diy solar

diy solar

Feedback request Xuba battery build (HS senior project)

I put a multimeter on the cell that has the capacity tester hooked up to it and it read 3.3. It still reads 2.85 volts on the screen and is continuing to run and is now at 108ah after 5.5 hours of running at 20.amps. I manually check the voltage pretty often to make sure I catch it when the actual voltage drops below 2.85 so I can be sure it will notify me when it hits 2.6 which is what I set the low voltage alarm at.

After it finishes I will try to do a reset and attach to another cell to see what it reads. Perhaps I messed something up when I was pressing buttons trying to set the low voltage to 2.6?
 
yes the wires are pretty warm.

I got the load tester that will used in his capacity testing video:

MakerHawk Electronic Load Tester USB Load Tester 150W 200V 20A Resistor Adjustable Constant Current Battery Capacity Tester Module Intelligent Discharge Resistance Power Tester

 
I reduced the amps on the load tester and the voltage shown on the screen of the load tester is closer to being accurate now. It sounds like I was trying to push too many amps through the small wires. I cut it down to 2 amps now and will use until I get thicker wire that I can use.
 
I made a new connector with 10 gauge wire and have it back and running at 20 amps. So far capacity reads 200 ah. WOW this is taking longer than I ever imagined.

I want to say thanks very much to everyone that has helped so far. I have really enjoyed every bit of this. I have done a lot of things this week that I have never done before, and learned a ton. The BMS concerns me to no end but one step at a time :unsure:
 
At last...I was not sure if the capacity tester would auto shut off or just beep as the reviews were a bit contradictory, but in fact it shut off once it reached 2.6. I checked the cells voltage with my multimeter and it read 2.777, not sure which is right, but the capacity read at 279.67ah. I had a hard time setting the low voltage disconnect on the capacity tester when I started and ended up resetting the test after a few minutes so I imagine it was right at 280 ah for the one cell. I tested the cell that seemed to be not equalizing well when I was charging all in parallel, so I am happy that it is ok.

I think my plan forward is to recharge the cell tomorrow, connect the cells in series and try to attach the BMS. I hope some of you are still around as I have a few questions....
 
Before I get to the BMS, quick question on capacity testing:

Running the capacity test this weekend on one cell showed me that this will take forever. I am currently charging the cell that was depleted. Next is there not two choices

1) wire all cells in series and attach the BMS; then use the capacity tester to test all cells since the BMS should give me the info on each individual cell. Is this going to basically take a week plus to do this? The one cell took me more than a day, so multiply by 8....is there a better way?

2) Presumably I could put together the "hand cart system" and then just use the completed project for a few days to run things in our house (eg., internet, tv, router, etc.). Is this more dangerous or less effective (give worse info) than using the capacity tester approach?

I know I was advised to do all together initially, but having zero experience with any of this, I felt much more comfortable risking one cell to understand the capacity tester before embarking on the BMS with the inverter, etc. TIA
 
I would charge the one cell to 3.5 and then connect all cells in parallel for at least a day before testing again.

If you get one cell out of balance and try charging with hi and low cells in the pack, the whole pack will charge to a target voltage...The cells with the lower voltage will cause the whole pack voltage to be lower and the cells with the higher voltage will spike up a ton and will bloat.....

<<<<DO NOT CHARGE A CELL in a PACK IF ANY OF THE CELLS ARE NOT ALL BALANCED>>>>

I know from experience.....very sad....
 
Re my concerns on the BMS wiring. The issues are more around tying the MPP Hybrid Inverter and BMS together. Adding rings to the BMS wires and connecting to the battery that is arranged/wired in series seems straightforward. Looking at Will's picture of the handcart system:


The main negative feeds directly to the Hybrid Inverter (HI) and the main positive feeds to the HI via a shunt, then a circuit breaker. The BMS is wired into the shunt and also wired into the HI switch. Is there an easier way to wire the BMS as opposed to directly to the HI switch

I was advised to include "a pre charge resister to charge the capacitors in the inverter, if you don't it will likely put your BMS into over current shutdown mode." Where exactly does that go in this setup?

Looking at the Detailed Assembly instructions for the Overkill BMS, the dual heavy gauge wires C- are bolted to the frame for a negative chassis ground system. Given I am doing the hand cart system, do I still do it this way?
 
I would charge the one cell to 3.5 and then connect all cells in parallel for at least a day before testing again.

If you get one cell out of balance and try charging with hi and low cells in the pack, the whole pack will charge to a target voltage...The cells with the lower voltage will cause the whole pack voltage to be lower and the cells with the higher voltage will spike up a ton and will bloat.....

<<<<DO NOT CHARGE A CELL in a PACK IF ANY OF THE CELLS ARE NOT ALL BALANCED>>>>

I know from experience.....very sad....
Thanks very much for the reminder. I knew that but sadly was getting ready to miss it...
 
Re my concerns on the BMS wiring. The issues are more around tying the MPP Hybrid Inverter and BMS together. Adding rings to the BMS wires and connecting to the battery that is arranged/wired in series seems straightforward. Looking at Will's picture of the handcart system:


The main negative feeds directly to the Hybrid Inverter (HI) and the main positive feeds to the HI via a shunt, then a circuit breaker. The BMS is wired into the shunt and also wired into the HI switch. Is there an easier way to wire the BMS as opposed to directly to the HI switch

I was advised to include "a pre charge resister to charge the capacitors in the inverter, if you don't it will likely put your BMS into over current shutdown mode." Where exactly does that go in this setup?

