Lithium expert help needed...

Photowhit

Okay, I'm NOT the lithium guy yet, I guess at some point I'll learn.

I'm trying to help someone on a solar forum with an off grid system she bought into.

I'm trying to figure out the battery bank she has. It has 4 sets of 16 of these batteries in series;


So as I read it it's a 8 ah 3.2 volt Lithium cell hope fully a LIFePo4 from the battery management system;



So math doesn't change with lithium batteries...lol.



It's still a string of 16 - 8 ah lithium batteries, for a string of 16x3.2=51.2 volts at 8 Ah or about 400 watt hours of battery each and 1.6 kWh for battery bank. 

BTW - it's hooked up to a 16,000 watt inverter.

BB.

16,000 Watt inverter? That is ~390 Amps @ 48 VDC.... That should be way more current than that Li Bank can support.

-Bill

Photowhit

Yeah Bill,
Not a balanced system, but they were buying for the future larger home. The inverter uses 45 watts in power saver mode and 225 watts at idle. It appears the battery bank was sold to them as a much larger battery bank.

I was just curious if Some how I'm reading them wrong, perhaps rated at peak load or something odd...

Perhaps someone who builds lithium battery packs/banks would see something right away.

BB.

Hmm... Looks like they have 4x of these BMS units:

https://dalyelec.en.made-in-china.com/product/kwTfYhcKZmWO/China-Daly-Manufacture-Hot-Selling-in-USA-Smart-LiFePO16s-48V-100A-Common-Port-with-Uart-Communication.html

So--They could be 400 Amp total.

Photowhit

I'm reading the cell in the first photo as stating 8Ah 3.2 volts.



Here's a Midnite Classic and the inverter for scale;

Photowhit

Another view of the cells, 16 in series;

BB.

If you go with (probably) a conservative max discharge current of 1 amp per 1 AH of capacity... Each 8 AH module would be good for 8 amps (1 hour discharge rate).

• 4 modules * 8 AH per module = 32 AH @ 48 volt bank
• 32 AH * 48 volts = 1,536 WH storage (exactly what voltage for bank capacity calculation)
• 32 Amp discharge * 48 volts * 0.85 AC inverter eff = 1,306 Watts max continous AC inverter max load (pure guess)

I wonder if those are 21700 cells (21mm diameter x 70.0mm long)... But seem to be around 3 to 5 AH max...???

You could have high drain cells--But what is the interest in draining a solar bank in 10 minutes or less...

8 AH does not seem to match the 21700 battery capacity (at this time anyway...).

-Bill

Horsefly

Those look like Headway cells, like these: https://www.ebay.com/itm/203092649653

From what I can find, the max continuous discharge is 15C for each cell, or 120A. You can probably find all the specs online. 

mcgivor

Looks to be an experimental setup, the battery capacity is probably just sufficient to satisfy the idle demand. There is nothing wrong with how the batteries are configured, there are pros and cons to doing series then parrallel, as it is in this case, or parrallel first then series, the latter needing only a single BMS.

This is not a functional system by any means,  there is enough equipment to build a substantially larger bank, the inverter however  is an energy hog even in power saving mode. What is the intent? the Daly BMS costs about  $50 without Bluetooth, each is rated at 100A, usually multiple BMS's are used to increase overall amperage, but the cells as configured would be incapable of delivering such a value which brings back the experimental statement.

Photowhit

...but just on capacity, they are 8ah cells, but made for hiigh drain situations?

BB.

Can't guess... That is one of the functions of the BMS--To limit maximum discharge (and charge?) current.

Using Horsefly's guess:

https://batteryhookup.com/products/4-headway-38120-hp-3-2v-8ah-lifepo4-lithium-batteries-25c-200a-super-cells

200 Amp discharge/80 Amp charge... An amazing cell.

-Bill

Horsefly

Yep, you can do a lot.... For a couple of minutes. 

Doesn't sound very practical for the inverter set up your friend has. 

Dave Angelini

The cell itself can only take 80A charge. A very imbalanced cell design also. A good learning lesson for the young lady I hope!