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Is my setup safe? I'm connecting two 7.2v 100ma solar panels in parallel using two 1N5817 then to a to 6v lead acid battery with a Constant Voltage Charge of 7.3-7.4v for cycle use.enter image description hereI can't use a MPPT or PWM solar charge controller do partly do to lack of space and partly because it with be in a water tight box with no ventilation.

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  • \$\begingroup\$ Welcome to EE.SE. Solar cells do NOT put out a dependable voltage or current, which is why we recommend you use regulator IC's for charging batteries. Without regulation you may overcharge and destroy the batteries. \$\endgroup\$ – Sparky256 Jun 25 '18 at 23:16
  • \$\begingroup\$ Thanks for responding, Do you know of an IC without many external passives, can run at a low temp (no external cooling required) and runs on 5 volts. \$\endgroup\$ – Jonathon Breitner Jun 25 '18 at 23:21
  • \$\begingroup\$ Please read the "Related" column to the right of the page. It contains previous Q & A's about solar cells and batteries. \$\endgroup\$ – Sparky256 Jun 26 '18 at 0:38
  • \$\begingroup\$ You certainly don't want to apply a high float voltage to a lead acid battery (your solar cells are above this) that is in a completely sealed container (your waterproof box) as it will vent explosive gasses. Even if you never exceed float voltage some amount of gas will be vented so really you should probably not be using lead acid for such an application unless you use a PTFE vent. \$\endgroup\$ – lucky bot Jun 26 '18 at 3:08
  • \$\begingroup\$ What is the open circuit voltage of the solar panels? \$\endgroup\$ – winny Jun 26 '18 at 10:26
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From what I understand my setup is safe. I have never seen my panel ever have an open circuit voltage greater then 7.2v which is 0.2v less then max Constant Voltage Charge for cycle use, so I don't see how I could ever over charge the battery.

But, I understand my battery will only ever charge at peak sunlight when the panel voltage is greater or equal to the current battery voltage plus the schottky forward voltage drop. So, a charge controller would capture more energy but in no way do I see it being dangerous without one.

Valve Regulated Lead Acids Do Not emit harmful gas when charged correctly. Otherwise, the SLAs in Uninterrupted Power Supply found under desks in offices would be very bad. Though they will emit harmful gas if the internal pressure is too high, But this is prevent explosions & fire caused by improper charging or sudden failure like a short circuit.

I'll be happy to hear why I'm wrong. @Sparky256 @HeneryCurn @luckybot

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    \$\begingroup\$ When it comes to solar cells and batteries, we take the side of safety, and always will. Especially Li-Po batteries. I am not going to tell you that you are wrong, simply because you are working with 'flea' power at best. These solar cells look too small to hurt anything, except your charging time. \$\endgroup\$ – Sparky256 Jun 28 '18 at 4:52
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    \$\begingroup\$ Ah, yes. All SLA's do tend to outgas - thats why they have that valve. If you charge them in a true pressure vessel it can be quite interesting opening it (expensive chemical sensor shot through two sheets of plasterboard by the pressure). The gas is not really a hazard normally, but overcharging eventually destroys them. Now you might fortuitously have just the right solar cells, ambient temperature, and sunlight, that this can't happen. But many people have destroyed starting batteries and gel cells by float charging with small (2W) solar panels. \$\endgroup\$ – Henry Crun Jun 28 '18 at 12:11
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    \$\begingroup\$ I suspect that if you cells can't reach overcharge voltage, they probably can't put any useful current into the battery at 6.9V+ . Do some measurements, and tell us what current you get at what voltage. If the current is low enough, it can't raise the voltage \$\endgroup\$ – Henry Crun Jun 28 '18 at 12:20
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No, you must have some sort of charge regulator.

e.g TL431 shunt regulator straight across the solar panels

see figure 30, http://www.ti.com/lit/ds/symlink/tl432b.pdf#page=26

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  • \$\begingroup\$ Like this? Circuit Diagram \$\endgroup\$ – Jonathon Breitner Jun 27 '18 at 20:36
  • \$\begingroup\$ put a diode from zener to to the battery. Don't want to try and zener the battery itself, and don't want to let battery discharge through the panels when it is let in the dark for a month. \$\endgroup\$ – Henry Crun Jun 27 '18 at 21:41

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