There is a short that is melting the negative terminal of battery . what cancause this

This sounds like it is trying to "Short Out". Turn off all the power to the chair and re-check your battery connections.
A "Short" is where the electricity is trying to flow the Wrong way around the circuit. This will badly Damage you batteries if you keep trying to connect them.
When connecting batteries together you ALWAYS connect a Positive (+) to a Negative (-). NEVER Positive to Positive or Negative to Negative.
Start at the Negative wire, connect it to the Negative terminal on the first battery, (Battery One). Then on Battery One connect the Positive terminal to the Negative terminal on Battery Two. You now connect the chair's Positive cable to the Positive terminal on Battery Two.
You should get NO sparks. Carefully turn on the chair's power. The batteries should NOT spark and should NOT get hot quickly.
**CAUTION** If it does spark or get hot quickly after you have done this TURN EVERYTHING OFF and disconnect the negative cable. This means there is a problem somewhere else in the chairs electrics.

From the description you have given, the only reason you're blowing fuses is a short circuit. To melt a battery terminal there must be a serious short circuit on your electrics. What work have you had done on it recently? Get a good auto electrician to check the electrics, as soon as you can, as a short is a BIG fire risk!

Battery and Negative Terminal - How to Disconnect a Battery

www.popularmechanics.com/cars/how-to/a1454/4213127/

Feb 28, 2007 - The positive terminal of your battery can deliver dangerous amounts of current if it shorts to. Pin ... the wrench prevented it from remaining connected long enough to melt. ... If you remove the negative clamp and inadvertently complete a circuit to ... What Happened When I Tried Ford's Drugged Driving Suit.Missing: parks

Damaged Cam Position Sensor May Cause Engine ...

repairpal.com > Problems > Chrysler > 300M

However as I got to sixty I tried giving the car more gas and it began to sputter again.... Clean the terminals connected to the positive jump terminal in front of the air ...Chafe wrap every sensor lead right down to the sensor and secure the chafe ... youdon't get melted wiring again anytime soon from radiated manifold heat.

Starter Troubleshooting - GoMoG

www.gomog.com/starter.htm

normally from the negative terminal of the battery to either the chassis,panel part or directly onto the engine

You have shorted out the battery and most likely melted the wiring. Check all wiring for signs of heat and replace, check all fuses and replace blown fuses. Recharge battery or replace, you are lucky it didn't go up in flames.

Sounds like a loose connection on the negitive terminal itself or on the reverse side of the connection causing heating due to high resistance.

The melt piece made a connection to the switch, to keep on instead above off. Scrape off melted surfaces and tape plastic coat it back.

It is called Battery Thermal Runaway. You are lucky that the battery did not blow up. You need a new battery.

It is caused by the battery charging itself and gets to a certain point and starts to over heat, melt, and blow up.

Remove Negative terminal first, then positive terminal. Battery will have some type of tie down or block at base with a bolt you will have to remove. Remember how it goes back together. Remove battery.
Install new battery and tie down or block. Install Positive terminal then Negative terminal.
Remember to never touch both Positive and negative terminals with a metal object (ie..ratchet, wrench etc) this can cause injury and/or permanent damage to a new battery.

When replacing battery in yamaha GP1200 when attach negative cable the unit seems as wants to turn over the engine while the unit is off and no key in place.. also notice in seconds that the battery terminal on negative side started smoking..actually melt the new battery terminal on negative side..yes, postive was connected to positive. Any ideas what why this would take place??