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diy solar

High DC voltage PV array to Battery

EcoWorrier

New Member
Joined
Mar 11, 2022
Messages
3
Hi all,
I have a 85 panel array on my house roof, south oriented at optimal angle for total annual production with 230 Wp, 37Voc panels. Total size 19.55KWp.
The panels are roof-mounted (and therefore not easy to re-configure) and currently arranged in 4 (I guess 22 or 21 panel) strings feeding to two Kostal grid tied inverters. I'm 10 years into a 15 year FIT with a rate that is (currently) above market rate. I live in Luxembourg where I export ALL production (about 21MWh per year) through an export meter at the FIT rate and import my own house consumption (11MWh + an extra 4MWh expected when an EV arrives in October) through a separate import meter at market rate (currently about 60% of the FIT tariff). There are no great time of use tariffs here - only a slightly cheaper night tariff.

I'm thinking about how I can take one string and use it to charge some 48V LiFePo4 batteries for house/emergency/ EV charging at night. My problem is the high string voltage from the serial connection. The smallest string would be 21 panels x 37Voc = 777Voc. All the Victron 48V chargers say they can use an input maximum of 8 times the float voltage (54?) which comes to 432V. SMA components show 600V maximum input.

My choices seem to be:
1) Take the highest Voltage battery charger I can find and be prepared for it to disconnect daily in summer when the input voltage goes too high.
2) Somehow down-step the DC voltage to within accepted range
3) Re-config the string from serial to serial + parallel (not easy because they are roof mounted at an angle and at height and would need some scaffolding)
4) Buy some new 320Wp (current tech) panels and make a new DIY array in the garden tied to Victron components and some big LiFePo4 batteries.

4 might be easiest but is the most expensive.

Your thoughts would be appreciated
 
1) Take the highest Voltage battery charger I can find and be prepared for it to disconnect daily in summer when the input voltage goes too high.
This will kill equipment in a hurry. Not an option.
2) Somehow down-step the DC voltage to within accepted range
This size of a converter will be cost prohibitive (probably more than #4)
3) Re-config the string from serial to serial + parallel (not easy because they are roof mounted at an angle and at height and would need some scaffolding)
This sounds like the most practical on paper. If you could reach the midpoint of any of the 4 strings, combining in parallel there would be a simple connection (combiner box).
 
Many thanks for the replies.
Option 4 it is... I might add some small wind to the new array in the garden to cover the baseload of the house as much as possible. If the noise does not upset the neighbours too much.
 
Why must you use the direct solar array dc voltage to charge your battery ?

Why not charge from the inverter ac output via a "normal" mains powered battery charger.
Let the grid tie inverter do all the voltage reduction.
 
Thanks Warpspeed.
With no grid (blackout conditions - which is currently rare) the inverters shut down (anti-islanding) so I would get zero production if there was a grid problem on a sunny day.

Currently I export all production through my export meter on a higher tariff and import all consumption through an import meter on a lower tariff. If the (floating) import tariff rises above the (fixed) export tariff or they cancel the export tariff (the contract lapses in 5 years also), I want to avoid the meters altogether by using one of the 4 strings myself on the house side of the meters to charge a battery/EV.
 
You could still charge your battery from imported grid power under normal conditions.
When the grid blacks out, your battery will be fully charged, and it should be possible to survive off that for a while until grid power is restored, even if it requires a second smaller inverter to do it.

Longer term, it should be possible to rewire one string with suitable switching to either work in full series assisting the other strings, or work independently at a much lower dc voltage to power a conventional off grid system.
 
Longer term, it should be possible to rewire one string with suitable switching to either work in full series assisting the other strings, or work independently at a much lower dc voltage to power a conventional off grid system.
I looked into this and decided a couple cheap panels to carry out of the shed/garage was easier (and cheap). And i would be able to move panels to follow the sun to maximize solar harvest if i needed more power.
 
That sounds like a much better idea.
For a backup when the grid goes down, if its only used for that, a very few panels could charge a mighty large battery .... slowly.
 
Hi all,
I have a 85 panel array on my house roof, south oriented at optimal angle for total annual production with 230 Wp, 37Voc panels. Total size 19.55KWp.
The panels are roof-mounted (and therefore not easy to re-configure) and currently arranged in 4 (I guess 22 or 21 panel) strings feeding to two Kostal grid tied inverters. I'm 10 years into a 15 year FIT with a rate that is (currently) above market rate. I live in Luxembourg where I export ALL production (about 21MWh per year) through an export meter at the FIT rate and import my own house consumption (11MWh + an extra 4MWh expected when an EV arrives in October) through a separate import meter at market rate (currently about 60% of the FIT tariff). There are no great time of use tariffs here - only a slightly cheaper night tariff.

I'm thinking about how I can take one string and use it to charge some 48V LiFePo4 batteries for house/emergency/ EV charging at night. My problem is the high string voltage from the serial connection. The smallest string would be 21 panels x 37Voc = 777Voc. All the Victron 48V chargers say they can use an input maximum of 8 times the float voltage (54?) which comes to 432V. SMA components show 600V maximum input.

My choices seem to be:
1) Take the highest Voltage battery charger I can find and be prepared for it to disconnect daily in summer when the input voltage goes too high.
2) Somehow down-step the DC voltage to within accepted range
3) Re-config the string from serial to serial + parallel (not easy because they are roof mounted at an angle and at height and would need some scaffolding)
4) Buy some new 320Wp (current tech) panels and make a new DIY array in the garden tied to Victron components and some big LiFePo4 batteries.

4 might be easiest but is the most expensive.

Your thoughts would be appreciated
Why not AC coupling?
 
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