[EVDL] re lithium ion balancing

1219 Views 3 Replies 1 Participant Last post by EVDL List, Feb 20, 2011

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Discussion Starter · Feb 18, 2011

>
> Message: 17
> Date: Fri, 18 Feb 2011 09:48:34 -0600
> From: Lee Hart <[email protected]>
> Subject: Re: [EVDL] Thundersky Lithium Batteries
> To: Electric Vehicle Discussion List <[email protected]>
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>

> Bill Dube <[email protected]> wrote:
> >> Don't use them in series.
> >> There is certainly a big internal transistor that disconnects the
> >> battery from the load/charger when the battery is full or empty.
> >> This will not allow the other batteries in the series to fully
> >> charge.
>
> Dennis Miles wrote:
> > Wow isn't that nice a built in BMS... Or is that just a great piece
> > of humor? Bill I love your Humor, er "BS" [:^)}
>
> I have no idea what Thundersky is using in their lithium replacement for
> a 12v starting battery. But, I designed the BMS for a client for just
> such an application.
>
> The 12v electrical system in a car can easily exceed the maximum safe
> voltage for the lithiums. People also routinely connect their own 12v
> chargers, which go even higher. They also forget and leave things on, or
> keep cranking an engine that won't start until the battery is way below
> the minimum safe voltage for the lithiums.
>
> Since the battery can *start a fire* if abused like this, I felt that
> you *must* include a BMS that can disconnect the battery from the system
> if an overvoltage or undervoltage occurs. Like Bill said, this means a
> big transistor or contactor in series.
>
> I chose to use transistors. I had six MOSFETs, three in parallel, and
> these two groups in anti-series to act as a switch that can stop current
> flow in either direction. These transistors can handle starting currents
> (hundreds of amps). But due to space and lack of cooling, they can only
> do it for seconds.
>
> Also, these transistor have to be low voltage (30v) to get the extremely
> low on-resistance needed.
>
> Such a battery can't be connected in series for EV use. The transistors
> would overheat from sustained high current, and if they had to turn off,
> they would be breaking the *total* voltage of the pack, not just that
> one battery's worth.
> --
> Lee A. Hart | Ring the bells that still can ring
> 814 8th Ave N | Forget the perfect offering
> Sartell MN 56377 | There is a crack in everything
> leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

I found this article with diagram to build an active balancing circuit for
ultracapacitors. they suggest a passive system but the active is much more
efficient. since ultracaps, at least the maxwell boostcap 3000 farad
2.7volt has the same voltage of lithium 2.7 cells, couldnt this circuit be
used on lithium cells just as ultracapacitors?

http://electronicdesign.com/article/components/shield-ultracapacitor-strings-from-overvoltage-yet.aspx
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Discussion Starter · #2 · Feb 19, 2011

Michael Clark wrote:
> I found this article with diagram to build an active balancing circuit for
> ultracapacitors. they suggest a passive system but the active is much more
> efficient. since ultracaps, at least the maxwell boostcap 3000 farad
> 2.7volt has the same voltage of lithium 2.7 cells, couldnt this circuit be
> used on lithium cells just as ultracapacitors?

Yes, it could. In fact, it is functionally identical to the ones used
for lithium cells. However, they used 14 parts to do what a single zener
diode could have done all by itself.

The other problem is that this design (like many for lithium cells) is
not fail safe. No fuses, no temperature or undervoltage or overvoltage
indicators, etc. The design is such that a small failure turns into a
big disaster.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Discussion Starter · #3 · Feb 20, 2011

>
> Message: 12
> Date: Sat, 19 Feb 2011 11:17:21 -0600
> From: Lee Hart <[email protected]>
> Subject: Re: [EVDL] re lithium ion balancing
> To: Electric Vehicle Discussion List <[email protected]>
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>

> Michael Clark wrote:
> > I found this article with diagram to build an active balancing circuit
> for
> > ultracapacitors. they suggest a passive system but the active is much
> more
> > efficient. since ultracaps, at least the maxwell boostcap 3000 farad
> > 2.7volt has the same voltage of lithium 2.7 cells, couldnt this circuit
> be
> > used on lithium cells just as ultracapacitors?
>
> Yes, it could. In fact, it is functionally identical to the ones used
> for lithium cells. However, they used 14 parts to do what a single zener
> diode could have done all by itself.
>
> The other problem is that this design (like many for lithium cells) is
> not fail safe. No fuses, no temperature or undervoltage or overvoltage
> indicators, etc. The design is such that a small failure turns into a
> big disaster.
>
> --
> Lee A. Hart | Ring the bells that still can ring
> 814 8th Ave N | Forget the perfect offering
> Sartell MN 56377 | There is a crack in everything
> leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

this kit is made by maxwell for their ultracaps to hook them in series. the
kit includes boards and bus bars for 6 caps.

It describes the kit will protect the caps from getting too much voltage.

http://www.rell.com/Pages/Product-Details.aspx?productId=842993
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Discussion Starter · #4 · Feb 20, 2011

Michael Clark wrote:
> this kit is made by maxwell for their ultracaps to hook them in
> series. the kit includes boards and bus bars for 6 caps. It
> describes the kit will protect the caps from getting too much
> voltage.
> http://www.rell.com/Pages/Product-Details.aspx?productId=842993

It's not very impressive. It seems like a lot of parts on that little
board for a simple function. It looks like it's intended for 2 to 6 caps
(i.e. not very many). I don't see any fuses, or output indicators if the
charging source supplies over 300ma (the most this device can bypass).
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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| Multiple-address or CCed messages may be rejected.
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