Hello from Illinois, US

Hello All

I am an engineer from Central Illinois, USA whojuststarted collecting batteries to build a power wall. I have been inspired by the youtube videos from
Jehu Garcia, HBpowerwall, Mikes diy powerwall etc.

Main goal in wanting to build a power wall is to learn. I hope to change jobs in the next couple of years to work in clean energy in some capacity.

Other projects are to power my home with solar power with a self installed system. I am planning for 15-20KW of Roof mountedSolar panels with backup capability from a power wall. Because 50-60 Volts is considered a "safe" voltage, I am planning to build a 48Volt system. I have not decided yet whether it will be a 12S, 13S, or 14Ssystem.

I'm also toying with the idea of paralleling in a super capacitor bank to handle the surge loads so I can baby the cells in the power wall when charging and discharging. Also, I am thinking of paralleling inone bank of lead acid batteries. There are some advantages tolithium Ion, lead acid, and super capacitors. They can complement each other in different ways.

I have a lot of experience in 24 Volt electrical system design as well as some experience inheating and cooling design.

I have built multiple houses, home additions and home remodels. I did most of the electrical wiring in those projects: single phase 240Volt / 120 volt home wiring.

Welcome m8!

Regarding 12-14s. No use in going lower than 14s as long as your equipment handles it! Most Equipment for 48v will handle 60v without issues. Dont forget that 14s can go down to 46V at its lowest point and on full load. 12s would be 39v... Not many inverters like that low voltage (This is just the extreme endpoints and im talking about voltage meassured under full load and at inverter)

If you going to have 15-20kw on roof i also suspect you want to have rather large battery bank (50kwh or more)
If so the capacitor bank is not very useful since that amount of 18650 will be able to handle the load just fine even if you surge 20kw On the other hand its not bad with a cap bank but not really needed.

Mixing LiIon with Lead acid is a bit tricky and i have been working on a system that will do that. Why i say its tricky is that you want to run a bulk/float charge on the lead acid where you take them up much higher and then let them sit on lover voltage. On Liion you do not do that. You go up to 4.2 and you done.
On the other hand if you use LifePo4 you could do that.

My idea is to disconnect the lead acid as soon as first stage is reached and then hook up a boost converter from the LiIon bank that will push the rest for x time. When done it will be connected again when the voltage have leveled out with the other bank.

This will though only work fine in an ideal world where you have alot of incoming power


And once again welcome! I look forward to see your setup and talk ideas!

Hi Daromer,

a 14S configuration gives 58.8 max voltage at the top but a safe charging voltage of 56 volts.
the discharged voltage of 42 gives 3 volts per cell which is also safe.

A 12S setup appealed to me because tesla battery modules are 12S. If I eventually got tired of harvesting batteries from laptops and paralleled in some tesla modules, it would be easier to use a 12S or 13S stack of harvested batteries to match up to the tesla pack.

The min max of a 12S setup is 36V and 50.4V respectively.

From the trojan website, a nominal 48V lead acid storage battery stack would charge with 59.3 volts with a float voltage of 54 Volts and an open circuit voltage of 50.93 Volts. The lead acid battery is for me experimental / optional. This matches up to the 14S stack about the best on the high side, but a 13S on the low side.

The cool thing about a Capacitor stack is that it could take the punishment of intermittent charge discharge all day long. I contemplate putting a slight resistance between the lithium ion pack and the other capacitor bank to protect or limit the Charge / discharge surge due to clouds going over and startup current with motors from washing machine or dishwashers, air conditioner, water heater heat pump and arc welder. It gets freezing in illinois, but the capacitor is not affected by the cold like batteries are.

I plan to put the battery setup in an unheated shed.

Hi Egam!

Seems like you have good check on the numbers!

Yeah if you going to hook up a Tesla pack i understand going 12s. Makes perfectly sense.

Regarding Lead as you see its a bit tricky. The resting voltage need to be on the same level as you want your topped up voltage on the Li bank are. Otherwise the Li bank may start to float the lead acid bank instead. (Not a big deal more than that you consume energy when you might not want to) Same with floating and bulk charging.

Going under 3.4V on Li doesnt give you much (Unless we talk about LiFePo4 or such chems) Dont forget eventuall voltage drop under load and on cables.
Some inverters have issues below 40V and become unstable and even shuts of. (Have seen people having issues with this and i have also had this on some cheaper ones when i started)
I started with 12S before i investigated a bit. The low voltage was my issues so i went to 14.

If you got the capacitor bank then add it. I would do that. Put it very close to the inverter.
Personally i wouldn't buy it but at same time my bank is so big that my cells will never see any high currents. If we talk about 10kw out or in to my system max cell current is whopping 30mA at most per cell.

Unheated shed does not work where i live. Down to -20C during winter. 68F

At what temperature C does your set up become a problem?

In Illinois, the ambient temperature ranges from -25C (maybe only a few days per year) all the way up to 40 C (Late July / August).
My understanding is that Heat kills Li-Ion more than cold.

I just don't want to chance burning down the house if some setting on the charge controller got changed by accident and overcharged the lithium ion batteries. Thats why I would put the bank in my tool shed versus my house.

My personal window is set to be between 10-30C. You loose alot of capacity going below.

http://www.mpoweruk.com/lithium_failures.htm

Its wise to not have it in your house. I will be keeping mine in my workshop/garage that is a bit from the house.