A lot of people buy new batterries after a short while d batteries develops faults and they get upset due to the cost of purchasing another new batteries
The fact is that deep cycle battery has a certain life span depending on the battery chemistry
Some stay more than a year and some stay up to two years while some stay up to 5 years before it get spoilt
These batterriea are like babies so the need to be taken care of properly for them to last and function properly, considering the cost of buying new set of batteries some would decide to go for used batteries and if you are lucky you might end up buying a very good battery because some of these batteries were used by our telecom firms at their cell cite(mast batteries and they always spend a lot of money changing these batteries after a short period of time,so these battries are sold of to resellers , but sometimes if your aren't lucky you get a bad one. So here I'm gonna tell you how to test the real capacity of your 12v lead acid battery/Vela/gel battery commonly used on inverters. You will need an inverter maybe a mini or a normal inverter ,you will need some 100w electric bulb(do not use energy saving light for the test ) u might need up to 5 of the bulbs depending on your battery capacity. You will need your battery fully charged. You will also need electric socket and lamp holder for your bulbs and your time piece (to check time) Connect your inverter to the battery and and connect the electric socket to the inverter
If you have a 100ah battery you might need 5 of 100w bulbs (5*100=500w) so we have 500watts of load
If you run d load on the inverter
The 500w should last up to 1hour 40minutea or there about
If the duration is anything less than that you might be having a weak battery
You can also increase the number of bulbs used on the test to reduce the waiting timetime
The same thing is applicable to 200ah battery

A good 100ah battery under 500w load should last as thus
(100ahx12x 0.8 )/500w = 1.92. ~115mins ~ 1hour 55 mins

So
The Real capacity of ur battery after the test,
With this formula
=(500w x time duration before battery low in hours)/(12v x 0.8 )


For example, let's assume that after the text, your inverter beeps low battery or shut down, in 30minutes
The the actual capacity of your battery is as follows

Note: 30minutes is equal to 0.5hours
So therefore. (500w x 0.5 )/ (12v x0.8 ) = 26ah

The truth is that your shiny 100ah battery real capacity is 26ah
So ,you would need to replace the battery

You have done a good justice to this. Kudos!