Can you mix lithium and lead-acid batteries on an energy storage project?
There are pros and cons associated with the two main battery chemistries used in solar + storage projects. 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.
Can one combine the pros of each chemistry to make one cost-effective, high-capacity battery bank?
Does one have to dismantle their lead-acid battery bank just to tap into the functions of a new lithium-ion battery? Can one add a few cheaper lead-acid batteries to their lithium system to meet a certain kilowatt-hour capacity?
All important questions with a less defined answer: it depends. It is easier and less risky to stick with one chemistry, but there are some workarounds.
Gordon Gunn, electrical engineer at Freedom Solar Power in Texas, said it is likely possible to connect lead-acid and lithium batteries together, but only through AC coupling.
“You absolutely cannot connect lead-acid and lithium batteries on the same DC bus,” he said. “At best, it would ruin the batteries, and at worst…fire? Explosion? A rending of the space-time continuum? I don’t know.”
K. Fred Wehmeyer, senior VP of engineering at lead-acid battery company U.S. Battery Manufacturing Co., provided further explanation.
“It can be done, but it wouldn’t be as simple as just adding lead-acid batteries to the lithium battery system. The two systems would essentially be operating independently,” Wehmeyer said. “The lithium battery system would still have to be controlled by its own BMS with its own charger and charge controller. The lead-acid battery system would need its own charger and/or charge controller but would not need a BMS. The two systems could be supplying the same loads in parallel but there might need to be some control to safely allocate load distribution between the two chemistries.”
Troy Daniels, technical services manager for LFP battery manufacturer SimpliPhi Power, does not recommend mixing the same battery chemistry let alone differing chemistries in a single system, but he does acknowledge it can be done.
“A couple ways to combine would be the route of having two isolated systems (both charger and inverter) that could share a common load or even split required electrical loads,” he said. “A transfer switch could also be utilized; however, this would mean only one set of batteries or chemistry could charge or discharge at a time and would likely need to be a manual transfer.”