, but not for all type of battery, it is a simple transistor circuit

diode will take care of above, when battery is overcharge then battery voltage will increase when it is greater then Vo - diode voltagedrop the diode will stop conducting.

thank you for your responses
please point out erorr in my ckt also

battery charge circuit is current source raither than voltage . That means you wont be able to control current properly at battery charge closed to max if diode to be used.

 

[nikhilele]

battery charge circuit is current source raither than voltage . That means you wont be able to control current properly at battery charge closed to max if diode to be used.

I donot know much about battery chargers,
but i read some where, there different types of chargers, for example constant current charges, constant voltage chargers etc.

diode is present in both the ckt.

 

[nikhilele]

In my second schematic
I think i have connected last med pwr pnp transistor in wrong way plz invert it