DIY Powerwall / Battery

[Sideways]

I'm sure youre aware of just how well DC can sustain an arc and how difficult it can be to break it (air blast circuit breakers in the steelworks LOL). Having 300v+ with a few thousand amps behind needs absolute care and respect.


Feb 2022 I was still plucking up courage to buy the parts LOL, it was nothing more than a thought in my head and sick feeling in my stomach, I did just flip one day on impulse and bought all of the parts then had the 'OMG, What have I done' moment just after.

My big maxeon solar panels make over 70V apiece and in strings of 8 they actually exceeded the real input voltage limit of my GEN24 inverter (supposedly 600V), so we had to reconfigure some of the strings.
The old tech 3kW inverter however is quite happy with 600V.
I know it's not the same as there's nowhere near the same ability to deliver current, but 600V would do more than make your eyes water !

That feeling sounds very familiar to me. Each round of searching and selecting a set of components seemed to be met with logistical problems and prices just crept up with fancier panels, 2 inverters instead of one, and £££ for the battery. In the end you bite the bullet.

In the weeks after we installed the basic PV without the storage (April 2022), Fronius emptied their warehouse and lead time for inverters went out to about 18 months.

 

[johnb80]

My big maxeon solar panels make over 70V apiece and in strings of 8 they actually exceeded the real input voltage limit of my GEN24 inverter (supposedly 600V), so we had to reconfigure some of the strings.
The old tech 3kW inverter however is quite happy with 600V.
I know it's not the same as there's nowhere near the same ability to deliver current, but 600V would do more than make your eyes water !

Aye the 600v will hurt, the danger with the high voltage and high current from the batteries is of course the ability to cause real damage in the event of a short or earth leakage. I'm comfortable with my 50v, that only gives a bit a tingle LOL. I do have a video somewhere from EVE where they short out a 280 Ah battery, IIRC, 57,000 amps, the 13mm metal bar they used glowed white in a very short space of time, there was a lot of gassing from the cell, it swelled and blew the pressure relief fuse but no fire resulted. The rate at which the temperature in bar shot up was scary, if it had been a spanner in your hand I'm sure it would have stuck to your skin.

That feeling sounds very familiar to me. Each round of searching and selecting a set of components seemed to be met with logistical problems and prices just crept up with fancier panels, 2 inverters instead of one, and £££ for the battery. In the end you bite the bullet.

It really was sleepless night stuff for a while.

In the weeks after we installed the basic PV without the storage (April 2022), Fronius emptied their warehouse and lead time for inverters went out to about 18 months.

Sounds like you had perfect timing there.

 

[throbscottle]

Bought house with solar panels, they are about 12yo so mid-life. We talked about getting batteries at the beginning but commercial systems prohibitively expensive, but it's still something we'd like - so this looks like a much more affordable option.
So anyway, I had a quick look at 280Ah EVE batteries - still £99 at Fogstar, but there are also some Envision 280Ah batteries, B grade, only 6000 cycles in them but also only £65.
Is the £544 saved false economy?

 

[johnb80]

Bought house with solar panels, they are about 12yo so mid-life. We talked about getting batteries at the beginning but commercial systems prohibitively expensive, but it's still something we'd like - so this looks like a much more affordable option.
So anyway, I had a quick look at 280Ah EVE batteries - still £99 at Fogstar, but there are also some Envision 280Ah batteries, B grade, only 6000 cycles in them but also only £65.
Is the £544 saved false economy?

John Ruskin Said:
“It's unwise to pay too much, but it's worse to pay too little. When
you pay too much, you lose a little money - that's all. When you pay
too little, you sometimes lose everything, because the thing you
bought was incapable of doing the thing it was bought to do. The
common law of business balance prohibits paying a little and getting a
lot - it can't be done. If you deal with the lowest bidder, it is well
to add something for the risk you run, and if you do that you will
have enough to pay for something better.”

All of my life I have always bought the best I can afford, on the few occasions when I havent I'm troubled wondering if the more expensive item would have been better, sleepless night stuff LOL.

There is quite a price difference and all I will say is it is that way for a reason. Probably not mission critical and maybe the saving is worth it. Only you can decide.

