I have a healthy12v 7Ah SLA and I want to add my new 12v 20AH

Lithium-ion Battery Pack to my electric bicycle. I know I can replace it and go ahead with simple circuit.
But this situation got me thinking why cant we have a SLA - Li_ion hybrid battery pack. The charging circuit can be simple and individually managed for SLA and Li_ion pack, but where as I am worried about the discharge circuit.
Can we build a current limiting discharge circuit with hybrid system ? Balanced discharge, peak current handling and discharge protection these are the things needs to be taken into account, so how will the hybrid battery management circuit will look like ?
Any idea(Brain Storming) on building the hybrid battery system ?
I am open to ideas and suggestions.​

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SLA is stone-age technology ... outdated and 4 times heavier than li-ion(for same energy) ...in addition it has a short dycle life ...

An e bike you want every think light , not heavy lead .....

Also matching SLA 12V to li-ion is impossible .... 3s is too low 4s is too high .... 24V matches well with 7s li ion

 

While not a good idea practically, if you want to do it for fun then the simplest possible solution I can think of (though probably not the best solution) is use diodes or p-fets to separate the batteries. The bike will pull from the battery with the higher voltage, so one battery will be doing all the work until it gets low enough that the other battery begins to drain. You would of course need separate charging circuits, and separate cut-off circuits to disconnect either battery once they reach the low voltage limit for that chemistry. A simple comparator and relay for each should do the trick.

 

Thanks for your valuable idea and suggestion. I just want to try to make this a hybrid system for out inverter as well. So is there a way to make the circuit to work with this configuration.