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LTO Powerwall
#1
Yep, not 18650 cells...
This is with the standard 66160 LTO cells that are in 30Ah to 40Ah capacity. The 30Ah cells are widely available as recycled units.



The cells are arranged in an 11s2p pack with a small space between the cells for additional air circulation and tollerances.

Additional small holes in the joining plates (2mm Aluminium) are to screw the bars into an outer wooden frame. and for connecting BMS cables.

The LTO cells have a working range between 2.7V and 1.5V and clipping the top and bottom of the voltage ranges to where the capacity lies with an 11s arrangement this gives the closest proxy to a standard 24V lead-acid battery voltage range and makes it more compatible with a standard UPS unit.



Each pack will then be 24V and a capacity of 2kWh (using 40Ah cells).

External connection will be via two XT90 connectors so that either more than one connection can be made or two connections to a cental busbar.
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#2
The busbar setup for the system will be a 12mm (or 18mm) Ply board with plastic stand off mounts for an aluminium busbar.

The plastic stand off mounts are actually part of a PVC window frame, which after taking appart for the glass some of the sections looked really useful. So, with a bit of cutting they are then the ideal material to mount the aluminium bar to, which then also prevents any issues with mounting it to wood and moistrue. With mounting directly to wood, at 12V your quite safe, however my system is going to be 72V and I'm starting to get cautious. This is comming from someone who used to play with 7kv transformers at the age of 12 seeing how big a jacobs ladder I could make, while blanking out the TV reception for around a quarter of a mile.... long time ago...

 

The busbars have earth blocks attached rather than drilling for lugs because it works out cheaper per connected wire and a simpler build process.... while allowing for future flexibility.

Using Aluminium also works out to be a lot less costly than copper and all you have to do is upsize by dividing your copper csa requirement by 0.65 and the aluminium resistance is then the approximate level of copper. Plus, there are not a lot of places selling copper bar at sensible prices in small quantities.

 

The busbars are arranged so that the bottom bar is 0V, next bar up 24V, then 48V and the top bar 72V.

The middle 2 bars do not need to be anywhere near as thick as the outer bars because the terminals will be taking the majority of any current flow and only pack to pack balancing/imbalance will flow along the middle bars.

The terminal blocks at the ends (may revert to lugs for these connections) are then the offtake points for the load via a 300A shunt then to secondary MOS arrays on a second single aluminium bar mounted at the end of the board...

The idea is to then have leads from the bars and separate leads from the battery packs, if the pack is at the same bus voltage then just plug and switch on.. or if they are out of balance, try and switch on and see if the packs 100A breaker trips or melts Smile
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#3
This is what the PVC window frames look like for the busbar supports and the thinner Aluminium bar (38mm x 3mm) that will be used. They are cut into 80mm lengths and then drilled for wood screws.

Free, fully insulated and strong busbar mounts... good for any conductors, copper pipe, coper wire, etc...

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#4
i like the approach to go with LTO cells very much! since storage space in physical dimensions is secondary for powerwalls (in my case) these cells should be a lot more uncomplicated in handling as well as less dangerous when misused. and last: great compatibility with existing systems designed to deal with lead-acid batteries.
the only drawback is hard availability in my area (austria)

waiting to see more progress on this!
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#5
With almost all of the parts now arrived the actual build process is set to star.... Still waiting for some DB15 conectors for the BMS hot plug for the packs, which is that last critical item that would prevent the packs from being used due to the wiring offtake positions.

Have altered the front panel slightly so that the breaker is now mounted on a DIN rail as I may add a smaller 20-60A breaker(s) on some packs for XT60 connectors that would allow individual packs to be used with a lower than 100A breaker ready to make magic smoke from unwitting devices. The smaller battery isolator switch would control these.

The rotory isolators are not rated for much above 24V so can't be used on the packs for isolation when in series connection, because the first switch could be trying to isolate upto 90V for the 72V pack and potentially a lot higher if many packs are connected together.

With the DIN rail the breakers can be easily added later fo all packs if needed. The DIN rail also made mouting the breaker a little more robust and allowed it to be sunk into the panel more so that the trip arm is less likely to be accidentally caught. The brekers are not intended to be used as regulat isolators/switches because I'm not entirely sure they would last a lot of use and expect them to only last around 5 fault trips



The ESP array was the test setup for the BMS system and the first unit will be a hot plug box with 12 units in (11 : one for each cell and one for pack overall shunt or volts).