J

 

[throbscottle]

Hmm, Ruskin and you both echo my own thoughts. Trouble is, lack of budget!

 

[johnb80]

Hmm, Ruskin and you both echo my own thoughts. Trouble is, lack of budget!

Yep, been there for sure.

FWIW, grade B cells dont make grade A because they have a higher internal resistance and/or lower capacity. None is really critical for home battery system and a decent active balancer can keep it all running nicely. You will probably experience a slightly lower capacity overall..

Have a read at this:-

Whats Grade B All About

I think it will put your mind at ease.

J
PS If you want a Victron Multiplus II GX 5000w Inverter, DM me, I have one for sale.

 

[Sideways]

Question for @johnb80 as your researches into DIY storage may have given you better info than mine.
Having gambled more money on a commercial battery that probably has lower performance cells in it than yours, I'm keen to maximise the life of what I bought.
16.6kWh battery is currently set to cycle between 20% minimum and 95% max.
That keeps the state of charge away from the extremes and still leaves me 12kWh of usable capacity.
Ample for most of the year.
In fact, for 6 months + we top up to 95% daily and rarely fall below 50%
In winter, we want to use most of our capacity as one day of sunshine may carry us through a couple of nights and a day of cloud and rain.

Have you come across any good sources of advice for setting charge limits that maximise battery life ? Including perhaps one set of limits when short cycling the battery in summer and a different set of limits for working it harder in winter without pushing too hard ?

Cheers

 

[Spectric]

What always seems strange is that we aim to generate a "mains" voltage from which we then convert down to other voltages and although modern conversion is very efficient it is really just making work. Eg we have a source of Dc from solar panels and use an invertor to obtain mains voltage and then in our LED lights there is another power supply that takes the mains and converts to a lower Dc voltage so why not just use low voltage LED lighting in the first place which could also easily have battery backup ?

 

[John Brown]

What always seems strange is that we aim to generate a "mains" voltage from which we then convert down to other voltages and although modern conversion is very efficient it is really just making work. Eg we have a source of Dc from solar panels and use an invertor to obtain mains voltage and then in our LED lights there is another power supply that takes the mains and converts to a lower Dc voltage so why not just use low voltage LED lighting in the first place which could also easily have battery backup ?

That would involve two separate circuits. I do have 24v LED lights in my solar powered shed, but to be honest, I never use them.
Can you get low voltage tumble dryers or induction hobs? I doubt it, but if you can they'll be made in such tiny numbers as to make them super expensive. Added to which the cabling would be rather heavy.

 

[johnb80]

Question for @johnb80 as your researches into DIY storage may have given you better info than mine.
Having gambled more money on a commercial battery that probably has lower performance cells in it than yours, I'm keen to maximise the life of what I bought.
16.6kWh battery is currently set to cycle between 20% minimum and 95% max.
That keeps the state of charge away from the extremes and still leaves me 12kWh of usable capacity.
Ample for most of the year.
In fact, for 6 months + we top up to 95% daily and rarely fall below 50%
In winter, we want to use most of our capacity as one day of sunshine may carry us through a couple of nights and a day of cloud and rain.

Have you come across any good sources of advice for setting charge limits that maximise battery life ? Including perhaps one set of limits when short cycling the battery in summer and a different set of limits for working it harder in winter without pushing too hard ?

Cheers

What I have found out is the 100% at the top only makes a small difference to longevity of the cells and it's only whilst it is at 100%. As soon as you start taking energy from it the damaging process stops. I charge mine up to 95% and solar then quickly takes it to 100%, I havent figured out why that happens yet, I set the charge limit to 95% on the scheduled charge but I cant find a way to stop it taking energy in when it sees an export taking place.

 

[johnb80]

What always seems strange is that we aim to generate a "mains" voltage from which we then convert down to other voltages and although modern conversion is very efficient it is really just making work. Eg we have a source of Dc from solar panels and use an invertor to obtain mains voltage and then in our LED lights there is another power supply that takes the mains and converts to a lower Dc voltage so why not just use low voltage LED lighting in the first place which could also easily have battery backup ?

