Para responder sua primeira pergunta:
De um modo geral, as baterias de iões de lítio modernas sofrem muito pouco da degradação.Commercial chargers do not allow changing the charge voltage limit. Adding this feature would have advantages, especially for laptops as a means to prolong battery life. When running on extended AC mode, the user could select the “long life” mode and the battery would charge to 4.00V/cell for a standby capacity of about 70 percent. Before traveling, the user would apply the “full charge mode” to bring the charge to 100%. Some laptop manufacturers may offer this feature but often only computer geeks discover them.
the worst situation is keeping a fully charged battery at elevated temperatures. This is the case when running a laptop off the power grid. Under these conditions, a battery will typically last for about two years, whether cycled or not. The pack does not die suddenly but will give lower runtimes with aging.
No entanto, a melhor maneira de manter sua bateria saudável é desconectar completamente a bateria quando estiver usando energia CA por longos períodos (ou seja, uma semana ou mais em energia AC) e armazenar a bateria com cerca de 50% de carga .
“Should I disconnect my laptop from the power grid when not in use?” many ask. Under normal circumstances this should not be necessary because once the lithium-ion battery is full the charger discontinues charge and only engages when the battery voltage drops. Most users do not remove the AC power and I like to believe that this practice is safe.
Também vale a pena notar que, com relação à segunda parte da sua pergunta, as baterias Li-ion são mantidas mais saudáveis quando são carregadas quando atingem cerca de 20-40% da capacidade.
Fonte:
Para ler mais, consulte este artigo em batteryuniversity.com, que também é a fonte das partes citadas esta resposta.