Several charging methods and schemes are proposed. In classic DC charging method battery is typically charged by current, rated at 10% of its capacity. For example, for 55Ah battery you would want 5.5A current.
There are two distinct modes of operation:
In storage systems charging is typically done in buffered mode - that system tries to maintain 13.8 voltage on battery. But battery would be charged to 80% and not used all of its active mass. That would lead to degradation over time, mainly because of sulfatation.
Here you charge battery to 14.2V-14.4V(this depends on ambient temperature, there are tables), then put it on use. While you using one battery you are charging another.
Main thing to remember. NEVER DISCHARGE IT BELOW 10.8V. Personally i recommend even not to discharge below 11.2V.
As for your conversion task.. For such low current you mentioned - 100ma - most that you may do - is to make current step-up device. leadAcid battery typically "hates" low currents, and "loves" pulsed charging methods - they are capable to taking insane currents shortly, without boiling.
You could give a try to the following current step-up scheme:
Idea is simple: You first charge some small capacitor with your turbine - a current source, to 40-100V (because of current from turbine has nowhere else to go). At some threshold voltage (which you set up by zener) tyristor opens and instantly discharges all accumulated charge in your battery, creating pulse of greater current flow. So use thicker wires from capacitor to your battery.
That could work, but you'd have to tweak nominals according to your generator setup and battery capacity. If that wont work - i think no other method would charge it.
PS. I forget to add inductor, where 50V is written, or you may use any of your wires from capacitor to battery, winding them on coils. When capacitor flash-discharges to battery, inductor would smooth the current and store some energy inside magnetic flux, thus provide some extra energy storage and increasing charge rate:
Dont forget to tune them to frequency, resonant with discharge rate to get most conversion efficiency.
More to read on wiki: