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Is it possible to have the MPPT not starting a new charge cycle every day? In fact i only want to start a new cycle if the voltage drops below the re-bulk offset. This to save my LFP batteries when there are no power consumers since they don't like te be at high SOC for longer time.
How is this taken care of with victron lithium batteries? I assume, since they have the same chemestry, they also don't like to sit at 100%soc for a longer time?
I would like to see the mppt recharge based on the SOC reading from the battery monitor. Perhaps this will be included in the next firmware that will allow synchronised charging using the ve smart networking. Eg if the SOC drops below 90% return to 100% and stop char.
@Swampbush, as mentioned below, you can use a VE.Direct non-inverting remote cable on the smaller MPPTs or use the built-in remote trigger pin on larger MPPTs to turn the charger on/off via the relay-out of a BMV-xxx, and program the relay to open/close based on battery SOC. This effectively accomplishes what you want to do, with a little bit of programming in the BMV; basically, just tell the BMV to close the relay (which turns on the MPPT) at 90% SOC and open it (turning off the MPPT) at 99% SOC.
I already use the relay for that to disconnect the alternators from the house bank, so I could do the same for the mppt. I think it would be better to keep the mppt in float to supply loads rather than cycle up and down. Is there a force to float trigger on the mppts?
@Yannick, it is not possible to change the charge cycle algorithm in an MPPT; each day starts a new cycle. Technically you could use a VE.Direct non-inverting remote cable triggered by the relay out of a BMV, set to close the relay at your desired SOC, to turn the controller on or off; this would effectively accomplish your desired effect. Or, if you would like your batteries to sit at less than 100%, the easiest way to accomplish this is to just change the charge voltage in your MPPT to a lower voltage than your battery's fully-charged voltage. For instance, for Battle Borns (charged voltage 14.4-14.6), you might set the bulk voltage to 14.2 or 14.0, and adjust the float voltages down accordingly.
As to whether or not LFP batteries "like" to be fully charged all the time, the data regarding this is inconclusive. Tesla's own recently-released long-term study across a vast dataset of users concluded that there was no substantial, identifiable difference in the lifespan of their batteries that had been regularly fully-charged and batteries that had been almost never fully-charged. Bottom line is that as long as you're not overcharging your LFP bank, keeping it fully charged will not adversely affect its long-term performance to any statistically significant degree.
All batteries like to be cycled, make no mistake about that, but there is no measurable adverse effect of returning to 100% SOC at the top of a cycle.
@Justin Cook, But there are study's that show decreasing capacity of LFP cells when held at 100% SOC (at the charger! that is what you're actually doing with solar). LFP cells reach 100%SOC between 3.41 and 3.42V per cell if charged long enough. So, starting a new charging cycle every day on a battery that is already at 100% SOC means overcharging, every day, over and over.
I've set bulk at 13.8V, and Floating at 13.3V. When absorption phase ends, i'm usually at around 13.6V (3,4V/cell). Without any load that is nearly 100% SOC. Starting a new cycle every day means eventually overcharging.
(Edit: Addition) When charging at higher constant currents, such a 0.2C rate or higher, you can detect the 100% SoC point by a tapering off the charging current. That detection can not be performed with low and varying charging current from a solar controller.
About the tesla study, i believe Tesla never used the LFP chemestry, or am i incorrect?
@Yannick That's technically correct; Tesla is known for using LiCoO2, NCA, and NMC chemistries for their higher specific energy capacity than LiFePO4; that being said, however, the basic charging principle remains. If your charge voltage is set lower than your desired charged voltage, it's impossible to overcharge the battery because the charger voltage isn't high enough to push current into the battery.
If your bank is sitting at 14.4 and your MPPT bulk voltage is set to charge at 13.8, the charger isn't going to charge your batteries. It'll start in bulk, fairly immediately see that your batteries are full, switch to the absorption cycle determined by vbatt when it woke up (so effectively nil) and then switch to float.
Current (A) requires voltage (V) to push it. If your battery voltage is higher than your charger voltage, no current will flow from your charger to your battery; thus, if your charger is set correctly, it is literally impossible for it to overcharge your batteries.
