Mixing AGM and LiFePo4

[FonTravels]

Hey!

I currently have on my van a 250Ah AGM battery that probably gives me 800 Wh of power... It sucks... and charges really slow to 100% which is awful for summer, not enough hours of sunlight. It's 1.5 years old and I keep it >12.4V 90% of the time and >12.15V some times.

I'm buying a DIY LiFePo4 150Ah and I was wondering if I could connect them together, I know they are different chemistry but my way of thinking is as follows:

I have a MPPT 30A and a DC to DC charger 30A that charges the battery using the alternator while I drive. I can set up the charging parameters for both to the LiFePo4 so that means:

The AGM will almost always be charged to 12.8V (the voltage of the LiFePo4 battery) and when the LiFePo4 battery drops to 12.6V a relay will disconnect it and start discharging the AGM battery which will be only a few times a month maybe, so normally I will only be using the LiFePo4 and the AGM will remain at 12.8V at minimum most of the time. It will be a back up battery.

Charging: They will charge up to 14.2V for 45 minutes, then at 13.5V which is what the LiFePo4 asks for and it's similar to what the AGM wants, so in a normal day the AGM will be already full (12.8V) and the battery that will be charging is the LiFePo4. I will put a 40A fuse between both batteries only for security. I only use 25A at most for LOAD.

What do you think about this set up?

Thanks in advance

 

[Steve_S]

MIxing battery types is NEVER a good idea, moreso with different chemistries.
I appreciate your wanting to keep the AGM's who wants to chuck money away... maybe just Bill Gates but rest of us aren't that foolish.

I suggest getting one of These Switches from Blue Sea (2 options: with or without AFD (Alternator Field Disconnect).

https://www.amazon.com/gp/product/B001V7TL10/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1

That will enable you to have one or both battery packs active and allow you to charge each type of battery separately as required and if you really need to use both to power your needs if in a pinch.

 

[grizzzman]

Well, I keep my CG2 bank tied to my Lifep04 (sorry my cat likes to lay on the keyboard) The LFP does the work while "tickle charging the FLA bank at night." Also on heavy loads, it load shares. (That was an unexpected benefit) That said, I remove the LFP after the trip fully charge it then discharge it to 50% tell the next trip. As a side benefit. if for any reason the LFP disconnects (And I have had this happen) The FLA will keep the ALT. and/or Solar controller from voltage spikes due to sudden load loss. (the DC to DC charger should keep the alt. safe) Sudden voltage spikes can kill electronics.

 

[FonTravels]

Thanks for your answers. I know that mixing batteries is not that good but I'm planning to configure all charging parameters to the LiFePo4.

I'm also thinking of keeping the AGM disconnected so that I can only use it in an emergency.

Anyway, my way of thinking is that the AGM always outputs less than 12.8V so the LOAD will come from the LiFePo4.

Once I get the LiFePo4 I'll do some controlled testing for all possible scenarios and post them here.

PS: Steve_S I've been using the alternator to charge the AGM battery for more than a year with a DC to DC Charger without any problems. It also takes 30A from the alternator which is not that much so it doesn't heats that much.

 

[FonTravels]

Follow up question:

Do you guys think that connecting a PWM 30A 8 dollars to a LiFePO4 battery can charge a AGM battery, the input PV will be the LiFePO4 with a 60A BMS and the output will be the AGM battery. I know that I should use a battery to battery charger but that are >250 dollars. My thinking is that this way the PWM can control the voltage profile for the AGM battery.

Another solution that comes to mind is to connect between the LiFePO4 and the PWM a 20 A DC Booster so that I can simulate the voltage of a solar panel (maybe 16V).

I know that this using a PWM and a DC to DC Booster is not efficient (heat) but I can compensate the efficiency losses by getting to use the AGM battery.

What do you think, I haven't tried any of this but even if it's weird. Can it be a substitute for an expensive battery to battery charger?

 

[DW SD]

I’m planning on doing similar with existing 660Ah lifeline AGM bank with new 300Ah LFP in my RV. Charging efficiency is a motivator. I have 900W of solar. But because of lead acid charging profile it wastes a lot of collected energy to get from 80% to 100%.
Did you read this thread?

Another Lithium battery thread

Yet another thread on LiFePo's. I'm getting a bit nearer thinking about putting some LiFePo's on board – purely to reduce the amount of hours I'm running the engine to charge the domestic bank. There were a few great threads at the end of last year and particularly https://www.canalworld.net/foru...

www.canalworld.net

One thought I have is to have a manual disconnect for the LFP battery and just equalize every few months the AGM pack. To make certain it doesn’t sulfate due to non optimized charging profile.
Also, like you I don’t want to throw out $1500 worth of batteries. And selling two year old AGMs doesn’t generate much return.

 

[DW SD]

In short, I plan to KISS as possible.

 

[FonTravels]

I've been with this set up for a couple of months now and it works great:

I have them in parallel and the charging generators (both the MPPT and Sterling Battery to Battery charger) are connected between them, that way, if I wanted to I can charge one or the other. It's working great. The AGM is always charged because the LiFePO4 is always >12.9V and that means that the AGM doesn't discharge (it's like keeping it at 100% SOC).

The charger are set up at 13.8V for 30 minutes an then 13.4V float so that the LiFePO4 charges up to 90% more or less. Once a month I disconnect the LiFePO4 and charge the AGM at 14.4V with the MPPT until it gets to float which takes very little because it's already charged.

