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Hi Allan,
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I waited a bit because I too thought someone on this list has probably dabbled in this. I have been thinking about this. Assuming there is only one LiFePO4 battery pack, and it's on the truck... New (healthy) cells and a well-controlled output voltage of a safe low (3.5v or 3.6v * qty of cells), it seems like maintaining balance would likely happen just fine. Perhaps do the first balance prior, and with proper equipment.
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Since it's a hot climate and he may be charging at or near the high charge rating for the cells, definitely use as many of the Orion BMS cell temp sensors as possible. Perhaps use the custom hollow bus connector bolts to get the thermistors deep into them for good thermal conduction from within the cells. As opposed to sensing somewhere on the plastic or Alu case.
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The BMS must be wired to enable the charge source (of course) when it confirms cell voltages and temps are ok. Use its backup enable too! "Charge Safety". Hopefully, the MPPT controller has a low current wire that you can use to have the BMS interrupt as the primary enable. Don't be tempted to have the BMS primary enable driving the coil of a contactor (to enable switching the MPPT DC output). That would be switching while the current is flowing. With current flowing during make/break, arcing will occur at the contacts. This may work for a while but would likely fail by welding the contactor "on" (not safe). Don't ask me how I know. So only use this as the BMS's secondary means to shut off charging "Charge Safety". In theory, it's only used if the primary method fails.
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The Orion won't care about the current. But it sounds like he already has the BMS? Be sure he bought the proper current sensor for the job. You'll program the BMS to whatever the max allowable charge amps should that your system can provide before it stops enabling the source. Assuming that's within your battery charge rate spec.
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If you could find the programmable controller you asked about, and its CAN, you may be able to get the BMS to control the current precisely, for the ideal finish charge. I am not experienced with customizing its CAN control yet. However, it may not be needed if you have a reliably constant peak output voltage and you can afford a few percent of capacity by setting the MPPT output voltage to be low enough. That is, low enough to allow some margin for a slightly unbalanced scenario to resolve itself without pushing the highest cell out of spec. The Orion has a relatively low bypass current for a high current charge rate. Even then, the worst case is that the BMS ceases to enable the charge, and sits for a while while the BMS continues to balance.
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PS - I wouldn't take this rather loose balancing approach if they weren't LiFePO4 chemistry.
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Absolutely no guarantees that this covers all the bases, but it's a start! If anyone spots flaws in my reply, please reply to all!
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Hope that's helpful,
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-Kirk
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On 01/31/2022 12:26 PM Allan Hesch <allanhesch@gmail.com> wrote:
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Hello all, I'm very out of my depth here so I'm asking for any information at all.
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An off-grid friend in New Mexico is building a conversion which will use 144 volts, maybe 25-30 kwh, of Lifepo batteries and an Orion BMS 2. He's wanting to use enough panels in series and a suitable charge controller to charge the truck. No ac charger. This makes sense to me except that I don't know a dang thing about it!
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Assuming plenty of sun, a charge controller that puts out steady regulated voltage (varies the amperage as the sun's output changes) and no daily commute, what are the drawbacks here?
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Some concerns:
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How will the Orion deal with often variable amp input and probable occasional low or no voltage? Damage? Ability to balance even though amps of input current will change? Interruptibility?
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How will the batteries deal with a DC input which is sometimes higher and sometimes lower amp and doesn't taper when it should?
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Anybody know of a programmable solar controller that would be up to this job, say 165-170 volts? He has a line on one but is looking for suggestions as well.
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I'm thinking plenty of people must have done this, maybe there's even a facebook group.
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Any ideas, suggestions of even spitballing would be much appreciated!
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Allan
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</div>_______________________________________________ Caevlist mailing list Caevlist@caevclub.org http://maillist.rdrop.com/mailman/listinfo/caevlist
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