This guy thinks outside the box adding lead to a LiFePO bank...

[Terrapin]

It seems to me correctly charging the system is problematic and I just don't know enough about the interaction on discharge to say if it's nuts or not.

I do recognize early in the video he is saying some things we don't hold as common knowledge about the specific benefits and disadvantages of the types. But I am hoping the focus of responses here will be regarding the concept of the discharge tendencies creating a long life bank.

...So I am wondering what the real gurus around here think (in simple english, LOL).

Is he a genius? or not understanding how this stuff really works?

The YT link:

 

[snoobler]

Didn't watch the video. This has been discussed many times. The lead bank contributes very little until the LFP bank is at very low SoC, and it tends to pull the LFP down at rest due to LFP's higher resting voltage "floating" the FLA/AGM/GEL bank.

Does it work? Yep. Is it worth doing? Depends.

 

[Horsefly]

I watched that video when he first posted it. He's living it, so I guess it really does work. I think @snoobler is right that the discharge profile is different enough (L-A has a slope, while LFP doesn't) will

My 24V AGM batteries (that I hope someday to replace with LFP) have a bulk/absorption voltage of 29.1V, and a float voltage of 27.1V. That's a little high for what I would want to do with LFP, but maybe OK if I can really trust the charge controller. The best-practice for AGMs to to hold them in absorption until the current drops to 1% o 2% of C (0.01C or 0.02C - I've seen both). That can be holding the charge at the absorption value for hours after the cells get to that point. I guess that may be OK for LFP?

I am curious about it, since I already have the AGM batteries. They may last another several years, so there's a good chance they will still be OK when I bring in LFP. So, I may have the opportunity to use them together, although my gut says I shouldn't....

 

[Terrapin]

I watched that video when he first posted it. He's living it, so I guess it really does work. I think @snoobler is right that the discharge profile is different enough (L-A has a slope, while LFP doesn't) will

My 24V AGM batteries (that I hope someday to replace with LFP) have a bulk/absorption voltage of 29.1V, and a float voltage of 27.1V. That's a little high for what I would want to do with LFP, but maybe OK if I can really trust the charge controller. The best-practice for AGMs to to hold them in absorption until the current drops to 1% o 2% of C (0.01C or 0.02C - I've seen both). That can be holding the charge at the absorption value for hours after the cells get to that point. I guess that may be OK for LFP?

I am curious about it, since I already have the AGM batteries. They may last another several years, so there's a good chance they will still be OK when I bring in LFP. So, I may have the opportunity to use them together, although my gut says I shouldn't....

Based on Snoobler I searched quite a bit under different words for more on this. I came up with some thoughts from others but not a ton. The reason It caught my eye was two fold. First was the possibility of extending the LA life. But I am coming close to maxing out the 120a rating on the single LFP12v BMS of my proposed Trailer upgrades. The inverter should fine at 1500w for running the microwave (1300w max) plus a few LEDs, safety monitors, and the LPG frig solenoid. So I am thinking/hoping the extra LA battery can boost the battery bank's amperage enough on the cheep to keep me on track at max need.

Does this sound like a possibility?

 

[Terrapin]

Didn't watch the video. This has been discussed many times. The lead bank contributes very little until the LFP bank is at very low SoC, and it tends to pull the LFP down at rest due to LFP's higher resting voltage "floating" the FLA/AGM/GEL bank.

Does it work? Yep. Is it worth doing? Depends.

Thx Snoobler. I am wondering specifically if adding the LA can boost the amps of the bank a little past the 120a rating of the LFP? I am so close to meeting my needs it could be very worth while in my case....if it does...

 

[snoobler]

Thx Snoobler. I am wondering specifically if adding the LA can boost the amps of the bank a little past the 120a rating of the LFP? I am so close to meeting my needs it could be very worth while in my case....if it does...

Not reliably or meaningfully.

FLA/AGM/Gel rapidly drop below 13V under load. LFP can hold 13.2V under load. The FLA/AGM/GEL are more likely to pull a very small load from the LFP bank to maintain the FLA/AGM/GEL voltage than they are to supplement current.

 

[Terrapin]

Not reliably or meaningfully.

FLA/AGM/Gel rapidly drop below 13V under load. LFP can hold 13.2V under load. The FLA/AGM/GEL are more likely to pull a very small load from the LFP bank to maintain the LFP voltage than they are to supplement current.

Thanks for the short concise answers and explanations. That would explain the comment in the video that it can increase cranking amps available (very short term). I think I need an understanding of if the 1300W max rating of my microwave is a locked startup amperage...if that's the case and it doesn't recur during normal operation I still may have reason for hope beyond just extending the capacity a little bit ....and longevity.

 

[snoobler]

I made a typo in the quoted text:

"LFP voltage" should be "FLA/AGM/GEL voltage"

Microwaves use about 60% more power than their rating.

1000W microwave uses 1600W. It's continuous. There is a VERY short mega-spike when the magnetron first energizes, but it's for 1/120th of a second. Most equipment won't even react to it. There's a smaller spike shortly after, but it's much lower and more typical of a small motor surge that a BMS could handle.

