# Automatic battery bottom balancer



## morse90 (Mar 23, 2010)

Am having an automatic battery bottom balancer made which I will probably sell on my website. Would EV builders be interested in purchasing?

Cheers
Theo

www.evbuilders.co.nz


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## e_canuck (May 8, 2008)

Hi morse90.

I don't know, yet, enough about lithium packs to make a call like that.

But I know enough to ask a few questions.

The packs have lithium cells in parallel and in series. We charge the series individualy. (Does this sound like English?)

How many cells could the charger handle? At 3.3v a pack of 144V would require 42 or 43 cells.

If I buy two or more will they talk to each other?

Will your unit work on North American house current 110V or 220V?

How about a charging station?









Edit:

What price range are we talking about?

Daniel


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## morse90 (Mar 23, 2010)

Hi Daniel, this would not be a charger but a battery bottom balancer. So what it would do is would discharge an individual cell to 2.75 volts which is just before the knee of the discharge curve or a standard lithium cell. You woud do this to every cell till all cells were sitting at 2.75 volts. Then using a charger of your choice you would charge to your pack to it top voltage. For example 144 volts. Basically bottom balancing projects your cells so if you discharge you pack completely so the car does not even move anymore. Which you don't really want to do none of the cells would go out of alignment and discharge below a safe recoverable voltage. Does this make sense?


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## Duncan (Dec 8, 2008)

Hi Morse

How much are you looking at?
I have a PowerLab6 that can do that (as well as everything else) it cost ~ $160

I even think my RC charger I bought for $17 can do that as well


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## e_canuck (May 8, 2008)

Hello morse.

As stated I am new to the lithium concepts.

I was thinking that bottom balancing means finding the lowest battery and depleeting the others to that level before recharging...

You wouldn't do that every time. Would you? Seems like a waste of power. Kind of the same as deep cycling nicads.

Daniel


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## morse90 (Mar 23, 2010)

Every battery has a different capacity I wouldn't take my batteries to much lower than the manufactures specs.At the top but the bottom is usually the same to my understanding. You should really only need to bottom balance when you first install you pack and then maybe check it every year or 6 months. Yes you do not end up using a small amount of power at the the top end but the only safe way to use that with out over charging some cells is to use a really good bms and they have also been known to destroy batteries in the past which is costly. I chose to bottom balance once a year and operate that way. No bms needed. Have been driving my EV for just over an year and half now with no problems. evtv did a really good piece on it a while back.


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## morse90 (Mar 23, 2010)

Hey Duncan,

Yes you can do with a power lab and those little rc units but they usually only discharge at 5 amps max.The power lab discharges more i know. But when I tired with power lab etc it did not account for the bounce back on the cell. So was not really automatic. It would discharge cell then stop. I would then need to wait for the cell to bounce back and run the whole process again. I wanted a system which was completely automatic which would account for this. So it would discharge cell wait for bounce back the repeat process till the cell was stable at 2.75 volts. Then once this process is completed simply clip it on to the next cell. Was, looking at it costing $80 to $90 NZ dollars at the moment but we are still working it out. Also will be configurable to you can choose the voltage you want to bottom balance really easily...no confusing menu's for beginners.


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## Duncan (Dec 8, 2008)

_no confusing menu's for beginners._

I see you have used a power lab!!


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## pm_dawn (Sep 14, 2009)

Well the whole line of iCharger has this implemented already.
They can be setup to taper off the discharge current down to 1% of the starting current. So that would pretty much do it for you, plus they do regen discharge if you want. I have their Biggest and Badest the i4010 Duo. it can do 40A charge and discharge and have two seperate channels. I'm doing capacity test right now with it.

Otherwise a very simpel solution is to use an Arduino like the UNO and a 5v relay. Then you have a fully programmable setup. It can sense up to 5v so you can use it right away for single cells. Just need a good relay and a Load. And maybe a little fan.....

Regards
/Per


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## pm_dawn (Sep 14, 2009)

With the iCharger version I have used they are very easy to use. especially for doing the same program over and over again. 
When the program is done, you press stop to get out into the other menues.
Move the cables to the next battery, then hold the start button down, and it will start the latest program that was run. Really easy.

I have not used the Power Lab.

I have used the i3010b and also a i1010, and now I use the i4010 duo.
plus the other stuff from Junsi like Cellog and Powerlog.
Also the programmable voltmeters that Jack has at EVTV.

But most flexible i guess the Arduino would be.
I think it would also be pretty easy to output voltage data to logview just as iCharger et all.
Regards
/Per


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## Tesseract (Sep 27, 2008)

morse90 said:


> ..Was, looking at it costing $80 to $90 NZ dollars at the moment but we are still working it out.


Heck, that's cheap enough I'd buy one. What kind of discharge current will this doohickey handle?

Also, consider making one that can handle high current (>100A) that can either be bought or rented. A bottom balancer is one of those things you don't need very often - possibly only once, if you set the finish voltage on your charger conservatively and don't have any cells in your pack that are significantly lower in capacity than the rest.


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## pm_dawn (Sep 14, 2009)

two of these 
http://www.newark.com/te-connectivi...p/24M2716?ost=1174280&categoryId=800000005308

thats about $16

one of these:

https://www.sparkfun.com/products/11021

$30

two of these:
https://www.sparkfun.com/products/11042

$16

And some random parts like heatsink, fan and cables.

with the two relays you can turn the two resistors on independently thus having two different power levels. 

would be able to do max 30A.

plus you would need a wallwart.

so about $65 plus random leftovers.

