Control system for electrochromic devices

1. A method of determining the transmission state of an electrochromic device comprising:

applying a voltage across said electrochromic device;

using the applied voltage and current for calculating leakage current of said electrochromic device;

determining ionic current as the difference between the applied current and the leakage current;

using the ionic current for determining the transmission state of said electrochromic device.

2. The method as claimed in claim 1 , wherein the calculating leakage current step comprises calculating internal voltage of said electrochromic device using the algorithm

V int =V app −R s *I app .

3. The method as claimed in claim 2 , wherein the calculating leakage current step further comprises using the algorithm

If V int =V T , I leak =V int /R 1 , otherwise, I leak =V int /R 1 +( V int −V T )/ R 2 .

4. The method as claimed in claim 3 , wherein the determining the transmission state step comprises integrating the ionic current to determine transferred charge (Q=∫I ion dt and correlating the integrated charge to the optical state of said device using the equation % T=f(Q), where f is a mathematical function or look-up table.

5. The method as claimed in claim 4 , further comprising:

applying a holding voltage for maintaining said electrochromic device at a desired transmission level, wherein the holding voltage is greater if said device is fully bleached before being colored to an intermediate state and said holding voltage is less if said device is fully colored before being bleached to the same intermediate state.

6. The method as claimed in claim 5 , wherein the applying a holding voltage comprises:

calculating an internal holding voltage for said device using the bleaching/coloring history of said device; and

adjusting external voltage applied to said device until the calculated internal voltage, V int =V app −R s *I app , matches the required value.