Running at 50% power only makes it worse. It just cycles it on at 100% and off at 0% periodically.

Your best option (size due to TT installation) is a 700W microwave that only uses about 1100W continuous.

Your second best option is an inverter microwave by Panasonic. These actually use a reduced power at reduced settings and are slightly more efficient. In the U.S., product selection is very limited. They tend to be full-size countertop units.

Replacing your current microwave is likely the easiest/cheapest solution for dealing with your 120A limitation.

 

[Horsefly]

You know, one of the reasons I don't want to dismiss the idea of putting AGMs and LFPs in parallel is that if/when a common port BMS shuts off connection to the LFP, the charge controller and inverter remain powered. This wasn't one of the things mentioned in the video. Another thread here was talking about the conflicting literature from manufacturers on just how disastrous it is (or isn't) to disconnect the battery suddenly from a charge controller while there is power coming from the PV array.

In my case I'm dealing with potentially very cold operating environments, so I would be only considering a BMS with a low temperature disconnect. Our cabin is visited by my siblings and their families, and I know - no matter what I may say or ask - that someone will fire things up without regard for the temps the batteries may get to at night. It might be reassuring to know that the AGMs could take over.

 

[Hedges]

Considering how good AGM is at delivering surge currents, and that it has longest life when kept fully charged, I would think a DC-DC charger from lithium to AGM would be a good way to go. Each would be charged according to its preferred profile, and when power was drawn from AGM, lithium would drain to supply the power and bring it back to float.

Inverter would be fed by AGM; no disconnect occurs.
Charger would go to lithium. Could add a few series connected diodes from lithium to AGM so if BMS disconnects, voltage can't fly too high without AGM absorbing the current. Voltage drop needs to allow any "cranking" or other surge current conditions from AGM without drawing any/excessive current from lithium.

(of course lithium is even better at delivering surge current, but usually without a BMS getting in the way.)

 

[Horsefly]

Considering how good AGM is at delivering surge currents, and that it has longest life when kept fully charged, I would think a DC-DC charger from lithium to AGM would be a good way to go. Each would be charged according to its preferred profile, and when power was drawn from AGM, lithium would drain to supply the power and bring it back to float.

Inverter would be fed by AGM; no disconnect occurs.
Charger would go to lithium. Could add a few series connected diodes from lithium to AGM so if BMS disconnects, voltage can't fly too high without AGM absorbing the current. Voltage drop needs to allow any "cranking" or other surge current conditions from AGM without drawing any/excessive current from lithium.

(of course lithium is even better at delivering surge current, but usually without a BMS getting in the way.)

Hmm... I'd have to think about it some more, but I can't imagine this as being workable. For a sizeable AGM bank, if you can find a big enough DC-to-DC charger I'm guessing it would be very expensive and very lossy / inefficient. Also, many (most?) of us with medium-sized or larger inverters have a charger built into the inverter, connected to our backup generator. Going the route you've described would have my generator charging my AGMs, but doing nothing for the LFPs.

Still, it's a thought. I guess I'll grab the Google and head down the rabbit hole. ?

 

[Terrapin]

I made a typo in the quoted text:

"LFP voltage" should be "FLA/AGM/GEL voltage"

Microwaves use about 60% more power than their rating.

1000W microwave uses 1600W. It's continuous. There is a VERY short mega-spike when the magnetron first energizes, but it's for 1/120th of a second. Most equipment won't even react to it. There's a smaller spike shortly after, but it's much lower and more typical of a small motor surge that a BMS could handle.

Running at 50% power only makes it worse. It just cycles it on at 100% and off at 0% periodically.

Your best option (size due to TT installation) is a 700W microwave that only uses about 1100W continuous.

Your second best option is an inverter microwave by Panasonic. These actually use a reduced power at reduced settings and are slightly more efficient. In the U.S., product selection is very limited. They tend to be full-size countertop units.

Replacing your current microwave is likely the easiest/cheapest solution for dealing with your 120A limitation.

Thanks for your input again Snoob. I read into that typo, no prob. ... I have noticed our house Micro does not turn on and off but runs steady. They cook very nicely. It's a huge commercial 12000w beast... I always wondered why all of them don't work that way. I can see why the cost wouldn't usually be added to small "dorm room" units Your thought may very well be the last resort. I did see a post about a 700w Panasonic Inverter one for a van build recently. I didn't understand the relevance though till now.

 

[Terrapin]

You know, one of the reasons I don't want to dismiss the idea of putting AGMs and LFPs in parallel is that if/when a common port BMS shuts off connection to the LFP, the charge controller and inverter remain powered. This wasn't one of the things mentioned in the video. Another thread here was talking about the conflicting literature from manufacturers on just how disastrous it is (or isn't) to disconnect the battery suddenly from a charge controller while there is power coming from the PV array.

In my case I'm dealing with potentially very cold operating environments, so I would be only considering a BMS with a low temperature disconnect. Our cabin is visited by my siblings and their families, and I know - no matter what I may say or ask - that someone will fire things up without regard for the temps the batteries may get to at night. It might be reassuring to know that the AGMs could take over.

