Balance at the cell low voltage limit

Dan - It may be problematic using the 4 balancers as connected for effective 16-cell pack. There are 2 balancers connected in parallel. My understanding of the balancer is that it is essentially a flying capacitor, which shuffle electron from cell to cell. With two balancers in parallel; running uncoupled; not sure the effect is what you want.

I think the easiest way to balance right now is to use a timer, as suggested else where, to cycle the charger on/off. Say 15/30 minutes on, then 30 min off so the balancer can catch up. If you know your pack is almost empty; you can set the on time longer for a set time, then go into the on/off mode to taper off.

The low cell voltage cut off is critical during discharge and the DB8 will beep; but you have to hear it !

For the time being, until Jack sent us a better system; I would err on the safe side and use shallow discharge/charge like the stock NiMh Prius pack (operating only between 40%-80% DOC).

Good luck,

 

The balancers are simple (relatively at least) shunt balancers. They bleed off energy from the highest voltage cell to try and get them all to the same voltage. There are no problems putting them in parallel and it increases the speed with which the pack can be balanced.

 

Dan,

Why bother fighting the loosing battle with the Chargery balancer. Most of the guys over the DYICar forum don't even bother balancing their (Thundersky) cells. They just try to keep DoD < 80%.

I know guys that have been running their cars for three months w/o any balancing issue. They do check but did not find any problem.

For high side balancing volt blocker works similar to the Chargery balancer but at single cell level @ $14/each it's pretty pricey. For a more cost effective solution maybe talk to evcomponents.com

I think banshee08's CellLog is the most cost effective way of for a "kit" at $60 total. You can use the cell LVC to trigger a buzzer or contactor to disconnect the kit.


BTW: I've been following ur progress blog. It's great keep those up.

 

Daox,

That was my understanding also. When you opened up the balancers what did you see? I don't think the balancers are actually transfering current from one cell to another??

-Chan

 

pbui,

The timer method may work but that again is balancing at the top which I believe is the problem. What the balancers are doing is taking charge away from the cells that have less capacity.Without effective cell monitoring during operation you will still have a problem weak or dead cells masked by the pack voltage. I am having some success with my initial balance at the bottom of the discharge curve, my cells after my discharge by two converters are < 10mV apart (static voltage).

Thanks,

Chan

 

There are a bunch of resistors on one side of the board, and the other side has a bunch of transistors. It doesn't transfer, it only shunts power as far as I could tell.

I'm starting to like the balance at the bottom option with the celllog too. I can make the arduino automatic shut down work just fine with that, and IMO these balancers don't seem to be trustworthy enough. They flake out too often, and when I saw the red light on the switch panel during my recent trip, I heard no alarms going off for low voltage. This really concerns me as it was my only method of low voltage protection. Looks like I'll be ordering the celllogs soon and tweaking the arduino automatic shut down to work with it. I need this thing to be reliable and hands off so I can get the most use out of it. The DB8s just aren't cutting it.

 

I think the balance at bottom theory is really balancing when all cells are close to the knee of the discharge curve. You have to be able to get all the cells somewhat close and you have to try not to Pass the 80% DOD point too often.

Dan is right, you have to balance at the bottom under load. So what you have to do is put a steady load on all the cells when they are all in parallel.. For my case that would mean (64cells* 39AH) 3V at 2496AH. If you were to discharge this at 20amps like I did it would take some 100 hours to get to 80% DOD. After I discharged the cells I let the cells rest for 2 days at 3.050V so that the buddy system could balance the cells closer. I then went about wiring the cells up as a 4 cell buddy pack of 48V and charged the pack without any regard as to the resulting voltages as long as I cut off when the first cell reach 3.625V. I then installed everything back in the car and ran for about 35miles to work and charged as usual. What I see now is the static voltages after my discharge cycle is very very balanced with a static voltage difference of < 10mV. Underload I am within 65mV delta between all my cells. I have not use the Chargery balancers in 4 days now without any side affect yet. Keep in mind I do monitor my cell voltage during my drive with two CellLog 8 devices.

 

Dan,

What you are purposing to do is to get all the cells down to the same charge level. That being 2.8V underload. If you let the cells balance under after your Step 4 by putting them in parallel then you have a balance pack at the bottom. I see what you are saying by budding as close as possible to equal total capacity and that may be a step that I missed in my setup. The cells may be close but they won't be equal so if you decide to balance at the top again, the same problems will arise over time. I think the key to balancing at the bottom is to not worry about the differences in voltages after the charge and when you charge the pack you are in turn balancing it since the current will go to the least charge cell first if my understanding is correct.

I would give it a try. I am not sure. I think from what I have been reading on the EV car forums is that Balancing at the top is just not a good idea if you don't have a cut off during discharge. As long as you can cut the load off the battery you can balance at the top. The goal is to get the most out of the pack and that would require that all the cells discharge equally and at the same rate and to the same state of discharge, but shunt balancing at the top will not achieve this and actually unbalance the battery pack even more. Balancing at the top will require a pretty robust BMS to get you out of danger of the low cell.

 

Excellent idea IMO.

