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Looking for advice on LiFePO4 car battery replacement for SLA Answered

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 a balancing and protection module enable the cells to work safely in the car? There are 100A and 200A 4s modules available, 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)

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PaulI48 (author)2018-05-02

Ah, good point, thanks. Yes, I'd missed that the battery capacity wouldn't add in series...

The drain on the battery is about 60mA I think. Checked it a while back as my old battery was running flat after a couple of weeks. So, yes, replacing a SLA battery with a smaller capacity bodged DIY job is probably not the smartest thing I've ever done. But I'm the only driver, so I can put up with a bit of risk here!

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Batterywizard (author)2018-05-02

If you wire batterys in series, the voltage adds up, but NOT the capacity, you only get 13,2 Volt and 5 Ah. While this is still enough to start the engine there is very little reserve for anything else as listening to the radio or interior (trunk-) lights without the engine running.

If you are aware of this and if you are the only driver everything might be ok. Someone unaware (careless) might get stuck with a empty battery...

Even small amounts of consumption add up considerably if you do not drive every day. If you only need 0.025A (25mA) for Car-Alarm, Radio-Presets, Clock, ... this will eat up half of your capacity in four days. I suggest to measure the "standby" consumption of the car to see if it is a factor.

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PaulI48 (author)2018-04-03

I was thinking maybe include a resistor between the battery and the caps... been looking at a similar project that someone else pursued, and they used that arrangement. 0.1 Ohms, 25W I think, though I wouldn't assume that I could use the same values without more research.

If I use a balance and protection board, I guess that would also protect against drawing too much current, but something more bulletproof seems smart.

As I'm looking into this more, I'm coming back round to thinking that using the super capacitors is probably the way to go, despite the added complexity - they'll provide the starting current more easily, and deal with the ups and downs of the car electrical system to which it's attached better than the LiFePO4 cells

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PaulI48 (author)2018-04-02

I'd have a separate balancer board for the super capacitors... but are you saying that adding super capacitors (basically in parallel with the LiFePO4 pack) would mess with the LeFePO4 balancing circuit?

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randofo (author)PaulI482018-04-03

The supercapacitors might be a large draw on the battery when they recharge (depending on the charging circuit). The LiFePo4 battery board might not like this. If the battery / charge controller board could source the current, I wouldn't mess around with super caps.

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Yonatan24 (author)2018-04-02

Where do you live/how hot is it where you live in the summer, and how hot does a regular LA battery become after a long drive, being heated from the motor?

I think there was also a Kickstarter called Ohm for a LiPo car battery a few years ago, might be worth reading about that.

Also, how about supercapacitors?

And I just realized you wrote LiFEPo4

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PaulI48 (author)Yonatan242018-04-02

London, UK, so summer heat not too much of an issue. And the battery location, behind one of rear wheel arches, means that engine bay heat gets nowhere near the battery.

The Ohm battery is absolutely the kind of thing I'm thinking of!

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randofo (author)2018-04-02

You need to have them balanced or bad things will happen to the batteries, and it could potentially be dangerous.

If you get a load balancer / charge management board with pass through charging, it might be okay depending on the output from your alternator and the specs of the board.

My best guess is adding supercapacitors is probably not a good idea either because of the balancing circuitry.

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