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Blue Solar MPPT leaves absorption too early

I'm having trouble automatically syncing the BMV with Blue Solar MPPT. In the MPPT manual it says:: "Additionally, the absorption period is also ended when the charge current decreases to less than 2A".


This design is not ideal with LiFePO4 batteries, and also if you have more than one MPPT charging the same battery bank.


In my case that's 3xMPPTs, thus I still have 6A "tail current" when the absorption ends, and due to that the BMV will usually not auto-sync.


Is there anyway this 2A limit can be made optional in the MPPT firmware, so it behaves just like a MultiPlus absorption with no minimum limits?


MPPT - Solar Charge Controllerabsorption
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Your absorption time limit of one hour will yield exactly a 1/6 × 60 = 10 min period, if the start battery voltage is above 12.5V, which it will almost always be with LiFePO4, and the amps stay above 2amps. If you want, say 30min absorb, set a 3 hour min limit. This works for me. The 2A tail current is not being invoked by your controllers as far as I can see.


@Leslieanne - excellent. Thank you for that. I've now set it to 6x of what I needed, and everything (including automatic sync) is working how it should. While I'd rather have Victron fix the inconsistency, at present I'd like to mark your comment as an accepted answer, but not sure how to do that.

I have similar problem, MPPT leaves absorbtion with 30 Amps. Acctually, there are 2 MPPTs, each at apeox 30 Amps, and BMV does't synch, because max SOC keeps around 85% end there's plenty of sunshine.


Why that happens?


Battery bank is 1000 Ah at 48V, lead GEL chemistry.

MPPTs are 2x BlueSolar Charger MPPT 150/60 in parallel.

There are 3 x MultiPlus 48-5000 in patallel.

BMV is BMV-600S.



4 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

I haven't seen those specific charging parameters before. Could you please share the document where you got that information?


I am also still not clear how that is different from a target voltage. You can charge the battery from 14.0 to 14.6 V, depending on if you wish to maximise cycles or usable Ah capacity. Once the battery hits that voltage, required current to hold it there falls off immediately and the battery can be considered 100% full. That 100% may be 460Ah or 495Ah depending on your preference. How long the battery is held at these target voltages only results in fractions of a percentage in capacity variations.


The latest info I have about charging requirements for Sentry batteries is here :

https://www.sealedperformance.com.au/wp-content/uploads/2018/04/Sentry-Lithium-Bluetooth-Instructions.pdf


In conjunction with a specific product spec for maximum charge currents such as:

https://www.sealedperformance.com.au/wp-content/uploads/2018/06/SNL12V24S.pdf



Charging

A standard Constant Voltage (Automatic) SLA Battery charger can be used to charge the Sentry Lithium batteries however the following requirements must be met:

• Charger must not contain Sulphation/ Equalisation setting. If so then this step needs to be turned off.

• Maximum charge voltage of 14.6V

• Recommended float charge voltage 13.9+/-

0.1V.

• Please ensure your charger is matched to the current rating of your battery and take note of the recommended charge current in the batteries specification sheet – Normally half the batteries rated capacity. For example SNL12V100S – Recommended charge current = 50A.

• A charger with LiFePO4 setting is recommended.




Someone from Victron with the ability to influence firmware priorities has seen this post. But I wouldn’t personally report it myself until I saw the necessity.


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I agree with both of you.

It's likely under a slow charge rate that going below 6A tail current will not increase charge very much at all with this chemistry. (There are some narrow situations where that could be the case, but they would be unusual.)

On the other hand, it is frustrating that the Victron hardware has arbitrary and varying parameter constraints -- for voltage thresholds, timeouts, and sync thresholds, e.g. -- across the product line. We need our trusted vendor to give us consistent (and broad!) programmability when we build up a system.

Thanks @ben - agreed that this won't increase the charge very much during these final stages, but that's exactly when the BMV detects tail current for auto-sync! I'm going with @Leslieanne recommendation - multiply the desired absorption time by 6. Seems silly? Yes.

gs_nz avatar image
gs_nz answered ·

Should've said I'm using Sentry LiFePO4 battery with integrated BMS.

