There are advantages and disadvantages associated with two main battery chemistries used in solar + storage projects. Lead acid batteries have been around for much longer and are easier to understand but have limits to their storage capacity. Lithium-ion batteries last longer and are lighter in weight but inherently more expensive.

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

Can the advantages of each chemistry be combined to create a profitable, high-capacity battery bank?

Do you have to dismantle your fleet of lead-acid batteries to take advantage of the functions of a new lithium-ion battery? Can you add a few cheaper lead-acid batteries to your lithium system to achieve a certain capacity in kilowatt hours?

All the important questions with a less defined answer: it depends. It’s easier and less risky to stick to just one chemistry, but there are workarounds.

Gordon Gunn, electrical engineer at Solar energy freedom in Texas, said it was probably possible to connect lead-acid and lithium batteries, but only through AC coupling.

“You absolutely cannot connect lead and lithium batteries to the same DC bus,” he said. “At best it would ruin the batteries, and at worst … a fire?” Explosion? A tear in the space-time continuum? I do not know.”

K. Fred Wehmeyer, Senior Vice President of Engineering, Lead Acid Battery Company US Battery Manufacturing Co., provided additional explanations.

“It’s possible, but it wouldn’t be as simple as just adding lead acid batteries to the lithium battery system. The two systems would operate essentially independently, ”Wehmeyer said. “The lithium battery system should always 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. Both systems could deliver the same loads in parallel, but some control might be needed to safely allocate load sharing between the two chemistries.

SimpliPhi does not recommend mixing brands of batteries or chemicals in facilities. Here, its lithium iron phosphate batteries were used in a solar installation at the off-grid private residence of former California Governor Jerry Brown

Troy Daniels, Technical Services Manager for Battery Manufacturer LFP SimpliPhi power, does not recommend mixing the same battery chemistry let alone different chemistries in one system, but recognizes that it can be done.

“A few ways of combining would be the way of having two isolated systems (charger and inverter) that could share a common load or even split the required electrical loads,” he said. “A transfer switch could also be used; However, that would mean that only one set of batteries or chemicals could charge or discharge at a time and would likely require manual transfer.

Separating loads and configuring two systems is often a more complicated task than many would like to take on.

“We haven’t been dealing with a lithium / lead-acid hybrid system at Freedom Solar because it wouldn’t be a cheap addition, and we’re trying to simplify our battery installations using just one battery chemistry and one. battery product, ”said Josh Meade, education and design manager.

There is a company that is trying to make combining the two chemistries a little easier. Manufacturer of portable power products Zero goal has a lithium-based Yeti portable power station that can also be used for partial home backup. The 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 extension module which allows the addition of lead-acid extension batteries. Yes, that’s right: Yeti lithium battery can be paired with lead acid.

A 1.4kWh Yeti lithium battery (top) with four 1.2kWh lead-acid batteries stacked underneath.

“Our expansion tank is a deep cycle lead-acid battery. This allows you to use the electronics of the Yeti [lithium-based system] but increase the battery, ”said Bill Harmon, CEO of Goal Zero. “At 1.25 kWh each, you can add as many [lead-acid batteries] as you wish. The customer can simply plug them in. Suddenly you have the portability of lithium battery and cheap lead acid batteries in the home.

The biggest issues when trying to bond lithium and lead-acid are their different voltages, charge profiles, and charge / discharge limits. If the batteries are not at the same voltage or are discharging at discordant speeds, the power supply will work quickly between them. When the power supply is running quickly, heating problems arise and decrease the efficiency of the battery cycle.

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

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

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

The Yeti Link device which allows lithium and lead-acid batteries to be coupled.

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

Goal Zero is now in its fifth generation of products, so it is confident in its lithium-lead combination capabilities. But for many others who are less comfortable with outright mixing battery chemistries, two isolated and independent systems can be installed in the same business or household, provided they are installed by a professional. electricity.

“A simpler and safer way to add storage capacity at a lower cost to an existing lithium system would be to split the charges and allocate them separately to the two battery systems,” said Wehmeyer of US Battery. “Either way, it should be done by a trained professional to maintain safety. “

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