# ev west bottom balancer



## grip911 (Dec 14, 2011)

I can lend you my bottom balancer if you want...michel


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## PStechPaul (May 1, 2012)

What specs are needed for such a bottom balancer? I made a simple cell monitor which could be programmed to bottom balance, and the parts are about $2/cell plus the PCB. It uses a 4.7 ohm 5W resistor but a 3 ohm would also work for 1 amp discharge. Higher currents would just need a lower value higher power resistor and a higher current BJT or MOSFET. The same PIC12F1822 can read the cell voltage and drain the battery to whatever bottom voltage you wish, and then it can go into low power mode perhaps flashing an LED once every 10 seconds to show that is has finished, and at that point will draw less than 100 uA.

My proposed 8-cell monitor and charge shunt controller can also do bottom balancing, if so programmed. 

Such things are easily designed and built as prototypes, but making them suitable for production and sale can be problematic. I can understand why Valery and now EV West have a hard time keeping up with customer demands. But if the product is designed well, there should not be very much need for after-sale service, and simple circuits like this can be machine assembled in large quantity for only a few dollars per board. 

I would think that a single cell monitor/balancer could be sold at good profit for $10/cell, and an 8-cell unit might be $50. Does that sound like a reasonable cost?

OK, found a video:






That's a pretty big battery, probably three 100 or 200 A-h cells, and the balancer is drawing about 18 amps or about 50 watts as it takes the cell down to its 2.6 volts target. A unit like that would be able to use the same basic concept and circuitry of the one I built, but of course needs some hefty power resistors and MOSFETs or BJTs rated at 50 amps or so, with a good heatsink. But the bulk of the cost will be the enclosure, heavy power leads, display, and other ancillary items. It could probably be built for about $50 in parts, and I think a reasonable sales price would be about $150.

One deficiency in that design is the cable clips and the cables themselves, which can cause an inaccurate reading. A better method is a Kelvin connection, which uses a power wire on one clamp jaw and a signal lead on the other. Then you can use thinner cables (#12 AWG is plenty for 20 amps). 

It seems like a hassle to bottom balance each cell individually, even if they are already at 20% SOC and can be balanced in 15 minutes. For an 80 cell 240V pack that would take 20 hours. If you could do 8 cells at a time, even at only 5 amps, you could connect the 8-cell discharger and let it go for an hour or more, and the whole pack would take 10 hours. Also an 8-cell balancer (and even a single cell unit) could run off the battery power and not need to be plugged in. It also might be more convenient to use while the batteries are still in the vehicle.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> What specs are needed for such a bottom balancer? I made a simple cell monitor which could be programmed to bottom balance, and the parts are about $2/cell plus the PCB. It uses a 4.7 ohm 5W resistor but a 3 ohm would also work for 1 amp discharge. Higher currents would just need a lower value higher power resistor and a higher current BJT or MOSFET. The same PIC12F1822 can read the cell voltage and drain the battery to whatever bottom voltage you wish, and then it can go into low power mode perhaps flashing an LED once every 10 seconds to show that is has finished, and at that point will draw less than 100 uA.


Once bottom balanced why would you ever need to rebalance?


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## Sunking (Aug 10, 2009)

Well you can take this FWIW, I would not buy that unit because it is a uni-tasker, and for less money can buy something more useful and will do a better job. 

One of my hobbies is flying RC planes and we use LiPo batteries. There are dozens of chargers on the market. Chargers is really a wrong term, it will balance charge, discharge, monitor, measure Ri, AH capacity, or multiple cycle any current or future battery chemistry. Forgot one it can transfer charge from one battery to another as it is bidirectional. The model I use is Power Lab 8 1344 watt 40 Amp. It will charge/discharge up to 8S @ 40 amps and run circles around the EV West uni-tasker. Not bad for $220. 

I used to bottom balance my golf cart 16 cells. All I did was hook up all cells in parallel, set to to 1S and discharge to 2.5 volts and let it run over night. Rested the batteries a few hours and rechecked voltage. All perfectly balanced at 2.5 volts.


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## pm_dawn (Sep 14, 2009)

I second Sunkings take on this.
I have been using the iCharger family which is very similar to the PowerLab.
And those machines are just so much more versartile than a discharger.
They can measure the actual ah capacity in a very good way.

And if you have powertool packs with Ni-chemistry you can cycle these and get very good results from that.

If you plan to mess with batteries in any way, a good bidirectional RC charger is the tool to have.

Regards
/Per


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## prensel (Feb 21, 2010)

Hi folks, 

I have been busy building a Arduino bottom balancer the last couple of days to bottom balance 16 GBS 40Ah cells.

Sure I could have bought the Cellpro or alike but building the balancer myself is more fun.

