Would it be possible to post the voltage curve graphs for the Lithium 12.8v batteries.
asked
Lithium Battery Voltage Profile Graph
It's just a LiFePO4 chemistry. They all have approximately identical characteristic curves.
Any manufacturer variation you see is either a marketing mistake or an oversimplification. Usually the latter, since voltage is dependent not just on SOC but also on both the immediate and recent charge/discharge rate and to a smaller extent the temperature.
Thanks Ben, I'd still be curious to see it. We love our Victron Lithium set-up however every so often I'm curious as to what the Voltage/ SOC should be (approximatly). We have a BMV 702 and Color Control Gx but every so often the house loads switch gets turned off and the SOC resets to 100%.
Also many LifePo4 battery set-up rest at around 13.3V, I often wonder if 13.5v float that Victron recommends is a bit high. None the less I have our multiplus 12/3000 set to the default Victron Li settings same for our Blue Solar MPPTs.
I think it's probably a little high, unless you need to have a high SOC at your disposal all the time. (That might make sense in an off grid scenario with variable and limited solar and no other charging option, for example.)
I float at 3.325Vpc and see steady state 70%ish.
One of the downsides with Victron is that there's no simple way to transition the charger back into Bulk from Float. So if you're okay day to day at 70%, but tonight you want to top off to 100% in anticipation of high demands tomorrow, you have to jump through a lot of hoops to get it to start charging again.
As said already the SOC vs battery voltage is quite hard to give as this also is depending on other criteria like age and temperature. What is the important part is to know when the voltage drops steeply when the battery is almost ampty (which is around 12-12,3Vdc) and at which point the voltage climbs steeply at charging (wich is around 14.0-14.2)
Hello, Charging a lithium doesn't really need a profile such as a lead/acid battery would need. As there it is a chemical reaction inside the lead/acid battery which needs to be controlled (to avoid gassing and such). At lithium its more that the battery charger act like a power supply bringing the Dc voltage to the required level, and then the battery is full. So then BULK is the charge phase to charge the lithium full, or at least until the BMS finds one cell with is at the required max voltage. The absorption phase can be skipped but is often used to give the balancing circuitry in the battery/BMS time to even out the cells across the battery, so that the battery becomes as full as can be.
Thanks for the reply Johannes, Sorry maybe I wasn't clear. I wasn't looking for the charging profile. Most Li battery producers publish a graph showing cell voltage vs SOC. Is this available?
This is something I found (after lots of searching) and modified for my own information...
If the float voltage is set (too) low the risk is there that the battery will be drained by DC loads after entering float, so you need to be sure that in float there is hardly any charge current (as thats not needed anymore) and that the charger acts as power supply for the loads.
What is see often is that a lithium battery is discharged to a certain level while only looking at the total battery voltage which is often causing unexpected switch-off events. The BMS looks at cell voltage and when lithium batteries are heavily used in combination with limited balancing periods the unbalance creates the sudden switching off. Advise here is to have the BMS to be there for emergency and have a BMV or such switch off loads far before the BMS needs to act. As far as the 100%, The BMV can be set to give 100% soc at startup (or not) in the latest firmware after the BMS has switched all off. Of course this switching of should be avoided!
@Johannes Boonstra (Victron Energy Staff): This thread touches 100% on a problem I am experiencing with an Island system with a 54V LiFePO4 battery (Blue Nova). The battery goes into Low Battery state (due to BMS kicking in) on the Multiplus(48/5000) sooner all the time - currently even at 40% SOC (~2.9V on some individual cells) with a 120W continuous load overnight. There are no peak currents drawn at that time of day. So the problem seems to be a bigger and bigger imbalance between cells. Absorption time was set till now for 1 hour long with 1 hr Intervals once a week (Default setting) and I have increased it now to daily 1Hr long at 30 minute intervals just to see if it makes a difference and also to allow the cells to get balanced out again.
My question is - what is safe settings (to prevent battery damage) for Absorption frequency, Interval and Duration for these batteries? And is imbalance of the cells to be blamed for the loss of usable capacity we are experiencing?
does the battery get charged to 100% SOC?
And what cell-voltages are reached then?
does the battery get out of balance using the factory advised voltages?
Well the aging will have a stronger effect on the higher charge period in absorption. Perhaps avoiding discharge levels which cause these imbalance is the better option. so as soon as the overall battery voltage is dropping below lets say 49Vdc , stop discharging further.
@Johannes Boonstra (Victron Energy Staff) Thank you for the feedback...
The battery protection does kick in, not on overall Voltage of say 49 Volts, but on individual cells that falls to below 2.9V. The Overall Voltage at that stage is often still above 50V.
Since I increased the absorption settings (Daily 1Hr long at 30 minute intervals) the individual cells are much more even in voltage now - both after discharge to 25-30% (overnight) as well as after full charge late afternoon. So the remedy seems to work well.
