End of absorption with tail current

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Elimac asked May 15, '20

End of absorption with tail current

Hello. I wonder if I am thinking this right... I am trying to end the absorption phase solely based on the tail current, but it seems system is ignoring that.

Scenario is: Charging LiFEPO4 bank with 8 cells (25.6V) with integrated BMS just for protection (no communication with Victron)

- Multiplus 24/ 3000

- 3x MPPT in parallel charging

- Venus GX

Customised charge profile:

- Absorption voltage: 27.6 V (I can monitor cells, and no cell is above 3.6V)

- Absorption time: Fixed, 1hour 30 minutes

- Tail current: 9 amps, battery capacity is ~180Ah

- Float voltage: 27.1 V

It ignored the tail current and the absorption time was exactly 1,5 hours. I would like to stop absorption when current decreases past 9A.

I didn't feel adaptive time was ok for lifepo4, as if bulk takes longer, absorption may not need so much time. I would like to make the criteria to stop Abs be solely the Tail Current (system!, not individually each MPPT)

How can I do this?

battery charging Lithium Battery mppt charging

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Answer 1 · 2020-05-15T13:10:37Z

Mark answered · May 15, 2020 at 01:10 PM

I have been using tail current to end absorption phase for some time now and it's been working very well.

I initially used the solar charger output current and then switched the DVCC - SCS when the feature was released with the BMV battery monitor as the tail current source.

However I have a single large MPPT, so it might be a compatibility issue when using the tail current feature with multiple MPPTs in parallel - linked by VE.Smart Network.

I will try to find out for you.

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Elimac commented · May 15, 2020 at 01:20 PM

Mark, is the presence of the BMV a mandatory condition? As System current is already known in my case, even without BMV, I was hopeful it wasn't needed. But, in SCS I can't detect any monitor...

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Mark ♦♦ Elimac commented · May 15, 2020 at 01:49 PM

I just got feedback that the tail current feature should work fine with multiple synchronised MPPTs and no battery monitor.

To trigger the end of absorption, the sum of the MPPT output/charge current from all MPPTs must fall below the tail current threshold set for 60s (while the charge voltage setpoint is also maintained).

Can you confirm that all of these conditions were met during your testing?

Note that without a battery monitor in the system and DVCC - SCS active, it will not use the actual current flowing into the battery, only the MPPT output current, so if you are also powering loads at the same time they will not be considered. (The battery current 'calculated' by the GX device from the Multi & MPPT cannot be selected as a source for SCS)

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Elimac Mark ♦♦ commented · May 15, 2020 at 02:24 PM

A-AH moment... if it considers the MPPT current and not battery current, then one must consider all the System load too!! That can explain it!!!

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mvader (Victron Energy) ♦♦ Elimac commented · May 17, 2020 at 07:52 AM

I confirm: even though your GX Device knows battery current (by calculating it from mppt-reported current and Multi-reported current), your mppt does not know the battery current.

it only knows its output current (or in your case total Output current of all synced mppts together) and thats what its using to evaluate the tail current threshold with.

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Answer 2 · 2020-05-17T11:47:21Z

john-hagtharp answered · May 17, 2020 at 11:47 AM

Are you running ESS & DVCC? If my understanding is correct, this would place the MPPTs under external contol using the charge algorithm from the Multiplus.

Unless I missed something in the changelogs, I don't believe you can set a tail current in the Multiplus. Adding the new charge algorithm capabilities from the MPPTs into the Multiplus is something I've been meaning to request some time.

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Elimac commented · May 17, 2020 at 01:37 PM

Only DVCC (mainly due to the charge current limit necessity). No ESS, it is a (nearly) offgrid system.

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