Hi Tony,
Do you have a multimeter handy? If not, something dead cheap
like this will help. Check the voltage value of the battery, when:
1. Standing alone (unconnected)
2. Connected to mount only (no movement)
3. Connected to mount (whilst slewing)
4. Connected to mount AND dew heater.
5. Measure the voltage at the end of the power cable, just before it goes into the mount (to check "the cable").
What sort of values do you get in each of the above?
Be sure to check the water level in the battery, if it offers access to that. Depending on the charger you have, the battery can get a bit parched. A cheap charger will boil the battery dry if left running overnight, as opposed to a nice digital "intelligent" charger. If water is low, fill with demineralised water to the proper mark. Without insulting anyone's intelligence, please do wear eye protection on this step...
Lastly, check for muck around the pins in the anderson plugs, or anything that can short out etc.
I've never dealt with (or even seen) a mount bigger than my Nexstar 6/8 AZ, not to mention an EQ6. Having googled whilst writing this, it seems the EQ6 appreciates a between 12 -15v (for which your battery is going to give a tickle more than 12 at best. You *may* find your rig is getting 11.x Volts and "browning out". See what can be done above, before worrying about a new (expensive) battery.
Best of luck
