question

Fra avatar image
Fra asked

How max current bulk charge affect absorption time?

My question is: with higher maximum charge currents does the absorption time have to be shorter or longer than at lower charge currents?

If my solar charge controller one day reaches the absorption voltage with max current 3A, or another day it reaches it with max current 15A, how should the absorption time vary, with the same depth of discharge?
Thanks
battery charging
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Francesco

Bulk and absorption are the same target voltage. The difference between the two stages is the current draw.

So if you are drawing a large current you are most likely still in bulk.

A smaller current is most likely absorption. This is the stage when all the cells in the battery are balancing their charges.

The time for absorption varies if on adaptive charging. Or fixed if set like that as part of a system. You can customise it and the logic is explained in the link.

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Fra avatar image Fra commented ·
Maybe I did not say it clear enough.
Victronenergy applies adaptive absorption time in its chargers and mppt controllers.
In battery chargers the absorption time is dependent on the first bulk phase time.
This means that for a 100 ah battery discharged at 0%, the absorption time will be longer with a 10A victronenergy charger and shorter with a 20A charger.
In fact, a 10A charger will have a longer first charge phase than a 20A one. And consequently a longer absorption time.
I thought it was the other way around, that is, higher maximum charge currents, longer absorption times, due to the lower efficiency.

In a generic mppt controller with settable absorption time, with same deep of discharge, Should the absorption time be longer or shorter when the charging current of the first phase is high, compared to when it is very low?
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JohnC avatar image
JohnC answered ·

Hi @Francesco

The standard Victron adaptive algorithms are different for an mppt and (say) a Multiplus. And neither really take account of the difference in charge rates. They will work ok to a degree from differing SOC start points. And efficiency isn't really considered either.

I've had solar days (mppt) when I've never reached Absorb, yet when I've hit up the genset to top off, the Multi goes straight to Abs and won't accept much charge - they're close to fully charged already.

The key to ending Absorb is the Tail Amps, the amount of charge the batts will accept. And that's not easy to quantify with varying loads, although if loads are fairly stable it can still be done. And the mppt's can finish Abs from the Tail reading. It's own or the real batt A from a shunt.

I grabbed a SmartShunt when they first came out to fix this, and networked across to the mppt I determined a Tail A of ~3% of batt Ah was the point to drop out of Abs. What surprised me though was that my floodeds were at 99.0% when this happens, so my historical concerns re not having enough Abs time were unfounded. I now see Abs times regularly of less than an hour, sometimes longer if I've drawn the batts down further than normal. And I use less water, something of serious concern to 'sealed' batt users.

Without that shunt I'd still be wondering..

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Fra avatar image Fra commented ·

Hy John

Thanks for the reply, I know when to finish the ABS phase.
I also own the victronenergy smartshuunt and an EPEVER mppt with adjustable ABS time.
I adjust it to end when the smartshuunt is about 2% current, with my victronenergy deep discharge agm. Or sometimes 3%.

My question was whether the charge current of the first phase affects the ABS time at the same deep of discharge.
In fact, the victronenergy chargers, with their algorithm, will have longer ABS times with low charge currents in the first phase. And shorter with high first phase currents.
For example, a 100 Ah battery discharged to 0% will have a first phase of 10 hours and an ABS time of 8 hours with a 10 Amp victronenergy battery charger.
But the same battery will have a first phase of 5 hours and an ABS time of 4 hours with a 20A victronenergy charger.
I thought it was appropriate to do the opposite.
That is, with high charging currents, increase ABS time, because the efficiency of the first phase is lower than at low first phase currents.
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JohnC avatar image JohnC ♦ Fra commented ·
@Francesco

I totally agree with you. It's not really efficiency though, it's just that the fast charge reaches Abs V earlier (and at a lower SOC), so really needs a longer time there to complete.

I only use a Multi to top off on bad solar days, and abandoned the adaptive algorithm. Just use Fixed in case I forget to turn off the genset.

And I find myself suggesting to others with non-solar chargers to look at using a less aggressive charge profile (lower V) if they're not in a hurry. That also will change the necessary Abs time in effect, but again will shorten rather than lengthen it under the Adaptive algorithm.

I had a cloudy day yesterday when I touched Float V just the once for a few minutes (3.2V lower than Abs), yet still finished the day >98% SOC. The batt monitor wins every time..

