House Supply With Battery And Solar
I embarked on my solar and battery path when energy prices went ballistic in 2021, my contract had 6 months remaining my energy supplier informed me that my monthly payment of £178 would rise to around £530. There was no way I would (or could afford) such an increase, it would effectively take my state pension and more. I had looked at solar and battery before, but the figures didn’t make it very attractive with a break even point a long way in the future, now with the imminent price rise it would be a very different situation.
I embarked on the road to getting solar and battery installed, I ruled out DIY at this stage because an MCS certificate was needed if I wanted to export excess energy into the grid. Cutting a long story short, my system comprised of 7kWh battery, 6kW inverter and 6.7 kW of panels split across South facing and West facing roofs.
The system worked well but had serious shortcomings which I discovered after a short time, the 7kWh of battery storage wasn’t enough and the 6kW inverter was fine until it was running on battery only then the output was reduced to 3kW which was a constant frustration.
I looked at solutions, bigger inverter, additional battery storage to existing system etc, all were very expensive and didn’t tick all of the boxes. I came across the concept of an AC Coupled battery, a system that was connected to my house supply, had it’s own inverter and ‘x’ amount of battery storage. Would it work with my existing inverter? Nobody would commit and the systems were still expensive (£12,000 for 10 kWh AC coupled battery with 3kW inverter built in). I spent a lot of time on DIY forums and there are a lot of very knowledgeable people in this area of interest. There are real enthusiasts on Victron forums and the Victron equipment is amazing. So, after months of research I bit the bullet and started to make my own AC coupled battery (often termed powerwall).
How does it all work
The power to our homes usually comes from the grid via overhead or underground cables. The cables feed into your electricity meter and the meter then feeds your consumer unit (fusebox) where the supplies to circuits are individually protected to a suitable level eg 6 amp lighting, 32 amp sockets, 50 amp cooker etc. When we add solar and/or battery we add an inverter to this. The inverter is a clever box of tricks that essentially converts DC (Direct Current) from the batteries or solar into AC (Alternating current) suitable for distribution round our homes.
The AC mains in our homes is in fact a sinewave, the Live conductor goes from zero to around 335v, back down to zero and the to -335v, then returning to zero, it does this following a sinewave pattern and in the UK 50 times per second (50Hz).
The installation of this is very simple, Live, Neutral and Earth to the inverter, battery supply onto the inverter and the magic begins.
The Inverter and it’s magic
The inverter does all of the clever stuff, first of all it synchronises its output to match the AC 50 Hz waveform of the incoming supply, having done that it sets it’s output voltage to be exactly the same as the incoming supply i.e. +/- 335v sine wave . Then the clever operation begins, using information from the CT (Current Transformer) it can see if electricity is flowing into your property or out. If it sees energy being imported it will increase it’s voltage which in turn means it starts to supply energy to your house. It continues to increase it’s voltage until there is no current flow in or out (import or export) to/from your property, the whole system is in balance. As the situation changes, you switch on the kettle for example, the inverter sees an import happening so again increases it’s output voltage to reduce the import to zero. When the kettle switches off, the result will be an export starting to happen due to the kettles load being removed. The inverter once again adjusts it’s output voltage down until the export stops and the system is once again balanced. All of this takes place typically 50 times per second. The inverter can also be programmed that any excess energy you have can be exported, ignoring any battery at this stage, if your house load is being satisfied by the output from the inverter and it can see more energy is available from the panels, it will increase it’s output voltage and therefore start to push energy into the grid. Your meter will record this and you will receive payment / credit from whichever energy company you choose to export to (it doesn’t have to be the same company you import from).
Battery operation
A battery can be added to Inverters (more correctly termed Hybrid Inverters). The inverter works in just the same way as above using power from solar and/or battery. The additional functions needed or a battery charging system. The inverter can use solar or grid import to charge the battery and use the stored energy to supply your house or export to the grid. At the moment Octopus are charging 7p kWh for import during off peak and they pay 15p to export, there are additional export times during cold weather where the export amount can go as high as £4 per kWh. Money can be earned by charging off peak and selling peak. The inverters have settings that allow priorities eg charge the battery before exporting to grid, export to grid to a maximum of xx kW, charge the batteries between hh:mm and hh:mm. and so on, if it’s needed, it will be in there.
