Mesuring a 12v lead acid battery

[schmitt trigger]

You are correct.
I was confusing the threshold voltage of other battery chemistries.

[NorthGuy]

Ball park figure of how long it takes to settle?

It depends on the battery and the accuracy you need. Couple hours might be enough. You can charge the battery then monitor the settlement process for, say, 24 hours. This will give you an idea of the settlement speed.

[towlerg]

@Northguy

Oh, thats a good idea, I only need to monitor the cranking battery (A) so a SPDT relay will allow that battery to be either the monitor curcuit or the controller. A SPST relay will either connect battery B to the conroller or disconnect it.

So my rule is now -
  a. Take a peek at A
  b. If A above 12.6v
             switch to B and monitor A, if A below 12.6v at any time main loop
      Else
             switch to A for 1 hour
      End
  d. Loop forever

This overcomes any potential problems with monitoring a battery being charged by controller, but leaves the problem of monitoring a battery being charged by alternator. If I could detect when the engine is running and disconnect the monitor, I only need to worry about spikes and nasties on Vdd not on ADC input.

Thanks again for that and all the other suggestions.

One more question, approx what current will the 10v-15v curcuit draw from Vdd and from battery?
I ask because if its only tiny I'll leave it connected and powered but if it's high I'll dissconect from the batteries and remove supply power to it except when taking a measurment.

[Zero999]

One more question, approx what current will the 10v-15v curcuit draw from Vdd and from battery?
I ask because if its only tiny I'll leave it connected and powered but if it's high I'll dissconect from the batteries and remove supply power to it except when taking a measurment.

About 1.3mA.

Whether or not that makes any difference, depends on how big the battery is. If the battery is small, say 1.3Ah, it will fully discharge in 1000 hours, with this circuit connected, but if the battery is much larger, 26Ah, it will take 20000 hours.

If low power operation is required, the TL431 could be replaced with a different IC, which requires less current and the resistor values all increased, but if that's not good enough, a totally different approach is required.

EDIT:
Where is the 5V supply coming from? If it's off the battery, via a regulator, that might be the main drain on the battery.

If the 5V is off, Q1 will turn off and no current will be taken from the input. To save power, the 5V supply, to the circuit I posted, could be an MCU output pin, which could be set to high, when a measurement is taken and low, when it's not needed.

[NorthGuy]

             switch to A for 1 hour

Be careful with that. If you overcharge, the water evaporizes from the electrolyte (producing hydrogen, by the way), the plates get exposed and then a little spark cause an explosion. Generac Guardian generators used to overcharge their cranking battery which caused real big explosions - you probably can find videos of damages on Youtube.

[helius]

If some system was consistently overcharging its batteries, it was using the wrong voltage. Gassing occurs at the equalization voltage, which should not be used to charge it normally.

[towlerg]

@Northguy and Helios - I'm using a PV controller. My project is to switch the controllers output between two batteries but with a priority on the cranking battery.

@Hero999 - I certainly will power the cuircuit via a uC pin. The battery will be min 50Ah. 5V supply will be regulated from the 12v battery. I guess to optimise I need to be able to detect when the controller is not providing power (at night) and go into sleep mode.

I also need to detect when the ignition is on so that I can disconnect the monitoring curcuit but leave the controller attached to the cranking battery.

Thanks again to all who posted.