For an in-depth examination of the various Li-base chemistries and their pros and cons, as well as somewhat of a debunking of the "don't fully-charge your LFP bank" misconception, I highly recommend this paper: https://www.mdpi.com/1996-1073/12/6/1074/pdf
It's a bit dense, but if you can get through it you'll have a far greater understanding of why, so long as over-charging is not occurring, the only things that are going to significantly impact your LFP battery health are temperature and depth of discharge.
If the bank is already sitting at 14.4 for more than a few minutes with a low rate charge source it is already overcharged. Default Victron float is 13.5 which maintains very close to full. A return to bulk each day by the mppt should not be needed in a situation when a boat is on shore power.
@Swampbush That's exactly what i mean. When there is no consumption during the night the MPPT is potentially overcharging the battery every day. Why is it not possible to use the re-bulk setting overnight?
Evidently my point was missed... 14.4 was an example. My point is: whatever your particular battery "charged" voltage is, set the MPPT bulk voltage to a few tenths lower than that, and the float voltage lower as well, and the battery can never be overcharged no matter what stage the MPPT is in.
Bottom line: don't use default settings if you don't have default batteries. Victron components are all extensively programmable to match your specific battery needs; as long as you have the charge controller correctly set, you can't overcharge your battery.
Your point was not missed, but there is much evidence to show that full charge can be achieved as low as 13.6 volts.
https://www.powerstream.com/lithium-phosphate-charge-voltage.htm
The mppts should be able to maintain a float voltage without having to return to bulk each day, as the multiplus can do.
"If the bank is already sitting at 14.4 for more than a few minutes with a low rate charge source it is already overcharged. "
No its not! LiFePO4 has a much wider overcharge tolerance above the charging voltage plateau than other lithium chemistry 0.7v up to 4.2v per cell so that equates to 16.8v with a 4s. verses like 0.1v over 4.2v
14.4v is not even outside of the recommended range up to 3.65v per cell for LiFePO4 so the term 'overcharge' is very misleading its within the recommend range!
You can use a lead acid charger on a LiFePO4 that applies 14.4v and its totally fine this is well established. Ironically far form over charging the battery it will not even fully charge the LiFePO4 because they will drop down to the lower voltage float where the battery is no longer being charged. The LiFePO4 chemistry requires a longer duration to establish that full state of charge.
Even if the battery has got a full state of charge LiFePO4 is just massively more tolerant because the exothermic heat is so much lower.
If you are gunning for longer life on the premise of reduced degradation at a lower state of charge then don't have your bulk voltage so high anyway! The difference between cycling at a higher state of charge and it sitting at a higher state of charge is just going to be pointlessly insignificant just cycle to a lower state of charge!
State of charge is also dependent on time spent at that voltage. Full charge can be achieved at lower voltages. Slow charging on solar can bring the batteries to full charge from anything above 13.6 volts. The 3.65 volts per cell is a recommendation from high rate charging. The page I linked shows clearly what state of charge can be achieved from lower voltages. After 5 minutes of absorption at 14.2 volts my charging current rapidly drops to almost nothing as the batteries are full. So just lowering the absorption voltage doesn't really deal with the problem of the mppts always recharging each day. It would be nice to not have to change the settings when on shore power versus at anchor.
I recognize this is an old post but am taking the chance that this is still an issue somewhere besides my system...
I must agree with the OP. Why would a new charge cycle start just because the PV voltage drops below a voltage that isn't configurable? Until the battery voltage drops below the re-bulk setting, I don't want my batteries hit with a bulk charge voltage every day. The "fix" for it is to manually turn off the PVs so there is no ability to charge but then I am exposed to shore power failures.
Two solutions come to mind (both in firmware/software):
While it is possible this is not immediately harmful to my batteries; it does require a significant change in maintenance as lead-acid batteries do boil off water when they are subject to a charge voltage. I shouldn't have to perform more battery maintenance just because I have a solar charging system. This happens every day there is enough sun to satisfy the bulk charge:
There are other less desirable solutions such as lower the bulk charge voltage but that creates other issues when the solar chargers are the only chargers online (i.e. away from shore power for an extended period of time...)
