If you have an old lead acid battery (mostly used in cars, bikes, home UPS and inverters) which is giving low backup, here is a small trick to increase its backup and make use of the battery until it gives up fully. When a lead acid battery is new, it takes longer to get fully charged and also gives long backup. When the battery gets old, it takes very little time to reach its charging cut-off voltage and hence your charger thinks that battery is fully charged and it stops the charging leading to even poor backup. If you check the acid gravity of these old batteries, you will always find it low.
Now the trick is to overcharge this battery so that it gets charged even when it reaches its cut-off voltage and acid gravity gets improved. Suppose you have a 12v lead acid battery and its cut-off is at 14v, connect a battery charger to it which can charger it at 14v and does not have any kind of cut-off. Also the current rating of this charger should be at least 10% of the battery connected to it. For example if you are trying to charge a 12v, 200Ah battery then charger should give 14v, 20Ah. Before you connect the charger and start charging, check the water level in each cell of the battery and if it’s low or overfilled then make the water level between minimum and maximum level. Too little water will damage the battery and too much water (till max level) will prevent battery from getting properly charged. You may need to keep an eye on the battery temperature, just check it every 3 hours and keep the charger connected until acid of the battery attains correct gravity. If battery overheats while charging then lower the charging current. Once battery is put on charge, remove all the caps of battery and in each cell look for bubbles. If at any of the cell you do not see these bubbles or very less bubbles then that cell needs magnesium sulphate treatment. For more information, look at this post on how to renew battery with dead cell.
I have tried this method on a 4 year old lead acid battery of 12v, 135Ah capacity connected to a home UPS and its backup time got almost doubled!