Regards
/Per


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## Tesseract (Sep 27, 2008)

pm_dawn said:


> ...
> so about $65 plus random leftovers.
> ...


Yeah, I could certainly build an automated bottom balancer, and probably entirely out of parts from my junkbox, but I don't value my *time* at nothing, and, besides, designing electronic stuff is what I do for work... Also, I don't generally build what I can buy, as long as what is commercially available does what I need for a reasonable price, why duplicate the effort?


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## morse90 (Mar 23, 2010)

pm_dawn said:


> With the iCharger version I have used they are very easy to use. especially for doing the same program over and over again.
> When the program is done, you press stop to get out into the other menues.
> Move the cables to the next battery, then hold the start button down, and it will start the latest program that was run. Really easy.
> 
> ...



What price do the icharger units go for. Do you use BMS as well?


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## morse90 (Mar 23, 2010)

pm_dawn said:


> two of these
> http://www.newark.com/te-connectivi...p/24M2716?ost=1174280&categoryId=800000005308
> 
> thats about $16
> ...


Yes of course you could do that but it still works out that same price for you in NZD dollars we are aiming to sell the automated unit for and you don't have assemble and program the boards etc, if you are not an electronics genius I know from personal experience after building my EV I wanted a quick fool proof solution that could quickly balance my pack so I could get on the road and burn some electrons quickly rather than programming a power lab and all the rest of it....

I also found that discharging at 30 amps at the bottom of cell meant and even bigger bounce back on the cell so ended up discharged at 10 amps when I was past about 2.9 volts. Well it worked well for me anyway.


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## pm_dawn (Sep 14, 2009)

morse90 said:


> What price do the icharger units go for. Do you use BMS as well?





Here :
http://www.progressiverc.com/chargers/battery-chargers/ichargers

Smallest one is $89:99 that does 10A at up to 6Lipo
Badass one is $349:99 that does 40A up to 10 Lipo and dual channels.

I have different setups, both with and without BMS.
Its difficult to find a BMS that can show some kind of figures for how even they load the different cells. Preferrably then dont load the cells at all of course. 
But when did that happen ?
On my next build I will be going up into the 300+v range and would really like to have a good BMS with good logging and monitoring. 
It just takes so much time to bottom balance.
I just wished that Junsi would do a real program on the iCharger series that would actually bottom balance to a specfic setable voltage. So that you could slap a series connected pack of 10 cells onto the charger and it would do max discharge amps until it came down to the right voltage of one of the cells and the tapered off to as low as the balancing max current .
Maybe that is how they work already, I have not tested that scenario.

REgards
/Per


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## pm_dawn (Sep 14, 2009)

morse90 said:


> Yes of course you could do that but it still works out that same price for you in NZD dollars we are aiming to sell the automated unit for and you don't have assemble and program the boards etc, if you are not an electronics genius I know from personal experience after building my EV I wanted a quick fool proof solution that could quickly balance my pack so I could get on the road and burn some electrons quickly rather than programming a power lab and all the rest of it....


Yes I know, that is why I bought the iCharger.




morse90 said:


> I also found that discharging at 30 amps at the bottom of cell meant and even bigger bounce back on the cell so ended up discharged at 10 amps when I was past about 2.9 volts. Well it worked well for me anyway.


Yes I know they bounce. that why I suggested using two relays and two resistors. That way you can do full Amps using both resistors until the first touch at 2,6v and then after that only use one of the resistors the rest of the balancing.

regards
/Per


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## EVfun (Mar 14, 2010)

I can't really say if I would buy or not without design being more fleshed out. 

I have considered building a simple shunting reg and using it to drive a relay. It would be a one shot deal, push a button to pull in a relay that starts the discharge and when the cell gets down to 2.5 volts the relay would open, disconnecting the whole reg/discharger circuit (zero parasitic load.) The problem is, my time is worth something. It would be easier to buy it.


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## Jerry Liebler (Feb 1, 2011)

morse90 said:


> Hey Duncan,
> 
> Yes you can do with a power lab and those little rc units but they usually only discharge at 5 amps max.The power lab discharges more i know. But when I tired with power lab etc it did not account for the bounce back on the cell. So was not really automatic. It would discharge cell then stop. I would then need to wait for the cell to bounce back and run the whole process again. I wanted a system which was completely automatic which would account for this. So it would discharge cell wait for bounce back the repeat process till the cell was stable at 2.75 volts. Then once this process is completed simply clip it on to the next cell. Was, looking at it costing $80 to $90 NZ dollars at the moment but we are still working it out. Also will be configurable to you can choose the voltage you want to bottom balance really easily...no confusing menu's for beginners.


FWIW: I'm building precision shunts set at 2.768volts+/- 0.002v that can be clip leaded to individual cells. The material cost is $3.15 each (including PWB) they draw 60 micro amps below the set point and act like a 0.47 ohm resistor to the set point voltage so they will take a while to discharge a large cell. (average current about 0.7amp or about 16 AH/day)
This is fine for me as I have 10AH cells and one shunt per cell.


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## thingstodo (Jul 16, 2010)

Any word on design progress?

Are there more detailed specs?


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