No one has convinced me that top balancing is even possible much less that a
BMS can achieve this.
If you hold a cell above the Open circuit voltage you are charging it even if
you are trying to shunt the current with a Mosfet
From: Lee Hart via EV <[email protected]>
To: Electric Vehicle Discussion List <[email protected]>
Sent: Thursday, June 18, 2015 10:46 AM
Subject: Re: [EVDL] Bicycle battery
David Nelson via EV wrote:
> That is a reason I don't use a cell level BMS. With a cell level BMS
> like the miniBMS there is a constant drain on the cells running the
> BMS boards and it is nearly impossible to make sure that each board
> uses the _exact_ same current regardless of voltage in the cell.
> Furthermore, boards like the miniBMS expect that you will balance at
> the end of every charge and last I saw, that balance voltage was at
> 3.6V or so which is over the theoretical 100% SOC level for LiFePO4
> cells which leads to potential overcharging of the cells on every
> charge cycle. If you stop charging without the balancing taking place
> and/or let the pack sit for extended periods of time then the
> different current draw of each board working 24/7 introduces an
> imbalance in the SOC of the cells in the pack.
Yes, this is a problem in the BMS market. Like quack medicine, it's full
of customers worried about a problem that they don't understand very
well. So there are marketeers only too happy to throw together some
piece of junk that purports to solve it. It doesn't have to actually
work -- it only needs to *sound* like it works, to separate the customer
from his money.
So some BMS work -- and some don't. The bad ones are so bad that they
give the whoel BMS market a bad name.
I've written about this over and over... check my old posts. It mostly
seems to be of no avail, so I won't repeat myself again. Anything I say
is overwhelmed by the noise of the internet.
--
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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