Replacing house batteries project

[Poey50]

RISK is a function of PROBABILITY OF EVENT and SEVERITY OF OUTCOME.

SEVERITY OF OUTCOME is conditional on the SITUATION.

The failure modes of LFP all tend to binary. The failure modes of LA tend to incremental.

If the SITUATION is a long passage or presence in a remote location then the RISK associated with LFP only is very high compared to LA.

How do you configure a system that mitigates the risk?.... while maintaining the benefits for LFP and keep it simple. In particular, better harvesting of charging sources and more time-flexibility due to better partial discharge performance.

...and you don't need to be physics genius to install and maintain it.

I agree with all that. There are several good solutions involving separating charge and load circuits. Good DIY systems and expensive integrated systems like Victron's do that. There is one straightforward solution using the other kind of drop-in hybrid system - lead acid to run the safety-critical circuits, LFP for the luxuries. Each on a separate circuit so chemistries are not combined. Charging usually to the LFP with the lead acid maintained on a B2B from the LFP. If LFP fails then all charging can be switched to the lead acid. It makes use of the advantages of both - high energy density and ability to take current of the LFP; the ability of the lead acid to take some abuse and remain reliable.

 

[ckris]

I just saw a comment from Rod Collins which shed a little light on why ISO and ABYC don't support LFP-lead acid hybrid. As well as general concerns about different battery chemistries not playing well together - if a lead acid cell should short and effectively become a 10 volt battery the LFP would then pour power into it with Big Bang consequences.

Do you have a link, would be interested to read (if in the public domain)?

Large lead acid only battery banks have similar risk, I wonder where the boundary situation is where you would judge LiFePO4 in the mix makes it more dangerous?

...and might the BMS discharge current overprotect mitigate the problem?

 

[Poey50]

Do you have a link, would be interested to read (if in the public domain)?

Large lead acid only battery banks have similar risk, I wonder where the boundary situation is where you would judge LiFePO4 in the mix makes it more dangerous?

...and might the BMS discharge current overprotect mitigate the problem?

I picked it out of a Lithium Batteries on a Boat Facebook conversation - it didn't say any more than that. There are many such discussions on the hybrid issue there if interested.

Because of the very low internal resistance of the LFP the flow of current would be much higher than with lead acid, although neither is a situation to relish. This is why LFP needs a main fuse with a much higher amperage interrupt capacity (AIC) with Class-T being the recommended type.

Possibly the current overprotect might do it but I wouldn't want to bank on it.

 

[PaulRainbow]

I just saw a comment from Rod Collins which shed a little light on why ISO and ABYC don't support LFP-lead acid hybrid. As well as general concerns about different battery chemistries not playing well together - if a lead acid cell should short and effectively become a 10 volt battery the LFP would then pour power into it with Big Bang consequences.

As ckris mentioned, this happens with a LA bank, i'v seen it several times.

It will make the battery gas and release hydrogen, but doesn't necessarily cause a bang, without something to ignite the gas. A well vented battery location should stop that, in most cases.

Personally, i don't accept Rods comment as a valid reason for not supporting a parallel hybrid system.

 

[Poey50]

As ckris mentioned, this happens with a LA bank, i'v seen it several times.

It will make the battery gas and release hydrogen, but doesn't necessarily cause a bang, without something to ignite the gas. A well vented battery location should stop that, in most cases.

Personally, i don't accept Rods comment as a valid reason for not supporting a parallel hybrid system.

Interesting. Rod Collins is very knowledgeable but not infallible.

 

[Beneteau381]

As ckris mentioned, this happens with a LA bank, i'v seen it several times.

It will make the battery gas and release hydrogen, but doesn't necessarily cause a bang, without something to ignite the gas. A well vented battery location should stop that, in most cases.

Personally, i don't accept Rods comment as a valid reason for not supporting a parallel hybrid system.

Happened to me twice, lucky i suppose, the battery culprit got very hot. The smell the first time warned us and the sevond time the CO alarm warned us

 

[PaulRainbow]

Happened to me twice, lucky i suppose, the battery culprit got very hot. The smell the first time warned us and the sevond time the CO alarm warned us

Last time it happened to me the CO alarm went off.

 

[ckris]

Last time it happened to me the CO alarm went off.

Does anyone have a sense of the relative current flow between LA and lithium in this situation?

eg if a 100AH LiFePO4 was in parallel with a 100AH AGM with a shorted cell how different would that be to a bank of 3 x 100AH AGMs with 2 discharging into the one with a shorted cell?

These two battery banks would be approx equivalent usable size and my guess is the outcome/risk would be similar....

 

[PaulRainbow]

Does anyone have a sense of the relative current flow between LA and lithium in this situation?

eg if a 100AH LiFePO4 was in parallel with a 100AH AGM with a shorted cell how different would that be to a bank of 3 x 100AH AGMs with 2 discharging into the one with a shorted cell?

These two battery banks would be approx equivalent usable size and my guess is the outcome/risk would be similar....

You don't need a lot of current, it's the fact that you're holding a 10v battery at 14v + that does the damage.

I'd agree, the outcome would be the same, whether the LA was connected to another LA, a LiFePO4, mains charger or even a solar panel of reasonable size.