Can Lead Acid & Lithium Battery Banks be Paralleled to 1 Inverter?

[Hotaznights]

I just built a 14s (48V) leaf battery bank of 14 stacks of 7 each. I have 16 cr-330 ah batteries and I want to know if I can use them in Parallel to the main busbars that go to a 12K inverter without any problems. Both Lith & LA banks have their own mppt charge controllers and both are set for a max charge of 57.1 volts. The lith ion banks are 2 individual banks that go to the same busbars - &+ and want to know if the LA battery can be used going to the same busbars as the Lithium ion battery banks to give me that extra power. The LA are just sitting there doing NOTHING and want to use them TOO!!! THX

 

[gnubie]

You'd have to prevent voltage flowing back into either bank from the other. The reason being they have very different charging requirements. Yes, some people do parallel them direct but they understand that either the lead acid suffers, or the lithium does. Typically they set up to preserve the lithium battery life.

High current diodes could be used to separate your two chemistries since each have their own charging system. You will get voltage drop across the diode especially with the very large currents you would be talking with a 12kW inverter.

example only, not a recommendation

https://au.element14.com/ixys-semiconductor/dsa300i100na/schottky-rectifier-100v-300a-sot/dp/3438334?st=300+amp+diode

You could use several cheaper diodes of course and there are things called 'ideal diodes' too.

Depending on the voltage each bank is at at a given time one will shoulder more of the load, effectively cycling it more than you might otherwise imagine.

 

[KMac55]

I have done a little research and it doesn't seem far fetched if you are able to design it carefully. Below are some of the more interesting things I found online about doing it. Some propose and one researches a pretty simple parallel connection with care given to voltage ranges and perhaps ratios of LFP to LA. Others suggest more complex solutions, and some of the commercial products might be simple or complex, I don't know.

This article researches doing it in remote medical clinics: https://juser.fz-juelich.de/record/829025/files/PE_070_Christiane.pdf

This forum discussion gets a ways down the path, but then drops off. I would like to know what they learned: https://community.victronenergy.com/questions/19981/mixing-lead-acid-and-lithium.html

This product is a LFP extension to LA battery, but they only do it in 12v apparently: https://www.bos-ag.com/products/le300/

Finally, I found the discussion in this article interesting in referencing a device by Goal Zero that combines them:

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

www.solarpowerworldonline.com

I am curious if the differences in charging voltage might be addressable with the LFP simply being limited in its range by its BMS. Also, couldn't the parallel LA battery save the charge controller from going to smoke when the BMS cuts off charging the LFP in too cold of weather?

 

[gnubie]

If the controller is well designed it won't go poof. I think there has only been one instance of a MPPT controller possibly going poof reported on the forum, despite several people, myself included, trying to provoke just that. In my own testing I found that some controllers produce output voltage spikes and other nastiness which may damage things connected to them though.

One way or the other something gives. The problem with lithium is that you can over charge it at a lower voltage. A typical BMS doesn't cut off on amp hours (or Wh) going in, only voltages and amps. It's possible to slowly degrade a lifepo4 battery by sitting it at 29.0V for hours on end, ie absorption stage charging for the lead acid. So you either set the BMS to cut off at a quite low voltage, topping the lifepo4 capacity and pretty much ensuring it never gets to cell balancing or slowly kill it as the lead acid slowly crawls its way to full charge. If its a dual port BMS, even if internally, charging will stop but discharging could still happen once the lead acid drops in voltage again.

Going the other way sees the lead acid always being under charged because it can't do a full absorption stage so it dies a slow death too.

The proper systems I've read about use DC-DC converters and some smarts to separate the two chemistries and ensure they are both properly charged but can both contribute to supporting the load when the time comes.

 

[snoobler]

LFP, yes.

Lithium NMC (leaf cells), no.

What BMS are you using?

 

[snoobler]

I have done a little research and it doesn't seem far fetched if you are able to design it carefully. Below are some of the more interesting things I found online about doing it. Some propose and one researches a pretty simple parallel connection with care given to voltage ranges and perhaps ratios of LFP to LA. Others suggest more complex solutions, and some of the commercial products might be simple or complex, I don't know.

