[Ben321]

I posted recently about my struggle with a broken BMS from an Enginer PHEV kit. I initially tried to fix the BMS myself but without success.

During the process, I did manually top balance all my cells one by one using a LiPro balance charger Imax B6.

That was before I decided to go without BMS, and that is unfortunate because now I need to go the opposite way: discharge the whole pack to about 2.7V per cell.... it is now fully charged at 3.5V per cell and we are talking about 90AH cells.

The LiPro small charger is able to discharge a cell, but it will do it at 2AMP. Quick calculation means it would take 45hours per cell, that is too long.

So I am looking for a safe way to discharge my pack, cheap if possible. The challenge is to have something that could be unnatended if possible and that automatically disconnect.

I have some ideasusing some diode. Maybe by placing a couple of power diode in series i can make a device that dischare a single cell and naturally stop discharging at around 2.7V (2.7 divided by .7 per diode, that seems legit but maybe too good to be true).

Annyway there are many experienced EV builder out there who bottom ballance regularily, so I am looking for your advice.

Thanks in advance

 

[onegreenev]

I posted recently about my struggle with a broken BMS from an Enginer PHEV kit. I initially tried to fix the BMS myself but without success.

During the process, I did manually top balance all my cells one by one using a LiPro balance charger Imax B6.

That was before I decided to go without BMS, and that is unfortunate because now I need to go the opposite way: discharge the whole pack to about 2.7V per cell.... it is now fully charged at 3.5V per cell and we are talking about 90AH cells.

The LiPro small charger is able to discharge a cell, but it will do it at 2AMP. Quick calculation means it would take 45hours per cell, that is too long.

So I am looking for a safe way to discharge my pack, cheap if possible. The challenge is to have something that could be unnatended if possible and that automatically disconnect.

I have some ideasusing some diode. Maybe by placing a couple of power diode in series i can make a device that dischare a single cell and naturally stop discharging at around 2.7V (2.7 divided by .7 per diode, that seems legit but maybe too good to be true).

Annyway there are many experienced EV builder out there who bottom ballance regularily, so I am looking for your advice.

Thanks in advance

JLD 404 setup with a switch and contactor connected to a motor to discharge the cells. Set up to discharge to like 2.7 volts. So when you have your meter setup correctly you can run a motor at high amps with a single cell and when that cell reaches 2.7 volts it will disconnect and when the battery bounces back up it will again energize the system and the motor will again begin to drain the cell and when it reaches 2.7 volts again it will again shut off. It will continue to do this until the cell can no longer bounce above 2.7 volts and at that point your cell is done. Do this for each cell until done. They will all be balanced perfectly.

I set mine so if they no longer trigger after an hour of rest then I consider it done enough. It may over a few days rise enough to trigger but there is so little to drain it once triggered it might as well not have done so.

So if you forget to check your cell will never go below 2.7 volts as long as all your setup is working properly. Works great and with a high current discharge like you get from a big motor then you are golden.

If you need a motor you can actually use the motor in your vehicle if it happens to be a DC motor. Other wise you will want to find another source to connect your cell up to for discharge. I just happen to have a few extra DC motors hanging around here that are perfect for the setup.

Don't do this with out the setup to shut it all down. I tried that and if your voltage is getting close to the end voltage you can't walk away. I did and lost a cell in a matter of 4 minutes when I went to pour and fix up a quick cup of coffee.

But with the JLD and all that I never lost a cell and all cells are well balanced.

Once I have the cells at 2.7 volts or for my case 2.6 volts I set my low voltage cut off on my controller to 2.8 volts. That way I will never go below my balance voltage on the low end. EVER. I have my current set so it will never go below that set voltage.

Pretty simple. Works and you can walk away without worry if it is all working correctly.

Pete

Hope this helps.

Might be time to revisit this topic.

 

[Sunking]

OK so what is the battery used for and voltage?

So first option is to put the battery into whatever i is used for and turn it on and let it do its thing until the batteries are almost completely discharged.

Not sure why you only charged one battery at a time if you had a Hobby charger that is good up to 6S. You could have charged 6 batteries at a time in the same amount of time it took you to charge 1-cell.

First I will give you a hint how to balance any pack at the Top or Bottom. First thing you do is wire all the cells in parallel and walk away for a few hours to allow all the batteries to Equalize Voltage.