I'm finding some great benefits:

- When the BMS disconnects because of full charge or one cell get's to 3.75V (yes, I haven't been able to top balance on the road), the AGM keeps connected and that means that the generators (both the MPPT and Sterling Battery to Battery charger) are not disconnected abruptly and the LOADs keeps connected.

- If the temperature goes down < 5ºC and the LiFePO4 gets disconnected, the AGM keeps working normally, charging and discharging.

- The AGM "backup system" works automatically because the AGM battery will only be used if the BMS disconnects or the LiFePO4 drops <12.6V

 

[DW SD]

Great feedback and validates some of my expectations.
Would you buy the B2B charger again? As mentioned, I figured I'd manually charge the AGMs via monitored absorb or equalization stage (with the LFP disconnected.).
I don't live in my camper. We use it 30 days / year. So at the end of each of my week-long trips, I'd top off the AGMs. Most of the year, the batteries are shut off and the unit in storage. And during, they ought to be @ 13.2 to 13.4 or so.
Thanks,
Doug

 

[FonTravels]

Well it depends on a lot of factors. I bought it because of winter here in Europe. My solar panels produced around 400Wh a day and I need 750W-1000W so I added a LiFePO4 battery 200 Ah. If I encounter ore than 2-3 days of clouds, I need to use the B2B.

In your case, I don’t think I would buy it, I would invest that money on solar or on batteries. If you really need more power on those 30 days, it’s way cheaper and better to go to a campsite and top up the batteries.

 

[DW SD]

Thank you. I do have 900W of solar and 300 days of sun. I rarely camp in the rain here in Southern California. Also have a 15kw generator.
In a day we presently use 100Ah.

 

[FonTravels]

WOW, then no, I don’t think you need a B2B ?

 

[FonTravels]

Hey guys. It has been six months now and the beautiful sun shines. The solar panel is really happy.?

I just wanted you to know that the system is working very well.

The AGM battery is paralleled connected with the LiFePO4 battery. It works great:

When the battery is fully charged the system generates 13.0-13.2V constant until the LiFePO4 gets to about 20%, then the voltage drops to 12.8-12.9V and then it drops to 12.3V. This is great because the AGM battery is only used when the LiFePO4 is at about 20% and that means that most of the time it's topped up and the one that get's the cycles is the LiFePO4. I only get to 20% of the LiFePO4 capacity maybe once every month.

I can also charge both of them to 14.4V which is the recommended voltage for both of them and float them at 13.5V which is also the recommended voltage for both of them.

There is also another benefit. If the BMS gets disconnected either by a drop in temperature or over charging, etc. the AGM is still always connected. This means that the LOAD is still connected and the LOAD devices don't get a sudden 0V that could damage them. Also, if the car alternator B2B charger get's a sudden BMS disconnect, the AGM absorbs the spike in voltage so that the alternator isn't damaged.

In my next system I will buy a LiFePO4 battery and a small, cheap Lead Acid battery too. It solves most of the problems that LiFePO4 has.

PS: please do your research. This is only my experience and research, not any kind of recommendation.

 

[BarkingSpider]

I have a similar setup, works great for me also, noone recommends it though, but i never get an exact reason why. Does anyone have any more information on the negative side of mixing these batteries?

 

[DW SD]

I am having a great experience too with my setup. I don't use it everyday, so my data is pretty limited. I rarely get below the voltage threshold where the AGMs kick in.
For me, I also plan a setup with a larger LFP bank and small AGM. Seems like a solar charge controller could self distruct if the BMS suddenly takes an LFP bank offline. (just as an alternator).

Downside of which I'm aware: extra weight. costs. the AGM static energy loss is higher than LFP. So part of your energy collection goes to keeping the AGM charged. This is pretty nominal, but it does have an effect.

Doug

 

[BarkingSpider]

The reason i have a mixed bank is because i had 4 big AGM deep cycle batteries just doing nothing, why not use them? I would prefer all LifePo4, but that's what i have, it's as simple as that.

 

[gnubie]

I can also charge both of them to 14.4V which is the recommended voltage for both of them and float them at 13.5V which is also the recommended voltage for both of them.

How do you manage the AGMs passing through their relatively long tail current stage and not exposing the lifepo4 to potential over charging by having them sit at elevated voltages for hours while that is happening?

 

[ScoTTyBEEE]

I've done the same in my campervan but use the agm's for 12v which uses very little power but essential equipment, and the lifepo4 for the 240v. It does mean I have 2 of everything else, but does offer redundancy and low temp charging.

 

[DW SD]

How do you manage the AGMs passing through their relatively long tail current stage and not exposing the lifepo4 to potential over charging by having them sit at elevated voltages for hours while that is happening?

I prioritize charging for the LFP battery and it is a compromise. But the AGMs remain mostly dormant at 100% SOC and float voltages when the LFPs are in most of their operating range. Unless a substantial load is delivered (like running an AC unit.). Most of my loads are simple 12V water pump, lights, etc.
The charging curve isn't perfect.

 

[BarkingSpider]

I do the same charging method as DW. I have the charge settings for lifepo4. In reality it works really well and the 2 chemistries appear to compliment each other. The small voltage range should give the agms a long cycle life, but since I already had the AGMs, the main advantage is the extra amp/hrs i now have without spending any more money.