Tyni powerwall project

Hello everyone. I am in the process of building my first battery pack and I need some help to answer my questions.

So first, what is my project? My project is to build a very small powerwall with a capacity around 200Wh to power a little cabin (very little house with no electricity, water and even no cellular network lol) to power some lights with solar 30W solar panel to charge it (small power because we dont stay more than 2 nights up there).

So I have the choice between Lead-Acid 12V 17 Ah which is cheap and 18650 cells which interest me a lot.
Firstly, I decided to do a 3s7p build but reading the forum I learned that we cant use it for 12V systems then I decided to make a 7s3p system but that brought questions:


1. What is the interest of having more in serial than parallel? I mean what if I build a 1s21p?


2. Ive seen a lot of 3s BMS on Aliexpress (same on ebay) and since 3s is useless what is the utility of them? same with 5s: these are very very cheap like 3 each but what is the utility of them, can I make a 5s system and use it as 12V with a converter?


3. Another BMS question: on a small system like mine is it useful?
Majority of Aliexpress 7s (and 3,4,5,14 s) BMS have too high overvoltage protection, too low overdischarge protection and 4.2V balancing parameter (and low current balance around 50-60ma most of them) so what if I dont wire a BMS? Ive searched a lot on Aliexpress but I dont find any BMS with good settings.


4. Safety concern: I really want this system to be safe since I will not be everyday. So, I really want to find a good BMS but at a fair price compared to the size of this system.
For more safety I thought of putting a switch on the solar panel to stop the charge when Im not there, but it loses the interest of being fully charged when I come. Am I crazy to want to build a system like that and leaving it alone? Will Lead acid battery be more safe or the danger is the same?


Im sorry for the long post and if my questions are not very smart Im new to this.
By the way please excuse and correct me if you want my English

1- the matter of serial is for the voltage you want to work with, with 3s you will add the voltage of every cell so will work between 4.2V*3=12.6V and maybe 3.2V*3=9.6V, the systems of 12 use to work with lead acid batteries that have a max voltage of 13.8V and will have problems with 9.6V, so you will have a low range of voltage to work with. I you build a 1s21p you only have a big cell of 3.7V nominal with maybe 42000 mAh, if your cells are 2000 mAh. So you have to put in series so many batteries or packs of batteries to get a voltage range your device can manage.

2- more or less the same it is a matter of voltage, and yes you can build a 4s or 5s and use a dc-dc converter but will lose efficiency.

3- BMS in your case is mandatory if your cells capacity are very differents, because you can overcharge or overdischarge a single cell and damage it,

4- of course lead acid is more secure, for the amount of current you will work wiith even a cheap chinesse will be enough

Apart of all you questions with the lead acid battery you comment above 12v*17Ah=204W you only can use 50% of capacity usually, even deep cycle lead accid batteries will decrease its life if you discharge more than 50%.

With the li-ion cells 21 cells * 2Ah *3.7V = 155.4W that means that you can only run 50 Wats of lights during 3 hours, maybe you must considere some more cells.

Regards

Welcome!

I think you should stick with lead acid batteries for this case. There is no immediate benefit of using lithium cells. If you are interested in lithium cells you can find another project or just experiment with them to learn all about them.

1. Higher voltage = less current for same Wattage = less losses on thinner wires and also easier to work with in general. There is no equipment out there taking 3.70V to 230VAC as example. Its 12, 24, 36, 48 and so forth. Where 12V is hard to archieve with lihtium with nominal of 3.7V

2. Yes you can use DC/DC but seems like extra step for nothing unless your plan is to run 12VDC appliances. There is BMS systems in all factions because there is uses for them.

3. Because its cheap crap. No one using those want it to cut out just because its a bit over or under.. That would just make the user angry. Even though its not good for the cells. When it comes to balancing you get what you pay. 50mA balancing current is more than enough for many of the applications (Not talking about giant powerwalls) since you should really not have to balance... If you have to do that you have issues with the packs...

4. Once again you get what you pay for.... Lead acid is alot easier and safer in generall. Thats a good choice for small system where you just use it as backup and stuff.

Thank you very much for all your answers ! That's clarify a lot for me

If all you want is lighting the simplest solution is probably to find some led lighting that runs happily with a 1S battery.
Then all you need to manage is state of charge from your solar panel rather than balance.

Jon said:

If all you want is lighting the simplest solution is probably to find some led lighting that runs happily with a 1S battery.
Then all you need to manage is state of charge from your solar panel rather than balance.

Click to expand...