This article researches doing it in remote medical clinics: https://juser.fz-juelich.de/record/829025/files/PE_070_Christiane.pdf

This forum discussion gets a ways down the path, but then drops off. I would like to know what they learned: https://community.victronenergy.com/questions/19981/mixing-lead-acid-and-lithium.html

This product is a LFP extension to LA battery, but they only do it in 12v apparently: https://www.bos-ag.com/products/le300/

Finally, I found the discussion in this article interesting in referencing a device by Goal Zero that combines them:

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

www.solarpowerworldonline.com

I am curious if the differences in charging voltage might be addressable with the LFP simply being limited in its range by its BMS. Also, couldn't the parallel LA battery save the charge controller from going to smoke when the BMS cuts off charging the LFP in too cold of weather?

He doesn't have LFP

 

[snoobler]

Here's what's going to happen:

Charge to 57.1. Lithium NMC is very happy to stay pretty much right there; however, the FLA wants to drop to 51.2 or so.

The NMC will discharge into the FLA overcharging it until both eventually settle to 51.2V where you will have maybe 50% of your NMC capacity available and most of your FLA capacity, but they will be damaged with every charge.

Lithium NMC plays well with Lithium NMC.

 

[Craig]

Here's what's going to happen:

Charge to 57.1. Lithium NMC is very happy to stay pretty much right there; however, the FLA wants to drop to 51.2 or so.

The NMC will discharge into the FLA overcharging it until both eventually settle to 51.2V where you will have maybe 50% of your NMC capacity available and most of your FLA capacity, but they will be damaged with every charge.

Lithium NMC plays well with Lithium NMC.

Or you could just charge to 51.2 and lose capacity of NMC but not damage anything. Real question is the trade off worth it?

 

[snoobler]

Nope. FLA will die if only charged to 51.2. likely never get above 50%, and they'll eat themselves to death in short order.

 

[Craig]

Nope. FLA will die if only charged to 51.2. likely never get above 50%, and they'll eat themselves to death in short order.

Ok so this is where I do not suffer from knowing too much. Your answers contradict themselves. What I do not understand is why the NMC will get hurt if the FLA sinks. Set the charge profile for the FLA then I doubt the NMC will have any serious issues sinking and charging the FLA a bit.

 

[snoobler]

Ok so this is where I do not suffer from knowing too much. Your answers contradict themselves. What I do not understand is why the NMC will get hurt if the FLA sinks. Set the charge profile for the FLA then I doubt the NMC will have any serious issues sinking and charging the FLA a bit.

Sorry, it's how I phrased it. "They" referred to FLA.

NMC rests very close to where the charging terminates.
FLA rests very close to the aforementioned 51.2V after being charged to 57.1

As the FLA wants to settle, the NMC will be keeping the FLA well above float by discharging into them. The FLA will be held well above float for extended periods. The FLA will be damaged from this arrangement, not the NMC.

NMC will suffer no ill effects aside from charging and discharging capacities that will be unusable which will ultimately reduce its long term cycle life.

 

[chrisski]

Only way I can see this working is to have both battery banks independent of everything, charged separately with two different SCCs, and then run each to a DC to DC converter that will step down the voltage to something useable for each bank, like stepping down to 48 volts.

That’s not paralleling them, but that feeds both banks to a single inverter.

 

[Jim Burrow]

Due to the discharge character of lithium batteries to Lead-acid batteries, where Lithium has a fairly flat voltage drop until all its capacity is used up. While Lead-acid batteries have a fairly sharp discharge voltage drop while it is being discharged. So it would seem to me that the two batteries connected in parallel would become out of balance quickly.

The Lithium battery with a higher voltage would prevent the Lead-acid battery from discharging or providing any real current to supply the load. In theory, the Lead-acid battery wouldn't begin to discharge until the Lithium battery is almost fully discharged. At that point, the Lead-acid battery would have a high voltage and would start to supply current to the load, but at the same time try to charge the Lithium battery.

It would seem to me that since the Lithium battery would have a much lower resistance with respect to the load, that most of the energy in the Lead-acid battery would flow into the Lithium battery versus going to the load.

So basically, I would think, you will have a big headache trying to manage the charge/discharge cycle with both batteries died in parallel.

Again, I'm only doing a brain thought and have not experimented to see what I posted is reality.

 

[snoobler]

Jim Burrow The OP stated he is using Leaf cells. These are not LFP. They are Lithium NMC. Lithium NMC does not have the flat voltage curve like LFP. Your post does not apply to this situation.

 

[Jim Burrow]

Jim Burrow The OP stated he is using Leaf cells. These are not LFP. They are Lithium NMC. Lithium NMC does not have the flat voltage curve like LFP. Your post does not apply to this situation.

ok, I miss understand. I thought he was trying to parallel Lithium to Lead-acid. In that case, obviously, my post is not relevant. I get confused with all these acronyms. OP, LFP, NMC, and so many more.

