This allows you to clearly SEE its performance, its exact capacity in Ah or Amp Hours, (or m/Ah hours) as well as displaying its internal resistance in m/Ohms.

This means that after a few months you can run the test again and display a new graph to see EXACTLY how much your batteries have deteriorated. In huge detail.

And you can save all these records/graphs for later on your PC.

If you have many batteries as I do then it helps to label each one with a new date and a number so you don't get confused!  And give the file the same number. 

It will help you decide the best time to replace you batteries. You can see how much capacity they still have, and more importantly you can see if the internal resistance has increased which is another big issue with powerchair batteries. (or any batteries) An example graph is below. 

The graphs below is from a used and abused 68Amp Hour (Ah) AGM wheelchair deep cycle Hawker Odyssey battery.

The voltage difference between on and off load and ohms law gives the charger an accurate internal resistance figure for any battery connected to it. This is potentially more important than the usual wheelchair tech guys "capacity test" if he even knows how to do that... As you will see below.

It will also tell you exact amp hours provided to a few decimal places from fully charged down to your chosen cut off voltage as it DISCHARGES your battery. And again as it charges your battery (or string of batteries)

And an average internal resistance figure over a charge/discharge cycle. That increases as the battery becomes more discharged. The figures given here are for fully charged batteries.

I tested a less than one year old Hawker Odyssey 1500T battery here, and here is the result. (this was from an old unloved chair that gets very abused! And not charged as often as it should be..

Internal Resistance measured at full charge was 11 m/Ohm (increasing to 18 m/Ohm as it becomes discharged.

It was just 2.5 m/Ohm when new under a year ago. So it is now 4x worse here!

That means 4x the voltage drop under load. That's why wheelies and steep ramps became a struggle. And why I tested it. New batteries needed!  At least for this chair that this battery came from.

Another lighter user with a lower powered and less aggressively programmed powerchair with less greedy motors probably wouldn't notice much wrong as his peak current draw is lower and less often used. And the battery would appear to perform normally  Since their chair wouldn't draw so much current (amps) and the voltage sag wouldn't be as severe and go un noticed. 

Capacity (storage capacity) tested at 61.5 Ah. Considering that it was tested over the 10 hour rate that's almost as "good as new" as far as capacity is concerned.  Was 67.5 Ah when new.

Capacity isn't much worse than new comparatively so range wouldn't be effected. That's why its not possible to say a battery is "good" or "bad" as it depends on usage, and what exactly you test for. Which specific powerchair and even programming settings change the requirements. For my purposes its now useless!

Gel batteries like MK batteries for example have 4.5 m/Ohm resistance when new. That means they start off twice as bad as the AGM Hawker Odyssey or Optima batteries I prefer.

What's more they are actually designed to increase their internal resistance as they become discharged. So that they "feel" flat as they become discharged and in fact become useless well before you actually get to use the real full capacity.

In other words they may "have" 70 Ah capacity if you just draw say 3.5 amps for 20 hours (that's how batteries are rated - say 70Ah @ the 20 hour rate) but when you try to remove great sudden "lumps" of power like a 100 amps or so to climb a curb they cannot access the reserves due to voltage sag.  Resistance in a battery is a bad thing!

They simply don't let you access their reserves!  This makes them perform worse but protects them from deep discharge damage by being starved of sufficient acid in the Gel (this weakens and is used up chemically during discharge) and as such they give better cycle life of typically 500 cycles. AGMs typically give 400 cycles. Both at the same 80 percent discharge level.  The difference is that at 80 percent discharge the AGMs still feel "strong" and can still produce many amps without huge voltage sag.