I had thought about the whole mixed LTO + other cell combinations for a long time.
The ideal option would be to have a small LTO pack as the default pack, connected to the inverter and then the other lithium cells providing a slower re-charge to the LTO (with DC-DC). The charging would simply be direct to the other lithium pack. The LTO pack is always aimed to be a full charge to handle the loads.
The big win for a mixed setup is the other lithium cells can be given a very easy time and never see any real surge requirements. The LTO can then be hit with some large discharge requirements.
The issues I came up with was a combination of things.
Losses : DC-DC is around 97% maximum, optimistically 95%, so you loose an additional 5% on a lot of energy in the pack
Discharge : Continuous high discharge would need a DC/Buck from the other pack to the LTO of equivalent capacity
Cycle Life : The other pack would still end up with a lot of cycles, unless large and then the whole cost increases
Pack size : The LTO pack would need to be large enough to cope with your largest continuous load less the input from the other pack, i.e. say you have a 3000W oven and a 1000W boost converter to charge the pack. The LTO cells would see the 2000W discharge requirement and if your LTO pack was only 1kWh then the oven goes cold after around half an hour (ok longer due to warming up, switching off once up to temperature, etc.)...
Components : Going 72V or higher is quite limited on choice, while boost converters over 1000W that can be run in parallel are rare or expensive.
24V / 48V combinations could work, the inverter/LTO being the higher connected voltage pack.
The mixed pack setup would allow for some drastically different options, like the other lithium pack to be at a completely different voltage to the inverter conected LTO pack. I was looking at using 72V or 96V initially and struggling for charge controllers at this voltage, while DC boost converters were ok for 90V at about 1000W. The setup would allow for a larger higher voltage, higher capacity (and cheaper) inverter to be used.
96V version of one of these would allow the transformer to be smaller and cheaper (less copper due to half the amps), however at 96V other issues occur, like death by accident. Around 120V and higher flashover and sustained DC fault arcs create some real issues and risks.
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