Replacing 12V Battery with LiFePO4 (Video)

[knightmb]

Price aside, this BMS requirement is the reason I have not swapped. And even if the 12v has its own reliable BMS, I'm left with the problem of cold spells in the winter when the 12v cannot charge.

@stanton, how did your LFP handle the cold week in Texas ?

That is a good point, if you live in an area that will ever get far into the negatives for temperature then the lead acid/AGM is probably the only battery that can live below those temperatures without damage. The one I swapped into my Leaf has a limit of -31F (-35C), so luckily it never gets to those extremes where I live, so I've had good luck with my lithium 12v swap.

[SageBrush]

That is a good point, if you live in an area that will ever get far into the negatives for temperature then the lead acid/AGM is probably the only battery that can live below those temperatures without damage. The one I swapped into my Leaf has a limit of -31F (-35C), so luckily it never gets to those extremes where I live, so I've had good luck with my lithium 12v swap.

-31F is the operating limit; 25F (-4C) is the lowest temp the LFP will charge.

I imagine the problem is made worse by the otherwise most excellent flat voltage curve during discharge. When the LFP drops into the 'charge now' voltage range the SoC is really low. If that happens early in the evening and another 12 - 16 hours have to pass until the 12v case heats up from ambient, it is a window for battery damage.

No doubt about it -- LFP has a lot to offer. But I'm awful leery about an Ad-hoc replacement in a 4 season climate that the EV presumes is PbA.

[bobkart]

This is the battery I went with:
https://www.ohmmu.com/product-page/12v- ... la-model-3

The link spec is 8.5 Lbs.
I was told by Ohmmu that they reduced the Ahr by half to ~ 20 Ahr so that their product would not cause alerts in the Tesla Model 3

I had a long discussion with them a few weeks ago that ended with them promising to perform a discharge test at 0.1C and to share the results with me. They did not keep their promise so I gave up on the idea of buying a battery from them for my Tesla

Looks like the specs drifted on the Model 3 version after they split off the Nissan Leaf version. The latter still has the 11-pound spec I saw. Odd that the Model 3 would have a problem with too much accessory battery capacity.

Sorry to hear that they dropped the ball on that discharge test. Actually I would have thought they would already have that data, but maybe not since lightening the battery. I still think they're good batteries (but on the expensive side), and the customer service I've experienced has been satisfactory. As far as I know, they work below freezing, but am not seeing a specific number in their specs. I know mine has been down to below 20F ambient a time or two, with no apparent ill effects. My impression is that they dope the chemistry somewhat to gain the lower-temperature-handling capabilities.

Regarding chemistries, LTO looks to have great service life and cycle count, and tolerates temperature extremes well (-40). Also has very high charge/discharge rates (~10C continuous). Somewhat less energy density than LiFePO4 though, with comparable safety regarding thermal runaway. At 2.3V nominal it takes five to make a 12V battery. With the right BMS I can see this being an even better automotive battery replacement option than LiFePO4. High cranking power (for ICE applications) should be easier to achieve than with LiFePO4.

[SageBrush]

As far as I know, they work below freezing, but am not seeing a specific number in their specs. I know mine has been down to below 20F ambient a time or two, with no apparent ill effects.

See my post above
viewtopic.php?f=37&t=29989&p=599863#p599853

[bobkart]

-31F is the operating limit; 25F (-4C) is the lowest temp the LFP will charge.

I see . . . are those numbers for 'generic' LFP, or specific to the Ohmmu batteries? Because my understanding is that it's possible to tweak the chemistry to improve various generic chemistry characteristics (often at the expense of other characteristics). Discharge rate is one of those. Low-temperature charging may be another.

[SageBrush]

-31F is the operating limit; 25F (-4C) is the lowest temp the LFP will charge.

I see . . . are those numbers for 'generic' LFP, or specific to the Ohmmu batteries? Because my understanding is that it's possible to tweak the chemistry to improve various generic chemistry characteristics (often at the expense of other characteristics). Discharge rate is one of those. Low-temperature charging may be another.

You can bet that Ohmmu buys generic cells and packages them.
Every time I read about LFP I come across the 25F charging restriction. While I doubt it is written in stone, for now it is pervasive.