If you Bottom Balance, leave the cells in parallel, and then discharge them to 2.5 volts and you are done. You really do not have to worry about over discharging a single cell LFP battery. With all the cells in Parallel makes it a single cell. Over discharging a cell occurs when you have more than 1 cell in series. Regardless if it is 2 cells or 100 cells in series. There is going to be that one battery that reaches 2.5 volts before all the others. It is those adjacent cells that cause the damage driving the empty cell into reverse polarity. With no cells in series, that problem is eliminated.

One last tip. When you bottom Balance you really want the battery rested voltage to be around 2.5 volts. If you stop discharging down to 2.5 volts initially and call it good is the incorrect way. The reason is 10 minutes later, the cell voltage creeps back up after rest. So you want to finish when all cells are rested and reading 2.5 volts and holds there over night.

If you had a good RC Hobby Charger makes this a piece of cake. For Bottom Balance just do the bulk discharge in an EV or whatever device you use the battery for. Take them out, put in parallel, connect the Hobby Charger and set it to discharge to 2.5 volts and walk away. Come back next day and every cell is rested at 2.5 volts. Wire them back in series and charge them up.

 

[Sunking]

Pretty simple. Works and you can walk away without worry if it is all working correctly.

Hey Pete, just watched your video, well most of it. Anyway I have a couple of comments I would like to share with you.

First is do not quit your day job because Tim the Toolman Taylor you are not my friend. Only way you can get to have your own hardware TV show is if that wife we hear in the background is another Pamela Anderson with great big (.)(.) That way no one will ever notice you in the picture.

OK enough giggles, time to get serious. I took notice you have a PL8 Hobby Charger. Don't you just love that thing? Anyway we are on the same page on Bulk Discharge. Only thing I can think of is to offer some other methods for Bulk Discharge that might be less expensive and smaller scale. Example you can use Car Headlights. the heating element coil in that old hair blow dryer that is never used.

Additionally with respect to PL8 reference after watching the video, 2 things occurred to me.

On the Fine Discharge using the PL8, you might of missed a feature that makes it a bit easier and precise. You can go into settings and check/change Discharge Termination. You select the current or percentage you want to finish and terminate at. Set it to as low as possible. Example 1 amp: Initially is constant current until voltage drops to 2.5 volts. then holds Constant Voltage until current tapers off to the set value of 1-amp. So if you have a 100 AH battery 1 amp is C/100. The battery voltage will not bounce back. At worse no significant Bounce. I came away with the impression you set your PL8 to terminate as soon the battery voltage = 2.5 volts. You had to keep coming back, and lower the discharge rate a few times. You want to terminate on Current Value. Do that and you really can walk away, come back the next day and you are done. Just like fully charging but in reverse. Sounds like you terminate at the end of the Bulk phase. You have to keep coming back to lower the charge rate amps.

Almost forgot. You can use your PL8 to discharge at 1330 watts. The catch is you have to use the Regeneration Discharge. That means have another battery to discharge a battery. The PL8 is bi-directional charger. You can use a battery to charge another battery. Just in case you don't have a motor laying around.

 

[Ben321]

Not sure why you only charged one battery at a time if you had a Hobby charger that is good up to 6S. You could have charged 6 batteries at a time in the same amount of time it took you to charge 1-cell.

This is my secondary charger, the main charger is a 15Amp 58.4 smart charger for lifepo4 (that charging voltage is too high i will have to figure out how to tweak it by adjusting some potentiometer, i will post another thread to get help for that too). So my only balancing cable are fore 8 cells. This small charger does not have a physical plug for 8 cells, top is 6 cells. It does not come with balancing cables and no store nearby sell these. I did not wanted to wait another 1 month of shipping, so without balancing cable = one cell at a time. That is the reason (I did tried to contact 6 different electronic store in the Montreal Area in Canada and nobody had a clue what a balancing cable was).

My first charging attempt was wiht the 15Amp smart charger and I was checking the cell voltage periodically. It was fine for the first 4 hours, but near the end One of my cell reached 4.2V. I hope i have not permanetly damaged it. Worse case scenario I do have 4 extra cells so if I need to discard this one i will. It was not warm at all though and the voltage settled back to 3.6V the next days. The other cells were still at 3.4V each so I used the small charger to top balance them one by one after that. That of corse was not very productive since I want to botom balance, but at that time I though I would still be able to fix the BMS. I have to learn from my mistake that is how life is.

