The customer can just plug them in. Suddenly you have the portability of the lithium battery and the inexpensive lead-acid batteries sitting at home.” The biggest problems when trying to link lithium and lead-acid together are their different voltages, charging profiles and charge/discharge limits.
There are ways to mitigate the issues but you should really not mix batteries that aren't built together and at the same time. You should only use "batched" batteries, this is true of all battery cells and it is especially critical and true of a Lithium installation.
From my understanding it is not good to mix Lead Acid batteries of different ages because their charge efficiency decreases and the absorb time necessary to fully charge them increases with age so to fully charge the old batteries means the new batteries will be overcharged or if we fully charge the new batteries the old batteries are undercharged.
Lead-acid batteries have been around much longer and are more easily understood but have limits to their storage capacity. Lithium-ion batteries have longer cycle lives and are lighter in weight but inherently more expensive. Storage installations typically consist of one battery type, like with LG Chem, here. Photo courtesy of GreenBrilliance
You should only use "batched" batteries, this is true of all battery cells and it is especially critical and true of a Lithium installation. Lithium Iron Phosphate surely is known for its safety but they still contain a lot of energy and issues can become very big problems if you aren't careful and thoughtful on the front-end.
Yes, that’s right: The lithium Yeti battery can be paired with lead-acid. A Yeti 1.4-kWh lithium battery (top) with four stacked 1.2-kWh lead-acid batteries underneath. “Our expansion tank is a deep cycle, lead-acid battery.