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I've had an idea of using LiFePO4 battery as replacement for car battery, since 4 of those in series make nominal 12.8V which is very close to standard 12V voltage. And depending on the cell we can parallel some to get required starting current. Since LiFePO4 have much longer life then Lead-Acid, such battery should last much longer.

But there comes few problems:

  1. Charging voltage. Since most alternators produce around 14V volts when charging it should be fine with LiFePO4 battery since maximum voltage for those is 3.6V which gives us little headroom up to 14.4V (4*3.6V=14.4V). Individual cell balancing can be done with simple dissipative BMS which dumps excessive charge to resistor. But we don't have possibility to disconnect alternator when battery is charged, so here is first Question: is it OK to hold LiFePO4 battery at floating charge with voltage which is close to maximum battery voltage? Or this will significantly decrease battery life?

  2. Charging current. Some LiFePO4 cells can be configured so they can easily take all charge current produced by alternator. (assuming 70A output and 3 parallel 40152S cells) So second question: Can it overload alternator or modern alternators are smart enough to lower voltage to avoid being overloaded?

  3. No over discharge protection. Currently have no idea how to deal with it besides not letting it to discharge completely.

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  • \$\begingroup\$ Some details here: barnson.org/node/1154, most importantly, I doubt a LiPo would hold up against the temperatures while leaving your car parked in direct sunlight. Also (from the comments): "don't fully-charge LiPos in very cold weather. If the ambient temperature is below 40-50 degrees and you charge to 4.2v/cell, you have over-charged the LiPo and its lifespan can be numbered in tens of cycles rather than hundreds." \$\endgroup\$ – RJR Nov 5 '14 at 1:58
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    \$\begingroup\$ @RJR LiPo <> LiFePO4. LiFePO4 are tolerant of 'higher' temperatures. \$\endgroup\$ – Russell McMahon Nov 5 '14 at 4:56
  • \$\begingroup\$ @RussellMcMahon ah, sorry. \$\endgroup\$ – RJR Nov 5 '14 at 5:12
  • \$\begingroup\$ @RJR Easily done. I have a good LiFePO4 focus at present as I am experimenting with them. About to have a remotely monitored deepish discharge application running shortly (or else) \$\endgroup\$ – Russell McMahon Nov 5 '14 at 11:51
  • \$\begingroup\$ If going down the route of making your own car battery, I would put some ultracapacitors in parallel for starting the engine, and just use the cells for operating the radio and other things while the car is off. \$\endgroup\$ – tokamak Mar 11 '16 at 2:56
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LiFePO4 vs LiIon vs LiPo

Some people have commented on LiIon batteries, but the question and this answer are about Lithium Ferro Phosphate batteries / LiFePO4 which I'll abbreviate in places as LFP4. .
These are related to LiIon and LiPo batteries but have major differences.
Notable, compared to LiIon and LiPo (which are chemically similar) LFP4 has an added internal inactive matrix which the Lithium resides within. This reduces mass and volume energy densities but also makes them free from the destructive melt down modes, gives them a much longer cycle life and a much longer calendar life, lower capacity, lower maximum voltage, better temperature operating and storage range, better overall energy efficiency - and a whole of life cost below to far below that of any lead acid battery.


LFP4 (LiFePO4) 4 cell batteries do make excellent replacements for normal 12V lead acid car batteries BUT proper attention to care and feeding is necessary to achieve the very long cycle and calendar lifetimes that they are capable of. They are far more "whole of life cost effective" than any lead acid cells in (probably) any application when properly used. Wrongly used they will die an early and expensive death.

A few manufacturers of cells or batteries suited to this use are

Thundersky, Winston & SinoPoly

There are others which I can list but a search on those names will get you into the right area. All those brands started out as Thundersky but there have been partings of the way and expensive litigation is happening.

Winston make 12V 4 cell batteries with no access to the inter-battery connections. Their 40 Ah and 60 Ah sizes are claimed to be well suited to automotive use. Sinopoly an Winston both make single-cell batteries in sizes from around 10 Ah up to vastly-huge_Ah and you can strap these together as required. I am currently experimenting with 2 x SinoPoly 12v, 40Ah batteries each comprising 4 x 40 Ah single cells with strapping to produce a physical pack. A 12V, 40Ah 4 cell battery is about the size of a Ford-Prefect battery and less weight but is equal to about the largest LA battery you will see in a say 4 cylinder car.

HOWEVER

Claims from various battery makers overlap but are not identical, charging specs are suspect, claimed lifetimes vary depending on sales path and even reputable sellers disagree. The Winston battery charging specs use suspiciously high end point voltages - higher than I'd expect for LFP4 but below LiIon- almost like LiIon being run super conservatively to get good cycle life. Sellers warrant most brands of 10 Ah + cells for 5 years or 10 years use subject to various conditions and probably subject to use of charge/discharge management electronics. Criteria for adequate monitoring varies - many claim that Vmax and Vmin are enough as long as C rates of charge and discharge are within spec BUT a local supplier demands a gas-gauge type monitor and a low voltage cutoff. (I'd consider the gas-gauge excessive and the cutoff or similar protection essential)

Charging voltage. Since most alternators produce around 14V volts when charging it should be fine with LiFePO4 battery since maximum voltage for those is 3.6V which gives us little headroom up to 14.4V (4*3.6V=14.4V).

