Balance at the cell low voltage limit

Hello Everyone,
On Setting up the Buddy cells, I got the 8 straps finished.
I charged all the cells, including the new ones. I took all the cells apart and checked them all. Most were at 3.345v a couple were at 3.35v-3.36v. None were low. This is all at rest.

I am now balancing all the cells at the top. I have 34 cells all in parallel. I have a charger on them putting 4 amps into them. I have a second charger that can give me a boost up to 13amps but it kicks the CB out after a couple of minutes. I am getting 3.37v with 4amps and 3.38v with 13amps. I plan to leave this charging for some time so that I am sure that I have a full charge on all cells.

More on my process later.

Thanks,
Dan

 

Yes, this is correct.

I have had all cells in parallel for 2 days with 8-16amps going into them for much of that time. They will sit static over night. Then tomorrow I will take them apart and assemble them in series so that I can discharge them while being sure that I have taken them down at equal power drains. Then the one that goes down to 2.8v first is the lowest capacity cell.

Thanks,
Dan

 

Hello everyone,

I left cells to sit over night in parallel. Then I broke them down and check individual voltages to be sure that they are all holding the charge equally. They were all at 3.451v at rest.



Then I connected them in series and did a controlled discharge.

Attached is a picture of the setup. 34 cells=100vdc. A 100w bulb pulled 3/4amp. A hair drier pulled about 7amps. A coffee cup water heater pulled about 2amps.
The surge protector on the power strip did not like the low DC voltage so I changed that out. As I remove the low cells the overall voltage will drop so I will need to add light bulbs.
I drained the battery pack. All Cells were in series so that they all had the same current going through them. I drained 15 amps for 2 hours and 25 minutes. That is about 36ah. 3 cells hit the low limit.

This is about right I think. So it confirms that I was having more of a balance problem than a weak cell problem.

I took out the lowest capacity cells 3 or 4 at a time. I marked each cell with the number of amp hours that it took to get a full discharge on that cell. I think I have a range of 36ah to 41ah.
When I put the pack back together I will pair the lowest cells with the strongest cells.


I now have the cells back in parallel to balance them at the bottom.

After sitting over night all the cells were at 3.09v. I am now further discharging them with resistors at a rate of 5amps. this is to try to get them closer to a balance at the bottom.
Thanks,
Dan Lander

 

Hello Everyone,
I got the cells paired up.

You can see the straps that I made. Each quad pack is two buddy pairs.

I will Get my next two Kits tomorrow so finishing these will wait till after I see what is in the new kits. I will be gone for a week so After that I will finish the permanent install on the Gen2 and then start on the Gen3 and Gen1.

Thanks,
Dan Lander

 

Hi Paul,
You can get 1ohm 1watt resistors at Radio Shack. But I also got a 2.3ohm coil resistor at an auto parts store that handles about 50-100 watts.

I put 2 of the radio shack resistors in series and put that in Parallel with the coil resistor. I measured the current with a clamp-on DC ammeter. But remember that this was after I had drawn down the pack with the hair drier and light bulbs.

What I did was not perfect. But it was the best that I felt that I could do.

Thanks,
Dan

 

Hello Everyone,
I am finally overcoming my laziness. I was out of town for a week. When I got back I charged my New Gen 3 kit. It charged evenly with no balance issues. The new kits are much more trouble free that what we have had in the past.

Then I worked on bolting my new box into my 2005. That was a slow process.

Finally today I started putting my capacity paired and balanced at the bottom cells into the box. I did this with 2 sets of balancer cables. I got this all done and got my CellLogs working including the alarms. Then I started charging.

WOW, Chan is right. The cells are charging with unbelievable balance. I am 1/3 of the way through the charge and I see less than 15mv of imbalance. I have never seen balance this good, even at rest. And this good balance is with the charger running.

It looks like things are going well. Maybe some time tomorrow I will get it all completed.

Thanks,
Dan

 

Hi Andrew,
The key to safely cycling the cells is to monitor all the cells and stop charging when the first cell gets high, and to stop discharging when the first cell gets low. The CellLogs with the obnoxiously loud alarms do that for me. I am balancing at the bottom but I am not relying on balancing at the bottom to be sure that my system is remaining safe to operate.

Clearly what I am seeing is better than anything I have seen in the past. The data at the end of charge will tell us how good it really is. We will see if the balance that I am now seeing holds up.

Thanks,
Dan

 

Balancing at the bottom does not mean that you discharge that far on every cycle. It does mean that if you do ever discharge that far the pack will be balanced at that point. I will run my pack towards the top. But I need to be safe if I ever get to the bottom.

Both both botom and top were origionally based on total pack voltage, but the new kits now have additional covercharge protection that is based on the voltage of each cell. Note that this protection will undo balance at the bottom if a cell voltage goes up to 3.7v.

The low voltage cutoff is 46v for the 16 cell pack. so that would be 2.875v per cell. So 2.875v under load is the best place to balance.

Thanks,
Dan Lander

 

Hi Paul,

Yes, all the CellLog does for the alarm is to switch the red wire to connect the red wire to the most negative cell wire. I do not use the black wire. I tap off of the cell 4 wire at the CellLog to get a 12v difference from the red wire and use that 12v to drive the siren. Also I have been removing the CellLog completely from the kit to do computer downloads because the USB power can get confounded with the pack voltage. The power distribution in the CellLog is not failsafe. Be careful with them.

I am Running 28.5/23 I do not know what the delta is so I left it high. I turned off the Time alarm. But most important are the cell alarms.

I had a high cell of 3.6v as the charger was lowering the current. Then it dropped back down and the charger turned off normally.
My time was about the same.

I am looking to get some extension wires and to move my CellLogs and sirens out from under the deck so that I will be able to monitor things an seldom never need to open the deck again.

Thanks,
Dan Lander