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: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