Looking at the Detailed Assembly instructions for the Overkill BMS, the dual heavy gauge wires C- are bolted to the frame for a negative chassis ground system. Given I am doing the hand cart system, do I still do it this way?
I connected my charge resister to the terminals on either side of my main circuit breaker/switch/disconnect.

I wouldn't ground the negative to the frame of the cart.

You might be able to put the Shunt on the Negative side if that makes your wiring easier. I have mine on the negative. I think the instructions wanted me to do it that way.
 
I would charge the cell you used for testing back up to 3.65V and then connect all of the cells in parallel again and charge all of them back to 3.65V to ensure they are balanced. You don't want to assemble your battery pack until all of the cells are balanced. Once your cells are balanced then assemble your battery. The exact connections may very based on your shunt/monitor. Do you have one, if yes which one is it. I would expect the following:

Neg battery terminal to BMS B-
BMS C- to Shunt Bat terminal
Shunt load terminal to Negative buss bar
Negative buss bar to Inverter negative Connection

Pos battery terminal to fuse/breaker
Fuse/breaker to Pos buss bar.
Pos buss bar to fuse/breaker for inverter
Fuse/breaker to inverter Pos terminal

If you will only ever have the inverter then you can leave out both buss bars and one of the fuses/breakers but I would recommend leaving then is as it provides a location to connect up other things like a charger or other loads.

I don't now if you already have all of he fuses/breakers but if not cableexperts seems to have the best price on the breakers by far hat I have found.

If you have a power monitor then you can do load testing of the completed battery using household loads via the inverter.

The precharge resister is used to connect the positive battery terminal to the pos input on the inverter. You can touch it across the inverter breaker/fuse for 30 seconds once everything is wired up before switching the breaker on. This will slowly charge the capacitors so when you trun on the breaker it won't have a high inrush of current.

You don;t want the resister permanently connected across the breaker as then if the breaker is off you will still have current flowing through the resister which will draw down your battery. You can either just touch the resister leads across the breaker or you can wire up a push button switch and press it for the 30 seconds or so.
 
Gotta say ... this thread is great. I will be referencing it often as I work through my project when things get here.
 
I agree. I have been very fortunate to have several posters be patient and try to walk me through this process...
I agree...I have screwed things up and we all make mistakes. I just don't want people to make the same mistakes I have.....

Good Luck to alllllll…...Ask lots of questions, their are a lot of good people on here...unlike other forums.....solarpanelT….hmmm
 
Hi, I'm in a similar stage of setting up an 8S battery using 202AH prismatic cells. I also have the Overkill Solar BMS. I haven't done top balancing yet - I don't have a similar bench power supply (yet) . So to bring all my cells to a high state of charge I wired up my 8S pack with the BMS, and charged using one 260Watt solar panel attached to one of my PWM solar charge controllers. The pack charged within 3 days of sun, and the BMS did its job and cuts off charging when the first cell reaches the cut-off I have defined as 3.65 volts. The Overkill BMS tries to balance the cells after the first cell reaches 3.4volts. However it doesn't seem to balance very fast. My pack has been in this state for several days, and all the cells still aren't balanced. I think I will have to disassemble it all, connect in parallel and do the parallel top balancing step that I skipped. So far I like the Overkill Solar BMS, however sometimes the bluetooth connection is a bit wonky, and it takes me several tries to get it to connect to my phone. I think I should just use an old phone and leave it near the battery, not use my main phone and have to reconnect every time I want to check it.

When you do your capacity test you will want to do it at close to a 0.2 C rate. In other words, drain such that the pack should be 20% depleted per hour and the entire discharge takes about 5 hours. For your pack that would be 280AH/5hours = 56Amps. So your load should be in the neighborhood of 25V x 56 Amps = 1400 Watts. But if your load is through the inverter, your inverter efficiency will probably be somewhere between 90-95% efficient, so you may want to aim more for 1300 Watts. If you find a good 24VDC load at 1400 Watts, you could do it without using the inverter. Generally if you do your load test at less than the 0.2C rate, you should get a bit more than 280AH. If you do it faster than 0.2C it should be a bit less. The capacity of the battery is rated at the 0.2C rate, so if possible that's how you should test it.

For my system I decided not to go with a hand truck. Instead I'm building my battery inside an ice chest. The cells weigh almost 70 lbs, but with the handles on the ice chest I can manage to move it by myself with just a little effort. I could have used an ice chest with wheels and a handle for better portability, but I plan to just leave this in my garage. For the inverter and charge controllers I am mounting them on a piece of plywood attached to a flat wheeled dolly. I plan to sit the battery on the wheeled dolly, and the whole thing will be pretty compact and have a low center of gravity unlike a hand truck system. I also 3d printed some 4mm plastic strips to act as spacers between the cells - to give them a bit of breathing room so that with heavy use they can dissapate heat just a bit better.

Another bit of advice is to learn all you can about proper wire sizing, choosing good quality copper wires. Learn how to properly crimp connections, ring terminals, MC4 connectors, Anderson Powerpoles, etc. I am north of 50 years old and have been playing with batteries and electronics off and on since I was a teen ager, and I had to recently re-learn how to make connectors for high amperage applications.
 
Instead I'm building my battery inside an ice chest.

Do you know which ice chest? Can you provide a link? And are you building a square pack or just going to make it long?

I am north of 50 years old and have been playing with batteries and electronics off and on since I was a teen ager, and I had to recently re-learn how to make connectors for high amperage applications.

Me too, me too, me too....lol. Although I am a little more north than you.
 
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