Battery
The actual battery consists of 2 major parts, the cells and the management system. The cells can be of various chemistry’s such as Lead Acid, AGM, LiPo, LifeP04 and more. There are many people making up storage solutions from scrap computer and powertool batteries, search youtube for more info on this. I chose LifeP04, it was one of the later chemistry’s, wasn’t as likely to go thermally unstable and offered a very good power to physical size ratio. I chose EVE cells, they were considered the very best at the time, high quality, good performance and a suggested 8000 cycles before their capacity degrades to 80%. It should be noted that 8000 cycles is 22 years assuming one full cycle every day. It should be good for my remaining life LOL. Each cell has a voltage of 3.2v a capacity of 280A/h and weighs a hefty 5.5Kg. Each cell has a capacity of 0.896 kWh. 16 cells were used to give a nominal working voltage of 50v with a capacity of 14.3 kWh.
Physical Build
I decided to make up battery modules in groups of 4 to make them easier to handle and convenient to make. I designed a battery tray to hold 4 batteries, have an Anderson connector for the heavy current and a smaller multipin connector for balancing (see later). The batteries physically had to be held in compression too, this was achieved with treaded rod between the ends of the modules. All of the batteries are connected in series i.e. +ve of the first battery connected to –ve of the second battery, +ve of the second battery connected to negative of the third and so on. Great care should be taken whilst doing this, the batteries have a very low internal resistance and have no difficulty is putting 40kW into a short circuit. Definitely insulate spanners etc whilst assembling. The output from the assembled cells is taked to the Anderson connector on the outside of the module, -ve from battery 1 –ve and the +ve from battery 4 +ve. This makes up a module of 12.8v @ 280A/h. The balance connections, a lead has to be taken from the battery 1 negative and every battery +ve so that the management system can monitor battery voltages and to balance them all.
Whats all this balancing malarkey
Each cell in a battery has slightly different characteristics in terms of capacity (how much energy it can store), internal resistance (determi9nes how much heat is made during charging and discharging) and internal leakage or self discharge.
LifeP04 cells have a strictly controlled maximum voltage of 3.65v during charging and an absolute minimum of 2.5v when discharging, exceed these parameters and it will rapidly cost you money. So, during charging the inverter will output a maximum of 3.65v x 16 = 58.4v. So it’s all dead easy, apply 58.4v the the battery will charge to 100%, nice idea but you cant do that. All of the batteries are in series and therefore the charging current passes through all of them equally. The cells with the lower capacity will reach 100% first and then go on to overcharge causing damage to the cells and further reduction in capacity and during discharge, the ones with lowest capacity will be at 0% first with further discharging taking place again damaging the cells. The weakest cells in this situation die very quickly and can under the wrong circumstances become a fire hazard. This is where another piece of equipment comes into play, the BMS (Battery Management System). This device is wired to every cell so it can see the voltage, it opens and closes charging and discharging routes so it can prevent damage and it communicates to the inverter to increase or decrease charging current and/or voltage. It’s an electronic doorman or bouncer controlling the electrons in or out of the battery nightclub. It also keeps a watchful eye on temperature, any hint of a thermal runaway starting, the batteries will be disconnected, LifeP04 should not be charged when the temperature is below zero, I have placed some heater mats under mine because theyre located outside and winter can see inside temperatures where the batteries are of -5 deg C. The BMS will switch off the charge or discharge if the temperature parameters are exceeded. During operation of the battery, charging or discharging, any cell whose terminal voltage is higher than the others gets picked on by the BMS, it discharger that cell slightly and gives the charge to the cell that has the lowest voltage. Parameters can be set of how close you want the voltages to be, mine are all within 5mV (5000ths of a volt).
Commercial batteries at the time were very expensive, in the region of £1000 per kWh storage. Looking around at the cost of the cells, they were much cheaper and better quality items could be sourced
That’s the technobabble done, here's the construction.
This is the physical battery module with 4 cells mounted in it. The MDF and acrylic cover were laser cut, the threaded rod to keep the cells in compression is sleeved in silicone tube. The large grey connector at the front is a 150 amp Anderson Connector which will carry the main current in and out of the battery, theyre robust and very reliable. The small circular connector to the right of it is the balance connections, a wire goes to each battery cell so that the individual cell voltages can be read by the BMS.
