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.”
It’s funny that people who are focused on money are so willing to sacrifice their credibility with claims that are absolutely inaccurate. For the last three years, I’ve had all of my 12-volt batteries (both FLA and LFP) paralleled on the end of my carport. They charge on the lithium setting of my charge controller. The only thing that is out of the ordinary is that my SLA are maintained at 13.2 volts when not in use. To my understanding, this is good for them. The only time I separate the two types is when I want to connect the FLA to my truck for jump starting. For all of my other uses, they do fine combined. They run all of my lighting and food refrigeration. The only reason I take the precaution for heavy draws is that I fear for the possibility that the BMS might not do its job of protecting the lithium cells. In short, FLA and LFP can absolutely be safely combined. just keep your lithium connected together rather than any alternating of the types.
Very nice info. However; combining li and lead acid on same load should be avoided. Separate and switch, charge separately. Be safe.!
Just to be sure, is it ok to connect my 200ah AGM lead acid battery to a 200ah Lithium ion battery with built in BMS? Is the built in BMS enough to make this setup safe? I would appreciate any insight or experience if someone has done this. Also, can I use the same solar controller on my lithium ion batteries that I have used on my AGM lead acid batteries? Thank you
I don’t get it. There are plenty 4 cell 12v (14.4V) lithium car batteries that work just fine with lead acid batteries. Both the charge and discharge load is greatly reduced on each individual battery. Especially if both batteries are connected right next to one another.
The only thing that might be an issue in my mind, is the lithium battery charging the lead acid battery for a while after the engine is turned off and voltage drops from 14.4 charge voltage, to 12.5 nominal voltage.
If the lithium battery is a 6aH discharge battery, it shouldn’t be a big issue.
Your concern of FLA batteries draining LFP batteries is not a real problem. FLA internal resistance increases with the state of charge. You can test this with an ammeter and battery charger by watching the flow toward the battery as it drops to about 0.2 amps or less, when full, on well-maintained FLA batteries.
My current system has a 24V power Jack inverter with two sets of DC input terminals that is 15kw pure sine wave and split phase. I also have 60 AGM 100ah 12V batteries connected in parallel 24V. To charge my batteries bank, I have 66 pieces of 12V 100 watts solar panels also connected in parallel 24V. This is an off grid system.
I am trying to upgrade my system with lithium ion batteries for which I have purchased the hsky elite batteries from big batteries. I have read almost all the comments on this chat about combining the two types of batteries and I am more confused now than before reading these posts.
Can someone tell me if I can use the two sets of batteries at the same time connected to my inverter and or separately? Also, can they be charged together using the solar panels I have or must be separated?
Combining parallel strings with LA and lithium is possible, if you fully understand the working of the lithium battery BMS. Newer BMS have the ability to switch the battery from discharge, idle, to charge only when the input voltage from the charge controller is within the specs of the battery. So the battery is fool proof, settings in charge controller is set for your gel or Lead acid batteries. In the morning when charging starts, Lithium takes all the current as voltages are still low. As the voltage rises you might get a condition where voltage rises above lithuim set point, but not yet at absorption Voltage for lead. The BMS will simply set the battery to idle state, by the time voltage rises to 52.5V (16s cell max safe voltage). Charge controller continues up to the set absorption voltage for lead, helds it there for the set time and continues to set float charge.
Normally the float voltage is again within the spec of the lithium battery, and if it was not full before entering the idle state, it will now charge the remaining balance.
So by sunset you a set of full batteries of different chemistries linked on a direct bus.
Discharging commences, for this I will recommend comparing the discharge curve vs voltage graph for each battery, but like many people said, lithuim will discharge first, up to a point of 70% DOD, and will shut down, leaving Lead to take over
Good aftenoon Mr. Deon vd Merwe. Ciukd you please if possible send me more detailed informatuin of the charge controler system ti which you refer to in you piece above. The reason for my enquiry is that I initial started my small soloar system witg four deep cycle 150 Amp hour grl batteries o a hybrid inverter. I now would like to expand my storage capacity and I would like to attempt to combine my lead acid gel batteries potentially with a LiPo4 battery bank. Your assistance in this matter would greatly be appreciated.
Hi Christiaan, I use microcare MPPT charge controllers, l was in the same situation as you, started out with SMF100 100 amp deep cycle batteries in 2015.
Charge voltage of 14.7 volts, float at 13.6V,than later gel came along, 14.2 to 14.4 charge voltage, 13.6-13.8 V float voltage. The pairing with this 2 chemistries is quite easy, you just limit your charging voltage to 14.4 volts, to protect the overcharging of the gel, which can lead to drying out of the gel and damage to the battery. Essential is to have a battery balancer on each series string, the sun pays sell them locally, to ensure that as charging begins, all 4 batteries in the series string stays voltage wise as close as possible. As l have a separate inverter, I can’t use the amp function of the charge controller(microcare) to revert to amps, I just use the 2 hours boost to float time.
As the float voltage of lead acid and gel are the same, the deficit charge in the lead acid(which will in any case be small at 14.4 volts at 2 hours) will be replenished in the float stage.
Now for pairing lithium to this setup :
Add your 48v lithium battery to your gel strings, watch your voltage rise as charging starts in the morning, Lithuim will absorb all the power form the panels, at about 52V your lithium battery will be full,and switch to idle, than your gels are still deprived of a charge.