I could plug the whole kit in my Prius, drive it in electric mode for maybe 20KM and that should discharge it. That is a viable option, i am just scared of discharging it too much. I would prefer balancing all cell in a more stable environment but you are right I could very much use the DC motor of my Toyota Prius 2008 to achieve that.

 

[Ben321]

JLD 404 setup with a switch and contactor connected to a motor to discharge the cells. Set up to discharge to like 2.7 volts. So when you have your meter setup correctly you can run a motor at high amps with a single cell and when that cell reaches 2.7 volts it will disconnect and when the battery bounces back up it will again energize the system and the motor will again begin to drain the cell and when it reaches 2.7 volts again it will again shut off. It will continue to do this until the cell can no longer bounce above 2.7 volts and at that point your cell is done. Do this for each cell until done. They will all be balanced perfectly.

I set mine so if they no longer trigger after an hour of rest then I consider it done enough. It may over a few days rise enough to trigger but there is so little to drain it once triggered it might as well not have done so.

So if you forget to check your cell will never go below 2.7 volts as long as all your setup is working properly. Works great and with a high current discharge like you get from a big motor then you are golden.

If you need a motor you can actually use the motor in your vehicle if it happens to be a DC motor. Other wise you will want to find another source to connect your cell up to for discharge. I just happen to have a few extra DC motors hanging around here that are perfect for the setup.

Don't do this with out the setup to shut it all down. I tried that and if your voltage is getting close to the end voltage you can't walk away. I did and lost a cell in a matter of 4 minutes when I went to pour and fix up a quick cup of coffee.

But with the JLD and all that I never lost a cell and all cells are well balanced.

Once I have the cells at 2.7 volts or for my case 2.6 volts I set my low voltage cut off on my controller to 2.8 volts. That way I will never go below my balance voltage on the low end. EVER. I have my current set so it will never go below that set voltage.

Pretty simple. Works and you can walk away without worry if it is all working correctly.

Pete

Hope this helps.

Might be time to revisit this topic.

Thanks a lot for all this info, I will review it all tomorow (getting a bit tired now) but I wanted to tank you right away.

 

[onegreenev]

Hey Pete, just watched your video, well most of it. Anyway I have a couple of comments I would like to share with you.

First is do not quit your day job because Tim the Toolman Taylor you are not my friend. Only way you can get to have your own hardware TV show is if that wife we hear in the background is another Pamela Anderson with great big (.)(.) That way no one will ever notice you in the picture.

OK enough giggles, time to get serious. I took notice you have a PL8 Hobby Charger. Don't you just love that thing? Anyway we are on the same page on Bulk Discharge. Only thing I can think of is to offer some other methods for Bulk Discharge that might be less expensive and smaller scale. Example you can use Car Headlights. the heating element coil in that old hair blow dryer that is never used.

Additionally with respect to PL8 reference after watching the video, 2 things occurred to me.

On the Fine Discharge using the PL8, you might of missed a feature that makes it a bit easier and precise. You can go into settings and check/change Discharge Termination. You select the current or percentage you want to finish and terminate at. Set it to as low as possible. Example 1 amp: Initially is constant current until voltage drops to 2.5 volts. then holds Constant Voltage until current tapers off to the set value of 1-amp. So if you have a 100 AH battery 1 amp is C/100. The battery voltage will not bounce back. At worse no significant Bounce. I came away with the impression you set your PL8 to terminate as soon the battery voltage = 2.5 volts. You had to keep coming back, and lower the discharge rate a few times. You want to terminate on Current Value. Do that and you really can walk away, come back the next day and you are done. Just like fully charging but in reverse. Sounds like you terminate at the end of the Bulk phase. You have to keep coming back to lower the charge rate amps.

Almost forgot. You can use your PL8 to discharge at 1330 watts. The catch is you have to use the Regeneration Discharge. That means have another battery to discharge a battery. The PL8 is bi-directional charger. You can use a battery to charge another battery. Just in case you don't have a motor laying around.

