On an EI system a low battery in flight will result in a few misfires and then a dead stick. On a twin cylinder many times one cylinder will drop first. On the ground you will get a few pops and possibly a short run before the flash voltage goes south. The I-4C would be the best product as it would give an actual voltage reading with the engine running. The bottom line however is start with a good pack and check it every few flights. The Onboard indicator is an option not an absolute necessity.
You can check the models under my profile and see that I am flying several sizes with EI,s and voltage indicators. What I have posted here is direct experience not conjecture or hearsay. For reference I started using a locally made onboard monitor in 1991 and have used various ones since just because I can.