Had to laugh at the family comment, I had been thinking this trailer would go on outdoorsy for rent.... nope...

Also I had not thought of the almost UPS benefit of some lead in the bank. Interesting...

 

[schmism]

Your second best option is an inverter microwave by Panasonic. These actually use a reduced power at reduced settings and are slightly more efficient. In the U.S., product selection is very limited. They tend to be full-size countertop units.

It was time for a new micro when I got my solar up and running. I went with a new Panasonic unit and I'm impressed. You can see the "soft start" of the new micro vs the hard hit that the old standard unit was. It also seems to do a MUCH better job at warming food up quickly and evenly.

 

[Ceberha]

You know, one of the reasons I don't want to dismiss the idea of putting AGMs and LFPs in parallel is that if/when a common port BMS shuts off connection to the LFP, the charge controller and inverter remain powered. This wasn't one of the things mentioned in the video. Another thread here was talking about the conflicting literature from manufacturers on just how disastrous it is (or isn't) to disconnect the battery suddenly from a charge controller while there is power coming from the PV array.

In my case I'm dealing with potentially very cold operating environments, so I would be only considering a BMS with a low temperature disconnect. Our cabin is visited by my siblings and their families, and I know - no matter what I may say or ask - that someone will fire things up without regard for the temps the batteries may get to at night. It might be reassuring to know that the AGMs could take over.

I have also been thinking about setting up a system like this, and was intrigued by having the lead batteries for continuity if the BMS cut out. However, one concern I had was if the LFP pack shut down due to low temp or something similar, and the lead cells kept powering the loads voltages would diverge. If the BMS kicked on again one could see large currents from LFP to lead...

 

[Horsefly]

I have also been thinking about setting up a system like this, and was intrigued by having the lead batteries for continuity if the BMS cut out. However, one concern I had was if the LFP pack shut down due to low temp or something similar, and the lead cells kept powering the loads voltages would diverge. If the BMS kicked on again one could see large currents from LFP to lead...

Yeah... but... Would that be a bad thing? I dunno. I think what @Hedges was pointing out is valid: LFP is really good at delivering current, and wouldn't mind propping up the LA. Sigh... It's too late, and two glasses of wine make so I can't think this through.

 

[Terrapin]

I have also been thinking about setting up a system like this, and was intrigued by having the lead batteries for continuity if the BMS cut out. However, one concern I had was if the LFP pack shut down due to low temp or something similar, and the lead cells kept powering the loads voltages would diverge. If the BMS kicked on again one could see large currents from LFP to lead...

Wow, this was one of the biggest cautions at start up. Since I am up here in New England serious cold is a real thing. You just put a big "what if" on my radar. Thanks.

 

[Terrapin]

Considering how good AGM is at delivering surge currents, and that it has longest life when kept fully charged, I would think a DC-DC charger from lithium to AGM would be a good way to go. Each would be charged according to its preferred profile, and when power was drawn from AGM, lithium would drain to supply the power and bring it back to float.

Inverter would be fed by AGM; no disconnect occurs.
Charger would go to lithium. Could add a few series connected diodes from lithium to AGM so if BMS disconnects, voltage can't fly too high without AGM absorbing the current. Voltage drop needs to allow any "cranking" or other surge current conditions from AGM without drawing any/excessive current from lithium.

(of course lithium is even better at delivering surge current, but usually without a BMS getting in the way.)

If you could calculate what diodes to do what is needed for this you are so far past my pay scale it's not even funny. But I appreciate hearing what is just out of my reach. I try to learn and what I get from this is... maybe not a smaller microwave, but just a slightly bigger BMS is what's in order. Maybe use this 120a BMS for a pelican case build aux. 100ah battery that I can use as a plug in add on for this trailer and also for my 12v trolling battery on the small bass boat. Then just bite the bullet on a >150a BMS w/ 240ah cells for the trailers fixed bank. I don't know if I can afford it but as I weigh options I appreciate all input.

 

[Terrapin]

It was time for a new micro when I got my solar up and running. I went with a new Panasonic unit and I'm impressed. You can see the "soft start" of the new micro vs the hard hit that the old standard unit was. It also seems to do a MUCH better job at warming food up quickly and evenly.

This was exactly what I was originally asking about RE: old school microwave Tech. Do you happen to have any hard numbers on what the original unit was VS those different types start up amps??? I did mention in another reply exactly what you are relating about how much better a variable powered magnetron is VS an old school on/off low power operation one is at heating evenly and effectively warming food. So this is yet another nod for going that direction like Snoob said.

 

[Ceberha]

Yeah... but... Would that be a bad thing? I dunno. I think what @Hedges was pointing out is valid: LFP is really good at delivering current, and wouldn't mind propping up the LA. Sigh... It's too late, and two glasses of wine make so I can't think this through.

In some of the earlier thread more senior/experienced folks had raised concerns about large currents such as this and the need to match voltages before putting the LFP and lead acid packs in parallel. At a minimum you would need to make sure that your BMS and wiring could handle it...