question

roly avatar image
roly asked

BMV712 resetting to 100% with no charging....

System:

4s2p LiFePo4, 2x BMS's, Argofet isolator, Victron BMV712 + smart shunt.

I top balanced the bank to <100mA @3.6V. in parallel, then switched off charging via BMS/Mosfet software to allow a SOC check vs capacity, with no charging. Discharge obviously still enabled.

SOC reduced to 88.x % after a days discharge. A couple hrs later I noticed it had reset to 100%?

I read previous reply to similar scenario:

So if it happens too early, you have to do one, two or all three of these:
- increase "charged voltage" value;

- decrease the "tail current" percentage;

- increase the "charged detection time".


Any suggestions please. Below were setting at time. I had tail current set at 1% and not 2%.

Roly





1690413801153.jpeg

SOC
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pwfarnell avatar image pwfarnell commented ·
Your settings screenshot has not attached correctly. You also need to confirm what voltage you charge at during absorption.
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kevgermany avatar image kevgermany ♦♦ commented ·

All three need to be satisfied.

Voltage drop on lithium is very low. If you did not drop below 14.3V in discharging to 88%, then the reset to 100% is to be expected. Suggest you check the actual from the battery and adjust charged voltage.


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6 Answers
roly avatar image
roly answered ·

victron settings.JPG






JPG Appears ok on my screen. Apart from being overly large.

Absorb = 14.4V



victron-settings.jpg (86.3 KiB)
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roly avatar image
roly answered ·

Thanks for the suggestions. Bank voltage was < 13.6V at auto reset.

As my "charged voltage" was 14.3 I can only reduce tail current and increase charged detection time. Could it be a back emf spike on field collapse of an inductive load? Fresh water pump or fridge compressor. Or combination of both at same time.

Balmar SG200 is doing same thing, (returning to 100% with zero charging) shunts are in series.

( As an early user already had the SG200 but wanted to replace with BMV712 because of its added functionality)

What I have discovered is there is a volt drop across BMS's when charging Mosfets are switched off.

I may try this again with charging switched off physically as opposed software.

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seb71 avatar image
seb71 answered ·

You might have some loose/corroded battery connection (especially check the inline fuse holder from the BMV positive wire) and this combined with the BMV setting to set SOC at 100% on reset causes your issue.

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roly avatar image
roly answered ·

Unfortunately no. All new and it was the first thing I checked. Besides you could replicate by turning charge on/off in victron connect..

I do like Victron gear though; just need to get competent with it!

Another data point is that if you started an inductive load with BMS's charge turned off volt drop would disappear on BMV712 / SG200 !?! Kill the load & would come back.

Don't care about V drop, but I would like to control 100% synch.

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seb71 avatar image
seb71 answered ·

Besides you could replicate by turning charge on/off in victron connect.

Not sure what you mean by that.

"Reset" in this case means disconnecting the BMV shunt from the battery (so it loses power), not turning off the charger.


Anyway, to rule that out, change the BMV "Battery SOC on reset" setting from "Set to 100%" to "Clear".

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roly avatar image
roly answered ·

Sorry, got distracted. Should read:

Besides you could replicate volt drop by turning charge on/off in BMS software .*. unlikely

to be high resistance junction.


I think I better read manual thoroughly. I guess I could have wiped the 100% reset and just entered previous value. I just don't understand why it could read a voltage higher than battery voltage when there is zero charge. Especially for 3mins.

Cheers.

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