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Connecting two TOTALLY different batteries and Panel - Problem?

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Arian40

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Jun 4, 2021
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Hi,

I made a previous post about my first project where everyone kindly helped and it seemed like I was way too short/low on my battery and panel - I am now upgrading a couple of things and I was wondering if I were to use my already purchased battery which was the 30ah lithium iron battery and add an extra battery of lets say HOPPECKE 12V DEEP CYCLE SOLAR PV Batteries 105AH "OR" a LEAD ACID Battery would I have any problems basically mix and matching them? would it actually work? or is it dangerous ?

Second question is would I be able to connect an additional 300w solar panel to an existing 100w panel? or do they need to be the same?


Many thanks for the replies in advance.


Regards,

​

 
I'm a little confused on what you are saying, but if you are asking if it is OK to mix some size A LFP battery with a size B LFP battery or maybe a size C Lead Acid battery, then STOP. You really do not want to do that. You really don't want to mix different battery chemistries. Everything that you do to make the battery work / last long is different between different batteries. Pick a path, and stay with it.

Adding more solar is different. Are the proposed new panels similar in Imp (current) and / or Vmp (voltage) to the others? If so there are ways to make it work. If you do it right, it is easy. You need to give us more info about the specs of your current solar and the panels you want to add.
 
Arian40 said:
Okay it seems like the battery mixing is a definite no no...

The solar panel I had purchased was: https://www.ebay.co.uk/itm/392627017636

and the one i am looking to add on is: https://voltaconsolar.com/solar-panel/solar-panel-320w-etsolar-pv-monocrystalline.html


Would these work along side each other?
Click to expand...
Unfortunately, the eBay listing (like most) doesn't give you any useful information.

On most solar panels, there is a sticker on the back that gives the STC (Standard Test Conditions) rating for voltage at maximum power point (Vmp) and current at maximum power point (Imp). It's not in the best interest for Ebay sellers to give you all this if they are just wanting to move the panels. A "12V panel" is an old term that means that the panel will probably work with a PWM controller to charge a 12V battery. Not very specific.

Edit: OOOPS! @labeeman saw it and I didn't. Yeah, they DID post the information, and NO it doesn't work with your existing panel.
 
Its no real issue mixing batteries in paralell. Just keep within limits of voltages.

13.6 max charge/boost/bulk/absorb/equilize volt for 12v system is fine for lead acid and lifepo4.

Once a month you maybe want to charge the lead acid seperately with 14.8v for an hour tho just as maintenance
 
Scph9002 said:
Its no real issue mixing batteries in paralell. Just keep within limits of voltages.

13.6 max charge/boost/bulk/absorb/equilize volt for 12v system is fine for lead acid and lifepo4.

Once a month you maybe want to charge the lead acid seperately with 14.8v for an hour tho just as maintenance
Click to expand...
I would respectfully disagree with all the above

As for adding a 300 watt panel to an existing 100 watt panel, your existing SCC might not handle the combined power and if so, you'd need to upgrade. Since your panels have different voltages, you really shouldn't be connecting them in parallel anyway, so the 'easy' solution is to buy a new SCC for the new panel and keep them separate. You can have as many SCC's charging the same battery bank as you like

Don
 
I do it all the time to keep all my random 12v batteries charged, just keep the connections fused to only allow as much current as the batteries rating.
 
Scph9002 said:
Its no real issue mixing batteries in paralell. Just keep within limits of voltages.

13.6 max charge/boost/bulk/absorb/equilize volt for 12v system is fine for lead acid and lifepo4.

Once a month you maybe want to charge the lead acid seperately with 14.8v for an hour tho just as maintenance
Click to expand...
Wow. Umm... I don't want to start a food fight, but I'm pretty sure the majority of experienced folks here would not agree with these statements, but to each his own. Maybe you @Scph9002 have not had any failures doing this, but the charge profile for lead acid is very different than for LiFePO4. Oh, and using 13.6V in the same sentence with equalize (as applied to lead acid) does not make any sense. Generally speaking, a 13.6V bulk / absorption voltage will not sufficiently charge a lead acid (especially an AGM) battery, and will cause the plates to sulfate far too soon.

