[Milsurplus] Li Ion batteries

Sat Nov 4 23:14:44 EDT 2017

In the discussion, there was very little discussion about charging; the main 
part of the discussion is regarding discharging.  Thus, when a protective device 
opens, it will then see the full battery voltage.  The opening on charge is 
trivial, which is why the two sets of FETs are not always the same part.  Lower 
voltage FETs have lower on resistance for the same cost.

However, charging is not always trickle!  My latest control system is charging 
26650 cells at a 10 amp rate, about 4C.  The cells don't even get warm.

When I designed 12 volt lithium batteries, the spec was that they could handle 
charging voltages through 16.5 volts.  Two of those in series is 33 volts, and 
also our batteries were designed to be able to series four for telecom 
applications.  You can't tell customers the battery can handle 16.5 volts and 
turn around and file with UL and claim for example 14.4 volts (And that's not 
even thinking of inductive transients when specifying FETs and snubbers, 
guaranteeing SOA when paralleling, etc.).  The system design code compliance 
guys (who lived by the NEC) tended to go by the UL recognized component spec.

Fuses are an interesting subject, especially when combined with solid state 
protection.  At what parts of the I2t curve do you open the FET vs. letting the 
fuse take over?  When you parallel MANY strings this becomes a complicated and 
interesting problem.  You can destroy the FETs if you choose wrong, and now your 
battery module is unsafe even if the fuse opened in that instance (and was then 
replaced).  Btw, look up pyro fuses.

Breakers were saved for the larger systems, e.g., 2500 amp 1200 VDC motorized 
ABB devices costing ~$25k each.

On 11/4/2017 10:46 PM, James Whartenby wrote:
> Peter
> What do you mean by "Real the full thread"?
>
> If you meant "Read the full thread", I did.
>
> Still would like to know your source of "code requirement".  Voltage systems 
> below 30 volts do not seem to under the NEC (AFAIK) other then wiring should 
> be of sufficient size to carry the expected current, i.e. use good practice.
>
> But if there has been a change in the NEC for 2017, I would appreciate a heads 
> up.
>
> Nothing has been mentioned so far about fuses or circuit breakers to prevent 
> short circuit currents.
> Jim
>
>
>
>
> --------------------------------------------------------------------------------
> *From:* Peter Gottlieb <kb2vtl at gmail.com>
> *To:* James Whartenby <antqradio at sbcglobal.net>
> *Cc:* "milsurplus at mailman.qth.net" <milsurplus at mailman.qth.net>
> *Sent:* Saturday, November 4, 2017 3:56 PM
> *Subject:* Re: [Milsurplus] Li Ion batteries
>
> Real the full thread.
>
>
> Peter
>
> On Nov 4, 2017, at 4:27 PM, James Whartenby <antqradio at sbcglobal.net 
> <mailto:antqradio at sbcglobal.net>> wrote:
>
>> By code, do you mean National Electric Code?
>>
>> As far as I understand the discussion, the charger is floating and is a 
>> trickle charger.  The FET switch in question only disconnects the charger 
>> from the battery it is charging to prevent a higher then normal battery 
>> terminal voltage.  It is not switching the battery load so should not see a 
>> voltage much higher then the battery terminal voltage.
>>
>> If it is a "smart" charger, when the terminal voltage is much lower then it 
>> would be when the battery is at it's lowest terminal voltage, the charger 
>> will not turn on.  If the terminal voltage is much higher then what would be 
>> expected, the charger again will not turn on.
>>
>> The system  described is below 30 volts, so if the NEC code still applies, 
>> could you cite chapter and verse?  It would be interesting to know what issue 
>> is being addressed.
>> Jim
>>
>>
>> --------------------------------------------------------------------------------
>> *From:* Peter Gottlieb <kb2vtl at gmail.com <mailto:kb2vtl at gmail.com>>
>> *To:* Bill Cromwell <wrcromwell at gmail.com <mailto:wrcromwell at gmail.com>>
>> *Cc:* milsurplus at mailman.qth.net <mailto:milsurplus at mailman.qth.net>
>> *Sent:* Saturday, November 4, 2017 1:52 PM
>> *Subject:* Re: [Milsurplus] Li Ion batteries
>>
>> I disagree. Draw it out. Any series switching element must be rated for the 
>> string voltage. This is also a code requirement.
>>
>>
>> Peter
>>
>> > On Nov 4, 2017, at 2:47 PM, Bill Cromwell <wrcromwell at gmail.com 
>> <mailto:wrcromwell at gmail.com>> wrote:
>> >
>> > An FET inside a 12 volt pack won't see or crae about and batteries in 
>> series above or below it.
>> >
>> > 73,
>> >
>> > Bill  KU8H
>> >
>> >> On 11/04/2017 01:01 PM, Peter Gottlieb wrote:
>> >> Generally yes. During normal use everything is fine; it is during charge 
>> and at end of discharge where things can go wrong. You already mentioned 
>> separate charging so that’s off the table. At end of discharge packs have 
>> electronics which open up a FET to keep cells from getting too and being 
>> damaged or posing a hazard. The FETs may not be rated for the higher voltage 
>> of series packs and fail shorted, posing the hazard. See if you can find out 
>> what their max voltage is.
>> >>
>> >> Also use iron phosphate cells, they are much safer.
>> >>
>> >>
>> >> Peter
>> >>
>> >>> On Nov 4, 2017, at 12:43 PM, John Watkins via Milsurplus 
>> <milsurplus at mailman.qth.net <mailto:milsurplus at mailman.qth.net>> wrote:
>> >>>
>> >>> Just wondering if you could series up 2 12 volt Li Ion batteries?  I need 
>> a 24 volt unit, but have seen 12 volt units complete with chargers and 
>> thought can I use those.  Would have to charge separately I think.  Any ideas?
>> >>>
>> >>> Thanks,
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