There would be greater losses in the wiring, the need for thicker cables and frankly, it's not worth it. SWMPSU's are so efficient the losses are minimal

 

[Sideways]

DC-DC buck and boost converters are pretty cheap and efficient.
Laptop and many electronic power supplies have a wide input voltage range.
They have a bridge rectifier on the input but would still work if fed from DC in their band.
A DC supply at 48, even 60 to 70 volts for example might be useful distributed throughout a home.
High enough to be useful and to keep the amperage modest, enough to tingle without being too hazardous.
I have a telco 48V / 30 amp power supply that is about 98% efficient. It could notionally supply a 30A DC ring without wasting any more heat than a 100W lightbulb. I can't imagine running more than 1.5kW of electronics in a house or workshop. You wouldn't use it to power heating loads.

 

[Sideways]

This is part of the argument in favour of hybrid inverters. Staying DC between the inverter and battery saves a percent or two. It's not really enought to worry about but technically there is a benefit.

 

[throbscottle]

Yep, been there for sure.

FWIW, grade B cells dont make grade A because they have a higher internal resistance and/or lower capacity. None is really critical for home battery system and a decent active balancer can keep it all running nicely. You will probably experience a slightly lower capacity overall..

Have a read at this:-

Whats Grade B All About

I think it will put your mind at ease.

J
PS If you want a Victron Multiplus II GX 5000w Inverter, DM me, I have one for sale.

Well that's quite mind-easing! And a very enlightening link, thank you. By the time I even look at inverters I suspect yours will be long gone, but thanks anyway.

 

[Sideways]

What I have found out is the 100% at the top only makes a small difference to longevity of the cells and it's only whilst it is at 100%. As soon as you start taking energy from it the damaging process stops. I charge mine up to 95% and solar then quickly takes it to 100%, I havent figured out why that happens yet, I set the charge limit to 95% on the scheduled charge but I cant find a way to stop it taking energy in when it sees an export taking place.

That makes good sense.

I've been refererencing info like this found through
https://secondlifestorage.com/index.php

20% to 95% SOC keeps you very well away from the bends at each end of the batteries charachteristic curve, but the real distinct bends in the curve are above 99 and below 15%

 

[johnb80]

That makes good sense.

I've been refererencing info like this found through
https://secondlifestorage.com/index.php

20% to 95% SOC keeps you very well away from the bends at each end of the batteries charachteristic curve, but the real distinct bends in the curve are above 99 and below 15%

View attachment 186436

I wonder why the dip and kick in cell voltage shown between 99 and 100% isnt reflected in the curves for 12, 24 and 48v packs?

20% and 95% does keep you away from either end of the discharge curve but it has little to do with the life of the cell. Time above 80% is the start of the accelerated deterioration, but it's almost insignificant anyway. IMHO theres more merit in monitoring temperature to detect when 100% is being exceeded, cell voltage is not a reliable indicator. In my system, the charge current drops to around 2 amps when 95% is exceeded and temperature monitored.

 

[Sideways]

This is typical enough of a day in the life of our system throughout the better 6 months.
As you see we don't work the battery hard at all because we have plenty of PV, and the battery does spend a lot of time up in the 90%'s.
It may make little difference, but it only takes a couple of minutes to change the max charge threshold and if I drop that to 80% then we'll just cycle between 50 and 80% instead of 65 and 95.
Just as easy to tweak it back up for winter to ensure it can take a full charge when that's available and the battery spends more time gradually discharging to the lower limit ....
Thanks

 

[johnb80]

View attachment 186437
This is typical enough of a day in the life of our system throughout the better 6 months.
As you see we don't work the battery hard at all because we have plenty of PV, and the battery does spend a lot of time up in the 90%'s.
It may make little difference, but it only takes a couple of minutes to change the max charge threshold and if I drop that to 80% then we'll just cycle between 50 and 80% instead of 65 and 95.
Just as easy to tweak it back up for winter to ensure it can take a full charge when that's available and the battery spends more time gradually discharging to the lower limit ....
Thanks

Absolutely agree, no problem at all. Can you see the BMS parameters? Some won’t start balancing until a threshold voltage is reached, typically around 90% mark 3.35v or thereabouts.