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Configuration to support 1000watt 24v inverter
#1
I am trying to design an 18650 battery to support a 1000watt 24v inverter and I want to make sure I have my maths correct.

The 18650 cells I have are rated for 5.2amp continuous draw

Am I correct is calculating that the 24v 1000watt inverter will draw about 50amps if run with full load?

Would a 7s10p configuration have a max draw of 52amps (using the 5.2amp cells)?

Would the 7s10p configuration support the 1000watt inverter? Or should I size the pack bigger (7s15p)?

TIA for the advice, these forums are a wealth of information!
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#2
The inverter have 1kw continues and what is the surge? Its often like 2*. If so you need 7s20p.

How Long runtime? 10 minutes or 2 hours? 7s10p would have given you max 25min at 1kw load. Even less due to losses.

Perhaps 20min.

The cells also dont last that Long so a bigger battery is better
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#3
You're on the right track math wise.   However - in my experience with ordinary cells (e.g. 5a max discharge variety) its much more realistic to plan for 1a (or less) continuous.    So you need 7s50p rather than 7s10p.     What happens is the cell may be able to deliver a max of 5a but the voltage will drop a lot - as in 4.2v full -> 3.7v as soon as you put load on and drain quickly down in just a few minutes.     

Nothing wrong with hooking up 7s10p and see for yourself... .

They do make high drain cells for ebikes etc that explicity have 10a or 20a discharge - but typically 5a is the more ordinary laptop type cell and not ment for continuous max drain.
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#4
I agree you want more closer to 50p to help keep each of the cells from hitting their peak. This increases their life cycles left
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#5
(06-15-2020, 02:01 PM)PJP-TX Wrote: Am I correct is calculating that the 24v 1000watt inverter will draw about 50amps if run with full load?

Not really. 1000/24=41.6 amps. 24v is almost empty for 7s. You'll spend more time closer to 25v so call it 40 amps. Everyone else's tips are great, so call it 7s40p if you only want to draw one amp per cell.

Beyond that 40p presumes you'll use 100% of your cell capacity, which you shouldn't. I charge to 4.1 per cell, about 94-95% of capacity and discharge to 15% min. That gives me 80% DoD, depth of discharge, which extends the battery's life greatly. So add 25% to 40p and you get 7s50p! Woah, mind blown, same answer different math.
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#6
Going a little further, not knowing your load demands, let's assume a few things. You have 1000w max, let's say you need 500w continuously for 8 hours per day. (A different assumption is at the very bottom)

500w divided by 25v is 20 amps. So a 20p pack would be the minimum you need in this example not to exceed 1 amp per cell, average draw. Your cells being rated for 5a continous (20p/5) you could do a 7s4p pack. Presuming your cells are 2600mah (5.2a sounds like 2c) they will run stuff for 2.6 hours. 8 hours divided by 2.6 = 3 so call it 7s12p for 8 hours. That's 7s36p for 24 hours.

You will kill your cells quicker using 100% capacity though. So let's go back to 36p for 24 hours, so 36p x 1.25 = 45p. Add 10% for efficiency losses and that's 49.5...hell, call it 50p!

You'll also shorten their life a little running them at 2c or 2x their rated capacity of 2600mah (presumed) or 5.2ah. As such if you discharge them at 1c instead of 2c you need a 7s100p to get 500w of power continuously for 24 hours. That's 8 hours a day with a 2 day backup!

Now let's say you have a 1000w inverter that you want to use occasionally on the job site or camping for a few hours or days. You needed 7s4p just to start. That's 2.6amps x 4 cells for 10.4 amps. That's about 15 minutes of run time at 1000 watts (1000wh/25v=40ah for 1 hr). Or you can draw 500w for 30 minutes, 250w for an hour.

From there just go up. If you need 1000w for 1 hour then you'll need 7s16p. 250w for 24 hours you'll need 7s96p. I've ignored DoD here because it's not as important for occasionally used batteries.

Hope this helps.

Disclaimer* these examples are estimates, my math is rounded and my a, ah, wHr, etc terms may be incorrect, so no sniping.
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#7
Thanks everyone for your thoughtful responses, this is great information. There are a lot of things to consider! I will probably scale back the build and learn more as I go. I will keep you posted.
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#8
(06-16-2020, 03:01 PM)PJP-TX Wrote: Thanks everyone for your thoughtful responses, this is great information. There are a lot of things to consider! I will probably scale back the build and learn more as I go. I will keep you posted.

Exactly what I did! I had grand plans of creating a couple of 6s74p batteries to compliment my Tesla batteries. Turns out that's a lots cells! So I'm gonna practice building a 36v 10s3p pack for my wife's ebike first.
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#9
Briefly, 24V means 7 cells in series.

The number of pairs give you the total current and capacity.

You need the cells to support 42+ Amps for 1000W. That would mean 5 pairs of 10A cells (like Sony Vs), 3 pairs of 15A cells (like Samsung Qs) or 25 pairs of 2A cells.

Of course nothing prevents you (except the cost) of using 25 pairs of 10 or 15 A cells - this will give you a lot of capacity with zero cell heating-up.
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