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Mixing LiFePo4 and AGM

Tanner Lynd

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So I've got a little beginner's system I put 3 x 35aH AGMs on in parallel for a total of 105ah. Panels are 200W for now, but not a factor for this question...

I can use a couple of those AGM's on another application, which would leave me one 35aH AGM. I want to upgrade the system to LiFePo4, so if I bought a nice and cheap 100aH LiFePo4 to replace the 2 AGM's, could I wire the LiFePo4 and the one remaining 35aH AGM just to get the addtional amp hours? Would the AGM drag down the LiFePo4 and ruin it?

Let's go one further: If I left the 3 AGMs inline and added the new LiFePo4 so the mix was more balanced would it have bad affects on the LiFePo4? 105ah AGM to 100ah LiFePo4?
 
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Well now, you will get a variety of opinions on mixing those two chemistries, but I think those against would say your more likely to cook the AGMs while charging the Lipo than hurt the lipo4s. If you search Lithium AGM you’ll find many strings discussing them

i do it, but I do it knowing I leave some of the top end off my LifePO4 and I don’t keep the AGM attached all the time
 
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So I've got a little beginner's system I put 3 x 35aH AGMs on in parallel for a total of 105ah. Panels are 200W for now, but not a factor for this question...

I can use a couple of those AGM's on another application, which would leave me one 35aH AGM. I want to upgrade the system to LiFePo4, so if I bought a nice and cheap 100aH LiFePo4 to replace the 2 AGM's, could I wire the LiFePo4 and the one remaining 35aH AGM just to get the addtional amp hours?

Yes - mostly.

Would the AGM drag down the LiFePo4 and ruin it?

Only if left paralleled with no charging for a very long period of time.

Let's go one further: If I left the 3 AGMs inline and added the new LiFePo4 so the mix was more balanced would it have bad affects on the LiFePo4? 105ah AGM to 100ah LiFePo4?

Generally, no.

Since LFP has a higher WORKING voltage, it will provide the bulk of its capacity before the AGM start contributing meaningfully.

When sitting, the AGM will pull a small current from the LFP to stay floated.

When you parallel the two, it's like you're adding an AGM backup to your LFP battery. AGM RAPIDLY drops into the high 12.XV range when discharging; LFP will give up most of its capacity BEFORE the AGM start contributing.
 
Just keep in mind that a huge danger of these types of configurations is if an AGM has a shorted cell (which will happen eventually), and your lifepo4 packs feed into that cell (considering their low internal resistance, can push a lot of current), your AGM will create a lot of heat. It will probably cause the over pressure relief valve to open. Battery could melt, or the lid could pop off, worst case. Really messy, really dangerous.

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.
 
Why not use both banks, but not simultaneously? Wire them through separate circuit breakers, to connect either bank to the system, one or the other, with the appropriate charging protocol set up and selected, for the different chemistries. Then you could also add to the lithium bank later without new problems.

This is what I do. I have an old, large, AGM bank that still stores lots of energy. I keep it charged for reserve power in case of a string of cloudy days. I use the large LiFePO4 bank for daily cycling. My Outback Power Mate3 lets me load the appropriate charging parameters, previously saved to an SD card, in less than one minute.

The AGM batteries have a significant self-discharge rate compared to the Lithium. Either keep them on a separate little trickle charger, or connect them to the system, perhaps once a month, to keep them “topped up”.
 
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Wow, thanks so much EVERYBODY. What great points all of you have supplied me to ponder! So much great critical thinking to turn the light bulbs on in the brain. The first best point is to migrate out of simple to a more robust and elevated system. Will supplies the best reason to consider safety first, and it appears there is plenty of reason to apply it. (Plus I'm very lazy and don't want to clean up any messes! :) ) That's exactly why I wanted to ask you guys before I did anything on this.

Awesome feedback guys. I'm very honored to have this place and all of you as a resource. Thanks so much again everybody.
 