I'm balancing 4 cells in parallel at a time and use an 12V heater as load. The heater draws about 3A from the cells at 3.2V. The heater is switched on/off by a BUZ11 Mosfet triggered by the Arduino. 

I have set the cutoff point at 2.5V and the balancer measures every second the I, V, Ah and Wh and puts these on a small 2x16 display.

The load on/off delay is set on 5 seconds. So it discharges until below 2.5V is reached, then disconnects the load for 5 seconds, when Vbatt is > 2.5V it connects the load again for 5 seconds and on and on until the cells resting voltages is 2.5V exactly.

If anyone is interested let me know


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## Ai! (May 9, 2014)

Hi Prensel! Interesting idea.

You are using arduino uno or due?


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## arklan (Dec 10, 2012)

need list of parts, arduino script, pictures and more information


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## prensel (Feb 21, 2010)

I'm using an Arduino Uno and the following items:

- ACS715 for measuring current
- BUZ11 MOSFET for connecting the load on and off
- LCD display 
- two resistors for driving the MOSFET
- Back Seat Heat Plus heater as a load

I'm planning on replacing the heater as a load for an 0.47R/50W resistor and replace the BUZ11 MOSFET for a single 5V relay board.

This will discharge the cells with 5.3A - 7.9A current so each 40Ah single cell from full to 80%DOD wil take about 5 hours.

I'm now working on some logging to the Uno's SD card, say every 5 seconds a line with A, V, Ah, Wh, time to a dedicated file per tested cell. 
This might give some information on how good/bad the cells are.
Resetting the filenumbers is simply by not connecting any voltage for say 10 seconds.


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## pm_dawn (Sep 14, 2009)

Hi !

Yes it is so much more fun.
Until you start doing some 100 cells and also want to do some cycling, to actually measure the real capacity of the cells.

Thats where those chargers really shine.

my 2%.

/Per


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## prensel (Feb 21, 2010)

I'm working on cycling now.

The idea is to first charge the cells to 3.5V, then decharge them to 2.5V and record/log time and Ah decharged. This should give a pretty good figure on the capacity of each cell I think.


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## PStechPaul (May 1, 2012)

This is a graph showing the discharge of a small (18650) LiFePO4 cell that was supposed to be 1800 A-h but actually proved to be only about 1000. The initial drop from about 3.4 to 3.2 volts was partially due to internal resistance. It is interesting that the cell exhibited a rather steep discharge curve to the 2.5V limit, and then "bounced back" to 3.2 volts with no load. Perhaps I could have squeezed a bit more from the cell at a lower discharge current, but it is already less than 1C.










Here is a video showing how I performed the test on a Li-Ion cell rated at 3600 mA-h. It was actually about 1/3 that:


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## Sunking (Aug 10, 2009)

PStechPaul said:


> This is a graph showing the discharge of a small (18650) LiFePO4 cell that was supposed to be 1800 A-h but actually proved to be only about 1000. The initial drop from about 3.4 to 3.2 volts was partially due to internal resistance. It is interesting that the cell exhibited a rather steep discharge curve to the 2.5V limit, and then "bounced back" to 3.2 volts with no load. Perhaps I could have squeezed a bit more from the cell at a lower discharge current, but it is already less than 1C.
> 
> 
> 
> ...


Paul you have a bit of missing information on your graph and test data to gain much useful information. 

Vertical scale appears to be voltage. Horizontal scale I do not know. One would assume initially it might be Amp Hours, but that is not possible because it exceeds the 1000 mah you stated. 

Missing is the Wh, AH, and discharge c-rate.


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## PStechPaul (May 1, 2012)

The color-coded legend to the right shows the four values to be voltage (red), current (green), amp-hours (violet), and watt-hours (blue). The horizontal axis is seconds. This represents a complete discharge cycle, with data taken at 5 second intervals and saved to a file as shown in the video. Sorry if it was unclear, but mostly I wanted to show how the voltage dropped when the load was first applied, and then remained fairly constant until it reached about 1 A-h, where it started to drop quickly. Thus a similar drop can be used to terminate a bottom-balancing discharge.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> The color-coded legend to the right shows the four values to be voltage (red), current (green), amp-hours (violet), and watt-hours (blue). The horizontal axis is seconds.


OK I understand that. But what is the Vertical Scale represent. I assume voltage which explains the Voltage curve. But how does one interpret Watt Hours and Amps you graphed?


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## PStechPaul (May 1, 2012)

All values share the same scale. At 2000 seconds, voltage is 3.1, current is 1.50, A-h is 0.8, and W-h is 2.6.


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## prensel (Feb 21, 2010)

Okay, shouldnt this Ah rating not be corrected regarding Peukerts law ?

