I've had one for about 15 years. One of the very first electronic types. Bought for accurate testing of lead acid batteries used on location filming. On a charged battery it does a pulse test and gives you a read out of the true capacity in amp.hrs. If the reading is below a certain percentage of the stated amp.hr, it is best replaced. Of course you might be able to limp on with one outside that range, but what it does do is tell you instantly if the battery is good.
There's no need to know the internal resistance of the battery as the pulse test does that as part of its calculation. Working out the true capacity is basically all the info you need. You can't have (near) the correct capacity with any other battery fault.