LiFePo Starter Battery In Mitsubishi Mirage 1.2
 

The original battery would appear to be on the way out in my 2014 1.2 3cyl Mirage.

After a 151 mile trip yesterday, the battery is measuring 12.4 volts today - approx 15 hrs later - and a hydrometer test indicates a specific gravity of 1.2 for all 6 cells.

The battery seems quite big for a 1.2 SI engine - 66AH and CCA 520 - but I assume this is because this vehicle has stop start.

During my trip, the battery was being charged approx 95% of the time.

I would assume that this is hurting my mpg though the average was a respectable 60.37 US # 72.5 IMP # 3.9 lit/100km.

Any advice on selecting a battery?

This is one candidate https://www.ev-power.eu/LiFeYPO4-bat...0AH.html?cur=1

http://ecomodder.com/forum/attachmen...7&d=1521643862

Just keep in mind LiFePO4 doesn't tolerate charging below freezing, so if you plan to use it in freezing temps it would be better to locate the battery inside the vehicle. This would probably require using a supercapacitor bank in the engine bay to provide the CCA the starter motor needs. You could downsize your battery if you go with a LiFePO4 and super capacitor bank. What is the electrical draw while the engine is off?

You'll probably never tell the difference in MPG though.

What size of LifePo and Capacitors would you suggest?

The weight reduction would be nice as this car is already quite light at 845 kg.

I would suggest none. Unless you plan to relocate the battery to the interior and bring it in at night.
LiFePO4 is bad for cool climates and under hood temp will kill them in short order.

As oil pan says, this is likely more hassle than it's worth considering the temperature sensitivity.

If I were to tackle this project I'd start by measuring typical electrical draw while the engine is off. This would determine what size battery to run, but I wouldn't go less than 20 Ah. I always recommend this 20 Ah pack:

LiFePO4 Prismatic Battery: 12.8V 20Ah (256Wh, 10C rate) (24.0) - UN38.3 Passed (DGR)

As far as capacitors go, you can probably start the vehicle using 6 series connected Maxwell 350 F caps.

https://www.ebay.com/i/302040184049?chn=ps&dispItem=1

You would connect the cabin located LiFePO4 in parallel to the caps that live in the engine bay, possibly with a very low ohm current limiting resistor. The resistor would force the caps to do the majority of the heavy lifting to prevent the wires and LiFePO4 from overcurrent.

Basic concept of the idea is in my video here:

https://www.youtube.com/watch?v=LX_o...ature=youtu.be

Remember I have been using LiFePO4 starting batteries since 2010, I live in new Mexico which has pretty mild winters and I know all about their cold sensitivity.
If someone in a cold climate switches over to them, learns everything the hard way they will be very disappointed and switch back to lead acid with in a year.

The battery has to be located away from engine heat, which means most people will have to relocate it and needs to be brought inside at night when the temperature drops down near freezing.

I don't think they are more hassle than they are worth but I was prepared to relocate the battery and bring it in on frosty nights.
The batteries have lasted far longer than any lead acid, but I take real good care of them.

Redpoint, Oilpan,

Any reason to be concerned that the OEM alternator was not designed for lithium? I understand such smart alternators will deliver voltage that is incorrect. I imagine if you kept the alt off most of the time and charger from the grid through a proper charger, using alt only as a range extender, one could get better life/service out of a lithium.

I'm on year 3 of running one in the Prius. The battery was already located in the cabin, so it was simple to replace. I don't bring it in when it's freezing and take no special precautions to protect it. Since the Prius doesn't use the battery to start the engine, the loads are relatively low. No supercap required.

The prius is ideal then.

I believe the smart alternators charge the LiFePO4 batteries to a more desirable level, to around 90%. Where dumb alternators can try to change them to 100% every time.

Max voltage for LiFePO4 is rated at 14.8v, so a dumb alternator charging to 14.3 - 14.4v should be fine.

I run the same battery in my jetski (3 summers so far) with no special considerations.