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Discussion Starter #1
We have just recieved our shipment of 49, 100-amp hour thundersky batteries. We have just finished measuring them all, finding them to be very consistent, all measuring at 3.327-8 volt range with the exception of one box that was in the 3.316-7 volt range. Should we top balance them?(and how?) or should we bottom balance them?
 

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We have just recieved our shipment of 49, 100-amp hour thundersky batteries. We have just finished measuring them all, finding them to be very consistent, all measuring at 3.327-8 volt range with the exception of one box that was in the 3.316-7 volt range. Should we top balance them?(and how?) or should we bottom balance them?
Are you looking to start another flame war? lol. There is lots written about this topic. Check it out. (Perhaps you should have done that first??)
 

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Discussion Starter #4
I have been reading up on the posts, and was hoping for something a bit more specific, also we have a zivan that we are looking to have reprogrammed.
 

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get a single cell charger, parallel them all, and fully charge them all. Use copper wire to connect all of the + terminals together and connect to the + of a single cell charger for lifepo4. Then connect the - terminals together and connect that to the - of a single cell charger.

This will slowly charge them to the same level and insure they're equalized. Then you can take them out of parallel, and wire as your project requires (I'm assuming series). This will make sure they're all the same voltage and on the top end of the charge, they'll be balanced.

Do you have any sort of Monitor/BMS/etc?
 

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Discussion Starter #6
We decided to go without a bms(mostly due to cost, we're just a high school program) but we may get one further down the road.
 

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If you top balance (no wars and flames, please, I personelly bottom balance), like the proposal from Frodus, you should have a realy good power supply.
It should provide a contant voltage and a huge output amperage.
You have to get max. 18kWh in your cells.
Normally they should be half charged, so eventually 9kWh have to be charged up.

At 3,8V and with a 5A output of a DC power supply it would take a while/month :)

By the way: the resting voltage (no load) is absolutly meaningless.
 

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I guess they are all at the same level. The CALB cells are delivered so. Perhaps someone with ThunderSky / Winston battery cells could give a reply?!?

My personal recommendation would be ~20 to 30 Amps. This results from my personal experience and opportunities.
Perhaps there is a better solution, but this works fine to me.

Under load, the voltage sags to a accurate level of voltage.
If you take of the load, it will rais up quickly.
Few days ago I drained my batteries to 2,85V @ 27A, took the load of and the voltage raised up to 3,22V, such as befor the drain.

Which Zivan did you get?
 

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Discussion Starter #10
We have a zivan NG-3 that we have had for about eight years, and its set to our lead acid batteries, we are looking into having it reprogrammed to the lithium algorithms.
 

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Do you have ANYTHING monitoring cells during charge and discharge?

I'd try to see if you can get something in place. It doesn't have to balance, but you should have something in place with that many batteries in series. If one goes too low (or too high) during discharge (or charge), you need to stop the controller (or charger). You should have some protection in place at all times, IMHO.

Whether it includes a balancing setup is up to you.
 

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You need something that monitors on a cell level, not pack level.

The CA only monitors pack voltage and current, not cellular. So thats a "no". It also doesn't do Cell level HVC and LVC.


Consider a paktrakr or MiniBMS for bare minimum protection.
 

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You need something that monitors on a cell level, not pack level.

The CA only monitors pack voltage and current, not cellular. So thats a "no". It also doesn't do Cell level HVC and LVC.


Consider a paktrakr or MiniBMS for bare minimum protection.
Come on frodus, he stated that he's decided to go without a BMS. Don't act/profess that it's an absolute necessity.
 

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Come on Icec0o1, Read my posts again, especially this one: http://www.diyelectriccar.com/forums/showpost.php?p=232724&postcount=11

I said a monitoring system that monitors the cell levels is neccessary. A FULL BMS is not required. I don't care if he balances or not, thats not what I'm arguing, but he's got nothing to tell him when a cell is high, or when its low. So how does he know when one cell hits full charge? or when one cell hits full discharge. Does he think he can just drive around while sucking 100's of amps and then measure them at the end of a drive and think they're ok? That tells you nothing. Batteries sag a ton at higher current discharge, and you wouldn't see that unless you're monitoring somehow.

You hit overdischarge and go towards 0V, you risk reversing a cell, but you wouldn't KNOW that because you are only monitoring pack voltage. Even the guys using no BMS are throwing on some of those Cell-log's and wiring them to an alarm for HVC and LVC.

Come back when you've got something worth listening to, Lurker. Or do you actually have some useful experience with lithium charging and discharging you wish to share?
 

