# help to balance my battery pack for the first time



## TEV (Nov 25, 2011)

Because your lower cell is 3.3V and the highest is 3.6V it would be a very slow process. I spend a few day balancing 100pcs of 100AH Calb's, I have the Elithion BMS. I also cheated and helped the lowers cells and discharged the highest but you realy have to know what are you doing if you try that.

Good luck.


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## TEV (Nov 25, 2011)

Basically the BMS have to discharge the highest cells until the lowest one get to the same capacity, in the process the shunts get hot and the BMS stop everything until the temperature drops. So a cold room and/or big fans would help reducing " down " time


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## frodus (Apr 12, 2008)

Can you parallel them and then charge? It's much easier to start with a balanced pack than it is to have the BMS bring them into balance. If you can, parallel groups and charge them with a power supply or single cell charger to 3.6V. It'll take a while, but do this before you assemble. It'll save you a ton of time.

Otherwise, you're going to use the shunt on the BMS to bring them into balance, and if the batteries have differing SOC's to start, it may take many charge/discharge cycles.


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## Rob_of_Waterloo (Nov 2, 2011)

My pack is already installed and not easily removed. I guess I will drain the few highest cells with a resistor. I believe one of the problems is the shunt resistor increases the temperature of the cell's bms modules which causes the BMS controller to stop the charging. The temperature of battery itself is fine or should be fine at 1A charging/discharging. 

I think the bms cell modules could have been designed better. The cells modules cannot measure the battery temperature directly and hence it rely on the thermal conduction from the positive battery lead to the thermistor. The shunt resistor contacts the PCB and has very large pads, hence the shunt resistor is a very good conductor of heat to the PCB where the thermistor is mounted. Not knowing all the details, it looks like shunt resistor should be thermally isolated from the PCB. By elevating the shunt resistor 2 mm off the PCB and optimizing the track/pad size, the heat from the shunt would have a reduced effect on the thermistor. Using the GBS battery covers does not help the situation.


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## frodus (Apr 12, 2008)

Do you have a single cell charger you can attach on each and charge them up fully? It'll take a while, but you can finish other things while you do so.


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## Rob_of_Waterloo (Nov 2, 2011)

frodus said:


> Do you have a single cell charger you can attach on each and charge them up fully? It'll take a while, but you can finish other things while you do so.


No such luck...maybe I should get one so i can charge the spare battery.


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## frodus (Apr 12, 2008)

Got an adjustable power supply?

You should get a single cell charger of some sort. Even if it's an RC Lifepo4 programmable charger, it's a necessity with a Lithium pack. Same as a 12V charger would be with lead.

Here's something you could use:
http://www.batteryspace.com/Smart-C...cells-LiFePO4-Battery-Pack-100-240VAC-CE.aspx

I use an adjustable power supply set for 3.7VDC and set the current to it's max 5A.


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## machineguy (Sep 4, 2012)

frodus said:


> Got an adjustable power supply?
> 
> You should get a single cell charger of some sort. Even if it's an RC Lifepo4 programmable charger, it's a necessity with a Lithium pack. Same as a 12V charger would be with lead.
> 
> ...


So i can just use a fancy power supply to charge my cells one at a time.?
thats pretty cool.

Mike.


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## frodus (Apr 12, 2008)

Yup. Just make sure it has adjustable current and voltage.

I use this:
http://www.mastechpowersupply.com/d...-30v-5a-hy3005d-3-triple-outputs/prod_10.html
although I'd recommend something like this:
http://www.mastechpowersupply.com/d...-dc-power-supply-30v-10a-hy3010e/prod_14.html

Note: The later is made for charging batteries and you can turn it off while connected to a battery. With my model DO NOT turn off power supply while the HY3005D-3 is connected, or current will flow back into the charger and potentially blow the transistors. I did this once and had to replace two of them. Disconnect the battery and then turn off.

I can put the outputs in series or parallel. I use parallel to get up to 10A and up to 30VDC. With no load, set the voltage to what you want (in my case, 3.65V). Then connect the battery. Slowly dial up the current to the max you want. Let it sit. The voltage will display the battery voltage. DO NOT ADJUST. The current will stay until you start to climb in voltage. Once it hits 3.65V, the current will start to decrease. Take the battery off once the current falls to a sufficiently small current (on a 10Ah battery, I let it fall to 0.25A).

That's all a CC/CV charger is anyway. It's pre-set to charge to a certain cutoff voltage and internally current limited to a certain current max.


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## mk4gti (May 6, 2011)

An adjustable power supply (with volt and amp display) is a bit better IMO, you can see when the battery is full when the amp drops.


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## machineguy (Sep 4, 2012)

Thankyou.! Great information. I have one of those and will put it to use.

Mike.


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## Elithion (Oct 6, 2009)

Rob_of_Waterloo said:


> How long does it typically take to balance a new pack.


There is gross balance (if you forgot to balance your cells before you built your pack) and maintenance balance. It sounds to me that your pack is experiencing gross balancing.

There is a precise answer to your question, and it is answered in this white paper, How much balance current?

Here's a graph form that paper:


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## cpct (May 31, 2012)

Rob_of_Waterloo said:


> Hello folks
> 
> I am have a challenge balancing my battery pack for the first time. The EMUS BMS commands the CAN charger to shuts down due to a high temperature limit or a high voltage limit. Mainly the temperature limit.
> 
> ...


We also use the EMUS BMS and had the same problem initially. But we just set the "final" voltage of the cells to 3.3V (which was then the lowest of the pack) and then let it shunt for some days at max 50% balance current. Then we set it back to 3.8V for charging.
If you let it shunt during charging an you're already at max voltage, the resistors get hot fast and the BMS shuts down the whole thing.


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