Don't go spending anymoney till you have given the Optimate battery plug and socket connectors a good looking at!
They are not all that good and the female pins (which are in the male connector) have a habit of spreading
My Optimate showed "weak battery" last weekend (batt 4wks old), a quick fiddle with the pins cured it, time taken 5mins, cash spent zilcho.
Even if the connectors need replacement, your local friendly Radio Control Car shop will flog you a new set for a few pounds, either take the old in or ask for "Tamiya" ones.
If you know anyone who seriously races RC, they will probaby have loads kicking around, not much cop at handling 70+ amps, tend to melt!!! Usually the 1st things that get junked on the path to RC victory.
Fine for 0.5A Optimate tho.