Feedback on simple isolating, high-safety solar charger - Page 1
Forum section: electronics / projects

author: kubark42
author_rank: Contributor
timestamp: September 15, 2023, 10:40:39 am
content: I have a glider in which I wish to keep a 26Ahr 12V SLA battery topped off with a solar charger. Efficiency isn't important because the ratio of flown time to insolation time is stupid high (avg 2hours/wk of flight, 84hrs/week of sun). Instead, what is important is weight, reliability, and data. Oh, and also fun.I wish to use a simple WiFi board (either ESP32/RPi Pico W) to control this system. This lets me experiment with MPPT, with datalytics, etc... The issue is that, done wrong, either the solar panel or ESP32 could kill the $$$ SLA by, respectively, overcharging and overdischarging the battery.I want to fully isolate both the panel and the ESP32 from the SLA. To do this, I would use a LiC/supercap to accumulate energy from the solar panel, and then subsequently discharge that energy into the SLA battery. And, in order to keep the solar panel always charging something, I will have two LiC so that while one is discharging into the SLA the other is charging from the solar panel.As seen in the Falstad screenshot (where the solar panel is idealized as a current source), using a DPDT relay works nicely for this idea. There is no direct connection between solar panel and battery, and yet the solar panel is always charging and the battery is always being charged.The ESP32 would be powered by the solar panel side of the circuit, so at nighttime when there's no power generation, or if there's a firmware bug which causes unexpected issues, the SLA is isolated by the relay and cannot be discharged by the ESP32. The only way the battery could be harmed is if the LiC shorts internally, and this can be addressed by having a blocking diode.Ignoring all the niceties about MPPT, ESP32 power consumption, supercap voltage rating and self-discharge, fuses, relay speed, relay energy cost, etc..., is this idea at its core valid? Is there something which would surprise me, or is this KISS system both isolating and high-safety in that it can never overcharge or overdischarge the SLA?Could this be done equally safely with a solid-state approach, e.g. MOSFETs instead of a relays?P.S. As much as I appreciate outside the box thinking and X-Y problems, that's a little outside the scope of my question. For my personal edification and fun I'd love to know if the proposed core approach has a critical drawback I'm overlooking.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2023, 12:45:45 pm
content: Use a solar panel with an open circuit voltage of less than 14V and a boost converter. Then have a failsafe cutout for the boost converter set to 14.4V.

author: kubark42
author_rank: Contributor
timestamp: September 15, 2023, 01:10:28 pm
content: That's a good workaround, and I wish it were an option. But for this problem the solar panel and the 12V SLA are already selected, and the solar panel's open-circuit is around 20V.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 16, 2023, 12:50:15 am
content: Quote from: kubark42 on September 15, 2023, 10:40:39 amI'm not quite following the logic here ?Just because you removed a 'direct connection' you can still overcharge or overdischarge, it depends entirely on how you manage the energy flows.What is the solar cell rating - you label 10mA, but that seems tiny ?You can have a simple shunt regulator to avoid over-charge and a battery-voltage sense to avoid over discharge.

author: moffy
author_rank: Super Contributor
timestamp: September 16, 2023, 02:45:48 am
content: If you want it isolated, use a transformer, but isolated from what? The Solar panel is fully floating, maybe what you mean is blocking so that power doesn't flow from the SLA to the ESP32, a diode will do that.

author: RLP
author_rank: Contributor
timestamp: September 16, 2023, 02:59:17 am
content: Conceptually I see one main issue which is that the supercapacitors will behave as voltage sources, and so will the battery. Putting the supercaps in parallel with the battery to charge it, is therefore putting different voltage sources in parallel. This is problematic because nothing controls the current flowing between them, except the voltage difference and the internal resistances of the various components (V=IR basically). So if the supercap is charged to a much higher voltage than the battery (e.g. 20V, if that's the solar open circuit voltage), then when you switch the relay there will be a huge current surge as the supercap drains/battery charges. I can't say exactly how big, but this likely isn't good for the components! It would be fine for charging though, since the solar panel is a current source so it can charge the supercaps to its open circuit voltage, or less if you switch it off before that point.Also, the supercaps can only deliver energy to the battery until their voltages fall to the battery's open circuit voltage, then the voltage difference is zero and no current will flow. The supercap will still be charged to 12V or thereabouts at this point, so actually lots of energy left in it - but you're not able to make use of that. So that would reduce the usable energy storage in the supercaps.I guess you could put a resistor in series with the relay to limit the capacitor to battery current, if as you say efficiency isn't important. But if you have a solar panel which is 20V open circuit... I would say a simple buck converter with appropriate under/overvoltage protections would work - and could be controlled to achieve MPPT as well!