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Fra avatar image Fra JohnC ♦ commented ·
I agree with you. And it all feels very personal.
I am becoming convinced that the smartshuunt tail current is the best way to determine the end of the Dell ABS phase

But why does each manufacturer have their own ideas?

This says rolls battery:

Longer abs time with lower max bulk currents!

And abs time always the same regardless of the DOD !!

"The Absorb time is only the last 20% of the Charge cycle ... So absorb timer setting does not matter if you are 75% Soc or 10% Soc.

Current is NOT constant, current is tapering, this is the .38 in the formula.

Absorb Target for our AGM batteries is 14.7, any voltage under this will result in dead batteries in less than 12 months.

Absorb Timer for a 110 Ah @ C / 20 rate battery with 10 amps of average current in the Bulk stage will be .38 X 110/10 amps = 4.18 hours. With every DOD.

Formula = 0.38 * (C20 capacity / max bulk current)

I'm very confused.
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Fra avatar image Fra JohnC ♦ commented ·
In addition to what was written before, I made another reasoning.
Maybe you can give me clarification on this.

THE LOWER THE MAXIMUM BULK CURRENT, THE LOWER THE ABS TIME SHOULD BE.
Let me explain with an another example
In a 100 ah agm battery with 80% DOD, recharged with maximum current 10A, when the Abs target voltage is reached, it will be charged to about 80%. And some abs time will be needed.

But if we charge the same battery with a maximum bulk current of 2A, when the Abs target voltage is reached (with a very very long bulk of course), it will already be at 99/100%. And no abs time will be needed.

Because if it reaches the target abs voltage with a current of 2A, it is exactly the ends amps current (or tail current), which indicates that the battery is at 100%.

I am wrong?
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JohnC avatar image JohnC ♦ Fra commented ·
@Francesco

You are right. And sometimes one may get the impression that battery makers know how to make them, but may not really understand living with them.

Let me throw you another thought.. All chemical reactions speed up with heat, and batteries are no different. So in summer, Bulk phase may actually last longer and take the SOC much higher before Absorb is even reached. Just because of increased reactivity, thus accepting the charge rather than developing V.

And while the Victron algorithms are quite clever, my batts always have some small load on them at sunrise, so the V is compromised in the calculation for Adaptive Absorb time. Even that's quite coarse, but far better than nothing.

A networked shunt is the real answer.

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Fra avatar image Fra JohnC ♦ commented ·
Yes John. However it is true that the charging algorithms are always a compromise, more or less refined. Lead acid batteries are complicated. Maybe more than lithium .. I don't know.
However, the victron algorithm is excellent, I am not saying the opposite.
However, I was only thinking about the limits.
I have many victronenergy things (battery, battery combiner, inverter, smartshunt, and soon the 230v charger), but the mppt controller is from another brand a little cheaper.
Despite having only manually adjustable abs time, I am satisfied with it.
In fact, thanks to the smartshuunt I can make the mppt controller work by manually programming very precisely.

Thanks you.
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rslifkin avatar image
rslifkin answered ·

The higher the bulk amps (relative to the size of the battery), the longer the absorption phase needs to be. With higher bulk amps, you'll hit absorb voltage earlier in the charge process (in terms of SoC).

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Fra avatar image Fra commented ·
Is exactly what I think!!!

But with the same battery and same dod, victronenergy algorithm do exactly the opposite.

With higher bulk max current, the abs time is shorter...


Here is another opinion.

Tecnical service of Rolls battery (high quality battery company) :

The Absorb time is only the last 20% of the Charge cycle... So absorb timer setting does not matter if you are 75% Soc or 10% Soc.


Current is NOT constant, current is tapering, this is the .38 in the formula.


Absorb Target for our AGM batteries is 14.7, any voltage under this will result in dead batteries in less than 12 months.


Absorb Timer for a 110 Ah @ C/20 rate battery with 10 amps of average current in the Bulk stage will be .38 X 110 / 10 amps = 4.18 hours.


They say that with 10A bulk current a agm need 4.18 hours abs time. EVER.

With soc 75%, 4.18 hours.

With soc 10%, 4.18 hours.

Theyr formula IS:

0.38* (CAPACITY OF BATTERY AT C20/BULK MAX CURRENT)

with every DOD.

Evry manufacturers have their own recipes!!! Not very scientific


But my agm tail current reach <2% after only 1 hour if DOD 10-25% for example.

Or 2 hours if DOD 30-40%.


Is 2% tail current the best parameter to ends Abs phase??

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