I'm looking forward to learning how Tesla deals with the limit, now that they are putting LFP in the Model S and Model X. My 2¢ wager is that they added a way to heat the 12v case. The other possibility is that Tesla charges the 12v when the ambient is getting cold as a preventative step. That would work in my 'hood, but not in SK or Scandinavia.
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LIke you, I have no idea why a smaller Ahr capacity battery solved Ohmmu's Tesla Model 3 problems. I asked, but was given a BS answer.
FWIW though, Tesla monitors the 12v battery in the Model 3 and 'knows' when it is EOL. It does not seem to me to be a far stretch guess that the LFP was confusing the Tesla diagnostics. It is beyond me though to say how or why.

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I'm looking forward to seeing the 25F charge restriction solved, and for 12v LFP to drop in price with some competition and scale. For now the price approaches $1000 per kWh while cells are in the $100 per kWh range.

[LeftieBiker]

The other possibility is that Tesla charges the 12v when the ambient is getting cold as a preventative step.

This is what I would bet. Heating the battery tray would be the next step for frigid climates.

[SageBrush]

I just found this vendor
https://relionbattery.com/products/lithium/rb50-lt

They include a heating element in the case

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I'm not going to spend that amount of money for our LEAF, but if Tesla does not come through with an LFP for the Model 3, I'll be inclined to buy one for our Tesla. As an aside, some Googling suggests that LFP has low internal resistance. Does anyone know the LFP SoC when the car BMS throttles down, and subsequently disconnects the DC/DC converter ? This is one of the tricky parts because the car presumes PbA

[knightmb]

I just found this vendor
https://relionbattery.com/products/lithium/rb50-lt

They include a heating element in the case

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I'm not going to spend that amount of money for our LEAF, but if Tesla does not come through with an LFP for the Model 3, I'll be inclined to buy one for our Tesla. As an aside, some Googling suggests that LFP has low internal resistance. Does anyone know the LFP SoC when the car BMS throttles down, and subsequently disconnects the DC/DC converter ? This is one of the tricky parts because the car presumes PbA

Interesting, so you can buy some off-the-shelf LFP with a built-in heater. Yeah, the capacity is overkill though, would probably need that to start a large engine, but if they made a smaller 20ah version for half or less the price that gets it closer to what I paid a decade ago for mine (without heater), though now the same battery/capacity I got is much cheaper now than what I paid. Isn't the original lead-acid that is used in the Leaf only rated at 30ah though? I'll have to check mine to see, but the actual power you get from the lead-acid is quite lower than the rating unfortunately, kinda like that for any lead-acid battery.

[SageBrush]

I just found this vendor
https://relionbattery.com/products/lithium/rb50-lt

They include a heating element in the case

--
I'm not going to spend that amount of money for our LEAF, but if Tesla does not come through with an LFP for the Model 3, I'll be inclined to buy one for our Tesla. As an aside, some Googling suggests that LFP has low internal resistance. Does anyone know the LFP SoC when the car BMS throttles down, and subsequently disconnects the DC/DC converter ? This is one of the tricky parts because the car presumes PbA

Interesting, so you can buy some off-the-shelf LFP with a built-in heater. Yeah, the capacity is overkill though, would probably need that to start a large engine, but if they made a smaller 20ah version for half or less the price that gets it closer to what I paid a decade ago for mine (without heater), though now the same battery/capacity I got is much cheaper now than what I paid. Isn't the original lead-acid that is used in the Leaf only rated at 30ah though? I'll have to check mine to see, but the actual power you get from the lead-acid is quite lower than the rating unfortunately, kinda like that for any lead-acid battery.

They sell smaller capacity batteries, for less money. Whether that is cheap enough for you is a ymmv.

I want a large capacity 12v for my Tesla because I think it would reduce "vampire" drain. That said, I may be barking up the wrong tree and instead should perhaps be looking for a battery with very low internal resistance. The best (I think) would be a battery that is *both* high capacity and low internal resistance, in a car that knows it is dealing with an LFP. Not there yet for the Tesla.

I agree that a 20 Ahr LFP battery that can handle the cold would probably work great in the LEAF and outlive the car. My LEAF is a 2013. It currently has a shitty walmart battery that is 3 years old that I am testing this week. I expect to either replace it soon or before the next winter. I figure that a better AGM PbA will last ~ 4 years, which is about how long I expect to keep our LEAF so the extra cost of an LFP has unclear benefit in my case. I've about decided on this replacement:
https://www.remybattery.com/batteries/b ... -html.html

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As an aside, LFP is cheaper than PbA on a kWh basis as a commodity. I don't know of any reason why LFP will not eventually be the cheapest and best option.