Lead acid and LiFePO4 together

[zeeman]

I agree with this - unless zeeman is storing the batteries below freezing, then SLA may make sense.

Space and capacity wise lithium is a no brainer at today’s prices. When adding the 2-4yr time to failure likely with daily-use FLA, LiFePo at “double the price” is going to be WAY cheaper in a ten-year view, and in use essentially LiFePo is twice the capacity. So in practice- practical terms- LiFePo is 25% as expensive.

thank you for your feedback!

 

[zeeman]

so, in scenario that LFP and VRLA batteries are connected in parallel (48V system) if LFP 48V battery pack max operating voltage is 57.6V and VRLA bank max voltage is 56.4V and operating voltage is 54V - how do you safely connect them together? Would that voltage difference be safe for VRLA battery bank? Or, maybe this can be set in inverter so that max state of charge is at safe level for VRLA battery bank?

 

[wattmatters]

Max top off charge is set at 56.4V

guess then that I would need to figure out how not to overcharge the VRLA batteries as LFP has a higher voltage state. Am i on right track here?

56.4 V is about perfect for an upper charge voltage setting for an LFP battery with 16 cells in series.

 

[12VoltInstalls]

I would need to figure out how not to overcharge the VRLA batteries as LFP has a higher voltage state. Am i on right track here?

Lead batteries need “~2V” over absorbed resting voltage to actually take any effective charge. So charging will probably be fine using the LFP charging parameters.

But Lead (FLA) batteries need periodic maintenance of fluid levels and (once a month or every six weeks-ish) need ‘equalization’ at a higher voltage to keep their capacity healthy. AGM/sealed Lead batteries do not require equalize cycles; they are a better fit ‘with’ LFP(LiFePo) batteries.
However, FLA batteries might not be good to have constantly parallel with LFP because they will constantly accept a trickle of current from the LFP battery bank.
A simple manual disconnect with periodic reconnection to ‘top up’ the lead batteries might be useful.
In cloudy periods, LFP won’t care if they don’t get fully charged every day of cycling, whereas lead batteries want to be at full charge as much as possible on a daily basis.
It’s not a problem, really, if you don’t mind dealing with observing the battery statuses and connecting/disconnecting as needed.

I don’t know if there is automatic equipment to handle this task or not. Perhaps there is.
I ran a FLA battery bank for several years without issue. However, when I made the LiFePo4 purchase I ent with straight up LiFePo and retired the FLA’s. I envisioned a set-it-and-forget-it situation and that’s what I’ve got- that fits my tolerance for fussing about with my solar. I’m 100% offgrid.
For a couple years it was fun messing about with maximizing my too-small basic component system, but I outgrew that. I now do whatever I want with lights etc. and really don’t give it much thought other than noting battery voltage in the morning when I get up. Your mileage may vary.

 

[zeeman]

56.4 V is about perfect for an upper charge voltage setting for an LFP battery with 16 cells in series.

great! thank you for that tip!

 

[zeeman]

Lead batteries need “~2V” over absorbed resting voltage to actually take any effective charge. So charging will probably be fine using the LFP charging parameters.

But Lead (FLA) batteries need periodic maintenance of fluid levels and (once a month or every six weeks-ish) need ‘equalization’ at a higher voltage to keep their capacity healthy. AGM/sealed Lead batteries do not require equalize cycles; they are a better fit ‘with’ LFP(LiFePo) batteries.
However, FLA batteries might not be good to have constantly parallel with LFP because they will constantly accept a trickle of current from the LFP battery bank.
A simple manual disconnect with periodic reconnection to ‘top up’ the lead batteries might be useful.
In cloudy periods, LFP won’t care if they don’t get fully charged every day of cycling, whereas lead batteries want to be at full charge as much as possible on a daily basis.
It’s not a problem, really, if you don’t mind dealing with observing the battery statuses and connecting/disconnecting as needed.

I don’t know if there is automatic equipment to handle this task or not. Perhaps there is.
I ran a FLA battery bank for several years without issue. However, when I made the LiFePo4 purchase I ent with straight up LiFePo and retired the FLA’s. I envisioned a set-it-and-forget-it situation and that’s what I’ve got- that fits my tolerance for fussing about with my solar. I’m 100% offgrid.
For a couple years it was fun messing about with maximizing my too-small basic component system, but I outgrew that. I now do whatever I want with lights etc. and really don’t give it much thought other than noting battery voltage in the morning when I get up. Your mileage may vary.

Per VRLA battery manual equalization is not needed and it is not recommended. And for the rest that you said - that is some really good info that is much appreciated! I may just do that for start (put them in parallel, do occasional disconnect put some load on it and then reconnect) and later I may get completely rid of VRLA bank.

 

[wattmatters]

Here's some data I posted a while back on the operation of my hybrid LFP-SLA battery:

Post in thread '“Mixed battery chems” or separate system'

Apr 24, 2024

Could I add these to my system in some way?

I run a hybrid LiFePO4 and SLA battery so it is possible. However it depends on few things.

Objectives?
Type of lead acid battery and the charge voltage specs? They differ depending on the lead acid battery type.
Relative capacity of batteries?
What is charging them?

In my case the SLA (~400 Ah) is my backup reserve capacity (and is what I had to begin with for power outage backup), and so they sit at float most of the time. The LiFePO4 (300 Ah) I added later on to do daily cycling. Nominally that's ~35 kWh of battery. I confine daily...

• wattmatters

• 

 

[zeeman]

wow! that is some awesome documentation of how this works in real life!
thank you. I will think about can I apply this to my scenario.