I run a hybrid LiFePO4 and SLA battery so it is possible. However it depends on few things.
Objectives?
Type of lead acid battery and the charge voltage specs? They differ depending on the lead acid battery type.
Relative capacity of batteries?
What is charging them?
In my case the SLA (~400 Ah) is my backup reserve capacity (and is what I had to begin with for power outage backup), and so they sit at float most of the time. The LiFePO4 (300 Ah) I added later on to do daily cycling. Nominally that's ~35 kWh of battery. I confine daily discharge to no more than the nominal capacity of the LiFePO4 only, i.e. 15 kWh (with 20 kWh / 35 kWh total being a total pack SOC of 57%).
Drawing down SOC lower than that would only occur during an extended power outage.
When discharging from 100% SOC, the LiFePO4 does all the work, with a little energy also being transferred into the SLA.
As LiFePO4's SOC falls to ~23% (total pack SOC ~67%), then the SLA starts to contribute some of the energy, and when LiFePO4 SOC falls to 10% then the SLA is doing most of the work, even supplying some charge into the LiFePO4 and keeping them from reaching a cut off voltage.
Here's some example charts of a test I did recently. This was a discharge during a power outage which started at 9AM, ending a little before 1PM. At the start of the outage the LiFePO4 batteries only had ~ 30% SOC (pack SOC 70%). Battery had discharged the night before and had received a little charge in the morning prior to the outage.
This chart shows the BMS SOC readings for my three LiFePO4 batteries, as well as the total hybrid pack voltage during this period of discharge. The lower chart shows the load (and pack voltage again for reference). Note the heavier loads towards the back end of the outage (people came home and started using stuff).
And this is the voltage and SOC for the combined hybrid pack which fell to 57% (a nominal 15 kWh of discharge):
View attachment 211406
The below chart is again the same time period and shows the relative proportions of the power being supplied by the SLA (orange as that's the colour of my SLA batteries) and LiFePO4 (grey), again with the pack voltage for reference:
Can see how the LiFePO4 does by far most of the work but as the pack voltage drops into the 51.x range the SLA begins to contribute, just a little to begin with but increasingly so as voltage drops.
With pack voltage dropping into the 49.x range the SLA eventually does the bulk of the work. And indeed under some loads it is supplying some energy back to the LiFePO4. If you look back at the first chart, note how the LiFePO4 SOC plateaus at ~8%, even though the hybrid pack is still discharging. In my case the SLA gives the LiFePO4 a "soft landing".
In terms of absolute energy flow, below shows the 5-min energy interval contributions by each:
View attachment 211405
So can it work?
Yes, however it requires a lot of ducks lined up in a row in order to be worthwhile. And the SLA need to be in very good condition, else self discharge will end up being costly. Appropriate battery safety and protection measures are required.
Would I use such a set up if starting from scratch? No.
In my case the SLA was already there, in good condition and has compatible charge voltages so I chose to try it out and it's working well for me. It's purpose is outage backup only but as it turns out it also helps the LiFePO4 out. I do not propose to discharge the hybrid pack beyond this level, excepting when required for long grid outages.
I charge up to 56.4 V and that works well for both chemistries based on the specifications of each specific battery. FLA would likely require too high a voltage for my comfort, and making that work would require a means to automatically disconnect the LiFePO4 in order to fully charge the FLA, and reconnect on the way back down. Solutions exist but that involves spending money which might otherwise just go towards more LiFePO4 instead.
In your case I'd be inclined to sell the lead and just get more LiFePO4.