Just keep in mind that a huge danger of these types of configurations is if an AGM has a shorted cell (which will happen eventually), and your lifepo4 packs feed into that cell (considering their low internal resistance, can push a lot of current), your AGM will create a lot of heat. It will probably cause the over pressure relief valve to open. Battery could melt, or the lid could pop off, worst case. Really messy, really dangerous.

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.
Thanks... Will add a fuse to my 100ah agm. I mainly leave it there in parallel with my five bb 100ah batteries because I'm afraid of a sudden drop out by bms will smoke my secondary alternator
 
I mix lead acid and lifepo4 in my RV. They are paralleled at the input to an inverter. However, the lifepo4 are hooked through a relay controlled by a solar charge controller with voltage cutoffs, and the lead acid are hooked up through a large diode and will eventually also be hooked up through another relay.

When i first put it together i had a small lithium and much more lead acid, and no voltage cutoffs beyond what the internal bms in the lithium had. In that scenario the lithium would drain down into the 11v range before the lead acid did anything, because the diode in line with the lead acid drops ~.7v and that circuit (lead acid, the diode, much longer wires) had much more resistance than the lithium battery and its short, direct connection to the inverter. At some point they would drain 'together' but the real utility of the arrangement was to pull from both sides during startup surge/inrush, which worked. When the lithium would charge to its higher resting voltage, the diode would prevent it charging dog slow because a lot of charge current would have fed into the lead acid if not for the diode.

Now the lithium are isolated by a relay and i have the cutoff set so that it actually disconnects before it would drop to the voltage where the lithium and lead would drain 'together'. I now have 3 lithium batteries in there so there is no real need to ever pull from the lead acid during surge/inrush because the lithiums will take it. The diode has become somewhat superfluous other than as a protection device such as Will Prowse was talking about. I am going to rewire it so that the lithium and lead packs are on the NO & NC contacts of a large relay/contactor and switch between them when the lithium hits low voltage cutoff, which is becoming less and less likely with the added lithium capacity.

But ostensibly i've been mixing the lead and lithium feeds into the inverter for months and everything has been fine. But i never did connect them directly in parallel with NOTHING in between. In my opinion that is not a good idea given that there are many better ideas that involve singular cheap/simple components (diode, fuse, relay, etc) to avoid some of the downsides of the direct connection.
 
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So I've got a little beginner's system I put 3 x 35aH AGMs on in parallel for a total of 105ah. Panels are 200W for now, but not a factor for this question...

I can use a couple of those AGM's on another application, which would leave me one 35aH AGM. I want to upgrade the system to LiFePo4, so if I bought a nice and cheap 100aH LiFePo4 to replace the 2 AGM's, could I wire the LiFePo4 and the one remaining 35aH AGM just to get the addtional amp hours? Would the AGM drag down the LiFePo4 and ruin it?

Let's go one further: If I left the 3 AGMs inline and added the new LiFePo4 so the mix was more balanced would it have bad affects on the LiFePo4? 105ah AGM to 100ah LiFePo4?
I'm running agm's with lith's & they work together just fine with a caveat, that being your charger needs to be set to 27.8v max output for absorption & float @ 27.6v. It slows down charging time but prevents overcharging the agm's & a bonus is when the lith's under load reach 25.4v, the agm's only then start contributing & slows down the rapid end stage discharge of the lith's otherwise the agm's remain dormant until then & the 27.6v float gives the lith's the ideal stop charge @ 3.45v per cell.
 
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Will made a good point above, make sure you fusebetween the different strings. My AGMs have faded to the point I have repurposed them to fishing motor and CamperVan duty so I am out of the mixnmatch game
 
Just keep in mind that a huge danger of these types of configurations is if an AGM has a shorted cell (which will happen eventually), and your lifepo4 packs feed into that cell (considering their low internal resistance, can push a lot of current), your AGM will create a lot of heat. It will probably cause the over pressure relief valve to open. Battery could melt, or the lid could pop off, worst case. Really messy, really dangerous.

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.
That's good advice Will, I hadn't thought of that scenario & my agm's aren't breakered tho my lith banks are so I'm pretty ok but the agm's will get 1 now too just for extra insurance.
 
