Voltage isolation

[Dumken]

Pls how can i isolate the voltage coming from Power supply unit to my battery from entering the voltage indicator. i want a condition where it would be measuring the battery voltage and not the one coming from the PSU, since the two circuits are in parallel ie the battery and the indicator?

 

[alec_t]

If the power supply (charger?) is connected directly to the battery then both are at the same voltage.

 

[crutschow]

If they are in parallel, then the only ways to measure the battery voltage is to either shut off the Power Supply (assuming it can tolerate voltage on its output when shut down) or connect a switch between them to provide isolation when doing the measurement.

 

[dr pepper]

We might be able to help you do what you want, but we need more information.

 

[audioguru]

You should NEVER connect the output of a power supply directly to a rechargeable battery without something in between that limits the current. The battery or power supply might explode or catch on fire. A power supply is not a battery charger circuit.

The voltage of a rechargeable battery slowly drops when it is removed from the charger even when the battery has no load.
EXCEPT a rechargeable Lithium cell stays at its fully charged voltage of 4.20V for years without a load.

 

[JimB]

The voltage of a rechargeable battery slowly drops when it is removed from the charger even when the battery has no load.

To emphasise AGs point, have a look at the attached graph.
It is a graph of voltage against time of a battery of eleven F size NiMH cells charging at about 1 amp.
Note how the voltage falls away when the charger is switched off, an exponential decay over several hours.

JimB

 

[Dumken]

Pls forgive me for my late reply. thank you guys for your contribution but it seems it wasn"t clearly understood by me. I will upload a circuit soon to explain myself very well.

 

[audioguru]

What is the chemistry of your battery? Lead-acid, Ni-Cad, Ni-MH or Lithium? They all charge differently.
Why do you want to measure the voltage of a battery that is charging? It is a poor way to determine how much the battery is charged. You need to store the capacity number of the battery (or measure it then store it) then measure its charging current and charging duration. A small processor chip can do the calculation. Maybe a battery charger chip can do it.

 

[Dumken]

audioguru? What's the name of the battery chip?

 

[Dumken]

Here is the block diagram of the circuit.

 

[audioguru]

audioguru? What's the name of the battery chip?

Look on the websites of IC manufacturers like Maxim or Texas Instruments for a battery charger IC.

 

[audioguru]

Your block diagram shows the charger directly connected to the battery and both are connected to the voltmeter. Then the voltmeter shows the actual voltage of the battery while it is charging.
A battery is never 7.1A but a lead-acid battery can have a capacity of 7.1Ah. Its datasheet will show its recommended charging voltage and current. Battery charger ICs might be made only for batteries that are more modern (Ni-MH and Lithium) than old lead-acid.

www.batteryuniversity.com shows that the voltage of a lead-acid battery reaches its recommended maximum voltage when it is about half fully charged in about 1/3rd of its total charging time.. Then its charging current slowly drops for the other half of its charging for the remaining 2/3rds of its total charging time. So your voltmeter will not show how much the battery is charged, instead it will show only that the battery is dead or that it is charging.