Query Moved and Approved.

But what vehicle is this? Car? Motorcycle? Scooter?
First, get your battery checked up.

If you still insist, yes, its possible. All you need is a 14 volt step down transformer/rectifier to get the battery charged. Typical example being your emergency lamp. Ideally speaking if your battery is in good condition, starting your vehicle for 5 minutes a week is suffice.

OT - I was wondering how this is possible? Do you've an all DC setup?

you can buy a battery charger from any decent electronics shop, that stocks stuff for cars/bikes. If you are in delhi, then chawri bazaar/daryagang are good places to scout for this stuff.

But remember, these chargers do not have a cut off, so you will have to be careful with that. Else go with the idea that aargee gave.

As people said, what type bike is it?

In any case 15 mins riding in day should give enough charge for keeping AC+DC setup bike's battery healthy.

Secondly it's ofcourse possible built a charger which you can plug in and there are two ways of charging it:

(1) Use a 15V, 1Amp Charger connected via rectifier for surge charging. Something which I use, for about 30 mins, which gives a boost current.

(2) Built a proper charger with regulator and if possible with trickle charging option, but it's only needed when battery is almost dead.

Thank you mod for quick approval and guidance to appropriate section .
Thanks guys for quick attention .
I presently have two bikes - Yamaha Rx135 and HH splendor plus .

Both batteries are 2.5mAh and the circuits are stock , no mods . I checked
battery charge with multimeter , it's 13.2v and 12.78v respectively .
Whereas building or buying a Mains-voltage charger is not a problem ,
I was worried about two things .. since I know nothing about a bike's
wiring circuit .

a) Is it ok to charge the battery while it is still in-circuit
on the bike ( i'e without removal or disconnection from the bike's circuit ) ?

b) How to monitor and cut off the charging at the proper voltage/condition of
the battery automatically ? What is the proper charged / discharged voltages in fact ?
A charger fixed in my garage with build in monitoring .. say a analog voltmeter / ammeter ?
I want it to be as simple as charging my mobile phone .

Are you guys sure 15-20 mins riding will fully juice-up a bike battery ?

Yes Rahul'ji .. a proper , permanently fixed to garage , reliable charger is what i am thinking ... trickle charging optional .
Also thinking of replacing these lead-acid one with 7mAh SMF ( UPS ) batteries .Maybbe a headlight upgrade on that damn
splendor later :-)

i dont think so they are milli-amp hour rated, i think they are rated in amp-hours.

You can upgrade the batteries, but you will have to see the output of the alternator to match the battery, so as to be able to charge the higher capacity battery, else the headlight will drain it faster than it can get charged.

Yeahh , sorry my bad ... AmpHour ( Ah ) is correct .. hehehe..

Ok , then say if I can fully charge my 2.5 Ah battery fully from mains daily . Shall the next day's use consisting of maximum of 3 hours night riding ,
drain it fully ?
I mean , if not , I can always recharge it for the whole night when I
am back home .
What about a fully charged 7 Ah SMF ( UPS ) battery ?
I am not planning to alter the alternator

Ok , I got an old 500VA APC UPS from a friend which has bad AC output / relay ,but battery charging is ok ... this should certainly charge a 7 AH SMF battery properly and safely overnight .
Now can anyone tell me .. if I need to disconnect the battery from the
bike's circuit to charge it , or it is safe to just connect to UPS charging circuit with additional wires ?

you can charge the battery while it is still in circuit of the bike. the output from the battery charger(yours) will be far smoother than the current from the alternator, RR of the bike; so no problem keeping the battery connected to the bikes circuit.
I replaced my 2.5AH battery in my ambition with the battery of my UPS(sealed Gel) and never had any charging issues with the stock alternator.
The charger from UPS is not a good idea I think, because its charging capacity is low, consistent and not high and rapid.
I used 12v 1.0 amp transformer and 2 diodes to charge my 4 year old Lead Acid battery after adding a little sulphuric acid and distilled water and the battery got recharged in one night from almost dead to life. even though it is 12v the actual output will be around 13~14 v open and nearly 12.8v closed. so no need to search for 15v transformers.
If you really want to make things simple, just buy a 12v 1.0 amp(min) battery eliminator(transformer type and not recent SMPS adaptor type; they output exactly 12v under load/noload and we need atleast 12.8 v to charge the battery) and go on with your male/female jack idea.
No need for smooth output of the current and trickle charging options like the UPS unless you tend to forget to disconnect the charger for long long time.

Thanks a lott sandeep , that clears up a lot of doubts and greys in my mind . .