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Few questions to those that made DIY batteries.

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Luk88

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Apr 5, 2024
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I'm building my first lifepo battery using 16x 280ah prismatic cells. I haven't yet decided if I put the battery in an insulated, but mostly unheated workshop, or in a basement beneath.

I'm going to use a heltec 8s-20s ht820s2a 150 (200A rated, 350A max, 1~2a equalisation) BMS.

120 sq mm (4/0 awg) wiring and sb350 plugs.

I'm currently wondering what should I do about an enclosure. I considered buying an ice chest box, but there are none as big available(maybe get two). My second choice is a thin-walled aluminium box that looks like this:Screenshot_20240408_171818_Chrome.jpg
It will definitely need a wooden base to not collapse with the weight and insulation. But I could fit 18 280ah batteries in one of these.

Which brings me to my question. What about building two smaller batteries and connecting them to a single bms? What opinions people have on this vs one big battery?
 
Luk88 said:
What about building two smaller batteries and connecting them to a single bms?
Click to expand...
In essense that's how batteries are built anyways. If you look at the guts of a 16s battery, physically it's just a pair of 8s batteries sitting next to each other with a connection between the two.

There's a big difference between how a battery is built Physically versus how a battery is built Electrically. In theory you could have 16 cells duct taped to the ceiling and down the walls in a pretty mosaic and as long as all the terminals and BMS leads are connected in the right places, then there is no difference Electricaly than if you had them in a single tall stack on a shelf Physically.

For simplicity's sake though, having all the cells in a single box makes things just easier. The aluminum box that fits 18 would be perfect as it fits all 16 of your cells and still has space in there for the BMS and wires and breakers and such.

As an example, when I built my camp battery I wanted extra features in there, so I built the box to allow space for everything inside. Physically it's a pair of 4s batteries inside, but Electrically it's a single battery.

Physically having multiple banks in physically different enclosures makes things a real PITA when you're trying to get all your BMS leads installed and if you knock one box around on accident there's not a lot to hold those thin little wires in place. So while you COULD do it, it would be a "Bad Idea" because physics is really unforgiving.
 
Luk88 said:
Which brings me to my question. What about building two smaller batteries and connecting them to a single bms? What opinions people have on this vs one big battery?
Click to expand...
Having built 2 X 8S batteries for 24 volts with enclosures from plywood, I would recommend just splurging on a container for them. There are ones you can order on Alibaba now. For my thrd 8S battery, I ordered a battery box from Amy.

Personally don't like the idea of a two 8S boxes with a Really long cable/busbar between them to make a 16S box. To me its gotta be one secure container.
 
Luk88 said:
Which brings me to my question. What about building two smaller batteries and connecting them to a single bms?
Click to expand...
You're not supposed to because the "long bus bar" that connects the two packs of 8S will affect the voltage reading on cells 9+.

It's possible to do on some BMS that allow pack splitting, which means they start a separate voltage count from 9 negative onwards. But the Orion JR2 is the only one I know for sure can do that. There could be others.
 
chrisski said:
Personally don't like the idea of a two 8S boxes with a Really long cable/busbar between them to make a 16S box.
Click to expand...
It also presents a resistance problem between the two 8S packs that could create balancing issues unless one has a very good BMS that can compensate for that.
 
So there seems to be a consensus building two 8S boxes is a bad idea. OK I just worry if I put my battery in the cellar as one 16s box I'll not be able to get it out without disassembly(due to it being heavy and me having a ladder like staircase). But these are all valid points, especially the resistance between packs one. I think I'll have to do it as one battery, just make some sort of a hoist system for it when it needs to come out.
 
Recommend contacting Amy for pricing on a battery case:

direct from Shenzhen Luyuan Technology Co., Ltd. in CN

Snacks from Shenzhen Luyuan Technology Co., Ltd.. Search High Quality Snacks Manufacturing and Exporting supplier on Alibaba.com.
szluyuan.en.alibaba.com szluyuan.en.alibaba.com

I have one order from her with another route. I built the 2 X 8S 280ah battery cases myself. FOr the second order of 1 X 8S 50 ah cells, I ordered the case from her with the batteries.
 
Luk88 said:
So there seems to be a consensus building two 8S boxes is a bad idea. OK I just worry if I put my battery in the cellar as one 16s box I'll not be able to get it out without disassembly(due to it being heavy and me having a ladder like staircase). But these are all valid points, especially the resistance between packs one. I think I'll have to do it as one battery, just make some sort of a hoist system for it when it needs to come out.
Click to expand...
If weight is an issue for mobility/lifting and you need a 48v system, try looking at 100ah cells (server rack style).

280 is def the best bang for the buck but mobility is a big concern

My build is also set up to be moved seasonally, I’m fine with sticking with multiple 8s as my peak load is minimal.
 
chrisski said:
Recommend contacting Amy for pricing on a battery case:

direct from Shenzhen Luyuan Technology Co., Ltd. in CN

Snacks from Shenzhen Luyuan Technology Co., Ltd.. Search High Quality Snacks Manufacturing and Exporting supplier on Alibaba.com.
szluyuan.en.alibaba.com szluyuan.en.alibaba.com

I have one order from her with another route. I built the 2 X 8S 280ah battery cases myself. FOr the second order of 1 X 8S 50 ah cells, I ordered the case from her with the batteries.
Click to expand...

Thanks. These prices are much better that what I saw available locally (half). But I need thermal insulation for my batteries so if I have to DIY that I may as well DIY the whole box.
 
Luk88 said:
I'm building my first lifepo battery using 16x 280ah prismatic cells. I haven't yet decided if I put the battery in an insulated, but mostly unheated workshop, or in a basement beneath.