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gs_nz avatar image
gs_nz answered ·

BMV

1. battery 500Ah 12v

2. charged voltage 14.0V

3. tail 1.0%


MPPTs (x3):

Absorption: 14.30V – max 1 hour
(BUT VICTRON MPPTs will end the absorption period is also ended when the charge current decreases to less than 2A – per MPPT – that’s 6A overall)

Float: 13.50V

Equalisation: off

Temperature compensation: Disabled

Low temperature cut-off: Disabled

Max current output: 30A


Multiplus:

Absorption: 14.30V – max 2 hours

Float: 13.50V

Equalisation: off

Temperature compensation: Disabled (by disconnecting the battery temperature sensor)

Max current output: 70A


DC-DC:

Max output voltage: 13.8V

Max current output: 25A

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What is the charged detection time for the BMV?


0.5% and 5 minutes would be worth a try.


You won't get a full charge of the sentry lithium battery from a 13.8 DC supply. So that shouldn't show 100%.

gs_nz avatar image gs_nz Guy Stewart (Victron Community Manager) ♦♦ ·

04. Charged detection time: 3 minutes

gs_nz avatar image gs_nz Guy Stewart (Victron Community Manager) ♦♦ ·

It's only the DC-DC that goes up to 13.8V. Unfortunately the Victron Buck-Boost DC-DC converter isn't as customizable as other Victron products as it actually is someone else's product simply rebranded as Victron. But that's another story, and I don't want to dwell on that here.

gs_nz avatar image gs_nz Guy Stewart (Victron Community Manager) ♦♦ ·

With BMV set to 0.5% (2.5A) tail current and 5 minutes, it won't sync either, it's the MPPT combo (3 of them in my instance) that drop the ball at 6 amps!

In my mind it still boils down as to why Victron hard-coded the minimum 2A limit on their blue MPPT gear when all other blue gear doesn't have what is seemingly an arbitrary limitation.

Is there anyway this 2A limit can be made optional in the MPPT firmware, so it behaves just like a MultiPlus absorption with no minimum limits?

Give the following settings a go.

5m

14.2 V

3% (500 x 0.03 =15A)

That should work and give you a reasonably accurate state of charge that is reset daily by the solar.

The battery will not fully charge from the DC converter, that is the actual situation, you are not fully charging that battery with a 13.8V supply.

gs_nz avatar image gs_nz Guy Stewart (Victron Community Manager) ♦♦ ·

The suggested settings will simply mean that what is currently 97% SOC will become 100%. That's a workaround I might consider.

You still haven't addressed my actual design problem of the MPPT hard coded minimum charging limit. Can this be addressed?

I am not sure I see the problem with the MPPT, only the BMV and that is already fixed with settings.


Lithium batteries are a “target” voltage charge, not a charge state profile charge.


As long as the batteries are getting an efficient, safe and reliable charge from the solar panels, the MPPT is doing its job.

Absorption is an important factor for lead acid batteries.

gs_nz avatar image gs_nz Guy Stewart (Victron Community Manager) ♦♦ ·

The battery supplier disagrees. On bulk charge (CV) the batteries will charge to around 95% unless the voltage is higher than 14.0V, which is supported up to 14.6V, but it reduces the lifetime of the batteries. These batteries once charged CV to 14.0V to 95% stated capacity, or 14.2V to 97.5% stated capacity, and then remain on 14.2V on CC to go to full 100% stated capacity. Long story short - absorption is still critical step for these batteries.

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Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

It would be easier to change the tail current percentage in the BMV settings.

This is very common to have to modify these BMV settings for different battery characteristics.

From the data I can see there,

Charged voltage of 14.2

Charged detection time 5m

Tail current - I can't extrapolate this from the data because I don't know you total battery bank size, so you will need to experiment.


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I've tweaked these settings with my dealer over few months so that it works with MultiPlus and DC-DC, assuming that MPPT will behave the same.

I have low voltage values to enhance battery life. On the graphs the battery is still taking 8A of current (~100W) for the next ~1.5 hours with MultiPlus, but stops short with MPPTs.

On my 12V/500Ah battery bank the tail current on BMV is set to 1.0%. So not much room to wiggle there. Setting a lower tail current is just a workaround.

Why is the min. 2A forced on MPPT? Consistency would be in order here, imho. Is this a hardware limitation?