This is a finished module, the busbars connecting the cells together, the thick red (+ve) and black (-ve) are the main supply cables to/from the module. The Thinner wires are connected from the first negative to all of the positive cells are the balance connections.
The 4 modules, 4 cells in each making 16 cells in total are housed outside in the lower section of a modified toolshed (bought from Amazon and is just about the perfect size.
The heater matts can be seen under the battery modules, the Anderson connectors can be seen connected placing all 16 batteries in series. You may have noticed another multipole connector hanging from the wires, this is for some extra monitoring added into Home Assistant. Note the insulation to the sides of the shed and on the door to keep the temperature up in winter time.
The 16 cells across the top, the heavy current cables down to BMS BATT + & - and the balance lines down to the battery inputs B1 to B16.
The Seplos BMS I used came as a bare PCB and display, I created a stand to hold it all and support the cables. I also made a front panel to dress it up a little.
This is the rear view of the BMS with one of the battery cables in place to test the design.
This is the front panel, I used laser engraving laminate to make a very acceptable outward facing cover to the electronics behind..
The little shed that was housing it all now had the 4 batteries in the lower half, the top half needed the inverter and a few ancillary parts.
Shown here is the Victron Multiplus II GX Inverter, A small consumer unit, A large fuse for the DC supply from the batteries and a battery isolator switch. The two units at the right hand bottom side are temperature controllers, the top one switches controls the cooling fan seen at the top of the photograph, the lower one controls the heating mats under the batteries.
This is the Multiplus II GX, the heart of the system, it’s not the cheapest of inverters but it is in my opinion one of the very best. The flexibility of the programming, the support worldwide from enthusiasts and Victron themselves is second to none. The weight is unbelievable. Only two wires for the battery supply (+ and -), three wires for the mains (L,N & E), a data cable plugged into the BMS and the CT plugged in and that’s the inverter done.
Apologies for any errors or spelling mistakes etc.
Part II to follow
I embarked on my solar and battery path when energy prices went ballistic in 2021, my contract had 6 months remaining my energy supplier informed me that my monthly payment of £178 would rise to around £530. There was no way I would (or could afford) such an increase, it would effectively take my state pension and more. I had looked at solar and battery before, but the figures didn’t make it very attractive with a break even point a long way in the future, now with the imminent price rise it would be a very different situation.
I embarked on the road to getting solar and battery installed, I ruled out DIY at this stage because an MCS certificate was needed if I wanted to export excess energy into the grid. Cutting a long story short, my system comprised of 7kWh battery, 6kW inverter and 6.7 kW of panels split across South facing and West facing roofs.
The system worked well but had serious shortcomings which I discovered after a short time, the 7kWh of battery storage wasn’t enough and the 6kW inverter was fine until it was running on battery only then the output was reduced to 3kW which was a constant frustration.
I looked at solutions, bigger inverter, additional battery storage to existing system etc, all were very expensive and didn’t tick all of the boxes. I came across the concept of an AC Coupled battery, a system that was connected to my house supply, had it’s own inverter and ‘x’ amount of battery storage. Would it work with my existing inverter? Nobody would commit and the systems were still expensive (£12,000 for 10 kWh AC coupled battery with 3kW inverter built in). I spent a lot of time on DIY forums and there are a lot of very knowledgeable people in this area of interest. There are real enthusiasts on Victron forums and the Victron equipment is amazing. So, after months of research I bit the bullet and started to make my own AC coupled battery (often termed powerwall).
How does it all work
The power to our homes usually comes from the grid via overhead or underground cables. The cables feed into your electricity meter and the meter then feeds your consumer unit (fusebox) where the supplies to circuits are individually protected to a suitable level eg 6 amp lighting, 32 amp sockets, 50 amp cooker etc. When we add solar and/or battery we add an inverter to this. The inverter is a clever box of tricks that essentially converts DC (Direct Current) from the batteries or solar into AC (Alternating current) suitable for distribution round our homes.
