Building a large Hybrid LFP/FLA battery.
I have ordered 17 EVE 280 a/h cells with a 24s BMS. I am planning to built a 17s, 15.2 kwh battery and parallel it with 16 - 420 a/h US Battery L-16 FLA batteries. The L-16s are configured for 48 volts (2p8s). They are 10 years old but are used primarily during the summer (6 months). They still provide the power that we need but since FLAs age like dogs, I know that they are closer to the end of life than the beginning. I have done a lot of reading on this forum and elsewhere but I cannot find any consensus on whether or not I should give this a try. The common objections are often a reaction like you "cannot mix chemistries", or "all the batteries need to be the same" or "the charge and discharge profiles are don't match" or "if a cell shorts out in one of the FLA batteries, bad things can happen". This is not an exhaustive list of the reasons not to do this, but seem to be the most common. I'm certain that the list can grow a lot and I'd like to know what I don't know. And the reasons sound a lot like the conventional management advice for lead acid battery banks. And they are right. I would never parallel FLAs with SLAs or mismatched LA batteries of any kind. But putting together the "hybrid" FLA+LFP looks on paper to be a very good match. I made the short table below to illustrate the voltage compatibilities of the batteries. I used the charge profile for the FLAs as recommended by the manufacturer (US Battery) and the recommended charge voltage for the LFP batteries.
I found that a 16s LFP battery would not be a great partner with the FLA bank because to get the FLA Bulk/Absorb voltage to a suitable voltage would require charging the LFP battery to 3.65 v/c, which is 100% maximum SOC. While it's ok to do that once in a while, I don't think I want to do that every day. Configuring a 17s LFP gets most of the voltages into almost the perfect range to match the FLA battery with the exception of the Float voltage for the FLA. I'll come back to that in a minute. If you look at the 17s charge voltages, charging to 3.45 v/c puts the charge voltage perfectly in the sweet spot for FLA. Even better is that in cooler weather, I cam bump the LFP charge rate to 3.55 v/c which acts almost exactly like temperature compensation on my charge controller (which will be "off"). Taking it to the extreme and bumping the LFP charge rate to maximum 3.65 v/c, I get close to an equalization charge rate for the FLA battery. So far so good?
But as I said above, on paper this appears to be a pretty good fit
except for the aforementioned Float voltage on the FLA battery. The charge controller float function will be turned off. The float effect on the FLA battery will come from the higher relative voltage of the LFP, which fully charged, is 54.4v at rest. The difference in recommended float voltage for the FLA is 52.10 volts. The LFP will try to equalize with the FLA. This is the one place that the "chemistries" don't match and I'm not sure how or if this will be an important factor. I have read in various places that there will be a low amperage flow to the FLA, but the high resistance of the fully charged FLA and the 24/7 house load on the battery will keep the amp flow to a mostly irrelevant value. The LFP battery will never be truly at rest.
I would very much appreciate commentary on this proposed hybrid configuration. If it works it would greatly improve the efficiency of my entire off grid system. I will also learn a lot and have some fun doing it.
Voltage Profiles for LFP and FLA Battery Bank - 48v Nominal | | | |
| | | |
| FLA (8 - L 16) | LFP (16 cells) | LFP (17 cells) |
| | | |
Fully Charged Bank Voltage (at rest) | 50.40 - 51.20 v | 51.20v @ 3.2 v/c | 54.40v @ 3.2v/c |
| | | |
Bulk/Absorb Charge Voltage | 56.40 - 58.80 v | 55.20 @ 3.45 v/c | 58.65 @ 3.45 v/c |
| | 56.80 @ 3.55 v/c | 60.35 @ 3.55 v/c |
| | 58.40 @ 3.65 v/c | 62.05 @ 3.65 v/c |
| | | |
Float Voltage | 52.10 v | 51.20 v | 54.40 v |
Note: the charger will be set to "No Float" | | | |
however the FLA'S will "float" based on the | | | |
voltage of the LFP bank | | | |
| | | |
Note: FLA Voltage range is based on | | | |
US Battery charging recommendations. | | | |