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Old 06-01-2021, 05:06 PM   #1
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An idiot's guide to building a big lithium bank

The following is how an idiot, namely myself, builds a 840ah 48v Lifepo4 battery. Follow at your peril. Or you could just learn from my mistakes and do it properly.

So your going to take the plunge and convert to lithium power. You've done the calcs, would like a few niceties, and realise that's it's time to go big or go home.

Jump on AliExpress, wait for one of their frequent sales and promo deals and order your cells. Of course, the price drops again two days after your order. Think that lithium is shipped as Dangerous Goods so order months earlier than you need them to allow for delays. But there aren't any and the cells arrive in a month to sit on your garage floor for ages until your ready. Open a box or two to admire your purchase. Even invite the neighbours around to confirm your brilliance and the scope of the project. Everyone thinks that blue tissue boxes are sexy, don't they? Don't worry about opening everything to check that you have enough of everything: busbars, bolts, washers etc. That can wait for a couple of months until your ready to build everything. Jimmy Wong will be so attentive then, if any parts are missing or you need more busbars...not! Of course you didn't think ahead and order a few extra busbars for cable connections, but you did save $3 by selecting those stupid little bolts that barely thread into the terminals. M6 * 25mm studs would have been so much better but it's ok if you strip a couple of the terminal threads, isn't it?

A big project like this is a good excuse to buy some more toys, I mean essential tools. A power supply will come in handy if you are going to be charging cells in parallel. The inverter(s) will likely only operate at full pack voltage. Lots of cables, lugs and terminals. You can never have enough. Don't check the actual diameter of the studs on the inverter, breakers, distribution busbars etc first. Just order lots of everything and then do multiple trips to the store when things are the wrong size. No need to buy quality components. Save 1% of the project cost by buying the cheapest crap you can find on eBay or AliExpress

Ok its B-Day: Build Day. Don't sit down and hash out a plan of what your doing and what's needed. You've watched a few YouTube videos so you've got this. Go for it. No need to write down target voltages: you'll be able to work out 16*3.65 in your head for the 800 times you will want to know it. Safety is for lib-tards so don't tape off tools or remove any electrically conductive jewelry. You will be super careful and the only guy ever who has assembled a big, powerful pack with no Oops moments.

It's a big system that's going to be full more than it's empty so let's top balance. All the cells were likely shipped about half full. Grab your busbars and bolts and wire all the cells up in parallel. What? They only ship enough busbars for series connections and parallel connection needs twice as many? Back down to the hardware shop to get Reward Points for aluminium flat bar to make some more. It's ok, you know everyone's kids names and school grades by now anyway. Link everything up in parallel, set your power supply to your target voltage for balancing and wait. And wait. And wait. You've got 48 cells being topped up ~100ah and your charging at 20a. Lucky you bought a big power supply. This should only take about 10 days!! Screw this. Pull everything apart. Again. Remember those easy to disassemble studs you didn't order? Wire the pack up in series and start charging at full voltage and power. Illuminate the sky with welders flash when you realize that the sequence is to power up the power supply BEFORE connecting to the battery and that connecting while it's turned off give a dead short. You can even jury rig your inverter to perform utility charging duties too. Now you are charging at 48v * 100a = 4800w not 3.6v * 20a = not many watts. Check your cell voltages. They are all really close. 3.31 or 3.32v. Repeat every 5 minutes just to check for any rogue cells. 2 hours later the cells are...3.31 or 3.32v. 4 hours later one cell has jumped to 3.33v. Progress!! Hours later we pass 3.40v and things are moving unbelievably slowly. Then in the blink of an eye all hell breaks loose. Most cells are 3.42 or 3.43v. A few others are 3.6v. A couple are 3.7v. Panic stations! Shut down the chargers! Thoughts of '000s of dollars of cells going up in smoke. Quickly wire in some bridging wires to connect the highest voltage cells with the lowest. Where did I put those damn crimpers? They balance out quickly so crisis avoided. No house burnt to the ground insurance claims. Disassemble the pack. Again. And reassemble in parallel, though not all at the same time this time. 16 near full cells charge and equalise pretty quickly. The power supply is slowly working its way down to 0.00a so go grab some dinner. After dinner it is showing 0.something amps. Close enough. Chicken's done! Move all the busbars (yes, those studs that you didn't order again!) to the next set of cells and check voltages. WTF? How does charging to 3.60v give cell readings of 3.70? Maybe you should have checked the power supply's calibration and watched the first set more closely? They soon drop to 3.59 to 3.60v at rest so all is ok. Drop the indicated charge voltage down a little and do the next two strings to get all cells balanced at 3.60 +/- 0.01v

Let the cells rest for an hour to check for any issues. There are none so tear everything down. Again. Reassemble in series strings and use the inverter to draw the pack down to the operating voltage of 3.3 to 3.4v. Stand back and admire your brilliance because no one else cares.

