Start her up and check voltage at the battery and the voltage at the alternator, that will at least tell you if you have voltage drop or not between the two. Obviously if you do get significant drop then your problem lies with the connections/cables/batterys.
If not and your voltage is low at the alternator connection then there might be a problem with the alternator. Your belt isn't slipping at higher revs is it? So worth doing a belt check too.
Just to expand what Erin was saying. You get a sulphated battery if it's left for a time on low charge/discharged, so it's important to get a fully charged battery.
Looks Slow but is Fast
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