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View Full Version : Word of advice for Headway users



mpipes
01 September 2015, 2148
So if you're building a Headway pack, I just learned a valuable lesson.

Try your best to orient the cells with the barcode/serial numbers so they are visible without too much disassembly in the event you need to see them, such as servicing/warranty or.... say you were shorted a few cells on an order but you had enough to cover your immediate needs so you bolted the pack together over the weekend before getting to contact the seller about the shortage LOL... and in doing so, you intentionally buried the barcodes for aesthetic reasons . Then the seller sends a list of serial numbers and asks you to ID the missing ones. :)

I've gotten pretty fast at putting a 192 cell pack of Headways together, still it's almost 400 screws to handle LOL.

670667076708

EVcycle
02 September 2015, 1511
Great info! Thanks!!

Not sure it will help in my packs, but a big help for the inspiring Headway folks!

6709

mpipes
02 September 2015, 1708
Yeah, your layout would be tough to set up so the s/n's are all visible. Even though my pack gives pretty good access there's a middle row within the stack that is hidden no matter what.

How do those headway bus bars seem to work out?

I was a bit skeptical using them for a high current application not to mention the prospect of needing so many to assemble a big pack so I had aluminum plates 1/4" thick made.

PaulWay
02 September 2015, 1934
IMO the other thing to do with Headway cells is write on them their internal resistance (if you measure that) and then try to group cells of like resistance. Keep a few spare in each resistance category so that when (not if) you need to replace one you can do so with a cell of like resistance.

The reason for this is that cells with dissimilar resistance that are in parallel will tend to use up the power from the lowest resistance cell first. This then pushes that cell down in voltage, and causes the entire group to lower its voltage quicker. Ultimately you end up pushing the cell too far and, because it's in parallel with another cell, it will be run at reverse polarity and the cell will be damaged.

AFAICS there's a logical fallacy with cells in parallel that somehow the cells will all be at the same voltage. That's not true: the voltage of the group is the voltage of the average of the cells. If one is at 0.1V and the other is at 3.3V, you'll get an average of 1.7V from that group. If that group is not under load at all, the 3.3V cell will be trying to recharge the 0.1V cell. But if it's under load, then more current will flow through the cell with lower resistance than the higher resistance cell(s), which in turn pushes the cell harder (that current is effectively discharging the cell). So under load, parallel cells have issues that I believe a lot of people don't address.

Hope this helps,

Paul

EVcycle
04 September 2015, 1738
The Bars work well. Have been 120 mph on it with no issues other than the motor that burned up.

Hard to beat headway's for making them fit in a tight spot.

Nuts & Volts
06 September 2015, 0533
Paul, it won't be the average voltage of the two cells. Voltage is a gradient. If there is a difference in voltage between two cells. Current will flow and the capacity of one cell will decrease while the other cell will increase. Voltage is a function of SoC (at constant temperature). So the voltage will eventually settle at a point halfway between the two cells original capacities. So if one cell is at 80% (maybe 3.2V) and the other is at 20% (maybe 2.9V) Then in the end both will hopefully come to about 60% SoC which could be 3.1V. It is definitely harder to tell with LiFePO4 cells because the SoC to voltage is very non linear, but the truth should still hold


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