Not at all a "Hands on" person but looking to change that, need help with battery math.

Submitted by FUCKPUTIN2022LOL t3_y4av1u in DIY

I'm dumb as a bag of bricks, and electricity was the bane of my existence in high school physics. I frankly have no fuckin clue how it works. With that in mind.. I'm looking to build a battery. Specifically a battery I can charge using solar DC power. I'm currently debating between 18650 and 32650 batteries. I want to solder them together ( I may have used wrong word I'm clueless) using nickel strip roll, and then have them output 12V through a Car auxiliary power port. (I can elaborate on why specifically that output format). How many cells would I need to to reach 200ah and 12v? My math got me 268 (18650) and I'm so bad I can't even retrace how I got it.. but if I'm right this would give me 2400WH which means I could run a 300W appliance for 8 hours. Did I get that all right? I realize that I could just buy something that fits those specs, but I really want to get my hands dirty. I've always liked building things like computers and other nerdy shit, but I'm really getting into tiny homes and alternative power as a broke 24 year old. (Building my own tiny home would be cool but 1 step at a time lol)

(3min after edit Not knowing anything about electricity If my target isn't realistic at 200ah, would 100ah still allow me to draw 300w? Just for 4 instead of 8 hours correct?))

Edit* much later 8ish hours* I realize that this idea isn't realistic or even potentially safe. I also realize saying this would probably get me downvotes, but the cost differential is insane here. For the price of a 2000+ wh jackery power unit, I could build a 28000wh system at cheaper using 18650 cells. (Refence on current LiOn and LiPo pricing https://www.18650batterystore.com/collections/18650-batteries?sort=price-ascending)

6000+ being the price point

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Diligent_Nature t1_isd7sk6 wrote

I implore you to buy a battery pack which suits your needs. I am an experienced electronic technician who has worked on high voltage and high current power supplies, but the thought of building a large pack from individual cells scares the shit out of me. You're correct that you are going to need 268 cells based on a 4s67p pack of 3Ah cells. You also need a BMS capable of handling all those cells unless you're a complete idiot. In reality you should buy a 12V battery (or batteries if you want to get modular) which have built in BMS. You still need to wire up the solar panels and a decent solar charger. Plus an inverter if you plan to use AC. That is challenging enough for a beginner who claims to be "dumb as a bag of bricks". We both know that isn't true. Don't try to prove me wrong by building it from scratch.

Edit: whatever you do don't use lithium ion. Instead use lithium iron phosphate (LiFePO) They are far safer.

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Thrug t1_isdm47y wrote

Elec engineer here. Listen to this guy please. The risk something goes horribly wrong resulting in major damage, injury or death far outweighs the additional cost of an ots system.

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YourFavWardBitch t1_isdrar7 wrote

>The risk something goes horribly wrong resulting in major damage, injury or death far outweighs the additional cost of an ots system.

This, but also for any situation where one of those outcomes is at all likely.

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Gorbashsan t1_isdt3y1 wrote

As a tech I absolutely second Thrug here on encouraging you to follow the advice posted above, I have done plenty of DIY when it comes to solar and battery packs in series, I absolutely 100% never assume I have done it safely until I consult with an actual electrical engineer and buy one of them enough beer to inspect my plans on paper before hand, and give a look after, if I can't get him over, I 100% buy a pre-built. Listen to me on this, I have burned down a shed by forgetting to use a BMS. I mistakenly bought several batteries assuming they had internal ones but I had been off by 2 digits in their product codes when comparing to a catalogue and ordering them online and got the slightly cheaper ones without.

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FUCKPUTIN2022LOL OP t1_isealga wrote

I responded above and won't spam with big message but I appreciate the concern. It's more about the fact that realistically this shit over the counter is triple or quadruple marked up.

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Kraakefjes t1_ises0ig wrote

Burning up in a self made fire is much more affordable.