Rebox everything and ship it to the boat. All 12 boxes of cells, 2 inverters and countless bits of hardware. Swear that next time assembly and commissioning will be done once and once only on the boat not in the workshop/garage.

Learnings:
Failing to plan is panning to fail. It's an oldie but true.
Big battery packs are immensely powerful. Take safety seriously.
You will not pick exactly when cells reach a certain SOC. Allow leeway.
Charge in parallel, using your biggest bulk chargers, to 95% full, then in parallel to balance at your chosen voltage, before any cells reach over voltage.
Don't leave cells sitting for an extended period at high voltages. It just promotes degradation.
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Old 06-01-2021, 06:31 PM   #2
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That's a lot of cells......
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Old 06-01-2021, 07:12 PM   #3
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That's a lot of cells......
It's the equivalent of nearly 3500ah in 12v speak. 😁

Cheers
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Old 06-01-2021, 07:25 PM   #4
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It's the equivalent of nearly 3500ah in 12v speak. 😁

Cheers
Does that calculation factor in the (say) 50% useful range of an FLA vs 80% of Li? If not, it could be the equivalent of 5,600 ah.
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Old 06-01-2021, 07:53 PM   #5
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Does that calculation factor in the (say) 50% useful range of an FLA vs 80% of Li? If not, it could be the equivalent of 5,600 ah.
Strictly speaking it's actually a little more. The battery is is 16S so at 3.2v nominal it's actually 51.2 not 48v. Call it 43kwh.

Perhaps that's why it took forever to charge and balance. 😁
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Old 06-01-2021, 07:59 PM   #6
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Old 06-01-2021, 11:42 PM   #7
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Strictly speaking it's actually a little more. The battery is is 16S so at 3.2v nominal it's actually 51.2 not 48v. Call it 43kwh.

Perhaps that's why it took forever to charge and balance. 😁
That's a lot of kWh! What's your use case? Running HVAC off batteries? Smelting aluminum?
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Old 06-02-2021, 12:40 AM   #8
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What's your use case?
Depends who is on board:

"My" boat has a full technical dive set up with high and low pressure compressors, gas boosters and freshwater wash tanks. I'm also keeping a 3phase 10hp anchor winch.

"Our" boat has AC, residential galley, induction hob, water and entertainment for teenage kids.

I've got a pathological hatred of having a genset blaring away in the back ground to power these when I'm trying to relax. The admiral doesn't want to play power Tetris when cooking or boiling the kettle. 10kw of inverters means items can be turned on at will.

Before anyone asks, recharge is predominantly 5kw of solar (hence battery capacity for a couple of days of clouds) with shore power or small genset available as required (hopefully as little as possible).

Cheers
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Old 06-02-2021, 12:54 AM   #9
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Depends who is on board:

"My" boat has a full technical dive set up with high and low pressure compressors, gas boosters and freshwater wash tanks. I'm also keeping a 3phase 10hp anchor winch.

"Our" boat has AC, residential galley, induction hob, water and entertainment for teenage kids.

I've got a pathological hatred of having a genset blaring away in the back ground to power these when I'm trying to relax. The admiral doesn't want to play power Tetris when cooking or boiling the kettle. 10kw of inverters means items can be turned on at will.

Before anyone asks, recharge is predominantly 5kw of solar (hence battery capacity for a couple of days of clouds) with shore power or small genset available as required (hopefully as little as possible).

Cheers
Very cool! What’s the vessel? Would love to see the 5kW array. I have 7kW on our house - is it a power cat?

I also despise generators at anchor. Are you heating water with electricity as well?
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Old 06-02-2021, 03:46 AM   #10
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Very cool! Whatís the vessel? Would love to see the 5kW array. I have 7kW on our house - is it a power cat?

I also despise generators at anchor. Are you heating water with electricity as well?
It's an old school converted prawn trawler: 54*16*7'. A major attraction was that it has a single pitch roof for most of the vessel with only the paravanes giving shade. 20*250w panels fit and I have them ready for installation after a refit starting next week. There is room for another 1000w if I utilise every square inch or roof space and a further 2000w as well if I cap the davit, but I'll probably leave those for now.

Yes the hot water is currently a 50ltr, 1800w, 230v electric model. It will stay but I'm considering adding a plate heat exchanger to make a DIY calorifier but that's down the list of jobs.

