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brbrv avatar image
brbrv asked

Second Controller Increased LiFePO Battery Bank Temperature

We just installed a second MPPT 150/85 controller to work with our solar wall of 6x 190w panels. The first controller is also an MPPT 150/85 and it handles our roof's 6x 180w + 2x 190w panels.

For both controllers, we set up the BMV 712 settings to match. Both are running the adaptive absorption setting, and 13.4v for float and 14.5v absorption.

We're incurring two issues ....

  1. The batteries hit 108°F on an 89°F day. Is that ok? We've never gone above 92°F with the first controller at a max v of 35v.
  2. The BMV app displays the second controller accumulating wattage, averaging 60v, when in float mode.

We're just concerned about the lack of syncing between the two controllers and the increased temps. :) Eric

MPPT ControllersLithium Batterytemperature
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4 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @BRBRV

-what brand / type of batteries?

-what capacity? what is the allowed continuous charge / discharge current?

-14.5 is -very- high for LiFePO4, this could also increase temperature.

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adev avatar image adev commented ·

Agreed, in fact a lot of the voltages on that spec sheet seem suspect to me. Even the nominal voltage which they claim is 12.0v, should be 12.8v really. 14.5v is too high for charging really, 10.5v is a bit low as a working minimum except maybe under a high load. BMS low voltage cutoff seems far too low (and is that recommended for external BMS, or set like that for internal BMS?)....


:/

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brbrv avatar image
brbrv answered ·

Hi Daniël. We have 8x Expion360 Viper 120ah batteries, in two groups of four batteries.

Expion360's recommended resting (full) voltage for the batteries is 13.3 +/- 0.3v. The recommended charge (full) is 14.5v +/- 0.1v. The maximum continuous discharge rate is 80A and the peak discharge (5 minute) rate is 100A.


viper-specs-may-27-2020-4-34-pm.jpg (233.8 KiB)
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adev avatar image adev commented ·

some of those specs seem a bit odd to me. For example minimum(empty) voltage is 10.5v but later ti says BMS low voltage turn off is at 8.0v. By my understanding 2.0v per cell is an absolute minimum and going lower can damage the cells, so for me it seems really bad to have the BMS low voltage set that low without any headroom. surely 10-10.5v would be better.


Similar with the high voltage protection turn off - seems a bit on the high side.

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Justin Cook avatar image
Justin Cook answered ·

@BRBRV Could you post a picture of how you have these batteries wired?

I notice that you're not only charging at the maximum recommended v but also well over the maximum recommended charge rate, which will, yes, raise battery temperatures... particularly if the batteries are connected in a less-than-ideal method.

I'll add that yes, when you double your charge rate it is expected that the batteries will get warmer than they did before, it's definitely critical that they're all connected correctly though, thus my request for a picture of how they're wired.

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brbrv avatar image
brbrv answered ·

Hi Justin. Thank you for the response. I attached a photo of the batts and components in the storage bay.

Per Expion360's recommendation, we lined up all 8 but split them into 2x 4 running parallel. The two positive cables run to an assigned positive bus bar, and the the two grounds hook up to the designated ground bus bar.

To avoid overcharging, what should our settings be for both controllers?

:) Eric


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adev avatar image adev commented ·

Since you effectively have all your batteries in parallel (12v right?) Would it not be much better, in terms of more balanced charge/discharge current, to connect all the battery terminals directly to your main bus bars with exactly equal length cables? As they are now there’s quite an imbalance in the current flow, although it could be worse.

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