That device sounds like a fancy battery charger that drops into a trickle charge mode when the battery is charged. For example one of their website FAQs states:
How is the Battery Tender Plus battery charger different from a trickle charger?
The Battery Tender Plus battery charger delivers 1.25 amperes during bulk charge mode, holds the battery charge voltage constant at 14.4 VDC during absorption charge mode until the battery charge current drops to 0.1 amperes at which time it then automatically switches to a float charge mode. During float charge mode, the output voltage of the Battery Tender Plus battery charger is 13.2 VDC, which is well below the gassing voltage of a lead acid battery. This keeps the battery topped off, while minimizing any detrimental effects to do gassing. The Battery Tender Plus battery charger is able to perform these complex switching functions because its electronic circuitry is controlled by an on board microprocessor.
I believe most commercial battery chargers that state they automatically go to a trickle charge mode when the battery is charged do a similar thing. Trickle charge typically means keeping the battery at a voltage lower than the normal charge voltage.
To build one you would need to program a microprocessor to do what it does. If you want to to that, go for it, but it's a lot of work and expense to reinvent the wheel. It'll likely cost you several times what a commercial unit costs.