Peukerts law is only valid regarding an 1Amp discharge current.
Battery capacity is mostly given for C/20 (20 hour discharge rate)

So testing if a 40Ah cell is really still 40Ah the measured Ah value should be corrected in some way if either the discharge rate is different then 20 hours or the discharge current is different then 1Amp.

Or I'm a wrong ?


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## pm_dawn (Sep 14, 2009)

Hi Prensel !

If you haven't noticed by now I will tell you that LifePo4 cells don't have much of a peukert effekt. You can find it if you search really hard.......

But to say that battery capacity normally is given as C/20 that is really mostly Lead Acid cells. If you search some info on your own GBS cells they seem to specify the capacity at C/2. 

So testing with a 20A draw on your 40ah cells would give you the right conditions for testing the capacity.

Regards
/Per


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## prensel (Feb 21, 2010)

I've read somewhere that the Peukert exponent for LiFePo is around 1.05 - 1.10. 

Wasnt aware of the C/2 rated capacity for GBS cells though. Discharging them other then C/2 should give other capacity ratings then ?


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## Sunking (Aug 10, 2009)

prensel said:


> I've read somewhere that the Peukert exponent for LiFePo is around 1.05 - 1.10.
> 
> Wasnt aware of the C/2 rated capacity for GBS cells though. Discharging them other then C/2 should give other capacity ratings then ?


Peukert Law does not effect lithium batteries at 1C or less and no significant effect up to 3C so it can be ignored.


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## PStechPaul (May 1, 2012)

Judging by the approximately 0.1V drop at 1.5 amps, the cell I tested had an internal resistance of about 67 milliOhms. This represents a loss of about 0.15 watts, so over the approximately 2/3 hour test, there may be an error of 0.1 watt-hour, which is only about 3% of the 3.5 W-h measured. This is similar to the Peukert effect, but Pb batteries have significant losses in addition to those caused by internal resistance, which accounts for the more complex formula. 

Most lithium cells, and especially unprotected LiPo, have much lower internal resistance for a given A-h rating. A good LiFePO4 18650 cell rated 1000 mAh will probably be rated as high as 10C, and may exhibit a voltage drop of 0.4 from nominal 3.2 to about 2.8, for internal power dissipation of 4 watts. The cell I tested would drop about 0.67V at 10C (10A) based on the measured capacity, for power dissipation of about 7 watts. For the rated 1800 mA-h it would be 18A and a voltage drop of 1.2 volts to about 2 volts, with internal power dissipation of 36W. Its short circuit current would be about 3.2/0.067 = 48 amps, and would then dissipate more than 150 watts.


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## Sunking (Aug 10, 2009)

Paul no offense offered but how many people do you know who are going to use LiPo and cylindrical small format cells on a DIY EV? I do not know of many and for those that do probable need a wake up call with an attitude adjustment upside the head with a 2 x 4. The expense and short life cycle just cannot be justified. 

Most use Chi-Com large format LFP cells costing 40-cents per wh with a 2000 cycle life. A 100 AH cell has an Ri of roughly 1 milli-ohm which is very low. Granted not as low LiPo used in RC models, but low enough voltage sag under load is very small.

For me that is the yard stick that should be used here to compare with not a $2 to $5 wh short cycle life battery no one would use. Well other than drag racers who can justify the expense of RC LiPo batteries. Those packs ar elimited to about 1.5 Kwh capacity made up of a hundred or more RC plane 50 to 100 C batteries. I got boxes full of them.


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## PStechPaul (May 1, 2012)

Well, this is getting off-topic a bit but I'm just showing the characteristic curves and internal resistances which probably scale up reasonably well. If a 100 A-h LiFePO4 cell has 1 milliohm resistance then a 1 A-h cell would likely have 100x that, or 100 mOhm. My test showed about 67 mOhm so it seems to scale quite well. I don't have any large format cells and for my present purposes I may use my 18650 cells in a battery pack for a drill and other power tools. My drill had 18V of NiCad cells and I would use six LiFePO4 cells as replacement. I have about 20 1250 mA-h (4 W-h) cells that I have only tested to a limited extent. The drill draws about 2 amps (36W) under no load and 6 amps (108W) under moderate load. If these cells have about 0.1 ohm resistance then the nominal 3.2*6=19.2 volts will drop 3.6V to 15.6V at the 6A (5C) load. 

This project also provides a small-scale model for my BMS and charger projects, which will also include bottom balancing by means of the shunt resistors. And I will probably include a Bluetooth interface for the battery pack which can monitor the charge and discharge current and individual cell voltages while the drill is being used and when the pack is being charged.


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## prensel (Feb 21, 2010)

Sunking said:


> Peukert Law does not effect lithium batteries at 1C or less and no significant effect up to 3C so it can be ignored.



Okay, so I can parallel connect some resistors of 0R33/50W and draw an average 10A current on each of them. That will speedup the decharging proces


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