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I have aabout 100 plants in pots under a light...... in my basement which is colder than the ambient temp upstairs. I bit the bullet and bought heat pads but they don't seem to be giving off much heat. The only plants that have popped are cold weather species. Is 45-50 just too cold for pepper, tomatos, spinach, basil, etc.?????

With bottom heat tomatoes can and do germinate in 3-5 days and 5-7 days is average in my experience.

Excerpted from Abstract for Characterisation of the germination responses to temperature of vegetable seeds. I. Tomatoes

Optimum temperatures occurred between 26 and 32°C; maxima for fifty percent rates of germination were between 35 and 38°C, and minima between 8 and 10°C. Seed of Lycopersicum pimpinellifolium and small fruited subspecies of L. esculentum germinated at lower temperatures, down to 6°C, and more rapidly at median temperatures.

https://www.sciencedirect.com/science/article/pii/0304423874900193

8°C = 46.4°F
So I guess tomatoes CAN germinate, but it will probably take weeks and at reduced total % germination. Peppers will have even more difficult time. But Spinach should do fine.

Average of 2 weeks until germination is normal when heat is not supplied.

BETTER METHOD would be to keep the trays upstairs and supply some kind of heat. Top of fridge is often mentioned, upstairs is warmer than downstairs, south facing rooms are warmer, etc.(I posted a whoe list somewhere just recently) (I proof my sourdough on top of the old TV....) And bring them down to the basement to put under the lights as soon as they germinate. Tomato seedlings can grow in 50's temperature. You'd be better off finding a way to provide light for the basil and peppers upstairs though.

Next time put your seeds in a plastic bag and keep them in your heated house for three days then pour them into the place where you start them! Another method is place the seeds in your boxes and place them in a sealed bag and put them in you mail living area for three days then return them to the 50 degree area under lights.
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You can also put the light on before the seeds sprout with the lights only a few inches from the surface of the soil which will add 5 or more degrees to the surface of the soil! Once the seeds sprout they will be fine under the lights in 50 degree temp! My seedlings are fine even when my temp goes to 35 for a few days!