Solar power chargers don’t have integrated battery, so there remains no concept of partial or full charging. Thus, you can attach your devices directly with solar charger and charging would start when there would be sufficient sunlight. However, concept of charging comes with solar power banks . Solar power banks are typically batteries which also have solar panels to top up integrated battery. As they work as traditional batteries so a load or device could be attached with them before battery of solar power bank is fully charged. Anyhow, to prolong battery life, it should be taken care of that charge of a battery doesn’t drops below permissible level of charge for specific battery type during discharge cycles.