Power Bank questions

First post and thought I'd jump right in.
I'm starting on a small project after spending about 70 hours (I'm retired) watching Adam Welsh, Jehuand HBPowerwall YouTube videos.
As I said small steps but with a view to a powerwall to power a caravan (trailer to our left hand drive friends).

We are Grey Nomads (Australian for spending most of our year on the road travelling this fantastic country) and I think a Powerwall in a 'van is feasible.
I use about100 amp hours a day from a 12 volt system, but would be only restricted to 12 voltsat output phase; aim to run my wall at or about 24 volt.
I have 500 watts of 24 volt PV's charging 960 ah of AGM deep cycles presently (ouch - 280KG of lead), and will needto replace them in the next 12 months.
Currentthinking is LiFePo cells but I like the idea of utilising 18650's.

We go off grid about 5/6 days a week with 1 to 2 days on grid power to use luxuries like AC.

Has anyone ventured down this path?
Is there a spreadsheet or online facility that assists in calculating the number of 7s 18650 cells I'd need to meet my power needs?

Now to my small existing project.
We have 2 mobiles, one tablet, 2 laptops, a Paperwhite and a charger for 18650 flashlight usethat currently add to my 100 ah draw from my AGM's and I'd like to switch that draw to 1 or 2 x 8 cell 18650 power banks. Not all get used or need charging daily.

I can access about 40 18650's from power tool battery packs and presently have 2 x 8 slot power banks.

My MPPT controller has a USB load output.
Could I just connect my powerwalls one a a time to that output for charging?
What about balancing?

Any advise would be appreciated?
cheers
emmsee

Welcome!

There is several out there doing what you do. I do sugest LiFePo4 too for longeivity. LiFePo4 do exist in the 18650 form as well. Note that the number is just the size of the cell and do not tell much about whats inside even though most of them on the market for us normal people is the ones from or the same as laptops.

There is plenty of places to calculate but we can do it simple. Lets say you want 1000Ah@24V and you are using 2.5Ah cells.
Then you need 1000/2.5 = 400cells in each pack.

Thats 7*400cells = 2800 Cells.

Total capacity meassured in kWh = 3.7*2800 *2.5 = 25.9kWh A decent size pack. The weight would then be roughly = 2800*45g = 126kG. Though you need to add 30-40kg to house them too.
Neverthe less you would have alot of capacity on above compare to the lead acid. Could easily do 5000+ cycles on LiFePo4 with that scenario.

Regarding your existing project:
Not sure i follow? You want to run lower voltage stuff on the bank? Use DC/DC buck converters for that. You could potentially have low voltage packs and use and just charge them from the big banks when needed.

Yes you can charge your smaller packs from the USB outlet but will take alot of time. Better to have a DC/DC charger for that. For instance an RC charger would work and it would also do balancing. Several of them take up to 50V input.

I highly recommend BMS including balancing function. This in junction with temperature sensors + disconnect switches. Since you are running this in a home i suppose you sleep in?

And if you go with Laptop cells that are 2nd hand be even more carefull and a proper casing with wents outside the place is what i would do!

Cheers!

daromer said:

Regarding your existing project:
Not sure i follow? You want to run lower voltage stuff on the bank? Use DC/DC buck converters for that. You could potentially have low voltage packs and use and just charge them from the big banks when needed.

Yes you can charge your smaller packs from the USB outlet but will take alot of time. Better to have a DC/DC charger for that. For instance an RC charger would work and it would also do balancing. Several of them take up to 50V input.

I highly recommend BMS including balancing function. This in junction with temperature sensors + disconnect switches. Since you are running this in a home i suppose you sleep in?

And if you go with Laptop cells that are 2nd hand be even more carefull and a proper casing with wents outside the place is what i would do!

Cheers!

Click to expand...

G'day Daromer,
Yes, I can see why you're confused.
We're talking 2 different timelines and projects here.
The long term project is a Powerwall for my Caravan......later.

My current, small project is a now project and is to use 8 x 18650 batteries, in a ebay purchased empty power bank (as opposed to Powerwall), to charge my electronic devices such as mobiles and tablets - https://goo.gl/X53euL - for view of power bank -


I figure I have 3 methods of charging the 18650's - plug into my 240v grid system, use the USB port on my caravans charge controller or use my 40w solar panel as a dedicated 18650 battery charger (my preferred choice).
I'm exploring the last option and am looking at the difficulties of charging 18650's that are enclosed in a power bank purchased for that purpose.
Incoming voltage varies when using a Solar Panel, so I want to regulate the input from the PV to the battery bank [DC/DC Buck converter??] ?
According to 2 youtube reviews of the power bank, the bank will keep the batteries in balance.
I hope I've made it clearer now.
cheers
emmsee

Ah ok!

