FUCKPUTIN2022LOL OP t1_iseaim1 wrote on October 15, 2022 at 9:05 AM

Reply to comment by Diligent_Nature in Not at all a "Hands on" person but looking to change that, need help with battery math. by FUCKPUTIN2022LOL

Thank you for your response. I was planning on LiPo from the get go. I've heard a lot of good things about them and few negatives. And before I sound like an idiot, I completely agree with every point you made. The issue that completely throws logic out the window though, (any my downvotes lol probably) is pricing. I wouldn't be here frankly if I hadn't already looked into that. In terms of cost, this project would cost me 2000 bucks max with 1000W worth of solar panels. The base Jackery 2000 (which is smaller in capacity to my proposed abomination) is still north of that price point. The full specs of what I would build for less than 2k would cost over 6k with them. (My reference is the jackery 2000 with 1200W of panels. Apparently their panels are made of gold and ambrosia or shit) and you may not think I am a dumb ass, but dumb ass question. Other than the fact I have no experience, how does building a battery pack out of individual cells scare you? You drive next to one every day. Tesla uses 18650 batteries in early models and now has switched to 32650 and larger batteries but that's literally what they used and stitched together lol. (I do know they use their own and manufactured cells)

jb32647 t1_isedsza wrote on October 15, 2022 at 9:53 AM

The main issue is that the one downside is that lithium batteries can easily catch fire if mistreated, and they cannot be put out (which is why most RC hobbyists keep a bucket of sand on their workbench). If you're not super familiar with lithium cells it is easy to make a small mistake that could start a big fire. Please just save your money for a bit longer and buy an off-the-shelf unit.

The_cogwheel t1_iseooyg wrote on October 15, 2022 at 12:10 PM

If you absolutely, positively cannot afford off the shelf lithium system, at least consider going lead acid (aka car batteries). They require more maintenance than lithium (you have to top up the water in them) and more ventilation (they produce hydrogen gas while charging) but carry less risk of catastrophic fire if mishandled.

It's still risky, and something that I still don't fully advise doing, but if you're gonna be bullheaded about it, then at least pick the option with less chances of burning your home to the ground.

As a bonus, you can later upgrade to lithium cells, so you can still get your system up and running soon, and upgrade to lithium whenever funds permit.

aminy23 t1_iseessj wrote on October 15, 2022 at 10:08 AM

> how does building a battery pack out of individual cells scare you? You drive next to one every day. Tesla uses 18650 batteries in early models and now has switched to 32650 and larger batteries but that's literally what they used and stitched together lol.

Early on a number of Tesla cars caught fire until they corrected it:
https://en.wikipedia.org/wiki/Plug-in_electric_vehicle_fire_incidents

When the Boeing 787 dreamliner was made, they had batteries on the planes catching fire:
https://en.wikipedia.org/wiki/2013_Boeing_787_Dreamliner_grounding

Companies like Tesla or Boeing rely on a team of engineers and advanced robots to make batteries. Even then they still had issues with fire early on.

If you will be welding 286 batteries - if you get one of them backwards or have a bad weld - it can cause overheating and lead to fire.

Running a 300 watt appliance depends on whether or not it's native DC. If you have to use an inverter to convert voltage, easily add 20% on top of that. If you want some safety margin, add another 20-25% to factor in battery degradation with time.

300 watts x 8 hour = 2.4 kw hours x 1.2 for inverter losses = 2.88 kw/hr x 1.25 degradation margin = 3.6 kw/hr.

3.6 kw/hr / (3.2 volts x 4) = 281.25 amp/hours at 12.8 volts.

3 deep cycles marine batteries is 303 amp/hours, easy to charge and wire, and costs $240:
https://www.walmart.com/ip/180350522

The main caveat is at 45.4 lbs each it will weight 136 lbs.

4 LifePo4 big boy batteries is $367 (random retailer for reference):
https://www.aliexpress.us/item/3256804647596547.html

A 300 watt load / 12 volts = 25 amps. If we over-spec to 350 watts / 10 volts low battery = 35 amps. Add $15 for a BMS:
https://www.aliexpress.us/item/2255800625909430.html

35 amps = 9 AWG wire.

A nickel strip is good for 5-7 amps. Stack 3 or so max if you have a high powered spot welder and you're at 21 amps.

Teslas work at 300-400+ volts, not 12 - that's how they do it.

18650s are possible if you target 5-7 amps.

400 watts (overspec if you ever want to charge your phone or something) / (5 to 7) = 57-80 volts = 72 volts = 24S battery with 18650s.

2.88 minimum or 3.6 kw/hr ideal / 72 volts = 40-50 amp/hour battery = 22-28 cells in parallel.

With 1,800 mah LifePo4 you're looking at 528 - 672 18650 cells.

With lithium-ion you'd be looking at 20S x 12-14 cells in parallel = 240, 260, or 280 18690 3,500 mah cells - consistent with your math of 268.

And at 72 volts, you could use anything below 22 awg wire.

A car aux power port typically has a 10-15 amp fuse and is good for 120-180 watts. 20 amps tops could push it to 240 watts. 300+ watts is beyond what that's rated for.

Diligent_Nature t1_isekrgb wrote on October 15, 2022 at 11:27 AM

There is a middle ground between building it from individual cells and buying an overpriced system from Jackery. Buy solar panels, charger, battery, inverter separately. You weren't going to build solar panels and a charger from parts. A Tesla battery has a professionally engineered battery pack which very rarely catches fire. ICE vehicles probably catch fire more often. DIY batteries catch fire much more often. Beginner DIY batteries are just about guaranteed to suffer a few shorts during or after construction. A 200Ah battery will make a huge fire very quickly if shorted.