charging via the balancing wires

[myzter]

Would it be a really bad idea to charge using the voltage monitoring wires from the CellLog 8 ?

my 27V 350 watt power supply toasted...
all I have left to charge is my 3.3v 150 watt power supply

I have negative lead wire connected to negative battery pack lead, then 8 wires to the connected to the positive leads of the pack..

 

[dnmun]

so you will connect this 150W charger to the sense wires at the plug?

i assume you will charge when it is discharged down to 2V, so 150W/2V=75A through those 26G stranded leads?

please have your video camera on when you do it so we can watch. i don't recommend it tho.

 

[myzter]

I have not yet drain my battery pack cell voltages under 3.10V / resting..
sure under load the voltage goes below 3.1v ..

Also about my PS, I made mistake it is only 100W
http://www.web-tronics.com/103siouposup.html

This charger on half depleted 3.1V 15Ah cell wouldn't output more than approx. 8A
so it is fairly safe...

 

[cell_man]

I've got something very similar that I use to charge my cells. It's a 3.3V PSU than I can increase to 3.65V, it outputs 25A. If you connect it directly it will very quickly fry the interconnecting cable. If you were to connect an in line resistor you could drop the current right down to avoid frying the cables. Maybe try a 10ohm resistor and see what current is running through the cable and adjust as necessary. It will take much longer of course, but it will get there in the end.

Edit: thinking about it more, it's gonna take a very long time to get any reasonable amount of Whrs into the cell with such an undersized cable. The voltage drop on the cable will simply limit the voltage delivered to the cell to too low a voltage to drive any current into the cell. Single cell charging is very effected by interconnection resistance. You really need a 4 wire active charge system to avoid this issue, as typical RC chargers use, failing that keep the cable and connections for the charging to the absolute minimum resistance.

 

[scoot]

... I have negative lead wire connected to negative battery pack lead, then 8 wires to the connected to the positive leads of the pack..

I guess I am not understanding something here... seems to me that connecting/combining all of the sense wires to the postive lead would simply create a dead short in 7 of your 8 cells. Will the cell to cell connections be discconnected when you do this? To use a single cell charger on multiple cells I thought it was required to change a series cell connected pack into a parallel cell connected pack for charging... at least that is how I understand it all. And yea, I agree that 3.3 volts won't be high enough to achieve the charging for LiFePO4 cells.

 

[tostino]

I would wait for your new PSU to arrive.

That 3.3v psu could single cell charge... if you were set up properly for it. You would need MUCH thicker wires going to each individual cell, and you would need to adjust the voltage output of the psu to the max desired voltage of each cell. Also, that psu is not limited to 3a at that voltage I promise you. It should be able to put out about 28-30a at that voltage. That is 2c charge for each parallel string, and really should be avoided.

Please, just wait for your 27v psu to arrive, and properly mod it to limit current, adjust the voltage to max pack voltage (8s at 3.65/cell = 29.2v), and do the fan mod. You will be completely set for charging after you do that. It will be rock solid, and you will be very happy.

 

[dak664]

Unless you break the series connections inside the pack that hookup will put a direct short on the charger. Look at those v connections - the same tap is connected to both + and - of the charger!

Or are you talking about just charging one cell at a time?

 

[tostino]

I think he is talking about one cell at a time, which is the only way to do it without a crazy amount of work on that pack.

 

[cell_man]

Sorry didn't look at the drawing properly. I thought you were talking about single cell charging 1 cell at a time through the balance leads. That wouldn't work for the reasons I gave above.

You cannot connect the balance leads as per the drawing, you will put a short across the pack and burn out the balance leads. Whilst the 15Ah pouch cells can probably take a 25A charge ok there is no point in putting them under that sort of stress for no good reason. When I use a similar PSU to charge cells I generally parallel at least 2 together.

If you are not going to use a BMS, use a conservative charge for the 8S pack (3.55V * 8 should still give you 95% or more SOC and hopefully avoid pushing any cells too high) or use a cell log to disconnect the charger when a cell hits HVC. You're talking about charging LiFePO4 cells to 100% SOC with no HVC and almost 1C charge rate, that is pushing your luck IMO.

 

[Ypedal]

I made myself one of these for my lipo packs..

I can charge at 2 amps safely and fill up each cell instead of discharging the high one's with a slow battmedic that can only drain milliamps..

Got a cell phone charger handy ? those are usually 1 or 2 amps, small gauge ballance leads can take that easily.

One cell at a time, then move on to the next one.