[SOLVED] A little problem is designing backup system

[tajiknomi]

A little problem is designing Power Backup System

I am trying to design a circuit for my router to be ON 24/7 without interruption. The place where I am living right now has some electric power issues so i want to power my router from batteries (if the main power isn't available for sometime).

Here is how I want my system to work.

When MAIN power is available, use it to power router and charge the batteries IF PD of batteries is below some threshold value. If MAIN power isn't available, use batteries to operate router.
For that purpose I have design a simple circuit , here is a rough sketch of it.


The RELAY is used to switch power b/w battery and Main supply. Comparitor (OP-AMP probably) will be used to sense the threshold value of battery, if it is below the specified value (selected by R1 and R2 divider), MOSFET will be ON to charge batteries otherwise it will be off.

I am stuck at position A (in the above screenshot). Suppose batteries are charging , then at position A the potential supplied through the mosfet will be 12V(to charge batteries) but that potential will also be seen by position B which is connected to the Comparitor and it will be turn off the mosfet and the result would be the ON/OFF loop of mosfet. How to handle this scenerio ?

Any help would be appreciated.

 

[BradtheRad]

You want certain events to happen based on volt levels in your system. You need to set the volt levels for each component, to determine what happens under all conditions.

You need to turn your block diagram into a working schematic.

Questions:

* Will your present hurdle be solved by adding another comparator somewhere? Possibly to create a window comparator?

* Will adding hysteresis serve as a substitute for a window comparator?

* Will your hurdle be solved (or simplified) by adding steering diodes?

* Do you have safeguards to make sure that your batteries shall not be excessively charged or discharged?

 

[tajiknomi]

Possibly to create a window comparator?

I don't think I need windows comparator here (or do I ?) because there is only one threshold (Battery Max PD) below which I will need to TURN ON the MOSFET (to charge battery).

Will your hurdle be solved (or simplified) by adding steering diodes?

That's a great idea, I will replace the Relay and use this instead to switch b/w MAIN and Battery supply.

Do you have safeguards to make sure that your batteries shall not be excessively charged or discharged?

(I think) The batteries will not be excessively charge because I will properly set the current and volts supplied to them via MOSFET.

But I would like to hear your advice/suggestion(s) on how can I safeguard my battery.

 

[BradtheRad]

I don't think I need windows comparator here (or do I ?) because there is only one threshold (Battery Max PD) below which I will need to TURN ON the MOSFET (to charge battery).

You also need to stop charging when the battery is full. Perhaps an easy way to do this is to make your power supply voltage equal to the battery's resting voltage.

But I would like to hear your advice/suggestion(s) on how can I safeguard my battery.

This is a popular project, to combine a power supply and battery and load, so they cooperate together. Look at various circuits. You should expect your system to be different, with its unique behavior. You need to compare volt levels in all situations, so that no device is overloaded. Also so current never flows in a wrong direction.

It's a good idea to keep a continual monitor on the battery, with an led or buzzer alarm. Both volt level and Ampere flow.

You need to make many decisions about how all sections interact. Example, do you want your power supply to drive the load and also charge the battery simultaneously? Is it powerful enough to do so, and maintain sufficient voltage? Etc.

 

[tajiknomi]

Perhaps an easy way to do this is to make your power supply voltage equal to the battery's resting voltage

Resting Voltage is battery PD (when is fully charged) and on NO-LOAD ? Li-ion datasheets says that max charging voltage should be no more then 4v2, So i thought I would place 12v (4vx3cells) across the battery for charging. And when the battery reach it's full potential, then ideally, there would be no current flow from Power Supply to the battery.

It's a good idea to keep a continual monitor on the battery

Agreed.

do you want your power supply to drive the load and also charge the battery simultaneously? Is it powerful enough to do so, and maintain sufficient voltage?

The load is actually a router, when I inspect its charger, it states that it can provide 12/1A (12watts). But the router can be operated on upto 9v and on inspection it consumes Max current of 450mA at 9V ~(max 4.5watts). So i thougt i can charge my batteries if i can provide around ~500mA|12v (6watts) to them(batteries) and keep the remaining i.e. ~660mA|9v (6watts) for my router (though it will not exceed 450mA anyway, so the charger will be safe), and both functions (charging & driving router) can be done at the same time. Am i missing something here ?