 

[snoobler]

There are multiple types of Lithium. Two most common are:

LFP - the favorite of this forum, 3.2V nominal, charges to 3.65V. Similar charging profile to lead-acid. High cycle life with full depth discharge. Highly safe, extremely low risk of fire.
Lithium NMC - Tesla, Leaf, almost all BEV, PHEV, 3.6-3.7V nominal, charges to 4.20V. Unique charge profile, operates from 2.5V to 4.2V with a mostly constant slope (it has legs at both ends). Much lower cycle life with full depth discharge. Longevity is pushed into the thousands of cycles by limiting to 20-80% SoC. Extremely sensitive to voltage range. Over-discharge causes violent cell swelling. Over voltage charging may result in fire or catastrophic damage to the cells.

 

[Craig]

I have experimented with a similar situation. I have paralleled marine/rv batteries with my LTO bank. This had been running for 3 months without issue. Now I will fully admit I don't care if the marine batteries do get destroyed prematurely as I was in the process of decommissioning them anyhow. They are all around 3 years old. I set my bulk charge voltage at 28.2 and my float at 26. After 3 months of light use no problems what so ever. Full disclosure I have never had bank below 23.5v or so and only then about half a dozen times. My absolute maximum discharge cutoff is at 23v

 

[Craig]

I have experimented with a similar situation. I have paralleled marine/rv batteries with my LTO bank. This has been running for 3 months without issue. Now I will fully admit I don't care if the marine batteries do get destroyed prematurely as I was in the process of decommissioning them anyhow. They are all around 3 years old. I set my bulk charge voltage at 28.2 and my float at 26. After 3 months of light use no problems what so ever. Full disclosure I have never had bank below 23.5v or so and only then about half a dozen times. My absolute minimum discharge cutoff is at 23

 

[snoobler]

Craig I don't know enough about LTO to comment on paralleling them. I know almost nothing about their specific operation.

I know a bit more about LFP, NMC and FLA.

LFP+FLA = minimal concerns.
NMC+FLA = Significant complications with no net positive benefit.
LTO+FLA = sounds like you're having favorable results.

 

[Craig]

But you said it wouldn't hurt the NMC so what difference does the Lithium chemistry make.

 

[schmism]

Ill be adding LFP to my NMC bank. I plan to run the charger at a LFP profile (3.2V -3.65V) which will cut off the ends of my NMC bank but also puts them right in the middle of their range for optimal life.

 

[snoobler]

But you said it wouldn't hurt the NMC so what difference does the Lithium chemistry make.

NMC will damage the FLA.
NMC will lose substantial capacity bulking/floating the FLA when charger is not active.

Net result:
Lose up to 50% of NMC capacity to use 100% of degrading FLA. Just doesn't seem worth it. I'd prefer to keep the FLA on a float charger powered by the system as a backup power source.

 

[Jim Burrow]

There are multiple types of Lithium. Two most common are:

LFP - the favorite of this forum, 3.2V nominal, charges to 3.65V. Similar charging profile to lead-acid. High cycle life with full depth discharge. Highly safe, extremely low risk of fire.
Lithium NMC - Tesla, Leaf, almost all BEV, PHEV, 3.6-3.7V nominal, charges to 4.20V. Unique charge profile, operates from 2.5V to 4.2V with a mostly constant slope (it has legs at both ends). Much lower cycle life with full depth discharge. Longevity is pushed into the thousands of cycles by limiting to 20-80% SoC. Extremely sensitive to voltage range. Over-discharge causes violent cell swelling. Over voltage charging may result in fire or catastrophic damage to the cells.

wow! thanks for the input. I still haven't bought any batteries yet due to funding. But I've got a lot to learn between now and then on what to buy

 

[Craig]

NMC will damage the FLA.
NMC will lose substantial capacity bulking/floating the FLA when charger is not active.

Net result:
Lose up to 50% of NMC capacity to use 100% of degrading FLA. Just doesn't seem worth it. I'd prefer to keep the FLA on a float charger powered by the system as a backup power source.

I think we must now agree to disagree.

 

[pollenface]

I have paralleled a 50ah lifepo4 with a 26ah in a camper trailer while running a couple of lights, fans and a 300w inverter, all charged from an 80w panel with pwm controller - no problems.

I should also add that the batteries were on paralleled occasionally for camping, most other times the batteries were separated and charged independently.