I agree most wholeheartedly with the pro-BMS group, such as Cor, Lee,
and David. You need a proper BMS on all multi-cell li-ion packs.
Without exception, all EV OEMs have a BMS installed. The automakers
wouldn't spend a dime that they don't have to, so a BMS is a very
necessary thing on a Li-Ion pack.
There is a wide variety of BMS's for sale in the home-builder EV
market. The few well-established brands, specifically made for EVs,
work very well because they have ironed all of the bugs out of them.
(They are not the cheapest option because they have done the R&D and
development.) There are many newcomers to the market that don't work
so well. Often, these are poorly designed and/or have poor quality
control. RC cell balancers tend not to work well at all in an EV
environment. You must read the reviews, install carefully, and test
after installation to be sure the BMS is working correctly. Trust but verify.
If you need convincing about the need for a BMS, simply watch the
cell voltages in real time on an OEM EV with the maintenance software
through the OBD port. For example, use LeafSpy Pro and a WiFi OBD
dongle on a Nissan Leaf. The BMS is always hard at work balancing the
cells and controlling the rate of charge, even in a new pack. It is
amazing to watch just how busy it stays doing its job.
If you don't have a BMS, you really don't know what your cell
voltages are doing, do you? You _THINK_ you know, but you don't. That
is the problem. Blissful ignorance, until the fire department arrives.....
Bill D.
At 12:43 PM 6/18/2015, you wrote:
Paul,
How familiar are you with electronics?
My wife's eBike has a nice pack with a built-in BMS
that simply starts shunting current above a certain voltage
and presumably (but I can't verify) also reduces charging current,
so you can take care that the shunting cells are receiving zero current
and no longer rise in voltage. The trick is to find a good voltage at
which your BMS will protect the cells.
That pack is very nicely top-balanced and I expect that many more BMSs
that you are not even aware of in daily appliances such as power tools
will all top-balance.
Regards,
Cor van de Water
Chief Scientist
Proxim Wireless
office +1 408 383 7626 Skype: cor_van_de_water
XoIP +31 87 784 1130 private: cvandewater.info
www.proxim.com
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-----Original Message-----
From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of paul dove via EV
Sent: Thursday, June 18, 2015 9:40 AM
To: Lee Hart; Electric Vehicle Discussion List
Subject: Re: [EVDL] Bicycle battery
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 <ev@lists.evdl.org>
To: Electric Vehicle Discussion List <ev@lists.evdl.org>
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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