Yes I just want to run some led lights but thez require stable 12v, I found good 24v to 12v but I don't for 1s battery

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By the way I found these cells on Aliexpress : https://aliexpress.com/item/4pcs-lo...Ah-Li-ion-3-7v-Battery-18650/32728833809.html

I've seen the videos about fake cells so I will not purchase them but they really don't seem to be fake ones what do you think ?
This is weird because there is no brand and the price is low but the capacity is standard unlike the fake which are like 9999 mah lol and what's surprising is that there isn't bad comments despite the number of orders!

So what do you think guys ? I'm just asking.

For my camper I'm running 7s and 36p for now with the ability to expand the parallel number. I then use a 7s Cheap from china bms that i check up on all the time as they are not to be trusted.... I know but it was cheap.

Then for my 12v stuff I use a buck converter that takes the up to 30v and knocks it down to a constant 12V. Now looking back I'd rather buy an adjustable one as 12V exactly is not the best for some pumps and motor driven loads as I have found out and 13.7 would have been better and more efficient but it works. the charge controller takes care of the max voltage and the bms keeps the cells in check and shuts off the power if there is a low voltage issue. Which I have tested and it works but.... The worst cell gets to 2.5V before it shuts off. So I'm not super happy with that. but it rebounds to 3.2V once the load comes off.

The whole system is more than I need for my camper with LED lights, radio, And Gas furnace that has an electric fan and Water pump.

PowerLio said:

By the way I found these cells on Aliexpress : https://aliexpress.com/item/4pcs-lo...Ah-Li-ion-3-7v-Battery-18650/32728833809.html

I've seen the videos about fake cells so I will not purchase them but they really don't seem to be fake ones what do you think ?
This is weird because there is no brand and the price is low but the capacity is standard unlike the fake which are like 9999 mah lol and what's surprising is that there isn't bad comments despite the number of orders!

So what do you think guys ? I'm just asking.

Click to expand...

The Samsung -26FM cells are still a mystery to me. Unlike the -26F you can't find any information on them. I got some from a laptop battery and they are fine, however mine look different from these ones on Aliexpress. Mine don't have a space between F and M, -26FM and not -26F M. Also the markings on mine match other Samsung cells (in layout and quantity) and these on Aliexpress don't. Does that mean they are a knockoff? I've no idea. But ~2.14USD per cell isn't a super special cheap offer so I wouldn't bother anyway.

jdeadman said:

For my camper I'm running 7s and 36p for now with the ability to expand the parallel number. I then use a 7s Cheap from china bms that i check up on all the time as they are not to be trusted.... I know but it was cheap.

Then for my 12v stuff I use a buck converter that takes the up to 30v and knocks it down to a constant 12V. Now looking back I'd rather buy an adjustable one as 12V exactly is not the best for some pumps and motor driven loads as I have found out and 13.7 would have been better and more efficient but it works. the charge controller takes care of the max voltage and the bms keeps the cells in check and shuts off the power if there is a low voltage issue. Which I have tested and it works but.... The worst cell gets to 2.5V before it shuts off. So I'm not super happy with that. but it rebounds to 3.2V once the load comes off.

The whole system is more than I need for my camper with LED lights, radio, And Gas furnace that has an electric fan and Water pump.

Click to expand...

Thanks for the feedback, like you suggest it I first wanted to use an adjustable buck converter but I was a little worried about the output stablity of these converter then I decided to use a 18-35v to 12v that I trust more for having nice 12v!

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DarkRaven said:

PowerLio said:

By the way I found these cells on Aliexpress : https://aliexpress.com/item/4pcs-lo...Ah-Li-ion-3-7v-Battery-18650/32728833809.html

I've seen the videos about fake cells so I will not purchase them but they really don't seem to be fake ones what do you think ?
This is weird because there is no brand and the price is low but the capacity is standard unlike the fake which are like 9999 mah lol and what's surprising is that there isn't bad comments despite the number of orders!

So what do you think guys ? I'm just asking.

Click to expand...

The Samsung -26FM cells are still a mystery to me. Unlike the -26F you can't find any information on them. I got some from a laptop battery and they are fine, however mine look different from these ones on Aliexpress. Mine don't have a space between F and M, -26FM and not -26F M. Also the markings on mine match other Samsung cells (in layout and quantity) and these on Aliexpress don't. Does that mean they are a knockoff? I've no idea. But ~2.14USD per cell isn't a super special cheap offer so I wouldn't bother anyway.

Click to expand...

Thank you DarkRaven for leaving your opinion on these cells