Some claim Winston LFP4 needs higher V than car systems provide. Others use them regardless.

SinoPoly OK on car voltages BUT ...

Individual cell balancing can be done with simple dissipative BMS which dumps excessive charge to resistor.

Winston cannot balance as sealed but claim superbly balanced cells are OK. Numerous dealers sell them and say this but at least one large site says not to use for deep discharge due to this inability. I'd be very wary. I made a Winston/SinoPoly initial choice for experience gaining based on this and strange Winston voltages and bought SinoPoly - despite apparently superior Winston specs.

But we don't have possibility to disconnect alternator when battery is charged,

Don't start. You must be able to manage your energy source - AND it is easy enough to do - but if your system insists on violating battery specs (which may not be the case) do not use LFP4.

so here is first Question: is it OK to hold LiFePO4 battery at floating charge with voltage which is close to maximum battery voltage? Or this will significantly decrease battery life?

Opinions vary
Small LFP4 - say < 1 Ah to few Ah MUST NOT be floated or they DO die early.
Some LFP4 large battery makers claim floating is OK.
Thou shalt not float LiIon or LiPo or else ... and while LFP4 has differences I'm uneasy.

Charging current. Some LiFePO4 cells can be configured so they can easily take all charge current produced by alternator. (assuming 70A output and 3 parallel 40152S cells) So second question: Can it overload alternator or modern alternators are smart enough to lower voltage to avoid being overloaded?

This deep-ends entirely on battery and alternator specs. Batteries have clear specs. Exceed them not. If your battery has NOT got good specs available do not buy it.

No over discharge protection. Currently have no idea how to deal with it besides not letting it to discharge completely.

This is essential. There are various ways of doing this but a LFP4 large Ah cell MUST NOT be taken under around 2.75V. If you cannot be SURE of that, do not use them. They cost far too much and are too light per volume to make good boat anchors.

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  • \$\begingroup\$ From what I remember a modern car alternator is self regulated for overload, that is if you try and draw too much current it just drops the voltage, if cooling is working them it will just make a bit more hot air while over loaded but not suffer any damage usually. The waste heat will reduced as the voltage is clamped more. \$\endgroup\$ – KalleMP Nov 5 '14 at 21:58
  • \$\begingroup\$ @KalleMP Yes - an alternator regulator limits maximum voltage - and would ideally do so intelligently allowing an over voltage boost charge then dropping back to a float voltage. The alternator itself is current limited by core saturation and can be designed to survive short-circuit indefinitely. There are several potential (pun noticed :-) ) problems. Lead acid is floated at 13.7V in standby mode and this voltage can be applied indefinitely. This may not be enough for what Winston say, MAY be more than what SinoPoly say, may not be what I decide is right when I become more expert and .... \$\endgroup\$ – Russell McMahon Nov 6 '14 at 4:20
  • \$\begingroup\$ ...the change over point from CC chg to float may be such that the battery is given substantial charge when it would best not have it || Possibly worse, and advice varies widely, LiFePO4 MAY be able to be safely floated but MAY be destroyed thereby. Small LiFePO4 in the AA (14500 cell = 14mm dia x 50mm long) to 18650 size (18mm x 65mm as in A123 and many LiIon laptop battery) cell range MUST NOT be floated when charged. Lifetime is drastically shortened if this is done. It is not ovious why it is OK to flat large batteries (say 10Ah up cells) but not small cylindricals. .... \$\endgroup\$ – Russell McMahon Nov 6 '14 at 4:28
  • \$\begingroup\$ .... Or IF one or other of these practices is wrong or unnecessary | Plus many Chinese sources are now selling claimed LiFeYPO4 where the added Y is Yttrium. What difference this makes if any is unknown. How Winston can use higher voltages in systems with notionally same chemistry is unknown. Why Winston deepish discharge home solar warranties changed from "5000 cycles) to "10 years" recently is unknown. Why Winston think they can safely seal a battery so it cannot be balanced is unknown. Why anyone would buy sealed Winston batteries is unknown :-). \$\endgroup\$ – Russell McMahon Nov 6 '14 at 4:32
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    \$\begingroup\$ @neverMind9 Carmakers do not care what it costs YOU over whole of life of car - they care about cost to them out the factory door. Consider the evil execrable "space saver" tyres. They save the car maker money and allow them to make a car SLIGHTLY smaller while achieving the same internal volume. Also SLIGHTLY lighter so SLIGHTLY better performance and SLIGHTLY cheaper. For this they sacrifice YOUR safety, YOUR long term convenience and utility and ... . LiFePO4 cost more for the same initial performance and lifetime is irrelevant to them so they are a non-starter (pun noticed). \$\endgroup\$ – Russell McMahon Nov 4 '18 at 10:49
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For what it's worth, I have floated a 4 cell, 10 Ah LiFePO4 battery pack that I built myself from flat 3.2V cells, for well over two years, with no problems. I've used it as backup power for a Mini-ITX computer with a pico-PSU power supply. Input voltage is 14.5V from a 75W DC power supply, and the battery provides additional oomph during the brief times the computer draws more than that. The battery is still doing just fine.

