Excerpt:In theory, all batteries are identical. If wired in series, you would
therefore expect them all to charge and discharge equally. But in practice,
there are differences. New batteries that are all the same brand, same model,
same date code (and without "lemons" or quality control defects) will still
have small differences. Each cell 's self-discharge rate, amphour capacity,
internal resistance, and charge/discharge efficiency will be slightly
different. This makes them drift to different states of charge. For example,
cells with a higher self-discharge rate run down faster just from sitting.
Cells with a lower amphour capacity get more deeply discharged on each cycle,
which lowers their efficiency (so they need a bit more current to fully
recharge). Cells with a higher internal resistance run a little hotter, which
affects their efficiency and self-discharge rate. These differences tend to get
larger over time. If not corrected, you can have a pack with some cells almost
full, and some almost empty!
Ok, here's the deal this is fiction. Self discharge is so low (milivolts over
years) as to be non-existant.Secondly, if they are in series then all current
passing through cells is equal. It is impossible to draw more current from one
cell than another in a series pack. It can short and pass all current or it
can open and pass no current but it cannot discharge faster or slower.
If it has higher resistance then it will cause cells to heat faster that is all.
Please show evidence that your theory is possible.
The only person I spoke with having issues with new cells is when they were
placed in parallel with old cells. In that case the wires melted because the
new cell was handling more of the current. You may damage cells if there is a
big difference in resistance only if you exceed the max current draw from the
cell. If one never approaches the max current draw this will not be an issue.
From: Lee Hart via EV <ev@lists.evdl.org>
To: Electric Vehicle Discussion List <ev@lists.evdl.org>
Sent: Thursday, June 18, 2015 10:53 AM
Subject: Re: [EVDL] Bicycle battery
Paul Dove via EV wrote:
> That was a very good summary. Cell balancing is something that was
> done with other chemistry a and many have tried to apply it to Li Ion
> chemistries but it doesn't work with Lithium since cells do self
> discharge or drift.
>
> If one wanted to balance them it has to be done below 3.38 volts
> otherwise you are still charging the cell. Thanks for your input.
Lithiums need balancing even more than other chemistries! You can only
get by without a BMS if the cells are so well matched that they
accidentally stay in balance. (Do you feel lucky?)
Look at my own balancer at http://www.sunrise-ev.com/balancer.htm
I built it to *use*, not to sell. It doesn't load the cells, or clamp
the voltage at some arbitrary level, and doesn't have failure modes that
murder cells.
It basically does what you would do yourself, if you had the time and
inclination. It measures the voltage of every cell, and charges the ones
that are low.
--
The greatest pleasure in life is to create something that wasn't
there before. -- Roy Spence
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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