Battery voltage level to ECU

What type of battery is this? If it's a lead-acid you can get a reasonable guess at the capacity remaining by measuring the open-circuit voltage. Most designs tend to switch in a dummy load resistor if they're not actually running off battery, though.

Right.

Right. The cap should be in parallel with the resistor that is in parallel with the ADC input.

None, if the two resistors are high enough so that the divider does not draw a significant amount of current from the battery. The higher the resistance, the lower the load effect, but also the higher the noise being picked up.

It is hard to accurately know the energy left in a battery. The output resistance of the battery is a better indicator (it grows as the battery drains), but it is also more difficult to be estimated (for this, you need to measure voltages and currents with two different loads).

This system is not for an engine. I'm not sure that I mentioned ECU but if I did, I meant MCU. The specific application and design of the system will not support realtime display of the voltage and/or current (or any other values for that matter). These numbers will be polled every so often, as needed.

I realize that this subject can get into some depth, but this application certainly does not warrant a full out battery monitoring system. As long as the voltage of the battery can be monitored, I will be all set. The exact amount of life remaining, and the such, is not necessary. If one minute the battery says 25V and two minutes later it says 20V, that is fine, I just need to know, and be able to report!

Thanks for your help though guys, I just needed some confirmation, because it seemed too simple.

If "ECU" means you are controlling an engine, then I think you are on the right track. Trust me, there is an awful amount of R&D involved if you want to get into true battery condition and battery charge monitoring - you can build a whole $100 million a year company on it, with dozens of engineers nailing down all the parameters.

For decent instrumentation of the electrics of an engine, I recommend you display battery voltage and battery current, in real time. That will cover all the following

- charge system faults (low voltage when on charge)

- low voltage during cranking, caused by battery fault, aging, corrosion of terminals or starter fault.

- refusal of battery to accept charge, even though charge voltage is correct.

I would not bother trying to relate battery OCV to state of charge - just display the voltage. OCV is an excellent indicator of state of charge if the battery has not recently been charged or discharged. Unfortunately state of charge is not all that much use - a 5 year old worn out battery still shows correct OCV when charged. I suggest you "don't go there" - just have good metering.

The basics are : 1) if your charge system delivers the correct (temperature compensated) voltage during charge, you are doing the right thing by the battery. 2) if battery voltage holds up during cranking, battery and starter and connections are OK. If either is out of whack you can display a warning, but you don't have to diagnose exactly what is wrong.

Roger