What is the charging capability of your Battery Tender? Is it just a couple amps or less, or is it a more robust charger? Batteries need a good solid hefty charge before the 24 hour rest period. A good charge should hopefully indicate at least a little activity in the battery acid with occasional small bubbles rising to the surface. If your Battery Tender is only a maintenance charger, it cannot perform such a charge.
Regarding the Hydrometer.... I find it necessary to tap the sides of the tube several times after sucking up the fluid .. to ensure that there are no bubbles attached to the float bulb. I finally changed to a battery refractometer to avoid that issue.