I've attached before pictures. I'm starting a refit (including pulling the gensets out) next week so everything will go in after this is complete

Cheers
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Old 06-02-2021, 01:55 PM   #11
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GD, if your water heater is a typical residential unit with two heating elements, then consider replacing one of them with a DC element. I've seen 900 watt 24VDC elements, but perhaps you can find a 48 volt unit as well. Then you can dump power directly from the solar panels into the water heater during the day and save 20% or so in power losses.
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Old 06-02-2021, 04:02 PM   #12
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GD, if your water heater is a typical residential unit with two heating elements, then consider replacing one of them with a DC element. I've seen 900 watt 24VDC elements, but perhaps you can find a 48 volt unit as well. Then you can dump power directly from the solar panels into the water heater during the day and save 20% or so in power losses.
Thanks for that. A good idea that I'll investigate.
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Old 06-07-2021, 01:44 PM   #13
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Old 06-07-2021, 02:38 PM   #14
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Amazing battery pack - makes our 800ah 12v system seem miniscule...

Your write up did not discuss a Battery Management System, what are you using to control charge/discharge and protect your huge pack?

Good luck with the boat install - that will be an amazing system once implemented!
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Old 06-07-2021, 03:53 PM   #15
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Very cool. Following with interest.

So is your house system 48v? Or do you step down for small DC loads?

Also isn't there some need for module management in the pack? It looks like you've got 3 48v banks in series?

I learned a lot rebuilding a couple of Prius HV batteries. Agree with your safety caution. Be careful.

My wife has a cousin in Cairnes who is a marine electrician. I'm sure you can find him if you need him, but I'll provide an intro from the other side of the world if you want.
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Old 06-07-2021, 04:30 PM   #16
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Amazing battery pack - makes our 800ah 12v system seem miniscule...

Your write up did not discuss a Battery Management System, what are you using to control charge/discharge and protect your huge pack?

Good luck with the boat install - that will be an amazing system once implemented!
The predominant charge sources will be 20*250w solar panels to give 5000w@75v nominal. Control will be provided by two 5000w inverter chargers c/W parallel links with over volt protection.

Discharge will be to AC house via the same inverters or 48 to 12v DCDC converters. The Inverters will have min voltage cutoffs and the DCDCs will be wired after adjustable voltage sensitive relays.

As a back up pack voltage will be prominently visible in the wheelhouse and I may wire over/undervoltage signals into my alarm system.

One of the main advantages of lithium in general and this large pack specifically is that it's easy to engineer in wide safety margins. Alarms will not be set at 2.50 and 3.65v. I can see 90% of usage to be between 3.30 and 3.40v. After commissioning, cutoffs will be set accordingly leaving a wide margin for error.

The vessel is being slipped now but I'll post updates as work continues
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Old 06-07-2021, 04:43 PM   #17
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Very cool. Following with interest.

So is your house system 48v? Or do you step down for small DC loads?

Also isn't there some need for module management in the pack? It looks like you've got 3 48v banks in series?

I learned a lot rebuilding a couple of Prius HV batteries. Agree with your safety caution. Be careful.

My wife has a cousin in Cairnes who is a marine electrician. I'm sure you can find him if you need him, but I'll provide an intro from the other side of the world if you want.
House will be 230vac and 12vdc. These will be powered by inverters and dcdc converters respectively. Exceptions will be 24vdc start for the mains (dedicated LA battery pack) and a 7.5kw/10hp 3 phase anchor winch (230vac input VFD).

The pack was indeed wired as 16S3P for the initial top balance. I wanted to be able to see individual cell voltages, heat etc. Final wiring will be 3P16S with the parallel strings linked and taps to individual cell readouts.

I'm sure I will have many questions but I have a Electrical Engineer providing adult supervision. It's important to recognise that I have enough knowledge to be dangerous. Lol

All the best.
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Old 06-07-2021, 04:59 PM   #18
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Awesome job.


I just got through the youtube show last week and thinking of the same thing.


Right now got 8 6v golfcart battteries making a 12v house bank.



Not sure how to get my best bang for my buck on doing a lifepo4 replacement system.
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Old 06-07-2021, 06:03 PM   #19
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Final wiring will be 3P16S with the parallel strings linked and taps to individual cell readouts.

What are you using for individual cell readings? And are the cutoff devices triggered by any cell running out of bounds, or will you be monitoring balance manually? Lots of people have been successful manually monitoring, but you do need to keep an eye on it. The pack voltage can be well withing range, but a single cell getting into the trouble zone.
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Old 06-07-2021, 06:22 PM   #20
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The pack voltage can be well withing range, but a single cell getting into the trouble zone.
What action does one take in that case? Does that call for some sort of pack balancing process?
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