This doens't stop you from balancing at the bottom, but it does pair dissimilar cells and will basically mean you can effectively use more of the pack before it needs to be recharged since the cells theoretically shouldn't be as unbalanced as they would be if they were randomly paired. (that was a long sentence)

 

I am new to the forum. I have been helping someone out with the issues being addressed in this thread, lots of good information here.

Looking at the diagram, I do see a problem with wiring a buddy pair as shown and that is that while under normal conditions the wires connecting the cells won't see much current, but I can envisage faults in which they could have large amounts of current flowing, which could cause the wires to overheat and start a fire. For example one of the cells could short and cause the other cell to dump all its charge into it very quickly. Better to have the buddy pairs next to each other connected with short, larger interconnects than some distance apart and connected with long, thin wires. Just my 2 cents.

Greg

 

Just wanted to update on my bottom balanced cells. I ran my 8kWh kit to autoshut off point today underload and the switchs turned off the converters. I then turned off the 2 converters and turned on only 1 converter continued driving until the pack voltage forced the one remaining converter turned off. This is a result of the 1hr 40min trip of about 41.6 miles in the City at 168mpg. The lowest cell I had underlaod was 2.715V and the highest cell was at 2.92V that I could see, really hard to see under load as it is jumping around. My pack seems very balance at this point. All the cells were registering about 3.095~3.140V after the system rested for 1 hour before my charge cycle began. I have two cells that are charging high and discharging low. Only these two cells reach 3.095V and 3.103V respectively, the rest of the cells were between 3.129V and 3.140V. The two bad cells are know problems since my infamous 16 cell failure prior to the buddy system and I will change them out if they become a problem in the future. I still have 2kWh of batteries as backup that I purchased form Jack.

This is the result of about 50 charge cycles since balancing at the bottom and not using the Chargery Balancers at all. It seems balancing at the bottom is definitely working for me. I am increasingly leaning to the side that the balancers may be the biggest culprit of the balance issue.

I unfortunately had the cellLog reset on me during the trip so no data was captured correctly. I lost a CellLog today. It just reset itself and will not register any voltages at all. That leaves me with three and I will have to see about a RMA. These cellLogs are letting me see what the cells are doing underload and letting me really put the battery to work. I can run the cells down to 2.9V without much of a scare since the pack is very much balance.

 

hi Banshee08,

did u have a bad experience with Chargery balancer (DB8) ? I was thinking that it can serve as a bottom balancer as well as a top balancer; assuming that it works as advertised.

Top Balancing : use the built-in DC in/out to latch a relay to shut off the charger

Bottom balancing: change the buzzer so that it is loud enough and annoying enough that the driver has to shut off the Enginer. The DB8, being a shunt shuffler, bleeds off charges from the high cells toward the lowest (weakest) cell - thus bottom balancing.

Is there anything wrong with my thinking ?

By the way; I discover by accident that temperature has a big effect on these LiFe. The sun was shining on half of my pack; the DB8 alarmed within 5 minutes of turning on the charger because the warm cells reached 3.7v so quickly while the cooler cells were still around 3.4 v

 

The goal of bottom balance is to get all the cells to the same discharge point or DOD. I am not sure if the DB8 is s shunt shuffler I think it is just a shunt bleeder, where it just bleeds off charge from the higher cells until it reaches the lowest cell. The DB8 can be used as a bottom balancer but you really need to put the batteries under load of at least 10A to see what there true discharge state is. Static voltages can differ between cells quite a bit depending on the quality and manufacturing lots.

I stopped using the balancers during charging and started using the CellLogs 8 as a alarm cause I don't want to unbalance the batteries by balancing at the top. I don't really care if the cells are unbalanced at the top of the charge curve as longs as they discharge to the same level. Every cell may have a slightly different capacity so if the pack is balance at the bottom they may end up unbalanced when they are fully charged but hopefully the charge current is distributed evenly to each cell in the pack. If I were to balance my pack at the top at this point, then what I would be doing is removing charge from cells with high voltages which would change the amount of charge held by the high cells.

I guess the main point is that the static voltages are not a good indication of SOC, you really need these batteries underload to really see the SOC. What I used to do is put the balancers after my discharge cycle to let the system balance at the bottom but I stop since I didn't really use the batteries up every time I ran the car. Now days I run the kit and let the system cool and rest for about 1.5 hours and start the charger for about 7hrs. At that point I will have 2 weaks cell groups reaching 3.6V and I will manually cut off the charger. Under a 100A load, my cells stay about 75mV from each other until about half the cells reach 2.9V where I would see up to a 120mV difference.

 

This is a good point.

So you only had to bottom balance your pack once so far ?

I guess the premise here, as well as from Richard @ EV Porsch, is that each cell has its own charge capacity and the capacity essentially remains unchanged.

 

Yes I have only bottom balance my pack once. It was pretty extreme in that the static voltages after the balancing was 3.050V and it took me a whole week to balance 64 cells. Jack Rickard is where I learned of this concept. His EV Porsche replica and EV Mini project is quite insightful and he has been quite friendly in answering some questions in email about some of my EV MR2 conversion questions.