General > General Technical Chat
Simple voltage cutoff schematic for solar charger, any tips?
Gyro:
--- Quote from: Zero999 on April 18, 2022, 10:22:50 am ---The end of charge voltage should be set higher than the float voltage to ensure it fulling charges.
You should set the cut-off voltage to 14.5V.
--- End quote ---
By the time you get to 50'C, the cyclic charge voltage would probably a fair bit lower than that. Even with a cyclic charge, you need to allow the current to decay to get to 100%. If you set the end of charge too high then you'll end up with venting SLAs or bubbling flooded LAs as applicable.
Zero999:
--- Quote from: Psi on April 18, 2022, 10:42:50 am ---The more I think about it the more i like the MCU option. That was until I looked at stock levels of sub $1 MCUs at digikey.
--- End quote ---
How many do you need?
--- Quote from: Gyro on April 18, 2022, 11:12:48 am ---
--- Quote from: Zero999 on April 18, 2022, 10:22:50 am ---The end of charge voltage should be set higher than the float voltage to ensure it fulling charges.
You should set the cut-off voltage to 14.5V.
--- End quote ---
By the time you get to 50'C, the cyclic charge voltage would probably a fair bit lower than that. Even with a cyclic charge, you need to allow the current to decay to get to 100%. If you set the end of charge too high then you'll end up with venting SLAs or bubbling flooded LAs as applicable.
--- End quote ---
Then you need to adjust the cut-off voltage depending on the temperature.
SeanB:
Simplest is to use the isolation diode to actually do work, and have your voltage comparator on the input operate a nice beefy MOSFET to short the panel output to ground. Diode isolates the battery from the short, and the panels do not mind running into a short circuit, they are energy limited anyway. Low on state resistance, so low dissipation, and so long as you put a few seconds of delay into the output of the comparator, ie a nice RC delay to a second comparator, with a lot of hysteresis, so the output is going to have to stay in one state or the other for the first one, before the output changes state. Battery gets fully charged, then gets a pulsed charge that compensates if you have any load that pulls charge off it.
Comparator and such fed from the battery side, and you can also add in a third comparator, to disconnect the load on low battery voltage as well. Another low Rds mosfet there. Can be done with a small MCU or some low power comparators and some CMOS schmitt trigger inverters, and a TL431 as voltage reference, and some high value resistors to divide down battery voltage to them.
floobydust:
Not advocating but there are shunt-regulator solar charge controllers out there, also some just using a relay to disconnect the panel.
Why such a primitive charge controller still exists i.e. Specialty Concepts ASC-12:
No PWM EMI
*Will initiate charging into a dead battery (hear that Morningstar, your crap runs entirely off the battery side). This is important!
OP I would only include temperature-compensated charging voltage, add a couple diodes to a TL431 or TLV431 (At -3% month self-discharge that's about 1.25mA steady drain.)
Power dissipation on a shunt reg at 1.4W is not much for a power transistor or mosfet with a small heatsink. For linear series-pass, it's the same actually when charging full tilt.
I think a 8W panel paired with 30Ah SLA is a bit weak for charging, but I'm not sure what sun and ambient temps, load is like for the project.
Psi:
Yeah, i've been looking at options for driving a N-FET low side switch using a comparator with hysteresis. Which is looking like it should work fine and be small enough to fit in the tight space.
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