BYD modules: adding 18650/headway LFP cells to correct for weak module cells

[Maast]

Just saw a post on the DIY battery facebook group about adding cells in parallel with individual weak cells in the BYD modules to correct for them not being up to par.

I'm thinking its a pretty good idea. There'd be a good bit of trial-and-error with adding a cell, cycling it to see the results, adding/removing cells based on that result, and cycling again, rinse and repeat.

I don't see any downsides to it other than the man-hours involved. Add cells to correct for gross capacity mismatch and let a BMS correct for minor mismatches. Sounds like it'd work a lot better than just a active BMS.

18650s are 1-3.6AH and headways are 8AH but I'm sure there a million other LFP cell capacities.

 

[Mtour]

Hi all...

I am having really good results with adding headway 8ah cells to my byd powerwall, just bottom balance and add headways to cells that go(high) out of balance at the top end. My power wall 12s2p, and is stable from 49v-54.4v charging up to .07c, I am really surprised how close these cells stay in balance 10-30mv range. Without the headways i was able to charge to 54v before I would get runway cells 100-500mv out of balance. I thought about going the route of paralleling all the cells, but decided against it, just too many wires. The headsways have only been in service for two weeks, so time will tell how long or if the wall will remain stable. These days i am pulling around 700ah nightly from the byd's heating my home with a couple of mini split heat pumps.

I purchased the headways and byd batteries from batteryclearinghouse.com, headways tested 100% capacity and the byd's tested like everyone else. Guy and i came to an arrangement, i will definitely be buying from Guy again.

 

[BarkingSpider]

Please post some pictures of your powerwall setup. I have 4 BYDs in parallel for 24v that are working well, but some cells are out of balance. Your idea sound interesting. I would want to add LifePO4 cells though, I have had bad fires with 18650s,.

 

[Mtour]

 

[Mtour]

 

[Mtour]

Headways are lifepo4.. Each headway added has it own 10amp fuse..

 

[BarkingSpider]

When you said 12s2p i assumed you were talking cells, not whole byd packs. That's 192 cells.

That must have been quite a workout lifting those, 4 was hard enough for me and i stripped my packs down to reduce weight, removing the heatsinks.

I trust your shelf is super strong! I have powerwall envy.

Have you tested your full capacity? It must be close to 75kw/h. Thats some serious power. Are you a bitcoin miner?

Here is my pitiful BYD powerwall, Im getting around 3.75kw per unit. It was feeling good until today....

 

[onemorebattery]

I have powerwall envy too but I'm also diggin' that BYD hoist. Is it motorized?

 

[BarkingSpider]

Are you getting clean split phase 240v power from the MPP inverters? Would you recommend them? How many watts can they put out in total?
I hope you got a bulk discount on all those BYDs, did they arrive on a giant pallet?
Did you encounter any bad cells or any other problems?

I see you drilled out the posts like I did to get a solid connection, in addition I also used a grinder to get a flat edge, but you could use a hacksaw or even a file if your patient as the posts are solid copper. I used the nuts/bolts that held on the big heatsinks to attach the lugs. The cells never get even slightly warm even under big loads. The fan and heatsinks seemed unnecessary with my low power needs < 4kw. This made the packs a lot lighter and easier to manage.

 

[Mtour]

I have powerwall envy too but I'm also diggin' that BYD hoist. Is it motorized?

Hi

The hoist has a $69.00 winch on top, makes it really easy to load the packs. First tried to put the packs up with help and a couple step ladders, not a smart idea...The wood is ash hardwood, its not going anywhere...
I haven't done a full capacity test, but I have tested random individual cells.. They all seem to be 100-115ah range, so i wou!d think the capacity would be around 1200ah usable..

 

[KBWaldron]

At 160 lbs each, that is 1.75 tons of batteries.

Got any pictures of how you added the headways to the BYD cells?

 

[Mtour]

 

[Mtour]

Cell 5 needed 2 and cell 1 needed 1...

 

[BarkingSpider]

Do you have more data on how you calculated which cells needed boosting with the additional batteries? Was it just with the bottom balance / top balance difference?

I found the BYD cells all charge and decay so differently. The low voltage cells at 100% capacity are the high voltage cells at 20% capacity And they all charge at different rates. Be interested to see how you determined what a/h to add. Did you see a measurable increase in capacity after you did the mods?

Joe.

 

[Mtour]

Eh Joe

This is what i have found with these batts.. There are no weak cells, there are cells that have an imbalance of capacity. So what I have witnessed is a pack nearing full charge, one or more cells will be high 100-400mv approaching 3.65v. If its over 200mv i have added 2 headways and 1 headway on any cell 100mv over the average the headway are at around 3v when added. I do not stop the charging, just add the headway...voltage settles down right away. There is an average of around 8amps traveling between headway and the byd cell when first connected. What also has been observed is once the headway is added, all the cells will even out somewhat across the battery. Then on the next full discharge i bottom balance the cell with the headway added.

Also i am using a pl6 and a gt1200 fma chargers, they can do regenerative discharging which means they dump the power back into the pack the charger is drawing from instead of using a power supply. This allows the chargers to be able to discharge a single cell up to 40amps. Sure speeds thing up for balancing, I discharge a single cell at 40amps, pull it down about 30mv past the target 2.9v and let it bounce back after discharge..

This has been working so far, but who knows what will happen in the next 100-1000 cycles... No bms is going to be able to balance these batteries, qnbbm balancers are $150.00 ..so far I have installed 8 headways at $8 each...

Is this the fix for these byd's, hard to say...they have made my wall stable to 54.4v bulk setting on lv5048 three hour absorbtion set. No cell is over 3.5v at end of charge..

I have to give Guy at BCH another big thanks, i started with the bakers dozen...

Hope this helps...

Dave

 

[zorlig]

Wow! Very well done!!!!

Power wall envy indeed, so nice to see someone doing with these packs something that will actually work as desired!!!

 

[KBWaldron]

I see you came up with the same method I had for connecting to the cell terminals. Quite a handy size those holes. I used them to put my two BYD's cells in parallel. Once I've topped my batteries up I'll see whether I need to go with your solution. Thanks for the idea.

 

[KBWaldron]

Well, it looks like I will need some headways to help balance some cells. The problem is that BCH and BHU are both sold out, and there seems to be a lot of other vendors selling repackaged crap as headway. Anyone know where I could pick up a half dozen or so?

 

[Mtour]

I have cycle tested about 10 different individual random cells, all are between 100-125ah capacity. Complete packs are averaging 110ah.

I am getting 1200ah usable charging to 54.4 for 5min and the floating at 53.8v. I had to drop the absorbtion to 5min from 3hr due to back to back sunny days. These days I am on float 8hrs a day, when the sun is high solar is charging at 240ah . By end of the day charge current will have dropped to 6amps total at 53.8v.

I am happy with these results, I am getting 83% of the total max capacity of the wall. This is with no interconnecting of cells or active bms. I have purchased the materials to interconnect, but just finding it hard to justify the added complexity.