Hi, I wonder if the smartsolar mppt (75/15, 100/20) support the inverter with solarpower even if the inverter is connected directly to the battery. This would save the battery's aging a bit in terms of cycle life of a lifepo4 battery. I just ask because i heard that the load-output is always synced to battery-Voltage so that solar power is used first if there is any available, so maybe for the bat-connection it would be the same.
Thanks
Hi @keves
I believe it will just by the nature of how power is being drawn from the system. The inverter isnt really choosing whether the power is coming from battery or solar. It's just drawing power from the DC system as a whole.
So if solar is providing 100w and inverter is using 100w. The 100w of solar isn't going into the battery and then back out. It's going from solar to inverter.
That being said you will need the battery connected as a buffer. ie if solar goes down the inverter will need power to sustain itself.
Hi @matt1309
Generally you are right, if there are 2 active power sources in a circuit. But if the bat-conection would be switched on by a mosfet just when charging, and else it would be switched off, then there would not be this 2nd power source in the circuit.
But as told i heard that on the load conection the voltage is synced with battery voltage by algorythm, thus maybe the bat-conection is as well "always-on" and voltage-synced and so solar power would be drawn as well in this case. But as both power sources would have same voltage, the consumer draws just 50% of the energy needed from solar power, but still better than 0.
Hi @keves
I think i see the issue, it's the word sync...
(I also stressed about this for ages but this is how i understand it now). Although the battery will balance the difference between incoming and outgoing it's not cycling like you suggest:
When the MPPT is charging (and not in float/absorption). It's said to sync with battery voltage however that doesn't mean it outputs exactly battery voltage.
If it did no current would flow from the charge controller to anywhere (as you need voltage difference for current to flow).
For the MPPT to output power to the system it would have to output voltage above battery voltage. You can see this when you have solar coming in, the battery voltage goes up but if you disconnect the MPPT it goes back down. The MPPT is keeping the voltage higher and hence current is flowing into the battery.
When you hit absorption phase this changes as the voltage is kept constant (it's maximum). In this scenario you can see current drop over time as the difference in voltage between battery what's output by MPPT decreases.
That make sense?
So in the scenario you proposed initially what is really happening is the mppt is increasing the system voltage as power's coming in. The inverter is decreasing the voltage as power going out. Assuming they are equal and opposite the solar will go straight to the inverter and not touch the battery.
If they're not equal and opposite the battery will balance the difference (unless you're in absorption phase then if there's an increase in inverter draw which would start to decrease the system voltage the MPPT would increase it's power output to to keep voltage at absorption voltage if there was sufficient PV)
Hope this helps.
EDIT: i thought about your mosfet suggestion more. ie battery disconnects when PV = load. I imagine in theory if you have one unit (so communication between inverter, battery mosfet and mppt was fast enough). You could do this, and have battery fully disconnected from load. However you'd have to have fast enough switching on of mosfet to avoid inverter running out of power to run if pv < load. Similarly if pv > load you'd want to turn on mosfet (and may have some dynamic inrush protection on the battery for the instant increase in power coming in).
And then you'd have all this additional code/switching but in theory just by the voltages you wouldnt have the current flowing if there was no swtich unless there was excess power.