As the lithium goes to idle, all energy form panels are driven into gels and charging continues as per set program of charge controller, only problem is your lithuim will still sense charge controller voltage, and might show a high voltage alarm.
At night the lithium will drain first, unfortunately up to the build in cut off point, 51.2V,and switch off, than loads will be supplied by gels.
All of these voltages fall well within inverter specs, so it won’t know where the power is from.
Downside is that every night the lithium battery takes 100 DOD
I have had a 100ah 12v lithium, and a leoch agm battery connected to a 140 watt panels and 1000 watt inverter. The charger is a cheap 30amp pwm box. The agm was dead not working. The lithium seems to have brought it back. I can leave the system on for a few hours.
One thing I forgot to mention is that my solar panels are divided into two sets with two 100A pwm battery charge controllers. I was told that I need to replace the two pwm battery charge controller with the MPPT charge controller
The life of Lithium chemistry cells suffer from high current draw from them.To save this for automobile application since the lead acid ells can be loaded up to 10 C for short duration to meat out the initial load, once the speed of the vehichle picks up this high demand comes down. Now the lithium cells can be loaded to save and extend the logitivity of lithium cells which are expensive. the short life lead acid batteries can be replaced as they are inexpensive.
If you can change the voltages and everything on the BMS I don’t see why you can’t hook it to lead acid batteries and charging discharge on like normal with a BMS what’s the difference between a BMS operating lead acid batteries and lithium iron phosphate one’s just different voltages have two separate inverters or a relay to swap the two back and forth simplest pie
There is some great info here. I have a 24 volt lead/acid battery bank on my small solar electric system (not grid intertied). I’m afraid of discharging the bank more than 75%. If I had lithium system leader, I could use more power for other tasks, reserving the lead acid for backup and/or emergency use (preserving their lives). I think I would need an automatic switching control for when the lithium ran out. I know lithium charges more efficiently so I think I could use more of the stored power most of the time, and still maintain the security system when the lithium runs out. Would anyone want to charge a car battery off of house battery. Might want to charge a swappable battery on the side.
Have you managed to do this? I have a large lead acid bank that i would like to top up during the night with lithium. Trying to figure out how to charge the lead acids without an expensive dc dc charger.
I have successfully combined 7-series lithium ion with 4-series ~6v lead-acid for a decent period of time with typical loads < 300w and PV-solar charging of < 1kW (typically 500-600w at peak).
If you check the voltage ranges they almost nearly overlap. The only thing I really did was cap the solar charger at about 30V, though I'm pretty sure if you have a BMS on the lithium-ions you can get around that cap (didn't have one at the time).
During the day I would get nearly a fully charged system and through the evening/night the lithium batteries would mostly take the load and end with the lead-acid batteries sitting around their float charge.
Was pretty cool.
I hope you see this reply, Blake, given the April-October lapse. What you described in your LA-Lithium setup is precisely what I am designing now for a AGM Tank, off-grid ranch system I just installed here near Tombstone, AZ. I know this has been done on boats and on shore. I have several 7-series LFP and Li-Ion packs I built so this would be a great use for them and to perhaps not having to buy more $700 AGM tanks. I would sure like to talk with you and compare your experience to my ideas. Any chance of that? Thanks!
Is there a battery type that can with stand an electro magnetic pulse explosion ?
Hi. We think the installer incorrectly set the inverter, to treat our Revov R100 LiFe batteries as lead acid. We have a Deye inverter. Another incorrect setting is the inverter is set to be a slave . This seems incorrect. As we have one inverter, should it not be main? The R100 batteries have built in BMS, so would we be allowed to manually change the inverter settings to read Lithium, without damaging anything? Its just a setting on the Deye.
We’d like to set battery percentages for grid charging but the options are currently only displayed as voltage which we do not want to change ourselves. (waiting for installer to come back).
The system is functional and we have not experienced problems but I’m nervous. Please assist!
I have combined several lead acid batteries parallel on the dc bus 48 v. Combined Narada rex 800 AH parallel with 5 parallel 16S packs each with own BMS and rs485 communication bus and addressing. Left the bulk / float setting according specs from Narada. The longest running system is now 2 years, Checked BMS last week with laptop and still SOH >90% while > 500 recorded cycles. Advantage also that lifepo4 discharges first and less cycling on lead acid . Also no wake up needed on sleeping BMS while discharged and discharge mosfet is interrupting the battery.
Hi kelly.
Great article, thank you.
I am right now trying to combine the two systems and struggling.
We have a stand alone system 5kw wind and 8kw solar with an control by SMA
The original batterys are 48kw lead acid and I have now fitted 28kw of LMO/NMC cells.
Can any one help me in finding the safest and best way forward with my project.
Any help would be greatly appreciated.
Roger
Dear Ms.Kelly
We appreciate for your valuable information and we agree with you that Lithium and Lead acid batteries may not be advisable to parallel because of different charging profiles.
In India we have carried out some experiments for Telecom applications where power shut downs varies from 30minutes to 300minutes and are frequently daily once on an average. We have adopted fall back system with electronic controls where short power shut downs are met through Lithium batteries and long power shut downs are met through Lead acid batteries.The experiment was very successful to meet the duty cycle and expected life of 3 years
I think it’s cool how you come up with this story angle- and find great sources. There were just like 3-4 sources in this one article alone! That’s awesome. What do you think would be the ultimate benefit of combining the two?