The AC mains in our homes is in fact a sinewave, the Live conductor goes from zero to around 335v, back down to zero and the to -335v, then returning to zero, it does this following a sinewave pattern and in the UK 50 times per second (50Hz).
The installation of this is very simple, Live, Neutral and Earth to the inverter, battery supply onto the inverter and the magic begins.
The Inverter and it’s magic
The inverter does all of the clever stuff, first of all it synchronises its output to match the AC 50 Hz waveform of the incoming supply, having done that it sets it’s output voltage to be exactly the same as the incoming supply i.e. +/- 335v sine wave . Then the clever operation begins, using information from the CT (Current Transformer) it can see if electricity is flowing into your property or out. If it sees energy being imported it will increase it’s voltage which in turn means it starts to supply energy to your house. It continues to increase it’s voltage until there is no current flow in or out (import or export) to/from your property, the whole system is in balance. As the situation changes, you switch on the kettle for example, the inverter sees an import happening so again increases it’s output voltage to reduce the import to zero. When the kettle switches off, the result will be an export starting to happen due to the kettles load being removed. The inverter once again adjusts it’s output voltage down until the export stops and the system is once again balanced. All of this takes place typically 50 times per second. The inverter can also be programmed that any excess energy you have can be exported, ignoring any battery at this stage, if your house load is being satisfied by the output from the inverter and it can see more energy is available from the panels, it will increase it’s output voltage and therefore start to push energy into the grid. Your meter will record this and you will receive payment / credit from whichever energy company you choose to export to (it doesn’t have to be the same company you import from).
Battery operation
A battery can be added to Inverters (more correctly termed Hybrid Inverters). The inverter works in just the same way as above using power from solar and/or battery. The additional functions needed or a battery charging system. The inverter can use solar or grid import to charge the battery and use the stored energy to supply your house or export to the grid. At the moment Octopus are charging 7p kWh for import during off peak and they pay 15p to export, there are additional export times during cold weather where the export amount can go as high as £4 per kWh. Money can be earned by charging off peak and selling peak. The inverters have settings that allow priorities eg charge the battery before exporting to grid, export to grid to a maximum of xx kW, charge the batteries between hh:mm and hh:mm. and so on, if it’s needed, it will be in there.
Battery
The actual battery consists of 2 major parts, the cells and the management system. The cells can be of various chemistry’s such as Lead Acid, AGM, LiPo, LifeP04 and more. There are many people making up storage solutions from scrap computer and powertool batteries, search youtube for more info on this. I chose LifeP04, it was one of the later chemistry’s, wasn’t as likely to go thermally unstable and offered a very good power to physical size ratio. I chose EVE cells, they were considered the very best at the time, high quality, good performance and a suggested 8000 cycles before their capacity degrades to 80%. It should be noted that 8000 cycles is 22 years assuming one full cycle every day. It should be good for my remaining life LOL. Each cell has a voltage of 3.2v a capacity of 280A/h and weighs a hefty 5.5Kg. Each cell has a capacity of 0.896 kWh. 16 cells were used to give a nominal working voltage of 50v with a capacity of 14.3 kWh.
Physical Build
I decided to make up battery modules in groups of 4 to make them easier to handle and convenient to make. I designed a battery tray to hold 4 batteries, have an Anderson connector for the heavy current and a smaller multipin connector for balancing (see later). The batteries physically had to be held in compression too, this was achieved with treaded rod between the ends of the modules. All of the batteries are connected in series i.e. +ve of the first battery connected to –ve of the second battery, +ve of the second battery connected to negative of the third and so on. Great care should be taken whilst doing this, the batteries have a very low internal resistance and have no difficulty is putting 40kW into a short circuit. Definitely insulate spanners etc whilst assembling. The output from the assembled cells is taked to the Anderson connector on the outside of the module, -ve from battery 1 –ve and the +ve from battery 4 +ve. This makes up a module of 12.8v @ 280A/h. The balance connections, a lead has to be taken from the battery 1 negative and every battery +ve so that the management system can monitor battery voltages and to balance them all.
Whats all this balancing malarkey
Each cell in a battery has slightly different characteristics in terms of capacity (how much energy it can store), internal resistance (determi9nes how much heat is made during charging and discharging) and internal leakage or self discharge.