Most of the banks do have charging port in form of USB. I would use that one or pick cells out and put into external charger.
Use the dedicated charge port or use an MPPT charge controller that have USB output.

There are a couple of cheap ones out there. Yes normal DC/DC should work too.

Most powerbanks has the cell in parallel since its easier and then step up. then you dont have to care about balancing.

emmsee said:

daromer said:

Regarding your existing project:
Not sure i follow? You want to run lower voltage stuff on the bank? Use DC/DC buck converters for that. You could potentially have low voltage packs and use and just charge them from the big banks when needed.

Yes you can charge your smaller packs from the USB outlet but will take alot of time. Better to have a DC/DC charger for that. For instance an RC charger would work and it would also do balancing. Several of them take up to 50V input.

I highly recommend BMS including balancing function. This in junction with temperature sensors + disconnect switches. Since you are running this in a home i suppose you sleep in?

And if you go with Laptop cells that are 2nd hand be even more carefull and a proper casing with wents outside the place is what i would do!

Cheers!

Click to expand...

G'day Daromer,
Yes, I can see why you're confused.
We're talking 2 different timelines and projects here.
The long term project is a Powerwall for my Caravan......later.

My current, small project is a now project and is to use 8 x 18650 batteries, in a ebay purchased empty power bank (as opposed to Powerwall), to charge my electronic devices such as mobiles and tablets - https://goo.gl/X53euL - for view of power bank -


I figure I have 3 methods of charging the 18650's - plug into my 240v grid system, use the USB port on my caravans charge controller or use my 40w solar panel as a dedicated 18650 battery charger (my preferred choice).
I'm exploring the last option and am looking at the difficulties of charging 18650's that are enclosed in a power bank purchased for that purpose.
Incoming voltage varies when using a Solar Panel, so I want to regulate the input from the PV to the battery bank [DC/DC Buck converter??] ?
According to 2 youtube reviews of the power bank, the bank will keep the batteries in balance.
I hope I've made it clearer now.
cheers
emmsee

Click to expand...

If you have simple "12 volt" solar panel then you can just use a standard car charger socket USB converter to charge your wee powerbank. Some car charger will accept up to "24" volts (probably more like 30 volts) as input. They can be quite slow to charge so you might be better to go for a DC-DC buck converter module link this:
https://www.aliexpress.com/item/DC-...er-4-5-30V-To-0-8V-30V-Power/32819195154.html
or
https://www.aliexpress.com/item/DC-...r-Replace-LM2596S-Adjustable/32799520043.html

Running normal step down on a solar panel is not really ideal. Why?

Because the DC/DC in that sense will assume it have all the current needed on input so it will drag down voltage until it shots down if needed.
You need to have a charger that will limit the output based on maxium power point or a pwm that just runs at set voltage.

If your DC/DC can have minium input voltage set that will work too. But just hooking up a DC/DC and run max current out to charge something else without having that power on the input wll cause issues.

As long as you just charge 1 phone for instance and that uses 10w and you got a 20+w panel it should theoretically work but if there is just slightest sun the input voltage will go to low.

Look at this on eBay http://www.ebay.com.au/itm/201674733896

I've played with these, they may be an option as they already have 2 x usb

Wattsup said:

Look at this on eBay http://www.ebay.com.au/itm/201674733896

I've played with these, they may be an option as they already have 2 x usb

Click to expand...

Thanks Wattsup,
I have one of those in a 20 AMP, so will give it a try.
cheers
emmsee

---

daromer said:

Running normal step down on a solar panel is not really ideal. Why?

Because the DC/DC in that sense will assume it have all the current needed on input so it will drag down voltage until it shots down if needed.
You need to have a charger that will limit the output based on maxium power point or a pwm that just runs at set voltage.

If your DC/DC can have minium input voltage set that will work too. But just hooking up a DC/DC and run max current out to charge something else without having that power on the input wll cause issues.

As long as you just charge 1 phone for instance and that uses 10w and you got a 20+w panel it should theoretically work but if there is just slightest sun the input voltage will go to low.

Click to expand...

Thanks Daromer,

I have one of the DC/DC buck controllers that Paul Kennett listed above - the first one and it has variable input and output voltage settings.
You suggest "If your DC/DC can have minium input voltage set that will work too." - what voltage should I then set it at?
I'm not looking to charge a phone from the pv, but a power bank of 8 x 18650 batteries.
thanks
ermmsee