I would just suggest that anyone considering putting lead acid in parallel with LiFePO4 do lots of research before doing so.
 
I would never connect different battery chemistry’s together.

Solar panels are another story.

I have connected wildly different panels together and it works fine. Just understand there are significant trade offs when doing so.

Here is a great explanation of what the limitations are for serial and parallel :

solarpanelsvenue.com

Mixing solar panels – Dos and Don’ts • SOLAR POWER SECRETS

Mixing Solar Panels: Discover Now Dos and Dont's of Mixing the Same Types or Different Types of Solar Panels. Act Now to Save Your Money!
solarpanelsvenue.com solarpanelsvenue.com
 
labeeman said:
Nope one is 18 vdc and the new one is 33 vdc.
Click to expand...

They definitely will work with limitations. I have used combinations with larger discrepancies than that.

The trade offs are detailed here :

solarpanelsvenue.com

Mixing solar panels – Dos and Don’ts • SOLAR POWER SECRETS

Mixing Solar Panels: Discover Now Dos and Dont's of Mixing the Same Types or Different Types of Solar Panels. Act Now to Save Your Money!
solarpanelsvenue.com solarpanelsvenue.com

I was stuck once with just two panels laying around to charge a 48v nominally battery which of course needs approximately 56 volts to charge. I believe my Victron MPPT requires 5 volts of headroom so now I need to generate 61 volts.

I have an oddball panel that generates 55 volts so I put that in series with an 18v panel and the MPPT took it from there.

Not the most efficient setup but it works fine in a pinch.
 
Last edited:
About the panels, 33V seems "a bit overkill" for a 12V system, but a decent solar controller should deal with it.
In any case, panels are cheap. Get two of the "300W" ones, they should be plenty for... how many Ah were you planning, in total?
Mind you, when I say "they should be plenty", I mean, they should. You should have as much extra charge power as you can to account for partially cloudy days and such.

As to batteries (please correct me if I'm wrong, but from what I gather), - which are definitely not cheap - if you have 100Ah LiFePO4, connect the charger and load to those, and then parallel some deep-cycle Pb ones - maybe with smallish wires, or a shunt, so they "act as a backup" to the Lithium ones rather than being in "total" parallel, charging shouldn't really be a problem.

If you charge to 14.4 (standard for a 12V system), the Pb ones will charge to 100%, the Li one to "only" 95%.
If you charge to 14.6, you get a full charge on the Li and a very slight overcharge on the Pb.
Just put a thermometer on the lead ones, if they don't heat up, you're OK.
If they do, and you only want to charge to 14.4... you're "throwing away" 5% of the maximum charge on your Lithium ones.
They shouldn't complain too much, should they? :)
 
Don B. Cilly said:
As to batteries (please correct me if I'm wrong, but from what I gather), - which are definitely not cheap - if you have 100Ah LiFePO4, connect the charger and load to those, and then parallel some deep-cycle Pb ones - maybe with smallish wires, or a shunt, so they "act as a backup" to the Lithium ones rather than being in "total" parallel, charging shouldn't really be a problem.

If you charge to 14.4 (standard for a 12V system), the Pb ones will charge to 100%, the Li one to "only" 95%.
If you charge to 14.6, you get a full charge on the Li and a very slight overcharge on the Pb.
Just put a thermometer on the lead ones, if they don't heat up, you're OK.
If they do, and you only want to charge to 14.4... you're "throwing away" 5% of the maximum charge on your Lithium ones.
They shouldn't complain too much, should they? :)
Click to expand...
I'm not sure I understand your wiring idea.