That's good advice Will, I hadn't thought of that scenario & my agm's aren't breakered tho my lith banks are so I'm pretty ok but the agm's will get 1 now too just for extra insurance.
I hadn't, either, and I'm glad I happened across this thread and that comment. I'd seen other threads mentioning that paralleling a LiFePo4 battery with Lead Acid would work (mostly), but the LiFePo4 would deplete completely before the LA battery contributed.

I connected a bank of 4p 12V LiFePo4 batteries to a WindyNation 3000w inverter, intending to use it to run the freezer, fridge, and other light loads during outages. But now I have one of the DC Solar trailers serving as an off-grid power station, and not much use for the LiFePo batteries and inverter. I considering wiring them in series for 48V, and then paralleling the FLA forklift batteries on the trailer. After reviewing the specs of the LiFePo batteries, I see that they are capable of surging to 900 amps, which could turn out badly in the scenario Will mentioned.

Plan now is to get at 60V bench power supply, and use it to gently supplement the FLA battery with the LiFePo/inverter I have sitting around. At max, it'll put out 300w for about 13 hours, which will nearly remove the base load of the whole house during the evening, and will hopefully extend the longevity of the FLA batteries by slightly cutting down on their depth of discharge overnight. The LiFePo batteries have a lifetime warranty, so it seems an awful waste to leave them laying around unused. I did look at DC-DC chargers, but I really like the simplicity and safety of the current-limited bench power supply via the inverter.
 
Just keep in mind that a huge danger of these types of configurations is if an AGM has a shorted cell (which will happen eventually), and your lifepo4 packs feed into that cell (considering their low internal resistance, can push a lot of current), your AGM will create a lot of heat. It will probably cause the over pressure relief valve to open. Battery could melt, or the lid could pop off, worst case. Really messy, really dangerous.

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.

What you're describing could happen to any types of batteries wired in parallel. Same types or different types or any mix and match configuration.

I've personally have had standard lead acid batteries experience shorted cells in the past. If the batteries have been connected for a time the shorts are usually weak conductors (sulfates bridges) and the good batteries will blow them like fuses before any real dangerous events can happen.

A shorted cell in an AGM battery is a highly unlikely event because the glass mat separator between the lead plates all but eliminates the possibility of any contact between the two, no matter how much abuse the battery is subjected to. https://coastalclimatecontrol.com/b...horted cell in an,the battery is subjected to.

Li batteries may be more prone to internal shorts than AGM. Hopefully the BMS will shut them down.

If there’s any question about battery compatibility in parallel configurations I’d recommend Schottky barrier diodes in series with fuses. The diodes will prevent the batteries from draining into another. And the fuses will blow if the diodes short. However after checking the price of high current schottky diodes, you might just choose to buy one larger capacity battery.
 
A very good point and one that should be remembered in any DIY configuration.
yeah, those things scare the absolute hell out of me, as much as climbing any rock face. In both cases you prepare, you focus, you learn, you take all reasonable precautions, and you know your damned limitations
 
yeah, those things scare the absolute hell out of me, as much as climbing any rock face. In both cases you prepare, you focus, you learn, you take all reasonable precautions, and you know your damned limitations

I thought I kinda had a sense of how much juice those LiFePo4 batteries could put out, but after reading Will's warning, I checked the specs and saw 900 amp surge (over 46kW!), let's just say my eyes got really wide... Hence opting for the 12V LiFePo4 -> inverter -> bench PS -> 48V FLA over some creative McGyver-ing.
 
I didn't read much of the posts on this but I'm combining agm's with lith's & they work together very good but I have my chargers limited to a max charge of 27.6v. I don't want my lith's charged over 3.45v per cell & 27.6v will charge both of them slower which I believe will improve the lifespan of both having a gentler charge & that's a perfect float for both. As you may have discovered by now, the agm's will not contribute any power untill the lith's approach 25.4v which is great 'cuz it slows down the discharge rate of the lith's & if you built your banks a/hrs above your usually drain then the agm's will get called on very little & extend their lifespan.
The other advantage of this is when you hit the lith's 25v near end of charge is a good place to shut them all down which leaves the agm's only meagerly drained & that'll add to the agm's lifespan.
The only issue I can think of with this combo is whether the agm's not getting it's recommended absorption charge has any effect on it's performance or lifespan but I haven't seen any evidence of that yet after more than a year in service.
 
