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RedRover avatar image

What should I set Lithium Battery Absorption Time to?

They say that Lithium batteries don't need to be charged in absoption mode. I've heard that when charging Lithium you should go from Bulk straight to Float. Will setting the absorption time to zero achieve this? What about setting the absoption voltage to a really low voltage that is never achievable? What do others on this forum know of this? What settings do others use when charging their caravan Lithium battery? MPPT Bluesolar 100/30, Sentry 300AH Lithium, 300W solar.

MPPT - Solar Charge ControllerLithium Batteryabsorptionfloat
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My understanding is that the term absorption is a legacy from lead acid days and the reason we use terms like bulk, absorption and float is those are the terms of most battery chargers. With Lithiums I charge at constant current (bulk) and as the battery gets to around 98% they are then basically full, but from time to time we need to balance the cells, so as Guy says we set a target voltage that the cells/battery should not go over and maintain that voltage (absorption) for about an hour as current drops towards zero to fully balance cells. We then set another voltage for after the hour (float) in case the charger doesn’t have a way to automatically switch off, to prevent the battery staying at the elevated (absorption) voltage.

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

LiFePO4 Lithium batteries like the sentry have a target voltage. They will absorb high current until that voltage is reached, and then that represents (more or less) the desired state of charge.

You can reduce the target voltage slightly (eg from 14.6V to 14.2V ) to increase cycle life, but at reduced usable capacity.

In my configurations using Sentry lithium batteries I still hold the target voltage "Absorption" for 30 minutes. Definitely disable EQ (and set it to target voltage to be doubly sure).

Test bench behaviour:

From a complete discharge the 8Ah battery absorbed a manufacturer specified maximum charge current of 8A (1C) in Bulk charge mode for 36 minutes and then Absorb charge (14.4V) mode for 29 minutes before reaching float.

The attached current logger recorded 8 Ah of charge. The attached Victron MPPT recorded 100 Wh of system yield from the solar.


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Guy, when you say target voltage are you referring to what Victron call “Charged voltage” in the battery screen of the 712?

Thanks

I am also curious to know if my Victron MPPT 150/45TR is basing it's target voltage off of the 712's "Charged voltage" setting. @Guy Stewart (Victron Energy Staff)

Esteban avatar image
Esteban answered ·

I will add that my understanding is that Lithium does not like to constantly topped off. It wears on the life to be running from say, 90% to 100% constantly. For that reason any float voltage will be set far lower - 13.4 or less.

As for ignoring all concepts of Absorbtion, Float etc, with lithium, the reprimand to forget these misses a key point: Almost every single charging controller you find today, if offering custom settings WILL require you to input parameters for these stages, whether your battery chemistry requires them or not. So you WILL need to have a clear concept of what parameters to set for each stage, in any case.

The question is very reasonable.

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And for most LiFePO4 batteries you can (/ have to) use the absorbtion voltage to make sure they balance themselfes.
So in your mind you could use 'CC', 'CV', 'maintain' or something to replace Bulk/Absorption/float

bitcrusher avatar image
bitcrusher answered ·

Is it true, that you can storage LiFePo‘s with a permanent refuduced floating voltage over longer times?

could this be done with the multiplus?

thx.

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yes, depending on what you want to accomplish (backup system that has to keep 100% charge all the time, or just to compensate from (self) discharge) you can set the float charge voltage on 12,8-13,4 V

btw you might want to post your question as a separate topic for better answers / search ability.

JohnC avatar image
JohnC answered ·

Hi Red (again, yeh I'm back). :)

And now serious. Red, you have Li batts, so let's please drop all of your preconceived bulk/absorb/float mindset. Pb is history for you, so push it aside and start again.

Your batts are expensive, include a bms, and I'm sure would come with a recommended supply V. Target that, nothing else. Observe their Vmax and try to do what they say. It's unlikely that Victron can't provide that.

And if that means flushing preconceptions down the toilet or vomiting in the sink, just do it. Go on, do it..

I did say serious, hey. But offered sincerely and in good faith.

Continue the discussion, please ask if you need, or disagree. C'mon, lets have it out. Plenty pundits here can help..

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Funny about that, I know nothing about agm either so dont have any preconceived ideas. Nor have you offered any solutions or advice on lithium.

One might wish to ignore Lead Acid, but the reality is that all devices, including Victron ask for parameters for Absorbion and Float. So we have to conform and try to find ways to set these parameters so they work best for Lithium.

Indeed re lead acid legacy terms. Adding to my other comment.

Charging to that (absorption) voltage only needs to be done occasionally to balance cells. I have both LFP and NMC Lithiums. The LFPs work on a standard Victron Phoenix lead acid type charger programmed accordingly. My NMCs are fully automated by the BMS and Color Control GX using DVCC (Distributed Voltage and Current Control) and I use a Victron MultiPlus, which I have programmed with appropriate lithium settings, more as a backup as DVCC and the BMS takes the central role rather than the charger settings.

Peter Polz avatar image
Peter Polz answered ·

You can set absorbtion time to 0 in Victron Connect if you want

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