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qdtk t1_isezbhw wrote

Just take into consideration that it might not be “marked up” but instead you’re paying for additional circuits that provide safety features. Then consider those features non negotiable. You could build a motorized 4 wheeled vehicle cheaper than a car by just making it an engine with wheels. You could save on brakes, airbags, crumple zones, seatbelts, and all sorts of other things. But you shouldn’t. Just use solar panels, a properly sized inverter, a charge controller, and deep cycle marine batteries.

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Sluisifer t1_isgiuuc wrote

It's not.

You think it is because you don't understand a thing about what you're getting into. I'm not trying to be mean or whatever; that's the simple truth.

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quadmasta t1_isdd4y5 wrote

Lithium iron phosphate batteries are also far more forgiving.

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Efffro t1_isel46e wrote

All of the above especially the LiPo part if you’re a novice. But seriously don’t do this, you can harm/kill yourself so quickly you’d be amazed.

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The_cogwheel t1_isendst wrote

As an in field electrician, I also echo the advice to buy a pre-built pack. Batteries can be little mini grenades, especially when you overcharge them or overheat them. Go ahead, Google "battery explosion" and have a feast of firey goodness.

Bigger battery, means bigger fire. And at 300+ cells... thats a lot of battery.

This most definitely not in the realm of "an internet search, a soldering iron and some gumption". The whole install sounds closer to "get at least three quotes from a local electrician or solar installer" territory.

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FUCKPUTIN2022LOL OP t1_iseaim1 wrote

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)

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jb32647 t1_isedsza wrote

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.

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The_cogwheel t1_iseooyg wrote

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.

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aminy23 t1_iseessj wrote

> 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.

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Diligent_Nature t1_isekrgb wrote

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.

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StickyThoPhi t1_isdbauf wrote

sometimes learning about DIY means that you cant DIY, but it means you learned something useful so its not so bad.

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aminy23 t1_isdq8pp wrote

Have you just considered buying a deep cycle marine batteries?

A size 24DC marine battery from Walmart is $80 and 12V 109 AH.

Two of them is 218 amp hours and $160.

Add $20 for a charger, $8 for a 12V outlet, and a little bit of wire and for $200 you got your target.

I considered building a KW/hr lithium battery once and realized it'd cost many hundreds and require specialized chargers, battery balancers, and protection boards.

Lithium batteries are spot welded, not soldered. Soldering them can overheat them and make them explode.

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wilisi t1_ise8w3o wrote

Lead acid batteries might require some modicum of ventilation (for Hydrogen released during the charging cycle), just something to keep in mind.

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Lehk t1_isei0ai wrote

What you are describing would be extremely dangerous to DIY

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MeasurementGrand879 t1_isd9p17 wrote

Batteries are very unforgiving. You cannot turn them off and if you make a mistake, there is a lot that can go wrong. With that out of the way, you should start small and learn as you go. There are a lot of tutorials online for building battery packs. To answer your math questions, battery cells in series add voltage, cells in parallel add current capacity. To get 12v (14.8v) you would put 4 cells in series (positive of one cell to the negative of another). This gives you a battery of around 3ah at 14.8v. You would then put 67 of these 4-cell batteries in parallel (all end positives to all end negatives). This yields 268 cells as you calculated. You will need a BMS and possibly cell monitoring for over/under volts/discharge as well as temperature monitoring. How portable do you want this? An AGM or lead acid battery is way more forgiving and tolerant to incorrect charging. You can find similar 12v batteries of many different chemistries for a similar or lower price than you will spend building it. Good luck. Edit: changed a series to parallel. Edit 2: just as easily as it was to make that typo, it is to make a mistake with a battery pack. Start small for sure. Lithium chemistry batteries can be monsters.

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Felaguin t1_ise0z67 wrote

I was with you until you said to put the 67 4-cell batteries in series. He wants them in parallel to increase current. Putting them in series (again) will just increase the voltage. Everything else was spot on.

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FUCKPUTIN2022LOL OP t1_iseb0ix wrote

Not portable at all. Shielded and bolted down with easy access for maintenance. Proper ventilation. This is more of a stupid pipe dream than anything. But the only reason youtubers, myself or anyone is considering this is because I can create a battery 10x cheaper and with more capacity than anything out there. A jackery 2000 with the max solar array is 3x more expensive than my Frankenstein and twice the solar power.