I'm going to use a heltec 8s-20s ht820s2a 150 (200A rated, 350A max, 1~2a equalisation) BMS.

120 sq mm (4/0 awg) wiring and sb350 plugs.

I'm currently wondering what should I do about an enclosure. I considered buying an ice chest box, but there are none as big available(maybe get two). My second choice is a thin-walled aluminium box that looks like this:View attachment 207917
It will definitely need a wooden base to not collapse with the weight and insulation. But I could fit 18 280ah batteries in one of these.

Which brings me to my question. What about building two smaller batteries and connecting them to a single bms? What opinions people have on this vs one big battery?
Click to expand...
He built several 8S into 16S wall packs.
 
I used the same banks as he did for a trailer build and did the same thing.
I ran #10 thhn to each cell, mounted them to a bussbar, and fed the bms from the bussbar.
Worked great, and was modular.
 
Thanks.

BTW, for those in cold climates, what insulation do you use? I'll most likely settle on vertical arrangement in a single file (so all 16 cells in a tower).

After considering various options (ice chests etc) I decided I'll just weld the container myself. The skeleton will be using square steel tubing, the outside will be 1mm sheet steel. But I'm not sure what thickness of insulation to use.

Would 5cm (2in) of rock wool do? (I believe rock wool is almost twice better than cheap styrofoam so that would be equivalent to 10cm(4in) of styrofoam. Perhaps I could get away with using less?

The lowest temperatures realistically my battery will see will be - 25C (-13 F), but that's extreme. More typically it'll be - 10C (14F).

What wattage of heating do people use for this size battery?

Then, I did some research on cables and I ordered 4/0AWG (120mm2) wire and a pair of sb350 Blue Anderson connectors. The longer wire will be 3.5m, the shorter 2.5m. I'm borrowing a hydraulic crimper to crimp the wire.

Edit: Also, is there some trick to dial in the compression (other with screws torque)? I'm leaning towards compressing my cells.

One more thing. It was suggested I can disassemble my battery for transport. But what about compression? These cells supposedly grow when charged. So if I made sure they were discharged to storage voltage and disassembled them. Would they essentially maintain the same size? Or am I likely to discover they bulge slightly when I release the bolts let's say in 2 years from now?
 
Last edited:
Luk88 said:
Thanks.

BTW, for those in cold climates, what insulation do you use? I'll most likely settle on vertical arrangement in a single file (so all 16 cells in a tower).

After considering various options (ice chests etc) I decided I'll just weld the container myself. The skeleton will be using square steel tubing, the outside will be 1mm sheet steel. But I'm not sure what thickness of insulation to use.

Would 5cm (2in) of rock wool do? (I believe rock wool is almost twice better than cheap styrofoam so that would be equivalent to 10cm(4in) of styrofoam. Perhaps I could get away with using less?

The lowest temperatures realistically my battery will see will be - 25C (-13 F), but that's extreme. More typically it'll be - 10C (14F).

What wattage of heating do people use for this size battery?

Then, I did some research on cables and I ordered 4/0AWG (120mm2) wire and a pair of sb350 Blue Anderson connectors. The longer wire will be 3.5m, the shorter 2.5m. I'm borrowing a hydraulic crimper to crimp the wire.

Edit: Also, is there some trick to dial in the compression (other with screws torque)? I'm leaning towards compressing my cells.

One more thing. It was suggested I can disassemble my battery for transport. But what about compression? These cells supposedly grow when charged. So if I made sure they were discharged to storage voltage and disassembled them. Would they essentially maintain the same size? Or am I likely to discover they bulge slightly when I release the bolts let's say in 2 years from now?
Click to expand...
Been there, done that!
 
Luk88 said:
Wow, this is real nice, but the weight.... ouch. I'm going to use 1mm (or perhaps 0.5mm - once I check my stock) for weight. I'd much prefer something like 3mm aluminium, but the cost...
Click to expand...
Yeah, lessons learned:

Use thinner steel.
Plan for tighter insulation
Use lighter steel!
Buy the parts BEFORE you start construction.
Use thinner and lighter steel!!!
😁😁
 
Also, to those that done it. How do you top balance without it taking weeks?

The usual method with a tiny 10A psu is going to take weeks.

Take my 16S 280ah battery. Let's say at 3.2 it came from the factory it is 40% charged. 60%*280*16 = 2688AH so the 10A psu will take 269 hours or 11 days to charge it!

Even with my Q8 30A charger it will take almost 4 days at 1S!

Or do you just wire it in series, plug a high power target voltage charger to get it to let's say to 3.5V per cell (+- whatever the cell imbalance is) which would be 56V in my case then you rewire in parallel, and charge from then on to full in parallel? The problem with this approach is that you'd need to reconfigure the cells from series to parallel while they are in a compression fixture without taking them out.

I wonder how do people resolve this?
 
I use a regular battery charger to do the bulk charge and then top balance the last bit when it's over. Most BMS's that I've worked with have a switch in the app to enable balancing all the time vs the default of only when charging so I csn bulk charge, let it sit, then bulk charge again and let the BMS do the balancing.
 
Luk88 said:
I wonder how do people resolve this?
Click to expand...
We Cheat!!
LOL - I put the pack together, and attach it to the Main ESS and turn it on, let the system on a sunny day bring the cells in the new DIY pack up close to full charge, the disconnect it from the ESS and balance with a bench source the last bit up to full pack voltage. Watch for any cells that might depart from the other's voltage and use an automotive bulb to pull just that cell down while the others are charging. Put on some music and work on something else in the shop for a couple hours, checking once in awhile on the new pack. When it is close, let the BMS do the rest and put it back into the ESS and let er go.
 
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