Knowledge and control of surface potential (or charge) is important for tailoring colloidal interactions. In this work we compare widely used zeta potential measurements of charged lipid vesicle surface potential to direct measurements using the surface force apparatus (SFA). Our measurements show good agreement between the two techniques. On varying the fraction of anionic lipids dimyristoylphosphatidylserine (DMPS) or dimyristoylphosphatidylglycerol (DMPG) mixed with zwitterionic dimyristoylphosphatidylcholine (DMPC) from 0 to 100 mol % we observed a near-linear increase in membrane surface charge/potential up to 20-30 mol % charged lipids beyond which charge saturation occurred in physiological salt conditions. Similarly, in low salt concentrations a linear increase in 1 charge/potential was found, but only up to ~ 5-10 mol% charged lipids beyond which the surface potential/charge leveled off. While a lower degree of ionization is expected due to the lower dielectric constant ( ~ 4) of the lipid acyl chain environment, increasing intra-membrane electrostatic repulsions between neighboring lipid head groups at higher charge loading contributes to charge suppression. Measured potentials in physiological (high) salt solutions were consistent with predictions using the Gouy-Chapman-Stern-Grahame (GCSG) model of the electrical double layer with Langmuir binding of counterions, but in low salt conditions, the model significantly overestimated the surface charge/potential. The much lower ionization in low salt (maximum fraction dissociated ~ 1-2 % of total lipids) instead was consistent with counterion condensation at the bilayer surface which limited the charge/potential that could be obtained. The strong interplay between membrane composition, lipid head group ionization, solution pH and electrolyte concentration complicates exact prediction and tuning of membrane surface charge or potential for applications. However, the theoretical frameworks used in the work can be used as guidelines to understand this interplay and establish a range of achievable potentials for a system to tune or predict the response to triggers like pH and salt concentration changes.