LFP + AGM

[norcal]

Hey!

I’m considering adding an LFP battery to an RV that already has house AGM batteries.

Voltages of LFP and AGM seem complementary - LFP will likely handle most of the load so that good.

However, I’d like to ensure to avoid high current rash when RV AGM is charging (via alternator, solar or shore power).

I was thinking of using a resistor, probably 1ohm, 100w. Assuming LFP should be in the 10-14v range and that alternator/solar/shore max at 14.4v (or so), the max difference would be 4v, and with 1ohm current should be limited to 4amp (according to ohms law, IIRC).

Thoughts?

Is this a really bad idea? If so, why?

Thanks! ?

 

[justgary]

The charge profiles of LFP and AGM are different, and you can't really expect them to play well together if you tie them to the same charger and loads.

If you plan to use LFP for separate loads, you can use a DC/DC converter to run a charge controller. You could also add a second alternator that has a regulator set up for LFP.

How many AGM batteries do you have, and what loads do they run? Would it make more sense to just dump them and go all LFP?

 

[norcal]

The charge profiles of LFP and AGM are different, and you can't really expect them to play well together if you tie them to the same charger and loads.

If you plan to use LFP for separate loads, you can use a DC/DC converter to run a charge controller. You could also add a second alternator that has a regulator set up for LFP.

How many AGM batteries do you have, and what loads do they run? Would it make more sense to just dump them and go all LFP?

2x 100ah, brand new, came with RV, so would prefer to keep them for now.

As for dc-to-dc, I considered that but that would allow energy to flow in one direction and what I’m trying to do is to allow LFP+AGM to act as one bank - that is, house will benefit from LFP and LFP would get charged by solar/alternator… with dc-to-dc it’s I will be able to charge LFP from alternator but AGM won’t get charged from LFP…

 

[wattmatters]

There are a number of threads about hybrid lithium and lead battery set ups, so have a search. In some scenarios they can work well but I would not recommend them as a first step. There are also devices which can help to isolate one battery or the other depending on the system state.

 

[Texas-Mark]

The charge profiles of LFP and AGM are different, and you can't really expect them to play well together if you tie them to the same charger and loads.

I have been running LifePo4 in parallel with AGM for months with no issues. Unlike the naysayers that claim something won't work without any actual first hand experience, I and others here have actually been doing it. As noted above, there are several threads related to the subject.

 

[time2roll]

The only way I would consider using these chemistries together would be to have a battery switch that indicates A, B, A+B, Off.
Use the switch to isolate the batteries appropriately.

I would mostly use the AGM really hard and the LFP mostly as backup. When the AGM gives up you can go all LFP and end troubles.

Skip the resistor and get a DC-DC charger to control alternator charging but I doubt it is needed. I recommend 20 amps rated unless the alternator capacity etc is evaluated.

 

[norcal]

The only way I would consider using these chemistries together would be to have a battery switch that indicates A, B, A+B, Off.
Use the switch to isolate the batteries appropriately.

I would mostly use the AGM really hard and the LFP mostly as backup. When the AGM gives up you can go all LFP and end troubles.

Skip the resistor and get a DC-DC charger to control alternator charging but I doubt it is needed. I recommend 20 amps rated unless the alternator capacity etc is evaluated.

Why stress the AGM if LFP can support it and has more cycles?

From what I’ve seen from people who tried it the whole idea is that the LFP has bear perfect voltage at its flat part of the curve to keep the AGM battery in good health and without stressing it.

 

[time2roll]

Why stress the AGM if LFP can support it and has more cycles?

From what I’ve seen from people who tried it the whole idea is that the LFP has bear perfect voltage at its flat part of the curve to keep the AGM battery in good health and without stressing it.

OK again my goal would be to have all LFP. I would be OK if I got just one more season, two max out of the AGM and let it go. I had perfectly good 4x GC2 that I sold locally to avoid the whole problems.

Charging the AGM at 14.4-14.6 in absorption for a few hours each day will stress the LFP more than necessary. While in use the resting voltage of the LFP (13.4) will be feeding the AGM and losing capacity. To really use the AGM the voltage may get down close to 12.0 and that might be a bit lower than the LFP needs to be. Yes it is correct that they will operate together. IMO not really the best situation. There is no immediate safety issue.

 

[justgary]

I have been running LifePo4 in parallel with AGM for months with no issues. Unlike the naysayers that claim something won't work without any actual first hand experience, I and others here have actually been doing it. As noted above, there are several threads related to the subject.

It's OK to color me naysayer.

Considering that AGM wants to stay completely full for longest life and LFP does not want to be full at all for longest life, it is hard to reconcile long life with the fact that LFP is full at a lower voltage than AGM wants to be. AGM will be stressed if you use it and LFP will be stressed if you don't use it.