Building a large Hybrid LFP/FLA battery.

I have ordered 17 EVE 280 a/h cells with a 24s BMS. I am planning to built a 17s, 15.2 kwh battery and parallel it with 16 - 420 a/h US Battery L-16 FLA batteries. The L-16s are configured for 48 volts (2p8s). They are 10 years old but are used primarily during the summer (6 months). They still provide the power that we need but since FLAs age like dogs, I know that they are closer to the end of life than the beginning. I have done a lot of reading on this forum and elsewhere but I cannot find any consensus on whether or not I should give this a try. The common objections are often a reaction like you "cannot mix chemistries", or "all the batteries need to be the same" or "the charge and discharge profiles are don't match" or "if a cell shorts out in one of the FLA batteries, bad things can happen". This is not an exhaustive list of the reasons not to do this, but seem to be the most common. I'm certain that the list can grow a lot and I'd like to know what I don't know. And the reasons sound a lot like the conventional management advice for lead acid battery banks. And they are right. I would never parallel FLAs with SLAs or mismatched LA batteries of any kind. But putting together the "hybrid" FLA+LFP looks on paper to be a very good match. I made the short table below to illustrate the voltage compatibilities of the batteries. I used the charge profile for the FLAs as recommended by the manufacturer (US Battery) and the recommended charge voltage for the LFP batteries.

I found that a 16s LFP battery would not be a great partner with the FLA bank because to get the FLA Bulk/Absorb voltage to a suitable voltage would require charging the LFP battery to 3.65 v/c, which is 100% maximum SOC. While it's ok to do that once in a while, I don't think I want to do that every day. Configuring a 17s LFP gets most of the voltages into almost the perfect range to match the FLA battery with the exception of the Float voltage for the FLA. I'll come back to that in a minute. If you look at the 17s charge voltages, charging to 3.45 v/c puts the charge voltage perfectly in the sweet spot for FLA. Even better is that in cooler weather, I cam bump the LFP charge rate to 3.55 v/c which acts almost exactly like temperature compensation on my charge controller (which will be "off"). Taking it to the extreme and bumping the LFP charge rate to maximum 3.65 v/c, I get close to an equalization charge rate for the FLA battery. So far so good?

But as I said above, on paper this appears to be a pretty good fit except for the aforementioned Float voltage on the FLA battery. The charge controller float function will be turned off. The float effect on the FLA battery will come from the higher relative voltage of the LFP, which fully charged, is 54.4v at rest. The difference in recommended float voltage for the FLA is 52.10 volts. The LFP will try to equalize with the FLA. This is the one place that the "chemistries" don't match and I'm not sure how or if this will be an important factor. I have read in various places that there will be a low amperage flow to the FLA, but the high resistance of the fully charged FLA and the 24/7 house load on the battery will keep the amp flow to a mostly irrelevant value. The LFP battery will never be truly at rest.

I would very much appreciate commentary on this proposed hybrid configuration. If it works it would greatly improve the efficiency of my entire off grid system. I will also learn a lot and have some fun doing it.

Voltage Profiles for LFP and FLA Battery Bank - 48v Nominal
FLA (8 - L 16)LFP (16 cells)LFP (17 cells)
Fully Charged Bank Voltage (at rest)50.40 - 51.20 v51.20v @ 3.2 v/c54.40v @ 3.2v/c
Bulk/Absorb Charge Voltage56.40 - 58.80 v55.20 @ 3.45 v/c58.65 @ 3.45 v/c
56.80 @ 3.55 v/c60.35 @ 3.55 v/c
58.40 @ 3.65 v/c62.05 @ 3.65 v/c
Float Voltage52.10 v51.20 v54.40 v
Note: the charger will be set to "No Float"
however the FLA'S will "float" based on the
voltage of the LFP bank
Note: FLA Voltage range is based on
US Battery charging recommendations.
 
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