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hopingforabetterpast t1_iseegtj wrote

There's a reason it's a fraction of the cost. Please listen to those who took the time to respond.

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tafrawti t1_isfgin1 wrote

If he won't listen to the other posters, I'm not gonna waste my time on him to try and keep him alive. I build battery packs for a living half the week.

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dyyd t1_isipxo4 wrote

Question to an experienced builder. Would such a pack be safer if built up from multiple 4s1p modules which each had their own BMS? If I am not completely mistaken then it should be possible to put packs in parallel without the BMS boards getting fried or going crazy.

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tafrawti t1_isiujxj wrote

Well I don't design them, I do the welding and heavy current soldering (radio guy here, this a power dude's design, or at least someone is his department)

One potential (pun intended, kinda) problem is that slight differences in characteristics between individual BMS boards could lead to strange things happening - if there is even a few mV difference between individual board calibration, placing several units in parallel could lead to things getting out of hand. This is especially true if the BMS has a lot of proactive monitoring on discharge (simple boards don't monitor the discarge so much, if at all - the cheapest are more corerctly called charge controllers)

Now, in a perfect world, good quality compnents, good design, attention to grounding to prevent EMC issues, low RF environment, stable temperatures, good quality cells all similar in characteristics, yeah, you MAY get away with such a multi-BMS pack. But I'm not sure I've seen that design commercially except maybe in EVs. So if you try it, be careful.

I have no idea if any given BMS chip would even tolerate that (check for reference designs on datasheets) and beware that most Chinese BMS boards sold are adequate-but-shoddy in my limited experience with them (solely from fixing up my friends power drills or converting old NiCad ones to lithium)

Like i say, I don't design them, I just spend my time on quiet shifts doing the tabs (bigass tab welder) and 0AWG soldering (gas torch) because I have years of experience with both procedures, But mainly because when they tried getting their usual contractors to do that kind of work they had several serious fires and two explosions that cost a lot of downtime and a lawsuit. So it was "hey old guy - can you help us with these?"

Batteries are dense in stored energy. Big ones are scary. Huge ones can kill. In many ways.

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frzn_dad t1_isg6i9v wrote

And when you burn your place down there isn't anyone to sue but yourself. Your insurance may even try to fight covering it because of what you were doing.

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MiteyF t1_isd68pl wrote

If you price out individual cells and materials, you'll likely find that you'll actually be spending more money by doing it yourself. Also, please consider that some lithium batteries if handled or charged incorrectly can enter thermal runaway... AKA, cause a huge fire with extremely toxic smoke.

With that said, if you want to learn how to do it, and how to do it safely, there's no reason you can't. You will need to choose a cell/chemistry first, and figure out how many cells you need in series (+ to -) to get your desired voltage. Then you can figure out your watt*hours depending on the capacity of those cells.

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FUCKPUTIN2022LOL OP t1_isear5s wrote

See larger comment. It's a third the cost. If that. 270 (to make it round) 18650 cells from the biggest distributor of them is 500 bucks. It's literally how tesla stays competitive because they literally use small rechargeable batteries to create larger cells instead of buying them. They make their own 18650 and 32650 cells

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FiveLobster t1_isd8nsc wrote

Go to Batteryuniversity.com and learn a whole bunch. Especially about charging. Learning how to safely charge/discharge a cell will force you to learn everything else.

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Echo63_ t1_isej4gw wrote

260+ decent quality 18650 is going to cost a fortune. I strongly suggest you have a look for a local “off grid” power retailer, or someone catering to ebike and EV builders.
Here where I am in Perth theres a few shops, but alt-tech is one I have visited, had great help from the staff there when I discussed my plans with them.

I can get 4x 100ah Sinopoly LiFePo4 cells, an 80ABMS and a box to assemble them all in, for around $600 Australian.

If you still want cylindrical cells, have a look for “headway” cells. You can get nice holders to clip them together, and they bolt together with busbars - much easier than trying to solder or spotweld them. Available in 8ah, 10ah and 15ah varieties.

