Peter VanDerWal via EV wrote:
Wouldn't it be better to have a 3.9V zener across the battery?
Unless thay have leakage current when off?
If I'm going to put it across the panel then I'd probably want a 4.3V
zener, and then drop ~0.4V between the panel and the battery?
The result doesn't have to be precisely 3.9V on the battery, just
somewhere in that neighborhood.
(Warning: Technical discussion to follow. May cause drowsiness...)
True zener diodes only exist below about 6 volts. True zeners have a
large change in their zener voltage versus current. For example, a
1N5335 3.9v 5-watt zener will measure about:
5.1v at 1200ma
4.5v at 600ma
3.9v at 300ma <-- rated current
3.5v at 150ma
3.0v at 50ma
1.0v at 0.3ma
In other words, it draws a *lot* of current even well below its zener
voltage. If connected right across a lithium cell, it would soon
discharge it below 1.0v, and murder it.
In contrast, "zeners" above 6v are in fact "avalance" diodes. A
different mechanism defines their threshold voltage, and they have a far
more stable voltage versus current. For example, a 1N5342 6.8v 5w zener
will measure about:
7.15v at 700ma
6.95v at 350ma
6.8v at 175ma <-- rated current
6.6v at 60ma
4.9v at 0.01ma
Notice that the voltage changes far less with current, and becomes
insignificant (10uA) below 5v.
In between the two (5-6v range), the characteristics gradually
transition from one behavior to the other. That's why you have to "test"
and pick the best parts for the job.
This means you want to use a higher-voltage "avalanche" diode across the
PV panel to limit its max voltage; then knock this down to the desired
cell voltage (and block reverse current) with 1 or more series diodes.
Lee Hart
--
When something bad happens, you have three choices: You can let it
define you; let it destroy you; or you can let it strengthen you.
-- Theodor Seuss Geisel
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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