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Advice on LiFePO4-based SLA battery replacement

Discussion in 'Power Electronics' started by Zogster, Apr 2, 2018.

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  1. Zogster

    Zogster

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    Apr 2, 2018
    I'm thinking of replacing my 12V lead acid car battery with a DIY lightweight battery based on LiFePO4 cells, but before I dive in I'd like to get some advice...

    My plan is to use 4 sets of 2 x ANR26650M1B cells in parallel, to give 13.2V output, and high output current, and about 20Ah capacity.

    (These cells seem to be used in commercial lightweight car batteries, so seem like a good choice.)

    Now, question 1: Can I just use these 8 cells as a drop-in replacement? With, presumably, occasional balancing out of the car.

    I think the answer is no: while my initial research suggests that some people are using off-the-shelf LiFePO4 cells in this way, a whole lot more people write about the protection and balance electronics they're using, and I don't suppose they're all wasting their time.

    Question 2: If Q1 = "No", would adding an off-the-shelf balancing and protection module enable the cells to work safely in the car? There are 100A and 200A 4s modules available (eg. https://www.aliexpress.com/item/4S-...r-4-Packs-18650-Battery-Cell/32819634220.html), so they should be OK with starting current (c. 120A in my case). I guess it's a question of whether it's OK to have such a protected battery pack constantly hooked up to c. +14.4V from the alternator, or whether I need to limit the time that the battery is charged from the alternator.

    Question 3: is it smarter to use super capacitors plus lifepo4 cells, to enable higher starting current while keeping battery discharging current low? (I started out thinking of combining an RC LiFePO4 battery pack with 6x400F super capacitors, but realised that I could get enough current with the ANR26650M1B cells, and fewer components should mean less to go wrong... if I used the caps I'd include a low-value high power resistor to limit the current that the capacitor bank could draw from the battery, and probably also a 200A circuit breaker on the battery as a short-circuit safety)
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Biggest question would be 'why'?

    What's wrong with the simple lead-acid battery arrangement that's currently used in 10's millions of vehicles with no particular problems and no immediate prospect of being changed?

    You will need 'special' charging, balancing and monitoring circuits (additional cost items) and gain...... what? in return?
     
  3. Zogster

    Zogster

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    Apr 2, 2018
    It's a good question!

    To save weight, partly. Lightness is a good thing. (The vehicle is a Porsche 944 Turbo, from which I've removed some unnecessary weight, eg air conditioning, rear seats, spare wheel replaced with sealant can. I'm not inclined to remove carpets and interior trim, but the stock battery is 18kg, and so there's a potential 15kg or so to be saved there.)

    Also because it would be an interesting project.
     
  4. BobK

    BobK

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    Jan 5, 2010
    Given the difficult electrical and climatic environment seen by a car battery, plus the propensity of Li batteries to catch fire, I would opt for a commercial unit.

    Bob
     
  5. kellys_eye

    kellys_eye

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    Jun 25, 2010
    A cranking handle would weigh even less.....
     
  6. Zogster

    Zogster

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    Apr 2, 2018
    I'm just not prepared to spend several hundred pounds on a battery, which is the cost of a commercial LiFePo4 battery - if I can make one for a good deal less, however...

    As far as the fire risk goes, I believe that LiFePO4 cells are a good deal less inclined to burst into flames than other lithium types. I always have a fire extinguisher in the car in any case.
     
  7. Zogster

    Zogster

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    Apr 2, 2018
    It would... when I had a Citroen 2CV, I started that a couple of times with the starting handle. But I think the effort required to crank a 2.5l 4-cylinder engine would be somewhere north of "bothersome"
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Think you will find they are the worst type as far as self igniting, same as was used in the commercial aircraft a few years back if I recall correctly.

    Don't know where you buy your lead acid batteries from at" several hundred pounds".

    I use top quality units (380cca) that last on average for 5 -7years for around Aus$140.00.

    Your 120a cca is way low also.
     
  9. Zogster

    Zogster

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    Apr 2, 2018
    No, the commercial aircraft fires were due to Lithium-ion batteries (Boeing 787), which go into thermal runaway at a much lower temperature than LiFePO4 batteries.

    The 120A I referred to is the current drawn by the starter motor, not CCA. I'm not 100% clear on how CCA is measured, but it's a very different thing, and is a test designed *for* lead-acid batteries. Working out equivalents for LiFePO4 batteries isn't entirely straightforward, I believe.

    I buy lead-acid batteries for about £70 – the hundreds is the price for a suitable commercial lightweight battery, such as those made by LiteBlox, Antigravity etc
     
  10. Bluejets

    Bluejets

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    Oct 5, 2014
    I think you need to do some reading.
    First line below is an example.

    The lithium iron phosphate (LiFePO
    4) battery
    , also called LFP battery (with "LFP" standing for "lithium ferrophosphate"), is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO
    4
    as a cathode

    https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

    As far as CCA, that is what the they allow for starter draw. Cold Cranking Amps.
    It is a calculation of the current draw available before PD drops below a set level.
    In the instance of a typical older model 4 cylinder car, it could be as much as 300-400 amp.

    Lithium are calculated (rather vaguely) on the c rating e.g.25c
     
    Last edited: Apr 3, 2018
  11. Zogster

    Zogster

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    Apr 2, 2018
    Thanks, I have. The Boeing batteries were Lithium-Manganese, not LiFePO4.

    Re. the CCA, I'm simply pointing out that CCA is not, as I understand it, a good way to compare Lithium-ion/LeFePO4 etc and lead-acid batteries. My lead-acid battery is rated at over 700CCA, but only 120A is needed to crank the starter.
     
  12. Bluejets

    Bluejets

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    Oct 5, 2014
    Interesting that you ask for advice then present an argument in support of your opinion.
     
  13. Zogster

    Zogster

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    Apr 2, 2018
    Interesting that you try and pick holes rather than offer any useful advice or knowledge.

    This particular conversation really is going nowhere.
     
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