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The two main battery chemistries used in solar + energy storage projects have their advantages and disadvantages. Lead-acid batteries have a longer service life and are easier to understand, but their storage capacity is limited. Lithium-ion batteries have a longer cycle life and are lighter, but they are more expensive.
Can the advantages of each chemical method be combined to make a cost-effective high-capacity battery pack? Is it necessary to remove the lead-acid battery pack in order to utilize the functions of the new lithium-ion battery? Is it possible to add some cheaper lead-acid batteries to its lithium system to meet a certain kilowatt-hour capacity?
The answers to all important questions are not clear: it depends on the situation. It is easier and less risky to stick to a chemical substance, but there are some solutions.
Gordon Gunn, an electrical engineer at Freedom Solar Power in Texas, said that it is possible to connect lead-acid batteries and lithium batteries, 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.”
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 in the same system, let alone mixing different chemistries, but he does admit that 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.”
“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.
One company tried to make the combination of two chemicals easier. Portable power product manufacturer Goal Zero has a lithium-based Yeti portable power station, which can also be used for backup in some households. Yeti 3000 is a 3 kWh, 70 pound NMC lithium battery that can support four circuits. If more power is needed, Goal Zero provides its Yeti Link expansion module, which can add lead-acid expansion batteries. Yes, that’s right: Yeti lithium batteries can be paired with lead acid.
“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.”
When trying to connect lithium and lead acid together, the biggest problem is that their voltages, charging curves and charging/discharging limits are different. If the battery voltage is different or the discharge rate does not match, the power supplies will run quickly. When the power supply is running fast, heating problems will occur and reduce battery cycle efficiency.
Target Zero manages this situation through its Yeti Link device. Yeti Link is essentially a complex battery management system for the original Yeti lithium battery, which can manage the voltage and charge between different chemical substances.
“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.”
Harmon says the Yeti 3000 may be smaller than traditional household lithium batteries-LG Chem, Tesla and Sonnen models usually have at least 9.8 kWh of power-but that’s the attraction. If someone can use some cheap lead batteries to expand it to the 9 kWh level, and carry lithium batteries with them at any time during camping or tailgating, why not?
“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.”
Target zero is now on its fifth-generation product, so it is full of confidence in its lithium-lead combination function. But for many others who are unwilling to directly mix the battery chemistry, two isolated independent systems can be installed in the same business or home, as long as this is 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.”