What is the "Cell Self-Discharge Rate" Parameter for the JBD BMS ?

dreuge

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The "Cell Self-Discharge Rate" is a parameter setting for the capacity configuration of a JBD BMS like Overkill/LLT SP04S020 (4S 120A LiFePO4 BMS). The iOS app gives the unit as a "%", but it is a rate so it should have a per unit of time. For example, ∆C/C is the percent change in capacity, so a precent discharge rate should be ∆C/(C*∆t). The SP04S020 BMS default value is "0.2%". The Overkill BMS manual just states, "3.2.5 Discharge Rate: Leave this at default setting."

My guess is that "Cell Self-Discharge Rate" is a "Daily Cell Self-Discharge Rate". I read somewhere that after 3 months, prismatic cells maintain ~95% SOC. So after some further search, I came across the EVE LF280 spec sheet which lists a reference to "Charge Retention at RT" giving "Capacity Retention >= 95%" after 30 days. This implies that "Cell Self-Discharge Rate" <= 5% per 30 days or (5%)/( 30 days) = 0.17% per day.

So if I understand this correctly, "Cell Self-Discharge Rate" = ∆C/(C*∆t) <= 0.17% per Day. Does this make sense? Does anyone have a good understanding of this?

In addition, does the BMS use this value to calculate/estimate a drop in capacity due to self-discharge?
 
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Zjohn

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I am writing software for my diy bms and would also be interested in the actual self-discharge rate of the LF280K cells. I suppose the BMS can/will calculate it by looking at the power difference of a discharge from full and a subsequent full recharge but it would be nice to find out what normal values would be. I believe that you are correct in that the LF280 spec shows max 5% per month. Hopefully that is really is the maximum because not even my old lead-acid batteries had that value but if your BMS default setting is 0.2%/day that would actually be even more. Perhaps they add a bit to account for the power use of the BMS itself.
 

Zjohn

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Specs are great but realitiy even better. You could just disconnect it and put a current meter in to find the real value.
 

Cal

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So if I understand this correctly, "Cell Self-Discharge Rate" = ∆C/(C*∆t) <= 0.17% per Day. Does this make sense? Does anyone have a good understanding of this?

In addition, does the BMS use this value to calculate/estimate a drop in capacity due to self-discharge?
Sounds reasonable. A 200 AH cell discharges by 200 AH * 0.0017 = 0.34 AH per day. That's probably spec'd at room temperature. Self discharge increases with temperature.

I doubt a coulomb counter needs to account for self discharge. A larger error source is BMS quiescent current. This current does not go through the current shunt. If quiescent current is 50 mA then that's an error of 50 mA * 24 H = 1.2 AH per day.
 
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HRTKD

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Yes, I've been looking for data on how much power the JBD BMS consumes while "idle", with no charge or discharge active.

The Overkill Solar folks covered this, maybe in the manual that they created.

I don't worry about the idle or self discharge rates. My cells get charged daily by solar, so there isn't enough time between full state of charge for it to come into play. Even if I had a foot of snow on my panels for a week, it wouldn't be enough to worry about.
 

kshopper2084

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The Overkill Solar folks covered this, maybe in the manual that they created.

I don't worry about the idle or self discharge rates. My cells get charged daily by solar, so there isn't enough time between full state of charge for it to come into play. Even if I had a foot of snow on my panels for a week, it wouldn't be enough to worry about.
Right. I'm wondering as my RV use means it will be stored for months at a time in the winter unused. I guess I could always unplug the monitoring/balancing leads from the BMS for the winter if it was a issue.
 

HRTKD

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Right. I'm wondering as my RV use means it will be stored for months at a time in the winter unused. I guess I could always unplug the monitoring/balancing leads from the BMS for the winter if it was a issue.

If you don't have a charge source, I would do just what you said. Discharge the battery to about 70% and then disconnect it. The voltage you see at that time won't differ that much from the voltage when you go back a few months later. LiFePO4 is very different from lead acid in that respect.

I'm in my RV even while at the storage lot to work on "projects" during the winter. So I use power 12 months of the year.
 
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