Hey, I designed a 9v alkaline battery capacity tester using an LM317L variable constant current load (125 uA up to 100mA, with a log taper) and 5 op-amps as comparators to check battery voltage. Since I'll probably use a quad and a dual op-amp, I've used the last op-amp as a buffer between the battery and comparators. I have 3 questions:
-I've separated battery ground and supply ground in my schematic. is this correct?
-should I protect the inverting inputs with resistors?
-will a battery's voltage react quick enough with smaller currents? my plan is to have a momentary button that you hold for a few seconds until the voltage drops.

 

You have shown a supply as V+ but you do not show what is generating V+.

-I've separated battery ground and supply ground in my schematic. is this correct?

No. You need to connect the two grounds.

-should I protect the inverting inputs with resistors?

No. Resistors are not necessary. However, I have found LM311 to be sensitive and having resistors seem to protect the inputs.
You do not need the unity gain buffer at the 9V battery.

-will a battery's voltage react quick enough with smaller currents? my plan is to have a momentary button that you hold for a few seconds until the voltage drops.

Yes.

You have a lot of variable resistors which are unnecessary. You can calculate the appropriate resistors. You can also breadboard the circuit and choose the resistor values by trial and error.

Better still, you only need three test loads, LOW, MEDIUM, HIGH.

In effect, you are not analyzing the capacity of the 9V battery. What you are determining is the internal resistance of the battery.
As the battery ages, the internal resistance increases. This is what limits the current capability of the battery.

 

You have shown a supply as V+ but you do not show what is generating V+.


No. You need to connect the two grounds.


No. Resistors are not necessary. However, I have found LM311 to be sensitive and having resistors seem to protect the inputs.
You do not need the unity gain buffer at the 9V battery.


Yes.

You have a lot of variable resistors which are unnecessary. You can calculate the appropriate resistors. You can also breadboard the circuit and choose the resistor values by trial and error.

Better still, you only need three test loads, LOW, MEDIUM, HIGH.

In effect, you are not analyzing the capacity of the 9V battery. What you are determining is the internal resistance of the battery.
As the battery ages, the internal resistance increases. This is what limits the current capability of the battery.

It's gonna be a 12v dc jack, and I've updated it to have a 1n4001 in series as reverse polarity protection. should I add bypass caps on the opamps? Also, the internal resistance of alkaline batteries increases as it gets used up, so wouldn't this technique still approximate the remaining capacity?

 

Yes. You should have bypass capacitors on the ICs, one 0.1 μF ceramic disc capacitor and one 10 μF electrolytic across Vcc and GND.

I would be inclinded to use LM339 quad comparator instead of op amps.

Battery capacity and internal resistance are two different things.
Battery capacity is a measure of how long the battery can sustain a given current.
Internal resistance determines the output voltage at a given current.

One is more likely to be affected more by internal resistance.