MPPT not getting the batteries full

For the past 3 days I have now witnessed this. My AGM’s 200Ah are allowed to discharge during the night to 70%.

At around 10H00 the next morning when all PV is out of the shade the MPPT (150 | 35) ramps up and charges up to 18amps (the most I’ve seen) but gradually throttles back as the batteries are nearing the full mark which I completely understand must happen due to internal resistance, absorption stage etc.

But, it is not reaching 100% SOC. It gets to about 95% and then the day is over and sunlight gone.

I don’t understand how this happens as my daily loads are no more than 600W, and I have a 2,4Kw array. 96V VOC and charging capability of 31.5A

I get the idea that the MPPT throttles back to quickly. Can this be heat problem? I have no external temp sensor connected to the batteries

Maybe a setting that needs to be adjusted somewhere?

This is today at 12h00

Again I know that it will never reach full SOC today.

Not my words, but copied and pasted from another user on another forum:

The next level is to fine-tune the BMV to your batts.

Set the tail current nice and low (about 1% of the Ah capacity, maybe 2% for some flooded cells), and the detection time should be set to something like 15 minutes.

“Tail current” is basically the current the bank will absorb at almost fully charged.

As your batteries charge they “Absorbs” less and less current.
The BMV will indicate 100% charge after looking at the following three conditions.
Once all three conditions are met, the BMV will indicate the bank as 100% charged:
Setting 2 - Charged Voltage: Bank voltage at “Float Voltage” or higher
Setting 3 - Tail current: Less than 4% (In my case 2.8%) of the total Ah rating of the bank
Setting 4 - Charged detection time: For a set time of ?? Min. (In my case 30min)

Ok so currently this is what my settings are:

Setting 2 - Charged Voltage: 54V Can this go higher? To where more or less? How do I see what my banks voltage should be at float?
Setting 3 - Tail current: 4% on the dot. I’ll lower this to 3%? Or 2% maybe?
Setting 4 - Charged detection time: 3 minutes. So this seems a tad low. Maybe up it to 20 minutes?

Once you see zero amps going into the batts, remember to set the SOC to 100% on the app.
This must also be check every once in a while.

Ok, the specs indicate the float voltage to be 2,25V per cell = 54V which is what mine is set at. Should I up this also slightly?

Ok with regards to the tail current.

On the BMV I have set it at 3%. This gives a tail current of 6amp from a 200AH bank

But I suppose I must make this adjustment on the MPPT also. So I have set this also at 6amp - no option of percentages given here.

The respective battery voltages are the only thing I am not entirely sure of. They are as follows:

Absorption at 55,2V
Float at 55V
Equalization at 57.6V

Does this seem correct?

I want to upload my spec sheet for the batteries but it seems the site wont accept PDF format.

Here is what it currently looks like. It will never reach 100% SOC at this rate of 1,6amps

Sync the BMV to 100% SOC on the App, or wait a little longer, see if the amps go lower.

MPPT and inverter must be set at the same charge and float volts yes.
Inverter set to the max the batts must be charged at.
MPPT, if you run ESS, must be set to charge at max amps of MPPT … in ESS the charge amps are then set, same as the inverter charge amps.

If that is what the doc says, then that is what it must be yes.
Set those on MPPT and inverter yes.

That’s the thing. Not sure if I’m interpreting the sheet correctly. The only thing it tells me for sure is the float voltage which is 2.25V

Then it gives equalization and cycle service values as 2.35 - 2.4V which I assume is then equalization voltage then at anything between 56.4V - 57.6V

But there is nothing that says anything about the absorption voltage.

See the screenshots please.

And where to I set this on the inverter? I have never seen an area where this can be adjusted.

Know only of the MPPT’s and the BMV’s section to do this.

Bulk = Absorption voltage with the specific Float voltages are all you need.
use the 2.4v, as you are cycling and get it done - it = the recharge.

Using a MK3 cable, you login to the inverter, and program it.
If you have a Venus then you can download the inverter settings via the portal, to your PC.
Then using VEConfig software, you can alter the settings.
And then you upload the saved settings, back onto the inverter via the portal.


Its been a loooooong day sorry.

Not sure what this means. Use 2,4 for the absorption voltage also?

So this will be the same as the equalization voltage then?

Sorry, my bad!

Set 57.6v as the Bulk/Absorb voltage.

And here in lies my problem with batteries and monitoring them in general. The BMV gets tweaked so that it shows the “Perceived correct value” now is this the correct value or the one you decided is the correct value. You will undoubtedly also notice at some time that the BMV will “jump” to 100% , most of the time on badish weather days. I ended up making my peace with the fact that you will not be a 100% sure when the batts are fully charged and that the BMV is there to show you when you abuse the batteries or if there is a problem with a cell (Mid voltage) the rest is up to you and how you tread them.
It does not matter what the BMV show’s , they will be charged if they are charged , unless you use the grid to charge them , then it is a different discussion :slight_smile: . I now a days prefer to limit how they get discharged per day but still use soc as a general indication on the less ideal days by setting the min Soc.
I knew/know a guy the would give his lead acid batteries hell during daytime , I mean welding , grinding every thing he could do he would to the extend that he would need to “top up” them in the late afternoon for evening usage.
I wonder if he still has that bank …

Last one , this is on Lead Acid , I would imagine the the Lithium batteries works on a different level…

Jip, what I see, they start at say 50amps charge. Goes “fast” down to 40, 30, 20a, 10a … 2a … then zero.

Settings are different:

@Swartkat, here are some more info on lead acid:

Each bank behaves slightly different, on my old Trojans there were settings that worked for them, but not for AGM batteries for example. You “tune” your BMV to your bank and it’s usage.

In my experience, AGM batteries come down really low on the tail current. 3% is good for flooded banks, but for an AGM battery I have seen it come down to 1% when it is well and properly full.

But there is really two issues here to watch. The first is to remember that SOC is always an estimate. If the charge efficiency numbers and all those things (charged voltage, tail current) is not perfect (and they never are, temperature, age, etc means they are quite fluid), your battery will either reach an indicated 100% before it is completely full, or it will stop at <100% even though it is full.

The second is that the important part for the battery is that it gets a full charge, ie it needs to spend a few hours at absorption voltage every day (otherwise the plates will sulphate). So the first thing you may want to do is check your logs… how much time is spent at absorption level each day. If that number looks good, chances are the battery is fully charged, the SOC estimate is just inaccurate.

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Is this the log you’re talking about? Or do I get it somewhere else?