A properly designed AGM system should get you roughly seven years of good service. A properly designed LFP system should get you probably double that. The fact that you have gone months without issues is not surprising. What about three or five years from now? Some part of the system is taking stress that it shouldn't have to.

I have plenty of firsthand experience with AGM, and I know that I was disappointed by the performance in a solar situation where I actually expected to use the batteries. At least with my wet cells I was able to test individual cells to see exactly what was going on. With AGM you never really know how the battery is doing on a cell-by-cell basis. With AGM you also have to oversize the system if you want to start big loads.

If you are getting great performance out of your hybrid system, I suspect it is from the LFP, not the AGM.

 

[Texas-Mark]

Considering that AGM wants to stay completely full for longest life and LFP does not want to be full at all for longest life, it is hard to reconcile long life with the fact that LFP is full at a lower voltage than AGM wants to be. AGM will be stressed if you use it and LFP will be stressed if you don't use it.

Read the other thread. it explains what people have actually experienced while monitoring charge and discharge.

 

[justgary]

Read the other thread.

Sorry, I didn't see a link to another thread. I see it now, so I'll go off and read it.

Honest, I'm not trying to knock you or anything. I sincerely hope you are getting the performance you expect from your system.

 

[Goboatingnow]

Why leave any lead in circuit LFp outperforms all lead

 

[norcal]

Why leave any lead in circuit LFp outperforms all lead

The agm are new and 200ah of lithium to replace them ain’t free…

 

[Texas-Mark]

Why leave any lead in circuit LFp outperforms all lead

They still have a lot of life in them. Makes no sense to toss the extra capacity. However, in my case, they will get replaced sometime in the next year.

 

[justgary]

The agm are new and 200ah of lithium to replace them ain’t free…

True, but at least you only need about 100-120Ah of LFP to replace them.

 

[wattmatters]

My home system (not a mobile system) is designed so that the 10 kWh of LiFePO4 does the daily cycling work while the 20 kWh of Enersys SLA batteries sit there at float with the sole purpose being as spare energy capacity for grid outage backup. The SLA is ex-telco backup, 1/4 the price per kWh and designed precisely for this purpose. It would be an expensive waste of a LiFePO4 battery capacity to be kept in reserve for backup.

I don't care that overnight there is a small leakage of float current from the LiFePO4 to the SLA. It's a few hundred milliamps. My inverter already uses many times that when doing nothing. And in the daytime that's what the solar PV is for.

Lowest I let my LiFePO4 get down to is about 10% SOC (often doesn't get down that far). Below that and my system runs from the grid using automations.

When discharging, the LiFePO4 provides all the energy until its SOC gets down to about 25%, then below that the SLA begins to supplement with a little energy. By the time the LiFePO4 gets down to about 10% SOC the SLA is at about 95% SOC (give or take).

Next day when the sun comes up, they recharge. The SLA doesn't need a lot and the LiFePO4 sucks up all the juice it can. The SLA is pretty much kept at float, with a brief top up charge during the day if it was required and is ready at anytime to supply energy should we have a long grid outage.

Lather, rise, repeat.

But each situation is different. For me the Lead battery is cheap insurance.

 

[Goboatingnow]

My home system (not a mobile system) is designed so that the 10 kWh of LiFePO4 does the daily cycling work while the 20 kWh of Enersys SLA batteries sit there at float with the sole purpose being as spare energy capacity for grid outage backup. The SLA is ex-telco backup, 1/4 the price per kWh and designed precisely for this purpose. It would be an expensive waste of a LiFePO4 battery capacity to be kept in reserve for backup.

I don't care that overnight there is a small leakage of float current from the LiFePO4 to the SLA. It's a few hundred milliamps. My inverter already uses many times that when doing nothing. And in the daytime that's what the solar PV is for.

Lowest I let my LiFePO4 get down to is about 10% SOC (often doesn't get down that far). Below that and my system runs from the grid using automations.

When discharging, the LiFePO4 provides all the energy until its SOC gets down to about 25%, then below that the SLA begins to supplement with a little energy. By the time the LiFePO4 gets down to about 10% SOC the SLA is at about 95% SOC (give or take).

Next day when the sun comes up, they recharge. The SLA doesn't need a lot and the LiFePO4 sucks up all the juice it can. The SLA is pretty much kept at float, with a brief top up charge during the day if it was required and is ready at anytime to supply energy should we have a long grid outage.

Lather, rise, repeat.

But each situation is different. For me the Lead battery is cheap insurance.

Insurance against what. The only cheap insurance is for very low risks. Ie the worst case situation won’t stop actually ever occur.

 

[wattmatters]

Insurance against what.

Against extended grid outages, which happen from time to time. I've used it for just that purpose quite a number of times and it's been exactly what we needed. I have no concern about my LiFePO4 being used to its full capacity for the regular daily cycling because I have a backup reserve of energy ready for outages. That's what it is for.