Ah... Well stated for a change.. Thanks.
So you have two cases.
The starter battery tester is kinda tough to just create because the power level of the test is large. Generally the best method is to lay hands on a real starter battery tester. The standard is called a VAT28. It is a nice big box with a large rheostat on it with two meters. You crank the rheostat up until a certain current is read. You note the voltage. Then you time it for like 15 seconds and note the voltage and current. Stop the test and read a table.
You could see if you can find a VAT28 on on ebay or any other tester for that matter. Search ebay for [battery tester].
Personally I would try other methods. Most battery tests really take a whack out of the battery.
For your starter situation I would get a nice cheap little full cut-off battery charger and just keep the battery fully charged. Then you don't need to test it. It is always ready to go. I mean what happens when you are ready to test a starter, you test the battery and its low? Now you have stop charge and wait. It is rather a "reactive" situation instead of a "proactive" one.
Here's one of the best hook it up plug it in FORGET it!
As for trouble shooting starters I use a DC clamp on ammeter probe that you plug into a DMM. Works really well because you instantly know the starter is pulling too many amps or too few amps. They aren't cheap though.
There are others, this is just an example. You need at least 1000Amps range though.
As for flashlight batteries. I really don't like testing them because you use an un-recoverable percentage of them with each test. If I want to test them I just turn them on. I know what you mean by them running down after a minute or two. If this is a concern I would just run the light for 1 minute.
I got sick of battery headaches and got one of these.
We use it in the farm yard every night. It can run one hour. Has never gone dead. We have never changed batteries. We just screwed the charger to the wall by the door and keep it there.
As for the few batteries we test [zinc and alkali] we do open circuit voltage test. Anything over 1.4V is considered "full" anything 1.2 to 1.3 we consider serviceable preferably not in flashlights. Under 1.2 chuck it.