The large power supply is a used 2800W Cisco Catalyst power supply that can deliver 50V at 28A - 62V at 28A or 113A at 12V. There are another two units rated at 4200W that are intended to be standby charger units from the grid/generator. Very cheap high amp power supply option... plus the outputs are fully isolated....
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#6
Wow Smile
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#7
Progressing with building the battery boxes (18 of them !) and cut just over half of the wood required for the build. Appart from the sheets all the other wood is from local skips throwing away a variety of good wood... still around 200 battons to cut and then final measuements for the width after the cells arrive.

9mm plywood is the main construction material, 3mm was too flexible for the weight and 12mm may create an issue with the bolts on the ends of the cells as there will be 3mm Aluminium cell connector + 12mm wood + 2mm washer + 8mm nut and I don't think the threads are long enough... reminds me I don't have enough washers.



The generator will be used when in a remote location as a backup for when there is not enough solar or wind. The 6kW charger until will be explained later in another post, recycled equipment costing less than £50 and will be CC CV with low upper V limit.

If it is not raining tommorrow (and sunny with solar power !) I will hopefully finish off cutting the battons, start fixing some of it together and start drilling out the holes for the cell mounts.
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#8
Bit more progress with the battery side supports for the boxes, which are now all drilled with 792 x M12 holes and now need the Aluminium cell joining plates, which is the next job to cut up. 180 cell joining plates plus 36 end connections and 1600 holes to drill.



The other 18 thin strips of wood in the image are for extra strengthening and going to be used to join the corners/edges together in the boxes and came out of two long floor boards that were hanging out of a skip I passed by.
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#9
Working on the pack balancing arrangement and on the front will be a DB15 connector with fused access to each cell within the pack. Axial glass fuses of 3A maximum (might de-rate to 2A) would provide internal pack protection from anything that is plugged in.

Small volt meter will be included in the pack and this will be swiitched with separate 1A fuse protection.




Connecting into the DB15 will be a connector that is separately fused and passes through a 0.27 Ohm resistor.

All of the connectors for a given parallel pack will then be connected together, i.e. in my instance 6 packs of one voltage will all be connected together.

The main balancing would then be done from the central connections, i.e. six of the above circuits would be connected together with the left hand DB15 connector. These connections would then be where the whole pack is balanced, current limited to a maximum 2A per parallel set. This is the diagram for just one of the 11 celll sets in the packs.



The reason for passing the connections through a nominal 0.27 Ohm resistor is to prevent the fuses from being easily blown on faults and excess over/under charge states. With a pack the cell to cell difference between two packs may be a maximum of 500mV. With a maximum cell to cell difference of 500mV passing through a 0.27 Ohm resistor the resulting current flow would be 1.85Amps or 0.93W. The aim is not to put all the cells in balance all of the time at every charge point, rather gently push or pull them into balance over days or weeks.

This interconnect arrangement will be a temporary solution ahead of the separate BMS wireless boards being plugged in once I finish them after the pack builds.
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#10
Bit of progress, cells arrived and have managed to mount most of them into the frames/boxes that I have been preparing. The packs are 22s2p and give a working range close to a couple of lead-acid batteries of 22.0V to 28.6V.



The cells appear to have been recovered from at least three different items as the voltages, labeling and consistent marking on the cells seemed to show three distinct groups of cells. The 66160H is the 40Ah cell that Yinlong produce and is th emost recent version of the cell, the 66160A is 30Ah and the first generation cell they made, the 66160F is second generation at 35Ah and I have not seen any of these in the used market so not sure where they were used if they were produced in any quantity.



Change of design on the fly as I ended up putting the connecting bars on the outside of the boxes, which is not ideal because of the connection. The threaded bolt will carry the current and then back through a spring washer (compressed flat), which makes the compression washer the narrowest part of the conducting are along the path and is around 20mm2 steel. I'm not planning to pull more than 20A on a continuous basis per cell from any of the packs (40A per pack or approximately 1kW at 25V) so it should not cause any problem.

 

Working on the balancing leads and connections, the image is part of a test board to help wiring up the rest of the leads to each of the packs with 14 wires to connect per pack to provide cell voltages external to the packs on a plug-play basis and hopefully not a plug and bang basis Big Grin 

 

The heat shrink on the back of the connector is just as much to prevent the cable from touching anything should it break off from the conector as the heat shrink will hopefully still extend beyond the bare wire. The Connection is arranged so that the maximum voltage between any two adjacent pins is only 4.6V so any bent pins or shorting will not result in a big flashover. Fusing, still having a think about internal axial fuse or external fuse as I don't like anything that is not on a fuse.

While the sun has been out each of the packs has taken a turn in a slow 10A / 250W charge to bring the voltages up, check the balances and try a few experiments on cross connecting cells and seeing how many Amps flow. Total charge so far is around 4kWh, just need to get the first UPS connected.
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