Adding grid power into PV system to 'shift' Economy 7 power to day time over winter.

Hi,
I have a small 'off-grid' solar PV system, 2 x 325W panels, Victron 100/20 MPPT charger, 4 x Exide AGM batteries giving me 24v and a Victron 800W Phoenix inverter.
This summer has been good for solar power, but now the solar power available is falling I'm looking at using the PV system to make better use of the Economy 7 electricity we have available. What I want to do is to charge the batteries overnight and make use of the power during the day, any solar power available will be a bonus. I can simply add a Victron grid powered charger in parallel with the MPPT charger, at about £170, but I already have a perfectly adequate charger in the Victron MPPT unit.
My engineering background tells me I should be able to connect a simple unregulated supply with a suitable current limit in parallel with the solar panels, a couple of 10amp power diodes can provide suitable isolation. I can build a nominal 60v unregulated supply with a 5amp current limit for a few pounds, which will give me ~300W of usable power. I'm assuming the MPPT charger would find the peak power point, i.e. just as the 5amp current limit kicks in.
Any one tried this, or got any experience with replacing the solar panels with a dc supply ? I'm wary of damaging the MPPT charger if, for instance, the MPPT algorithm doesn't function properly with a dc supply rather than solar panels.

Regards
PaulM

Will (who started the forum and youtube channel) frequently substitutes a power supply for solar panels when testing SCC and batteries.

In place of is fine. In parallel with, not to sure that would be good. The panels could take some of the power.

That would be the reason for including a couple of power diodes. If a forward biased diode is placed in series with each of the panel connection and power supply they can be connected together to the charger input. The one with the highest voltage will prevail while the other will be reverse biased and block any reverse current flow. It's a common way of providing redundant power from multiple supplies.

Thanks for the replies. I have a dual 30v 2amp bench power supply which will give me 60v at 2amps, which will be good enough to try this and see how it works under controlled conditions. I've ordered some 100v 10amp schottky diodes (lower forward voltage drop than standard rectifiers, hence lower power dissipation) to place in series with the panels and power supply. If this goes to plan I'll build a suitable 5amp power supply to fit permanently. I'll let you know how it goes.

It took a while but I've now got this working (see graph attached). I ended up using a chinese 60v 5A bench PSU as by the time I'd bought the components for an unregulated PSU (capacitors especially) and built it, it wouldn't have been much cheaper. The PSU is currently set at 50v 4A (200W), I turn it on just after midnight and leave it on until some time after the solar voltage takes over after daybreak, the schottky diodes handle the voltage switching. I have a DIY automatic mains contactor box that gives solar power priority so only need to turn on the inverter to make use of the power from the batteries.

I already have an Arduino monitoring the voltages and currents (which produced the attached graph via InfluxDB and Gafana) which will also control the PSU and inverter switching via a couple of relays, making the whole thing automatic. The plan is the Arduino will enable the PSU at midnight, then disable it when the battery is fully charged (~27.5v, <~1A charge current). The inverter will be turned on when the battery is charged and the solar voltage exceeds ~50v (daytime), then disable it when the battery voltage drops below ~24v, which should equate to about the half power point to maximise the life of the batteries. No doubt it'll need some tweaking to get the most from the system, and probably a different scheme during the summer when solar power is more abundant, but at present it's fulfilling my goal of making better use of the Economy 7 tariff.

At present it's only powering a few Raspberry Pis that run my home monitoring system, my media system and my desktop computer but intend to scale it up to include the house lighting and other low(ish) powered systems eventually.

PaulM

You could also consider

A Victron charger

Setup ESS on a Pi

That allows you to use schedules and VRM

@PaulM47 in my small solar power plant, I connected the power supply in parallel to the mppt controller to the battery, the power supply setting is 13.1 volts, the mppt setting is 13.6 volts

when there is little solar energy and the voltage on the battery drops to 13.1 volts, the power supply starts to help the sun and gives out exactly as much as the load currently requires, within its power of 30 amperes

I have done a little playing around feeding AC to dc supply in to the PV input,
its hard to keep the mppt unit from trying to run at it's max power when doing this.
so it's going to warm up pretty fast most of the time.