author: Someone
author_rank: Super Contributor
timestamp: September 16, 2023, 03:40:18 am
content: Quote from: kubark42 on September 15, 2023, 01:10:28 pmBoost converter across the solar panel, but reference the battery -ve to the high side of the panel (and +ve input of the boost regulator).

author: kubark42
author_rank: Contributor
timestamp: September 16, 2023, 03:58:01 am
content: Quote from: PCB.Wiz on September 16, 2023, 12:50:15 amI appreciate the comment, but I don't understand what you're saying. Unless I'm missing something, it would not be possible to overdischarge the battery, as the ESP32 is not in parallel with it. If there's no solar energy than the ESP32 can't run, which means that it cannot trigger the relay, so there's no way for the charger to move energy out of the battery.Quote from: PCB.Wiz on September 16, 2023, 12:50:15 amI would have hidden all the values from the Falstad screenshot if that would have been possible as the simulation is wildly incomplete for an overall system. It was only to show that a DPDT relay would have the desired effect of establishing an open-circuit as appropriate. The advantage of the DPDT is that it's all but impossible to have shoot-through.Shunt regulators and voltage sense can fail or be mishandled by firmware. One requirement for this design is it over a span of decades it must be (all but) guaranteed not to fail in such a way as to compromise the SLA. Another requirement is that no matter what I do, there's no possibility for a firmware bug to compromise the battery.Quote from: moffy on September 16, 2023, 02:45:48 amYes, I guess I was thinking of blocking and isolated as different things. If there's an open-circuit at all times, then in my mind this is isolated. But if the correct word is blocking then I'll happily use that instead.I'm concerned about a diode's leakage. After all, the solar panel is by its nature diodes, and they can drain batteries over a long enough period. Someone killed my sailboat's battery by putting its maintenance charging solar panel inside, and then leaving the boat that way for a winter.I know that in this schematic the relays will not leak no matter how old or abused is gets, which is comforting when designing something which is guaranteed not to have a leakage failure.Quote from: RLP on September 16, 2023, 02:59:17 amYou're exactly correct about the supercaps. That needs to be managed, possibly by a boost circuit, possibly by an inefficient direct dump strategy like you describe. Since my worst case scenario is a 50:1 ratio of charging time vs power draw, this inefficiency is fine.It's also worth pointing out that the average power flow from the caps to the battery will be equal to the average power flow into the caps from the solar panel, so it's fine to switch the caps when they're only a suitably small margin above the battery. The downside is the relay will switch more often, so that's something to consider. I'm happy to have a relay failure due to overuse, though, as if my approach is a good one a failed relay can never damage the system.Considering the potential extremely high cost of failure, I'm not keen to experiment with MPPT unless I can be guaranteed that the battery can never be hurt. I could buy a Genasun or other equally well-tested MPPT controller and get exactly what I want in terms of safety and reliability, and exceed the performance of anything I could build, but where's the fun in that?

author: kubark42
author_rank: Contributor
timestamp: September 16, 2023, 04:00:59 am
content: Quote from: Someone on September 16, 2023, 03:40:18 amFor reasons stated previously, I want to explore the topology I detailed in the OP, but that being said I'm really curious about what it is you're suggesting so this is worth a detour. I can't quite visualize how your suggestion would work; I know it's a fair bit of thread drift to ask, but do you have a link to a schematic?

author: moffy
author_rank: Super Contributor
timestamp: September 16, 2023, 04:50:59 am
content: Quote from: kubark42 on September 16, 2023, 03:58:01 amThe diodes leakage will be much less than the self discharge rate of the SLA battery. The breakdown voltage of a blocking diode will also be much higher than that for the solar cell.For example a UF4004 1A/400V diode has an absolute worst case reverse leakage of 0.05 mA @ 100C.P.S. It would take about 60 years for the worst case diode leakage to discharge your battery whereas self discharge is around 3%-5% per month.

author: kubark42
author_rank: Contributor
timestamp: September 16, 2023, 12:38:40 pm
content: Quote from: moffy on September 16, 2023, 04:50:59 amThis is great info, and I appreciate the learning, but I'd love to get to my core question about whether there are unforeseen risks inherent to my proposal. I'm not arguing your X-Y point, but I don't know much about diode aging when installed in the outdoors and I'm not keen to find out. There must be a solid reason why even new airplanes still use mechanical 50A contactors rather than 50A SSRs, so rather than rediscover that reason myself I should stick to what's tried and true.

author: moffy
author_rank: Super Contributor
timestamp: September 16, 2023, 01:14:07 pm
content: Quote from: kubark42 on September 16, 2023, 12:38:40 pmI would use a contactor/mechanical switch as an isolator i.e. completely break the circuit, but I would use a diode if I want to control power flow in one direction only. They are two separate purposes and uses not to be confused with each other. You should have an isolator switch for the battery but that doesn't preclude a diode to control power flow into and discharge from the battery