Mixing Lead and LiFePO4

[wattmatters]

I'm just very uncomfortable with it, because far too many of my friends have said "overblown" before an incident with either their wallet or the hospital.

How many of these involved a well set up and designed LiFePO4 / lead acid hybrid battery storage system? Can you elaborate further on what happened with these set ups?

 

[KMac55]

Not overblown - like playing chess, think one move beyond the immediate:

If your boat burns down, and the inspectors see a hybrid-bank, even though it wasn't the cause, blame it and deny your coverage? Does the ABYC or any other worldwide marine entity condone this type of use?

Use at your own risk. It's a risk you are comfortable with, and not my job to sway you. I'm just very uncomfortable with it, because far too many of my friends have said "overblown" before an incident with either their wallet or the hospital.

I understand you think I haven't thought a move beyond the immediate, and that a superior chess player (which I'm obviously not) could look ahead and see what is faulty with my system. But while it seems to be obvious to you, I'm not aware, technically, what is wrong? Instead of educating about this important topic, you persist in withholding any substantive explanation. I don't understand why, since I've asked for one?
I would appreciate insight into the technical aspects, which I'm sure you could provide, but instead you just mention how you think there are no commercial products along these lines, and now, again without bringing me new knowledge as requested, you simply recite a parade of horribles, including others' tragedies, who you obviously tried to warn (but they just would not listen and then lost their boats!)
Please, I don't want to lose my garage at my cabin! I would appreciate a bit of a technical electrical and/or electrochemical explanation from your knowledge so I might be convinced. TYVM

 

[Texas-Mark]

I'm confused as to why anyone would want to in the first place.

Because I have some perfectly good lead batteries and want to get everything out of them that I can.

I have been running my hybrid system for over two months now and it is still working exactly as I expected. All of my Lifepo4 batteries have BT BMSs, and quite frankly I have grown bored looking at them everyday expecting something to be "wrong". Eventually I will phase out the lead, but I see no good reason to give up 40% of my storage capacity. Also, size and weight is of zero concern for me.

 

[Substrate]

Kmac55 - I'm not trying to win any argument here. Do what you like.

I have already presented the reasons for not doing it, and why manufacturers and other entities don't recommend it either. In this thread. I do have experience believe it or not, down to the EI layer in lead-acid. (unlike the SEI layer in LFP, except for it's location between the plate and electrolyte).

In your situation, they don't apply to you. Fine, it's a free world - do as you please.

 

[derekja]

Canada in winter.

Option 1: heat lifepo4
Option 2: hybrid system
Option 3: physical switch between systems when freezing

(Biased towards lIfepo4 because of charging rate, though, so not dipping into agm is preferred.)

 

[Texas-Mark]

As with other threads and topics (not just solar related), there are always the naysayers who say something can't or shouldn't be done based on theories. And then there are those who have actually done it, and have real life data and experience.

 

[Substrate]

I would appreciate insight into the technical aspects, which I'm sure you could provide, but instead you just mention how you think there are no commercial products along these lines, and now, again without bringing me new knowledge as requested, you simply recite a parade of horribles, including others' tragedies, who you obviously tried to warn (but they just would not listen and then lost their boats!)

Guys - I'm just trying to keep it friendly, and do what you like. You all seem quite capable of takin' care of business.

It's just that I go back to the Nigel Calder (boater, not science writer) marine days, when things about AGM's in particular - mostly big ass Lifeline 8D agm's - and such - smoking alternators, not obtaining full charge and all that fun, was a novelty for many transitioning from flooded to agm. While I was EV'ing it with Enersys pure-leads, he was doing the mariner thing so I followed his adventures closely.

Gosh, so many years ago now...

 

[Boron]

Actually, in the video he connected LiFePO4 and FLA in parallel.

But let's discuss particular situation ...

First you claim, "a LiFePo4 requires a higher voltage than an AGM"
Explain why you can't use 14.4 volts for the LiFePO4 battery?
That looks like the exact same voltage for AGM at 70°F.
You think 14.4 volts is too low for LiFePO4 battery - you are joking, right?
You need to specify the voltage you think is "required" for LiFePo4, because you have not.

Secondly you claim, "you won't be able to utilize the true deep cycle cycles of lithium".
Actually, we can and will get 80+% DOD from the LiFePO4 battery.
The Low Voltage Drop-Out voltage will be that of the AGM.
And the TOTAL amp-hours is the SUM of both battery banks.
You need to do some actual research regarding this issue.

Third you claim, "lithium has a high efficiency of 99% where lead acid efficiency is about 70%"
So what?
This causes no problems.

Nevertheless, you still have not given a single valid reason WHY this cannot be done.

yes you are quite right. These arm chair speculators dont know Jack Schwepps. 14.4v is good for a Lifpo4 - 4s you dont lose much Ah<5%
and its less stress on the cells = longer cycle life.

You can take Lifepo down to 2.5V/cell = 10 V for 4S but thats a little low for AGM (only likes >50%DOD for longer life) so stick to 11V give 2.75v/cell, just fine for a Lifepo but lose some Ahs maybe >10%. So overall you lose maybe >15% with those settings. Only thing is MAKE SURE your charger doesnt have a recondition setting (like the Victron) as this will push much higher volts (15.5V = 3.875v/cell) and you dont want that on your lifepo cell - certain damage if left. A dumb agm charger CV with 14.4V max (normally preset to prevent gassing) should do nicely. Of course makers will tell you otherwise - they want to sell new ££££ kit.

I am in the process of hooking up Labs and Lifepos. I might fit a pair of blocking schottky diodes (as they do on solar panels) in case one side fails (wont bring down the other side - needs thinking about.

BTW be careful to match Vocs closely when first connecting or you might get very large current flowing ie 1mohm with say 0.1V difference could give 100 Amps flowing. Use a power resistor first of say 0.1 ohms to balance Vocs. Take care or you will be sorry.

 

[grizzzman]

yes you are quite right. These arm chair speculators dont know Jack Schwepps. 14.4v is good for a Lifpo4 - 4s you dont lose much Ah<5%
and its less stress on the cells = longer cycle life.

You can take Lifepo down to 2.5V/cell = 10 V for 4S but thats a little low for AGM (only likes >50%DOD for longer life) so stick to 11V give 2.75v/cell, just fine for a Lifepo but lose some Ahs maybe >10%. So overall you lose maybe >15% with those settings. Only thing is MAKE SURE your charger doesnt have a recondition setting (like the Victron) as this will push much higher volts (15.5V = 3.875v/cell) and you dont want that on your lifepo cell - certain damage if left. A dumb agm charger CV with 14.4V max (normally preset to prevent gassing) should do nicely. Of course makers will tell you otherwise - they want to sell new ££££ kit.

I am in the process of hooking up Labs and Lifepos. I might fit a pair of blocking schottky diodes (as they do on solar panels) in case one side fails (wont bring down the other side - needs thinking about.

BTW be careful to match Vocs closely when first connecting or you might get very large current flowing ie 1mohm with say 0.1V difference could give 100 Amps flowing. Use a power resistor first of say 0.1 ohms to balance Vocs. Take care or you will be sorry.

This is true but my observation of this is that it is short lived. In my system when I connect LFP to the FLA bank through a 500 amp kilovac contactor. ( This can be true temp high amps even when you are close in voltage matching. )