Ok, connecting 2 batteries in series give you twice the voltage and the same Amp-hour rating available, so you have, effectively, a 24 Volt, 35 AH battery.

Ok, a 100 Watt motor, (or light bulb or other load), will pull 100 Watts. Let's use ohms law here, where:

P = Power in Watts,
E = Electromotive force in Volts,
I = Current in Amps and
R = Resistance in Ohms.
(Multiply going up, divide going down)

P = 100
E = 24
I = 4.166 Amps
R = 5.761 Ohms

So we have 4.166 Amps draw.
35 / 4.166 = 8.4013 hours or ...

60 * .4013 = 24.078

8 hours and 24 minutes until the fully charged batteries are completely exhausted.

Over 60 years of training and experience in electronics.

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