diy solar

diy solar

32650 8p8s build with minimum insulation

DThames

Solar Wizard
Joined
Nov 22, 2019
Messages
2,669
A little more than a year ago I built an 8x8 where two 4p8s groups were set top to top with insulation between. While this worked and was compact, it don't like the idea of depending on insulation between cell groups. Physical separation is more attractive.

Getting an idea or two from some Youtube videos, I used a different configuration. Most of the cells are end to end much like in an old D cell flashlight. Not end to end connected but just assembled that way. The result is that the top of one group and the bottom of the group above it are the same voltage point. The bottom of the stack, group #4 and group #5 are bridged with nickel strips. Cell groups are connected with nickel strips that stick out the side. Welded with Kweld brand welder.

Not shown, stranded fiberglass tape around it vertically reduces movement and takes any stress off the connection tabs. Will likely cover in a heat shrink tube.

Photo description:
PackTop, what each of the 4 pack tops looked like before adding the 10mm tabs to connect the packs.
BottomGroup4to5 shows cell groups 4 and 5 connection with 10mm strips, while inside each cell group, cells are connected with 8mm strips.
GroupToGroup-TabConnection shows setup ready for welding. Trying to weld without a backing surface was a bit of a pain but built a little spacer tower to help.
Stack, welded stack, waiting for the wiring to the BMS and positive terminal.
 

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Thanks for sharing what you're doing with the cells. I hope you receive many years of use from your battery pack.
 
Here are a couple of final photos. I added a little close cell foam padding around parts of the BMS to keep stress from the heat shrink off of the small wire connections on the BMS and on the end to protect from small bumps. I used 2x10ga silicon leads and XT90 connectors so I could surge more amps in case this is used on an inverter with a motor load.
 

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Thanks for sharing your build, I appreciate it.

Just wondering. Is this a smart BMS that you can change settings and get a view on the condition of the cells, temperature, etc.?
I know that the packs that are used on an E-bike operate at an elevated temperature even if they're not pushed too hard.
I purchased the cells because I was interested in converting an old bike to electric, but I didn't get the weld quality to a point
that I felt it was safe.
 
Thanks for sharing your build, I appreciate it.

Just wondering. Is this a smart BMS that you can change settings and get a view on the condition of the cells, temperature, etc.?
I know that the packs that are used on an E-bike operate at an elevated temperature even if they're not pushed too hard.
I purchased the cells because I was interested in converting an old bike to electric, but I didn't get the weld quality to a point
that I felt it was safe.
Yes, it is a JBD 100amp with Bluetooth. I have 7 or 8 of them on various projects. Great to be able to manage all of them with the one phone app. I really like to be able to turn them off so the leads are not hot when you make connections or disconnect them.
 
Thank you for the reply.

Because of what you stated about turning off the leads before making or breaking a connection, I now have a concern about how I've been switching between the two battery banks I have in my emergency backup system.

I turn on my chargers, so they'll power all my loads on the battery pack. Then, I switch off the pack I'm currently using, the chargers are taking the entire load at this point. Then I switch on the other battery pack and turn the chargers off.

I use a resistor when hooking up the chargers and inverters to pre-charge the capacitors, but I've not figured that the BMS may have a similar condition happening. I'll have to do some studying.

Thanks again.
 
Thank you for the reply.

Because of what you stated about turning off the leads before making or breaking a connection, I now have a concern about how I've been switching between the two battery banks I have in my emergency backup system.

I turn on my chargers, so they'll power all my loads on the battery pack. Then, I switch off the pack I'm currently using, the chargers are taking the entire load at this point. Then I switch on the other battery pack and turn the chargers off.

I use a resistor when hooking up the chargers and inverters to pre-charge the capacitors, but I've not figured that the BMS may have a similar condition happening. I'll have to do some studying.

Thanks again.
Sounds like your current procedure is okay. One think "bad" about turning off the BMS output is that if connected to an inverter where the inrush is an issue, I don't like to turn it off and leave it off (for long) without a pre-charge concern. I have a small 24v battery (3p8s 18650s) that I use to precharge my 24v inverter with. It don't really even need a resistor as its current is not huge. So I can just clip the little battery across the inverter DC in, let it charge, remove it, then turn the BMS back on. I have two batteries in parallel and I can leave one BMS off until the first battery is at the same voltage as the second one and then turn the second one on. Really love those BMSs.
 
Sorry for the late response.

My inverters are never without power unless there's an issue that requires that I disconnect them. I have 4 Battle Borns 200ah for one pack, and 8 280ah cells that make the other pack. Each pack has a switch to connect and disconnect it from service. I like having two packs in case one needs work, I can stay online without disruption. Before switching to the other pack, I use Victron IP 67 chargers to supply a little more power than is needed to keep things running. I connect the chargers, the pack is switched off and the voltage readout rises to 28.8v immediately, the voltage of the chargers. I wait a few seconds, then I switch on the other pack, watching the voltage go lower to its SOC. Then I disconnect the chargers.

I know there's an inrush of current going from the chargers to the battery, and I can't adjust this on these chargers. The switches are rated at 150 amps and the BMS, on both batteries, is maximum 100 amps. I never have both batteries on at the same time, the chargers take that roll between switching battery packs. I keep the 280ah BMS's "charging" and "discharge" switches in the ON position. The Battle Borns don't have a "smart" BMS, so I can't even observe their condition.

I'll have to look into this, as I've never really thought about it. I know about the inverters needing their capacitors charged but I neglected researching whether the BMS may take a voltage/current "hit".

I'll research this and make any necessary changes to my system.

Thank you, have a great day.
 
When your new battery helps get you coffee on a road trip.
 

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