Can you mix lithium and lead-acid batteries on an energy storage project?

By Kelly Pickerel | August 17, 2020

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.

Storage installations typically consist of one battery type, like with LG Chem, here. Photo courtesy of GreenBrilliance

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.”

SimpliPhi does not recommend mixing battery brands or chemistries in installations. Here, its lithium-iron phosphate batteries were used in a solar installation on former California Gov. Jerry Brown’s off-grid private residence

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.”

Separating loads and setting up two systems is often a more complicated task than many want to take on.

“We haven’t dealt with a hybrid lithium/lead-acid system at Freedom Solar because it wouldn’t be a cheap add-on, and we try to keep our battery installations simple by using only one battery chemistry and one battery product,” said Josh Meade, P.E. and design manager.

There is one company trying to make combining the two chemistries a little easier. Portable power product manufacturer Goal Zero has a lithium-based Yeti Portable Power Station that can also be used for partial home backup. Yeti 3000 is a 3-kWh, 70-lb NMC lithium battery that can support four circuits. If more power is needed, Goal Zero offers its Yeti Link Expansion Module that allows for the addition of lead-acid expansion batteries. 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. This allows you to use the electronics in the Yeti [lithium-based system] but expand the battery,” said Bill Harmon, GM at Goal Zero. “At 1.25-kWh each, you can add as many [lead-acid batteries] as you want. 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. If the batteries are not at the same voltage or are discharging at mismatched rates, the power will run quickly between each other. When the power runs quickly, heating issues arise and decrease the efficiency of the battery cycle.

Goal Zero manages this situation with its Yeti Link device. Yeti Link is essentially a sophisticated battery management system suited for the original Yeti lithium battery that manages voltages and charging among the different chemistries.

“Yeti Link is regulating that power transfer between the batteries,” Harmon said. “We control that in a safe way, so that lithium battery doesn’t even know it’s married with a lead-acid battery.”

The Yeti 3000 may be smaller than traditional lithium home batteries – LG Chem, Tesla and sonnen models typically have at least 9.8 kWh of power – but that’s it’s draw, Harmon said. And if someone can expand it up to that 9-kWh mark with some cheaper lead batteries and also take the lithium battery with them when camping or tailgating, why not?

The Yeti Link device that allows for the pairing of lithium and lead-acid batteries.

“Our system is for all the people in the country who don’t have $15,000 to spend on an energy storage installation. And then when I’m done, all I have is something permanently installed in my home,” Harmon said. “Yeti is for those who are sensitive to what they’re spending money on. Our system is $3,500 total installed.”

Goal Zero is now on its fifth generation of product, so it’s confident in its lithium-lead combination capabilities. But for many others who are less comfortable mixing the battery chemistries outright, two isolated and independent systems can be installed in the same business or household – as long as it’s set up by an electrical professional.

“A simpler and safer way to add lower-cost storage capacity to an existing lithium system would be to divide the loads and allocate them separately to the two battery systems,” U.S. Battery’s Wehmeyer said. “Either way, it should be done by a trained professional to maintain safety.”