Electronics > Mechanical & Automation Engineering

DC Failover for Solar Power

(1/3) > >>

Lukas52:
Hello!

Im a bit stuck at my biggest project so far.

I am currently installing some solar panels, with the goal being to power my home servers and 3D printers straight of my massive 24 Volt LiFePo4 Battery. I already figured out the charging, the BMS and the power distribution plus a lot of safety stuff.
What prevents me from sleeping at night is the question: "What happens if my battery gets empty". Theoretically i don't have enough solar power during the winter months, so its a real problem.
I have come up with two plans, but don't know which one is the "proper" of doing it...

Plan A:
Use a AC powered battery charger that gets switched on via a relay if the battery drops bellow 20%. Possible by using a programmable output on my solar chargers, but I'm not sure if that stays on when there is no solar power... (I am planning in using two Victron MPPT 250/70). The efficiency would be kind of bad tho and could possibly put unnecessary cycles on the battery, also it would need to be a pretty big charger.

Plan B:
Use 24 Volt DC Power Supplies. Switched using the same programmable output as in plan A. Problem with that: I am struggling to find high current DC relays that don't cost a fortune, i would also need to add considerable capacitance to prevent the voltage from dropping to far down during switching, so i don't overwhelm my power supplies. 

How is this done in commercial installations?
Any recommendations?

Thank you :)

rteodor:
One simple and reliable option is to use 2 beefy diodes: one from the LFP battery, another from a mains power supply.
Use a mains PSU with tunable voltage and set it to ~25V (that is for 3.1V/cell). In this way you have a soft power drain transition from battery to mains. Cheap solution and works satisfactory if done right. The BMS should protect the battery when it is empty and there is no mains.

The problem with this solution is that you have no automated control of the transition. You need the SoC value, either from BMS or from a Victron Smart Shunt or something similar. With that you can automate turning turning ON/OFF the mains PSU so you can properly control the remaining SoC (in case you want to control the depth of discharge and/or keep reserve for power outage).

Avoid high current relays especially for continuous use. Leave them for battery protection only and use MOSFETs. Or use power switches (Victron, 123Electric, etc.) yea, yea expensive ... I know.

Lukas52:
That is elegantly simple!

Would i even need control over the remaining battery capacity?
If i understand this correctly the diodes would prevent reverse charging, so all i need to do is turn the AC PSU on using the programmable relay on the victron charger, and do some math regarding voltage drop at the diode and cell voltage.

For example: The Cells im using a rated for 6000 Cycles when charged between 3,65 and 2,5 Volt. I set my BMS to be a bit gentler to hopefully squeeze some more life out of them by using 3,50 Volt and 2,7 Volt instead. Should still give me 80-85% capacity.
To keep some reserve i would want to switch over at say 3 - 3,1 Volt. That would be 24 or 24,8 Volt, add too that the Diode Vfr of 600 mV -> Set PSU to ~25,2 Volt.

As soon as my battery voltage goes bellow this limit the diode should prevent current to flow from the battery shouldn't it? Since the rail Voltage would be higher thanks to the power supply than the battery voltage.

The only problem i can see is a scenario where the sun comes up on a cloudy day, causing a lot of switching once the battery voltage rises again...

Maybe a combination of both methods would make the most sense... as in disconnecting the battery and Solar Chargers using something like the 123\PowerSwitch until the battery has recovered a bit.
I would probably need two of those, assuming i can put them in parallel (unlike mechanical relays).


Is there something wrong with using my original Plan B, but with 123\PowerSwitch (es) instead of relays? It would eliminate the diodes as constant power draw (@ 0,6 Volt drop they would eat up almost 50 kWh per year). Capacity is a bit of an unknown to me. In higher voltages i would be very careful, but at 24 Volts the worse that can happen (in my mind) is some accidental short results in some welding, and potentially a tiny bit of fire. But i do have fuses in everything, including main battery fuses, wire protecting fuses for the distribution layer and another layer of fuses for the actual loads themselves.

rteodor:

--- Quote from: Lukas52 ---Would i even need control over the remaining battery capacity?
--- End quote ---
I can not tell you from experience that you need the SoC because I do this thing with diodes and lead-acid batteries. That works very well but LFP is different.

From what I know about LFP you would get 6k cycles when cycled at 50% DoD (depth of discharge). But it really depends on your specific goals, requirements and cells specifications.

My example: hybrid installation with 5KWh LFP. I am cycling it at ~50% because of two reasons: (1) to get reasonable cycles and (2) have some reserve for occasional mains power loss during night storms.


--- Quote from: Lukas52 ---If i understand this correctly the diodes would prevent reverse charging
--- End quote ---
Yes that is their purpose.


--- Quote from: Lukas52 ---To keep some reserve i would want to switch over at say 3 - 3,1 Volt. That would be 24 or 24,8 Volt, add too that the Diode Vfr of 600 mV -> Set PSU to ~25,2 Volt.
--- End quote ---
Two remarks here:

* 1. Schottky diodes have lower Vfr depending on current. What diodes I use, have 0.1...0.6V. That makes for a smooth transition of power consumption when both sources are available.
So the diodes works also as equalizers, they will have variable Vfr depending on the current consumption on each branch. Unless the loads have a problem with that Vfr drop, I would not factor in the Vfr but consider the voltages before the diodes. It keeps things simple.
* 2. 3-3.1 V could mean 5% or it could mean 20%. You could be fine with that large variation or go back to the problem of getting/measuring SoC.

--- Quote from: Lukas52 ---As soon as my battery voltage goes bellow this limit the diode should prevent current to flow from the battery shouldn't it? Since the rail Voltage would be higher thanks to the power supply than the battery voltage.
--- End quote ---
As mentioned above, it will be a smooth transition: current from battery branch decreases and current from PSU branch increases.
That is still the case but in oposite direction when the Sun comes from behind the clouds.


--- Quote from: Lukas52 ---Is there something wrong with using my original Plan B, but with 123\PowerSwitch (es) instead of relays?
--- End quote ---
Technically both of your plans are correct but ...
... with Plan A you can not maximize the use of solar
... with Plan B (assuming the use of 123\PowerSwitch-es) the cost is quite high. (BTW: if my memory is right they mention on their site that you could paralel them). 50[KWh/year] that you have calculated * 0.4[EUR/KWh] = 20 EUR per year. One 123\PowerSwitch is ~120EUR+VAT.

With diodes you can turn off the PSU on the AC side (with a relay or better, a SSR) and avoid the 123\PowerSwitch altogether.
I do not know how well it would work to command the SSR from MPPT charger because the charger does not know the SoC. You might get too much on-off cycling on the PSU ....
The problems I see in your setup are this:
* (1) getting a proper 24V tunable voltage PSU,
* (2) choosing proper diodes and
* (3) more complex: getting the SoC and use it to optimize cycling of the battery and reduce idle/redundant power consumption from the PSU.
Many things to consider but I hope it helps.

rteodor:
Something to clarify: I mentioned the power switches because I did not want to hang to the diodes idea not knowing your project specifics. Also I wanted to point out to whoever might read this thread, that mechanical switches are better for the abnormal use cases (protection) while the solid-state power switches are better used for normal use case switching.

Navigation

[0] Message Index

[#] Next page