Julian, OH8STN.org Has some great videos on building lithium powerpacks. His videos are aimed at amateur radio operators, and working in the field but are very relevant for what you want to do

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FireWireBestWire t1_isf6wka wrote

If you are as dumb as you say you are, you should not DIY electricity. Solar and batteries use DC, and if you fuck it up, you're dead.

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riesdadmiotb t1_isdzqai wrote

To add my 2c.

TL:DR buy a deep discharge 12V 250 amphour battery(wt or agm) and an applicable mains charge, a suitable solar regulator and a suitable mains convert(12V to mains if you need it). Gain experience with those first. Practically, I buy 2x6V 250amp hour batteries as I can(could) easily lift those if I needed to move them.

​

A battery is made up of cells, e.g your 12volt car battery is made up of 6 2.2V cells to give 12volts nominal for car use.

According to a quick web search, your 18650 are cells with a nominal voltage of 3.7volts, so you are going to need 4 of these in series(joined positive to negative) to give 12v (nominal)

Now you have a nominal 2,600 milliamphour of capacity. Note the milliamphour. and convert to straight amphours of 2.6 amphour.

Now to work out the number of cells, it is a simple matter of dividing 200 by 2.6 is 77 strings of cells, then multiplied by 4 of get the total number of cells you need. About 308 cells.

This figure comes from tying everything to a 12volt battery (and 12volt solar panel?), because there is a mass of stuff out there(RV/camping field) using 12 volt batteries.

Now to make this work, you need a regulator(jargon) between your solar panel and a string of cells. This regulator basically takes whatever voltage and current the solar panel is outputting and converts it up/down to the ideal voltage to charge your string(s) of cells. Note, each cell, depending on its type (wet, agm,sla, nicad, lithium, etc) has a required/optimum charging voltage or current. For the first three it is voltage, and for the others it is current and both optimally vary according to the state of charge.

The major problem you are going to have is those 77 strings of cells are not going to perform equally and if you are using second hand batteries will almost certain fail continually if you try to deal with them in bulk.

Cells/batteries in parallel failing are usually not a major problem. The total voltage drops and the internal cell resistance increases and stuff just stops working.

If a cell in a parallel set up fails, it can slowly<->disastrously bring down the whole array.

If the cell fails and its voltage drops, then it can slowly/rapidly bring down all the other parallel strings. Being the lowest voltage in the array it draws power from the rest of the array and gradually discharges the whole array.

Now, a lot of people will tell you all sorts of batteries can be run flat. This is only at a great cost and it will shorten the life of the cell in stuff like capacity, life cycles, charge/discharge rates.

Now that you know about only some of the pitfalls, you are just a little bit wiser if you want o go ahead, scounge a few cells and stat experimenting.

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GypsyToo t1_iserxbl wrote

I'm kind of in the same boat, but I realized that the battery is definitely not where you want to start this learning process. After reading all I could get my hands on and watching lots of videos, I plan to start with buying a LiFePo4 battery and then putting together the components to safely charge that battery using solar power. You will still save a chunk of money and it will be way safer.

One of the resources I used was Will Prowse's book which is just about $5.00 and has a lot of basic, simple but at the same time detailed, information.

Please stay safe. Batteries are very dangerous when you don't know what you're doing.

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elohesra t1_isexjvp wrote

Now you see why paying attention in school to those subjects that you think you'll never use in real life is good advice. Lets hope we don't see you as a nominee for a "Darwin Award".

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CaptainPoset t1_isfa6n5 wrote

Just one question:

If you want to use it stationary and on a 12V-car-level, why would you not just buy a car battery? They last longer, they are far safer and they are comparatively cheap.

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daHavi t1_isg7r5x wrote

>It's more about the fact that realistically this shit over the counter is triple or quadruple marked up.

So what I think you're saying is... You don't like that the retailer is making money while selling to you... even though every product you've ever bought from any store EVER has been marked up.