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yes, we have a cycle analyst to monitor our pack.
Just ignore frodus. If you top balance, be careful not to go below 80% DOD. If you bottom balance, you have to spend extra effort in setting up your Zevan charger. Either way, having wires from every battery terminal going through your vehicle, even fused, is dangerous and is more likely to kill your batteries than not having cell level monitoring. Plenty of evidence that if you properly balance and set up your charger, you can charge them 300-500 cycles without much diversion between the batteries.
 

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ZANDARRR,

Some things to consider:

First, do not use a PakTrakr unless you have a balancing BMS. The PakTrakr will unbalance your pack! It draws its power from only the first three cells connected to each remote and the remote which powers the display draws even more energy from those three cells.

The charging parameters given by the TS documentation is for a charging procedure. This is important to note. This means that if the ending current is lower than the standard procedure you will want to end your charge at a lower voltage than specified in the standard procedure. If your NG3 follows the same profile that my NG1 does it will end with less than 200mA of charging current for 45-60 minutes. If your cutoff voltage is set too high you may over charge your cells. Resting voltage for my 2009 cells is just under 3.4vpc so charging at low currents at voltages above this can overcharge them. What I have found with my Zivan NG1 and my 200Ah 20 cell pack is that it only takes about 18min to go from the point that the charger first starts to cut back current until the cells are full. I am charging to 3.485vpc. The voltage measured is dependent on the current. If charging stops at a higher current, the ending voltage would have to be higher to get the same charge in but I don't have any data to show what the change would actually have to be.

If you cant get Zivan to program your charger for a specific cutoff voltage then have the charger configured for a smaller number of cells than you will actually use. This will effectively lower your cutoff vpc value. For example, if the charger will only be configured for a 3.65vpc cutoff voltage and you are going to use 45 cells in your pack have the charger programmed for 43 cells at 3.65vpc which will give a cutoff voltage of 156.95V. When you use it to charge up 45 cells they will be at an average of 3.488vpc. This should give you a long life with no problems. So far I'm not seeing the cell "drift" that so many warn me about and I've been running with no balancing for over 8 months.

On balancing: With no BMS or cell level monitoring you need to balance at one end or the other.

The advantage of balancing at the top is that it is easy to do and you can easily monitor how the cells are doing at the end of charge. I check mine during the last stage of charging. I watch for when the charger is only pulling 4-5W from the wall and then take my measurements. I've been doing this each month so I have some data on what happens over time. If I didn't care about the data I would only collect it 2-3 times per year as a precaution. Your pack will be fine as along as you don't go beyond 80%DOD (20% SOC) but if someone pushes the limits and goes until the voltage drops it will be a miracle if you don't kill off at least one cell.

If you balance at the bottom, say 2.8V open circuit after resting for a few hours, you will protect against a reversed cell if someone pushes the limits on range because all the cells will drop in voltage together. The problem is just moved to the other end of the charge curve. If you go with the bottom balance approach you will want to baby sit the cells during the first couple of charges in series. Charge to 3.5vpc x number of cells and be monitoring the voltages of each cell to make sure none of them go too high. This will happen very quickly even at low currents. If all the cells are relatively close then you should be ok.

With either balance method carefully record the individual cell voltages at the very end of charge against the cell serial number or hand number the cells and use that. Periodically check the voltages at the end of charge so you can see what is going on with your pack. This will also give you a good handle on how these LiFePO4 cells behave.

On the CA I'm finding that the Ah out is very close to the Ah put back in by the charger depending on how many days I go between charges or how deep the discharge was. The "extra" going in is generally less than 2% of what was taken out.

I'll send you my data in a PM so you can look at it. Maybe it will help you in your decision. [Edit: I sent the data to your teacher since I didn't have your email address.]
 

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Discussion Starter #19
Thanks Gizmo our instructor printed off the e-mail this morning, and as always your information has been incredibly helpful! We have looked into modifying our zivan via the potentiometer, but ten percent is not enough to get the top voltage we need.
 

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I check mine during the last stage of charging. I watch for when the charger is only pulling 4-5W from the wall and then take my measurements. I've been doing this each month so I have some data on what happens over time.

Gizmo, I would LOVE to hear more about your pack balance over time. How many months have you collected, and have you had to 're-balance' at all.... or what your criteria is for out of balance enough where you want to re-balance?

Just going back a few posts here, it sound like you may be using very much the same process I am in waiting for end of charge, a brief settling period while still plugged in to slow the settling out of the the knee, and then trying to get all cell measurements while they are up in the top knee and showing any balance issues....


It sounds like you are a top balancing, non-shunt BMS person?

oohhh, and I am using a CA as well. Bad news for me is that with a 120v nominal pack voltage, the internals are 'oversaturated' according to Justin at ebikes, which is why I cannot get the amp-hr counter to zero and stay zeroed, and also why I cannot tune the voltage reading to match voltmeter... it 'pegs' and aparrently needs n internal resistor changed from 220 ohm to 330 ohm? Have you done this?
 
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