Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

RE: Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

Jan Cumps — Thu, 20 Jan 2022 12:58:43 GMT

The e-Peas harvesting circuit is the solution for a Cartier solar harvesting watch:
Cartier's first solar energy harvesting watch [...] is driven by e-peas' [...] power management solution.

image: cutout from the press briefing listed above - fair use

I'm not affiliated. But e-Peas is from Belgium. That adds to my excitement.

RE: Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

Jan Cumps — Sat, 01 Aug 2020 18:43:58 GMT

Note to self: excellent application note from e-Peas about provisioning the right cell and capacitor for a given load - with calculations and guiderails:

https://e-peas.com/wp-content/uploads/2020/06/AppNote_EH_Photovoltaic_AEM10941.pdf

RE: Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

jc2048 — Sat, 01 Aug 2020 08:23:51 GMT

I leave it to the coulomb experts to calculate the charge.

That's easy. Even I can do sums like that!

Q = C x V

so 0.4F x 4.5V = 1.8 coulombs of charge

The charge is directly proportional to voltage so, unlike a battery, you can determine the state of charge simply from the terminal voltage [at low currents, that is - at higher currents the internal resistance will also play a part].

The energy stored is

J = 0.5 x C x V^2

so, once charged to 4.5V, that will be: 0.5 x 0.4F x (4.5V x 4.5V) = 4.05 Joules

1.8C isn't a massive amount of charge. Even if you could contrive to use all of it to power a circuit [not realistic], that would be

1.8A for 1 second or
1.8mA for 16 minutes or
180uA for 2.7 hours

So, for an extended period of darkness, that suggests either very low power operation or a sleeping processor waking occasionally for a bit of activity.

But even powering from the panel, if it's scavenging internal ambient light of a few hundred lux it's not going to be producing all that much power, so the supercap probably matches that reasonably well. Anyway, you'll see when you start to do your measurements.

Where is the output converter connected? Does it take its input directly from the supercap?

RE: Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

DAB — Fri, 31 Jul 2020 19:13:23 GMT

Good update Jan.

UV would be very bad for the Bio-polymers they use, so if you want to put them outside, pick up a UV filter.

The circuit looks very similar to one I picked up from Texas Instruments five or six years ago. They had the same idea, solar cell to super cap, but it never did work well when I tested it.

The supercap looks huge, what is the energy storage specs on it?

DAB

RE: Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 2: Circuit Analysis

genebren — Fri, 31 Jul 2020 18:51:21 GMT

Nice update on an interesting product/evaluation board. I am currently working on some smart solar lights for my backyard, so I have been digging deep on similar parts.

Keep up the interesting blogs!