LifeP04 cells have a strictly controlled maximum voltage of 3.65v during charging and an absolute minimum of 2.5v when discharging, exceed these parameters and it will rapidly cost you money. So, during charging the inverter will output a maximum of 3.65v x 16 = 58.4v. So it’s all dead easy, apply 58.4v the the battery will charge to 100%, nice idea but you cant do that. All of the batteries are in series and therefore the charging current passes through all of them equally. The cells with the lower capacity will reach 100% first and then go on to overcharge causing damage to the cells and further reduction in capacity and during discharge, the ones with lowest capacity will be at 0% first with further discharging taking place again damaging the cells. The weakest cells in this situation die very quickly and can under the wrong circumstances become a fire hazard. This is where another piece of equipment comes into play, the BMS (Battery Management System). This device is wired to every cell so it can see the voltage, it opens and closes charging and discharging routes so it can prevent damage and it communicates to the inverter to increase or decrease charging current and/or voltage. It’s an electronic doorman or bouncer controlling the electrons in or out of the battery nightclub. It also keeps a watchful eye on temperature, any hint of a thermal runaway starting, the batteries will be disconnected, LifeP04 should not be charged when the temperature is below zero, I have placed some heater mats under mine because theyre located outside and winter can see inside temperatures where the batteries are of -5 deg C. The BMS will switch off the charge or discharge if the temperature parameters are exceeded. During operation of the battery, charging or discharging, any cell whose terminal voltage is higher than the others gets picked on by the BMS, it discharger that cell slightly and gives the charge to the cell that has the lowest voltage. Parameters can be set of how close you want the voltages to be, mine are all within 5mV (5000ths of a volt).
Commercial batteries at the time were very expensive, in the region of £1000 per kWh storage. Looking around at the cost of the cells, they were much cheaper and better quality items could be sourced
That’s the technobabble done, here's the construction.
This is the physical battery module with 4 cells mounted in it. The MDF and acrylic cover were laser cut, the threaded rod to keep the cells in compression is sleeved in silicone tube. The large grey connector at the front is a 150 amp Anderson Connector which will carry the main current in and out of the battery, theyre robust and very reliable. The small circular connector to the right of it is the balance connections, a wire goes to each battery cell so that the individual cell voltages can be read by the BMS.
This is a finished module, the busbars connecting the cells together, the thick red (+ve) and black (-ve) are the main supply cables to/from the module. The Thinner wires are connected from the first negative to all of the positive cells are the balance connections.
The 4 modules, 4 cells in each making 16 cells in total are housed outside in the lower section of a modified toolshed (bought from Amazon and is just about the perfect size.
The heater matts can be seen under the battery modules, the Anderson connectors can be seen connected placing all 16 batteries in series. You may have noticed another multipole connector hanging from the wires, this is for some extra monitoring added into Home Assistant. Note the insulation to the sides of the shed and on the door to keep the temperature up in winter time.
The 16 cells across the top, the heavy current cables down to BMS BATT + & - and the balance lines down to the battery inputs B1 to B16.
The Seplos BMS I used came as a bare PCB and display, I created a stand to hold it all and support the cables. I also made a front panel to dress it up a little.
This is the rear view of the BMS with one of the battery cables in place to test the design.
This is the front panel, I used laser engraving laminate to make a very acceptable outward facing cover to the electronics behind..
The little shed that was housing it all now had the 4 batteries in the lower half, the top half needed the inverter and a few ancillary parts.
Shown here is the Victron Multiplus II GX Inverter, A small consumer unit, A large fuse for the DC supply from the batteries and a battery isolator switch. The two units at the right hand bottom side are temperature controllers, the top one switches controls the cooling fan seen at the top of the photograph, the lower one controls the heating mats under the batteries.
This is the Multiplus II GX, the heart of the system, it’s not the cheapest of inverters but it is in my opinion one of the very best. The flexibility of the programming, the support worldwide from enthusiasts and Victron themselves is second to none. The weight is unbelievable. Only two wires for the battery supply (+ and -), three wires for the mains (L,N & E), a data cable plugged into the BMS and the CT plugged in and that’s the inverter done.
Apologies for any errors or spelling mistakes etc.
Part II to follow
