i need to charge a lead acid 12vdc battery and for that i need two chargers
1- 12VDC 4A
2-12VDC 20A
first one is for normal charging daily and second one is for fast charging battery once a 4-5 months to maintain its gravity now the problem is that in my setup i need to install both chargers but i dont know how to do switching with a simple switch
i am attaching diagram which is a rough idea what i need and suggest me any solution

as you can see in this diagram that i have ofcos one terminal but if i give it 4A it will send it will start charging battery ofcos but on other hand it will give that current back to 20A charger which is also connected to same terminal so what can i do to prevent that or how can i connect these two chargers but also put a button to select my Amperes whenever i want ?

 

you could use a DPDT switch
DPDT=double pole/double throw

 

you could use a DPDT switch
DPDT=double pole/double throw

but the thing is how to stop it from giving that 4a back to other 20a charger ?
and what if i wanna control it with arduino what will i do then ? i mean in arduino i can only use relay to change switching.
how will i put two relay and connect it accordingly ?

 

Use the switch to connect or disconnect each charger from the terminals..
If you use an arduino use a DPDT relay the same as the regular switch..

 

You could likely use just a SPDT relay, and make the grounds for the battery and the two chargers common, unless the two charger commons can't be connected together for some reason.

 

I am just curious, but why do you need two chargers? Is the 20amp one going to be just sitting around for 4-5 months?

Is 'maintaining its gravity' the same thing as giving it an equalization charge?

Not all battery chargers have the ability to provide this function, does yours?

http://batteryuniversity.com/learn/article/equalizing_charge