on board charger
 

I bought an onboard charger but it will only charge one battery ,if i run another 2 wires from the existing fittings to run to the seond battery (to charge both batterys @ the same time)will this work or am i doin more harm than good????

If it's not a dual output charger you could have problems. What make & model charger do you have?

Basically, yes, it will work. However, it will take longer to charge the batteries.

Two 12v automotive type batteries linked together in parallel (positive to positve, negative to negative) will be the same voltage as a single battery. Charge them using a 12v battery charger linked across the negative terminal of one battery and the positive terminal of the other battery. Connect the charger, switch it on and monitor at regular intervals.

Two 12v automotive type batteries linked together in series (positive to negative) produce 24 volts. The charger voltage must match the battery voltage, so you cannot charge a 24-volt battery with a 12-volt charger. If this is how you are using yur batteries, then just buy a 24v battery charger. Connect it across the terminals at the ends of the two linked batteries and charge as you would for a single battery. :head:

on board charger
 

yeah it is a 12 volt system but when im only using 1 batt will the other one be getting used too because of the wiring goin from one to the other??:head:

It will work, but the battery with the higher initial charge will get overcharged once it's fully charged (until the battery with the lower initial charge is topped off). I asked the OP for the make and model of the charger to know how many amps it's putting out.

Best thing to do would be return it and get a 2bank. Got a 3 bank on mine and love it. Depending on what the batteries are running they are all in different conditions and require specific charges.

There is little cost between a 1 and 2 bank.

That's where the "Connect the charger, switch it on and monitor at regular intervals". sentence comes into play.

As to overcharging, not necessarly true. It depends on a lot of factors...age of both batteries, condition of same, amount of drain they receive, float voltage, etc. Also, don't forget that if they are wired together in parallel they will draw down and recharge together at the same rate.

That depends on how they are hooked up. If you just have them wired together in parallel, then yes, they will both get used together. But if you use a battery isolator switch then you can just use battery one, or battery two, or both, depending on where you manually set the position of the switch.

The answer to this specifically should be NO. The wires coming from the charger (4 instead of 2) will not connect the batteries together at all, so if one is draining, the other will not, during use. BUT, when charging, it will charge both, perhaps a little slower, but as stated in other replies, must be monitored. Just change it out for a 2 bank smart charger and you will sleep better.