So instead.... you want to play with chemicals that are toxic as shit, will f*cking kill you or severely injure you in ways that will disable you for the rest of your life, AND once you put electricity to them could explode (literally) if your home-engineered design isn't perfect.

You already said electrical theory wasn't your thing even at the his school level (very basic).

THIS PROJECT IS OVER YOUR HEAD. The chance of severe injury far outweighs the slim possibility of the battery successfully working and you saving some money.

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hijinks t1_isd4txs wrote

If you are looking to add a second battery like overlanders do, if you do it wrong you risk destroying your computer which can be very pricey.

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FUCKPUTIN2022LOL OP t1_isd57tx wrote

The battery would actually be more for lighting and or a heater/AC. That's why my wattage threshold is so high. Even low wattage ones draw like a mf

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UntakenAccountName t1_isdbjzs wrote

Running heat/AC off of batteries feels unrealistic to me. What will you be charging them with?

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FUCKPUTIN2022LOL OP t1_ise9icv wrote

Solar. Anywhere from 1000-2500w depending on budget. The general idea is tiny home+super insulation+ small heater and AC. I'm aiming for the completely unrealistic effort of a 250w heater and ac. (Not combined) that's why I'm aiming for 300w, so I can run either max power (low-power max power you will) and still be "good". The rest of the power will be stored for fridge, lights, etc. (The general basic WH/T variable shit I get, just not conversions because my autismo brain goes brrrr)

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John-C137 t1_iseawne wrote

For heating and cooling I'd recommend something like a diesel parking heater or a gas powered AC unit. Mate I think what you have to take away from this thread is your initial fact finding mission was a success. You found the initial project is a dangerous, expensive and just the wrong application for what you want to achieve.

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FUCKPUTIN2022LOL OP t1_isebnnj wrote

Except it's literally how tesla builds their cars. Their batteries are built literally with 18650 or 32650 cells. I realize it's dangerous (I mean electricity and fire are lol) but it's not like I wouldn't be using spacers and proper wiring. (I did forget to ask about gauge of wiring) I want full electric. I'd rather not destroy and pollute more than we already do. I mean, we have a giant nuclear constant reactor in the sky and we burn old dinosaurs instead. K.

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throwawhatwhenwhere t1_iseem1l wrote

> Except it's literally how tesla builds their cars

Yeah dude no. You're not qualified to do this and it will go wrong.

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John-C137 t1_isef774 wrote

Tesla also have spent millions of dollars and had teams of expert engineers design their manufacturing process, test procedures and quality control. For example they will have dialed their spot welding down perfectly so the cells won't be heat damaged but still have a low resistance connection, can you really say you can hand solder hundreds cells with no I'll effect to the cells? I could go on about batteries and UPS's all day, is a real iceberg of a subject.

Good on you for going all electric and have an interest in renewable energy. Save yourself some pain and start at the start, work out your requirements using leisure or marine batteries, design and assemble your own charge system using available parts and research DC power systems and batteries as you go. Starting simple will help you develop the skills and techniques to go on to more ambitious projects in future.

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Dr_Sigmund_Fried t1_isd6u40 wrote

So, are you looking for higher amperage or higher voltage? If amperage then connect batteries in parallel and you can add the amperage of each battery together to give you a sum total. If you are looking for higher voltage then connect the batteries together in series to get a sum total voltage.

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Xeno_man t1_isdsuiz wrote

Honestly, like everyone else has said, building a backup battery out of cells isn't the best place to jump into DIY with little knowledge about electricity.

What I would recommend is something like this.

https://ca.ecoflow.com/products/river-portable-power-station

You can charge it from solar or the grid and provides you with 5v, 12v and 120v and probably cost less than what it would take to buy the cells, let alone the equipment and parts you need to assemble said battery.

If you want to get into battery building, start with repairing cordless tool batteries. You only need 5-10 cells, not 260 plus. That is asking for a fire.

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aminy23 t1_isef4up wrote

That has 288 watt - hour capacity.

OP needs something in the 2,000-3,000 range.

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Xeno_man t1_isg3zm5 wrote

That's just the base model. Larger units and extra capacity is available.

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