I see now Larry and Alex explained one of my problems was low capacty early shut down. I had the cc set to absorb volts below 100% and over two years the battery seemed to lose capacity. Voc 80 volts solar array The cc absorb 55 volts i was not charging full. The BMS. would never top balance. After i removed the battery and put on my power supply at 59 volts for a couple of days top balance occurred i have returned battery to service and works fine. Alex's point about cells not being balanced. Larry's point my system never disconnected but ramped down as i approached absorb volts therefor no BMS disconnection and no voltage spikes.
On Tue, Jan 8, 2019, 12:15 PM Alex MeVay <[email protected] wrote: > Hi All, > > As Larry said, when a lithium BMS disconnects, this causes a "load > dump" just like disconnecting a lead-acid battery. Well-designed > chargers (solar or otherwise) should be able to handle this without > damage, though a failing or poorly-designed lithium battery may cause > this to happen many orders of magnitude more frequently than the > once-in-a-blue-moon that may occur with Pb systems. I'd like to share > some pitfalls we learned back when we made lithium batteries in case > they are helpful to Dan in making his decision about the Li bank. > > 1. Load dump can mean different things to different chargers. Solar > MPPTs, (boost MPPTs excluded) won't put out more than the solar Voc. > For mobile installations that may include alternator charging, a > "dumped" 12 or 24V alternator may put out 100V plus before the > alternator has a chance to respond and the field current can decay. > Mobile systems should have a mechanism to cut the field current before > popping the BMS relay. > > 2. Most chargers are not power supplies, and are designed to have a > battery attached as the proverbial 900lb gorilla to stabilize the > output voltage and buffer load transients. BMSs should be configured > never to allow chargers and loads to be directly connected without a > battery present. With no battery, load dump transients can destroy > sensitive loads, and brownouts or bus voltage instability is likely. > Best case is if the BMS has separate controls for charge and load > relays, but even with one control output, separate relays for chargers > and loads can prevent many problems. > > 3. Related to #2, most modern MPPTs use synchronous rectification in > their power stage. This means that, fundamentally, they can pump > power into the array from the battery just as well as in the "normal" > direction. It's only the controls that keep power flowing the correct > direction. Let's say a one-relay 24V Li BMS disconnects, leaving > chargers and loads connected. Say things brown out to 10V on the bus, > with the MPPT doing its best to keep things powered, and running from > 75Vmp on the array. BMS decides it's time to reconnect, relay closes, > bus voltage jumps back up to 26V and instantaneously, the MPPT power > stage is still set for a 7.5:1 ratio, so power will get pumped back > into the array, trying to push it up to 26V*7.5=195V, and if the MPPTs > control loop or protection circuits can't respond fast enough, pop go > the MOSFETs (depending on input rating of the MPPT). Separate relays > for chargers and loads should prevent this, as most MPPTs will either > shut down or go to absorption when the battery is disconnected, > preventing the high buck ratios and problems on BMS reconnect. > > 4. As a side note, we get a fair number of tech support calls from Li > battery users complaining of over-voltage LED indications, etc. from > our MPPTs. Almost invariably, this turns out to be a "don't shoot the > messenger" situation, where the lithium battery was never balanced at > the factory (etc.), and is disconnecting, and our controllers are > notifying the user of the problem. Usually a little customer > education (and patience during cell balancing) solves the problem. > > Thank you as always, Wrenches, for all the knowledge you share on this > list. > > Alex MeVay > > Blue Sky Energy * http://www.blueskyenergyinc.com > Genasun * http://www.genasun.com > _______________________________________________ > List sponsored by Redwood Alliance > > List Address: [email protected] > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://www.mail-archive.com/[email protected]/maillist.html > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > >
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