<div dir="auto">I see now<div dir="auto">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. </div><div dir="auto">Alex's point about cells not being balanced. </div><div dir="auto">Larry's point my system never disconnected but ramped down as i approached absorb volts therefor no BMS disconnection and no voltage spikes. </div></div><br><div class="gmail_quote"><div dir="ltr">On Tue, Jan 8, 2019, 12:15 PM Alex MeVay <<a href="mailto:alex@blueskyenergyinc.com">alex@blueskyenergyinc.com</a> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi All,<br>
<br>
As Larry said, when a lithium BMS disconnects, this causes a "load<br>
dump" just like disconnecting a lead-acid battery. Well-designed<br>
chargers (solar or otherwise) should be able to handle this without<br>
damage, though a failing or poorly-designed lithium battery may cause<br>
this to happen many orders of magnitude more frequently than the<br>
once-in-a-blue-moon that may occur with Pb systems. I'd like to share<br>
some pitfalls we learned back when we made lithium batteries in case<br>
they are helpful to Dan in making his decision about the Li bank.<br>
<br>
1. Load dump can mean different things to different chargers. Solar<br>
MPPTs, (boost MPPTs excluded) won't put out more than the solar Voc.<br>
For mobile installations that may include alternator charging, a<br>
"dumped" 12 or 24V alternator may put out 100V plus before the<br>
alternator has a chance to respond and the field current can decay.<br>
Mobile systems should have a mechanism to cut the field current before<br>
popping the BMS relay.<br>
<br>
2. Most chargers are not power supplies, and are designed to have a<br>
battery attached as the proverbial 900lb gorilla to stabilize the<br>
output voltage and buffer load transients. BMSs should be configured<br>
never to allow chargers and loads to be directly connected without a<br>
battery present. With no battery, load dump transients can destroy<br>
sensitive loads, and brownouts or bus voltage instability is likely.<br>
Best case is if the BMS has separate controls for charge and load<br>
relays, but even with one control output, separate relays for chargers<br>
and loads can prevent many problems.<br>
<br>
3. Related to #2, most modern MPPTs use synchronous rectification in<br>
their power stage. This means that, fundamentally, they can pump<br>
power into the array from the battery just as well as in the "normal"<br>
direction. It's only the controls that keep power flowing the correct<br>
direction. Let's say a one-relay 24V Li BMS disconnects, leaving<br>
chargers and loads connected. Say things brown out to 10V on the bus,<br>
with the MPPT doing its best to keep things powered, and running from<br>
75Vmp on the array. BMS decides it's time to reconnect, relay closes,<br>
bus voltage jumps back up to 26V and instantaneously, the MPPT power<br>
stage is still set for a 7.5:1 ratio, so power will get pumped back<br>
into the array, trying to push it up to 26V*7.5=195V, and if the MPPTs<br>
control loop or protection circuits can't respond fast enough, pop go<br>
the MOSFETs (depending on input rating of the MPPT). Separate relays<br>
for chargers and loads should prevent this, as most MPPTs will either<br>
shut down or go to absorption when the battery is disconnected,<br>
preventing the high buck ratios and problems on BMS reconnect.<br>
<br>
4. As a side note, we get a fair number of tech support calls from Li<br>
battery users complaining of over-voltage LED indications, etc. from<br>
our MPPTs. Almost invariably, this turns out to be a "don't shoot the<br>
messenger" situation, where the lithium battery was never balanced at<br>
the factory (etc.), and is disconnecting, and our controllers are<br>
notifying the user of the problem. Usually a little customer<br>
education (and patience during cell balancing) solves the problem.<br>
<br>
Thank you as always, Wrenches, for all the knowledge you share on this list.<br>
<br>
Alex MeVay<br>
<br>
Blue Sky Energy * <a href="http://www.blueskyenergyinc.com" rel="noreferrer noreferrer" target="_blank">http://www.blueskyenergyinc.com</a><br>
Genasun * <a href="http://www.genasun.com" rel="noreferrer noreferrer" target="_blank">http://www.genasun.com</a><br>
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