Got 113 attached screen. What is different or what should I look for
please? --Dave
On Tue, 8 Jan 2019 10:15:40 -0800, Alex MeVay <a...@blueskyenergyinc.com>
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
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--
Dave Angelini Offgrid Solar
"we go where powerlines don't"
http://members.sti.net/offgridsolar/
e-mail offgridso...@sti.net
text 209 813 0060
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