Tool Man Taylor Im not, Just better. Yes, there are many ways to discharge your cells. Yes, you can parallel them then connect that set of parallel cells to the motor and JLD and let it run with the little setup that I have done. Then after the set has not rebounded I'd disconnect then hook up the PowerLab8 and do a fine discharge using the same type of algorithm that is used for charging. Constant Current then Constant Voltage until you are like 1 amp or even less. At that point if you choose 2.6 volts you should be just fine. You don't want to just discharge bulk to 2.6 volts and say you are good. That won't work. You need to hold that voltage until those batteries no longer rebound. Then set your cut off above your voltage you used for your bottom balance. So I did say in the video 2.8 but that was for a bulk. I actually do 2.5 volts as my balance voltage.

I have done this with my Leaf Modules and have had great success.

I really love the PowerLab 8 but for hard 40amp discharging or charging you need to have some serious cables because the stock ones just suck and will melt the coating.

For bulk first drive your vehicle so you don't have so much to discharge. Then do my JLD setup and if you use 2.5 as your low voltage you will do just fine. Connect them in parallel to balance them all together and then leave them together while you drain them to 2.5 with the JLD. Be sure to have a safe load that can handle long heavy drains. I use a small motor in the video but I usually use a spare 9" GE motor because I get a consistent 100 amp drain under no load with that motor. The smaller motor does not drain at such a high current. I have even used three motors in parallel for a seriously fast drain on the cell.

 

[onegreenev]

Just to let you know, you can destroy a cell by over discharging it by having it connected to a reasonably high discharge current. I lost a cell when I was using the motors before I began using the JLD404. I stepped away to fix up a quick cup of coffee and when I got back it was slightly bloated and at zero volts. Never did recover. Up until then that cell was just fine. Held a constant voltage for years. So, yes, you can ruin a cell if you just let it go to zero under a situation like this. Use a JLD and contactor so you can allow it to never go below your set voltage but you do need to be sure you don't let the battery that powers the JLD go dead. I have an external power unit that I can now use for my 12 volts. That way I don't rely upon my battery for that.

 

[e^2]

I am expecting some batteries to be delivered today or tomorrow. What voltage would you recommend to bring them to before hooking them in parallel? I'll have 18ea 5S 60A cells to balance!

 

[pdove]

http://store.evtv.me/proddetail.php?prod=250wresistor

The economy bottom balancer/hand trimmer.

When bottom balancing, at the end of the day we are taking out very small amounts of energy from each cell to get it to hit an exact voltage. This small 250 watt 0.1 ohm power resistor is the perfect thing to manually trim LiFePo4 battery cells.

Just connect this across two terminals while observing cell voltage with a multimeter. It's low tech but it draws 18-20 amps from a single cell and quickly gets the voltage down to where you want it.

The extra long format helps dissipate heat and the entire rig is very light and portable. Insulated clips make it easy to connect to cells without shorting or sparks.

 

[Sunking]

I am expecting some batteries to be delivered today or tomorrow. What voltage would you recommend to bring them to before hooking them in parallel? I'll have 18ea 5S 60A cells to balance!

Anywhere between 2.5 and 3.6 volts

 

[dcb]

What voltage would you recommend to bring them to before hooking them in parallel?

As long as they are the SAME voltage, it doesn't matter, so whichever voltage takes the least work (i.e. they are all 3.5v except 1, make that 1 3.5v).

If they aren't the same voltage then you can get significant current flow when you parallel them.

 

[e^2]

Thanks. I wasn't sure if the goal was to get every cell to 10% capacity and then charge to 90%, or just all the same. After a bottom balance, would you then avoid a 100% charge unless you have a BMS?

 

[EVfun]

Thanks. I wasn't sure if the goal was to get every cell to 10% capacity and then charge to 90%, or just all the same. After a bottom balance, would you then avoid a 100% charge unless you have a BMS?

Yes, you only want to charge until the first cell approaches full. All the cells will hopefully be very close. If you install a BMS and charge until it goes to work it will be top balancing your pack. If the BMS cell level connections don't draw exactly the same current they will unbalance your pack. A bottom balance pack is generally used without a BMS. I think I'm in about the same area as you and run a top balanced no BMS pack of LiFePO4 cells. They have been working for me since 2010.

 

[e^2]

Thanks! Once I get my motor mounted I'll be doing this step next.