If you set your charger to charge to 14.4V (that 3.6V per cell) your BMS is going to quite frequently hit HVD (high voltage disconnect) on one or of your cells. Oh and by the way, 3.6V / cell is not 95% charged. It is 99.5% charged. If you do a capacity test on a cell that starts at 3.6V, you'll find that it drops very quickly to 3.4V. My 230Ah cells indicated only 0.4Ah between 3.6V and 3.4V.

Here's the thing about mixing these chemistries. Lead acid batteries drop down in voltage more linearly as they are depleted (see the attached picture - I don't know where I got it), whereas LFP batteries have a very flat voltage curve "between the knees" (3.4V to 3.0V per cell). So if you have lead acid connected in parallel with LFP, as energy is drawn out of the combined battery the lead acid battery will be trying to drop down in voltage. The LFP will have a higher voltage, so current will flow from the LFP into the lead acid. Flooded lead acid is at around 50% SoC when it gets to 12V, and by then the LFP will be down to more like 15%. So this means your nice LFP battery will mostly be cycling to try and keep the lead acid charged.

It's worse if your lead acid batteries are AGM, as they want a higher bulk / absorption voltage, like 14.6V or even 14.7V. If you don't periodically get an AGM battery to a full charge (absorption at 14.6V until the tail current drops to 1%) the battery will age more quickly, causing sulfation of the plates. If you try to charge your LFP to 14.6V you will definitely have a HVD virtually every time.

Finally, charging of lead acid batteries must be temperature compensated. That means at a lower temperature you will charge at a higher voltage, making it even worse for the LFP.
 

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Horsefly said:
... so current will flow from the LFP into the lead acid .... by then the LFP will be down to more like 15%. So this means your nice LFP battery will mostly be cycling to try and keep the lead acid charged.
Click to expand...

Very good point. I hadn't thought of that.
So maybe wire them the other way around... load and charger to the lead-acids ones, lithium shunted as "backup"... I'll have to think about that one :)

About HVD... aren't LiFePOs supposed to be charged at 3.65V? Which x4 makes 14.6?
If that is the case (it's what I read), use AGM for lead, you should be good, right?
 
Personally I would not mix battery chemistries for a solar battery storage system but I know some do like David Poz with Lithium manganese cobalt and Lithium Iron Phosphate with number of cells matching as close as possible in charge parameters but you would definitely not be utilizing the same potential storage as both battery types used separately.
I also have seen an article (cant find it) on adding a "little lead based battery" to a majority Lithium Iron Phosphate system for RV /Marine etc starting system especially when dual large lead acid batteries would normally be used . Apparently that setup prevents alternator damage unless you are using one of the newer redesigned electronics alternator. There were other methods but I digress.
 
Don B. Cilly said:
About HVD... aren't LiFePOs supposed to be charged at 3.65V? Which x4 makes 14.6?
If that is the case (it's what I read), use AGM for lead, you should be good, right?
Click to expand...
In practice, there is no reason to try and charge to 3.65V per cell, for a couple of reasons. First, as I mentioned earlier the energy curve for LFP is so steep on the upper end that there is virtually no additional energy stored between 3.4V and 3.65V. Second, cells would have to be perfectly matched for them all to reach 3.65V at the same time. More likely, one or two cells will get there first, and then they will try to rise above that before the other cells get to 3.65V. That triggers the HVD. I haven't deployed my 8S 24V pack yet, but I've been running a few cycles here to find their "happy place". I've found that I can charge to 28.0V or 28.2V (3.5-3.525V/cell) consistently every time. I may go with 28V for my bulk/absorption, but if I ever have a HVD I know I can drop it down to 27.8V (3.475V/cell) and not be losing anything. I'll probably set the float to something around 27.0V-27.2V (3.375V-3.4V/cell).
 
Well, on (smallish) boats, we just use totally separate batteries for starting engines and utilities. Automotive for the engine.

As long as 30 years ago I remember switching the utility bank on a 52' from automotive to deep-cycle. 2V elements - 12 of them for 24V.
The results were somewhat disappointing... but then, it was 30 years ago.

Thing is, the OP already has the LiFePOs. Small, but... nice :). And expensive...
 
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