From what I understand: It's not great to fully charge the battery, and then keep the voltage on it. This is because the thermal dissipation of the internal charge resistance will still wear away at the necessary oxide layer in the battery. LiFePO4 is less sensitive to this than LiPo, and floating at lower voltage levels causes less of this problem than floating at max level.

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Do not use LiFePo4 as a car battery, at or around freezing you will start doing permanent damage as the alternator tries to charge it. The solution is Lithium Titanate. you can purchase them from AliExpress (Alibaba is for businesses, they will sell it to you but use AliExpress first). Titanate can handle the extreme temperatures and has an insane charging/ discharging rate. the only problem is they are large cylindrical cells, you will need to relocate them through the firewall and maybe install it under a seat or build a box so someone doesn't kick it. for a 12v system usually 6 cells minimum are required, but since it's for a car you MIGHT need 1 extra cell because the alternator pumps out 14.4 volts I think. You will want to add balance wires to connect your battery bank to the BMS and you will want insanely large cables, I would try 4/0 first WITH a fuse and if it's not enough you will need THHN cable, it starts at 250 MCM and goes up really high. THNN is for buildings. for the proper fuse you will probably need a Class T fuse rated for the wire size. Consult a professional, you are playing with a battery chemistry more powerful than lifepo4 and larger capacity than most lithium ion except for vehicles. Almost forgot Titanate has an extreme cycle life, like 20,000, it will outlast your car. for comparison lifepo4 has an insane life cycle compared to Pb lead at 2,000 to 4,000 cycles. that translates to about 10+ years so do the math for titanate you can see what I mean

-an addendum: for lifepo4 you do not want to keep the cells at 100% SOC state of charge for a long time. it is harmful long term, and while it can survive going down to 0% SOC it is also not advised. it is usually set in the BMS battery management system as 2.5v for the cut-off voltage and if you limit the max capacity to around 90% or less your cells will last much longer. so the lower limit is about 10% SOC or more. again that's for lifepo4, TITANATE probably has another set of rules to follow, firstly follow the manufacturer specifications, it will be listed on the aliexpresss product page. also do a cross reference match for specs of similar cells. I have seen one manufacturer list lifepo4 as being able to tolerate temperatures well below freezing, that is an outlier and wrong, lithium is in it's infancy, even many manufacturers do not know what they are doing and their recommendations change like the weather. use common sense, cross reference specifications and read what other people on forums do, many are knowledgeable! also I suppose this is a "fun fact" (I hate that meme) lithium titanate is used in car sound competitions because it can handle the abuse, they also fill the vehicle walls with concrete for the sub-woofers

2nd addendum: forgot to mention you will burn out your alternator unless you introduce a charge controller of some sort. what you must do is find a device to limit the amps coming from the alternator because lithium batteries will attempt to pull as much current as the alternator can supply until it burns out unless the battery is already nearly full which than the BMS will cut off the charge according to your upper voltage limit that you chose as per the SOC chart for titanate. remember titanate has completely different voltages so do not attempt to use the same lifepo4 voltages examples I gave

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Lithium motorbike batteries are readily available at the place down the street. They are in a standard battery case, same terminals, and have all the necessary circuitry inside the box. They are also so light they feel like an empty store display unit. The price is about 50% more than a lead-acid battery.

Given that lithium batteries failure mode often involves a vigorous fire, do you really want to put one of your own design inside an expensive machine right next to a supply of gasoline and several litres of oil?

High-energy battery engineering is best left to the engineers. They have the experience and budget for the necessary experiments, and they don't care if the test unit goes boom - it's almost expected to do so a couple of times.

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    \$\begingroup\$ LiIon <> LiFePO4. You can drive a steel take through the heart of a LiFePO4 cell and it will give you an accusing look and may keep on working. LiFePO4 make excellent car batteries but DO still need proper care and feeding. \$\endgroup\$ – Russell McMahon Nov 5 '14 at 4:59
  • \$\begingroup\$ For those who read this: You can't drive a steel stake through a LiFePO4 battery and expect it to work. It may even cause smoke and heat when you do that, and probably stop working. It is, however, much more robust than LiPo and LiIon batteries. \$\endgroup\$ – Jon Watte Apr 8 '17 at 18:18

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