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.
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.
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.
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)
You can set absorbtion time to 0 in Victron Connect if you want
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..
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.
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.
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
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.
Not satisifed with the thoughts above, my view is that we have to adopt a target charge voltage as mentioned above, that represents the daily SOC we want to adopt. In my case (7S NMC) thats 27.6V, which i know from my cycle tests is 85% SOC. To achieve this with the bluesolar, i set all three voltages: absorb, float and eq to 27.7V. When the battery reachs that voltage the charge current will slowly taper until it reachs zero, and there is not a need to worry about over charging them. Some people use a float voltage 0.1V lower just to give some comfort that the charge current indeed stops. However we have to factor the loads on the system, if we use a lower float voltage, and then the SOC happens to reduce in the afternoon, we will go into the night at a lower SOC. Typically with lead acid they lean on the rebulk feature, but really its not necessary if you leave all three voltages set the same. Thus you can set absorb time to 0, (or leave it set to a token amount like 1 hour to indicate that it is int he taper stage) and turn off rebulk.
