Fuel economy in a car: Charging battery the Arduino way: 2nd and last part

26650 LifePo4 A123 batteries (2.3 Ah)

After analyzing the advantages and disadvantages of changing the car battery for a homemade LiFePo4 one as we discussed, I've finally discharged it for the following reasons :

1. High cost of the battery; unless we build one of only 20 Ah capacity, high-capacity batteries discharge and safer LiFePo4 double in price with respect to cobalt ones; a 12V with only 20Ah home costs more than 200 €.
2. Low reliability: Having to add a charge control (BMS) to avoid possible rapid deterioration, we are adding an electronic element that can fail at any time (it is assumed a life span of over 50,000 hours, but being made in P.R.C... I have many doubts). The only possibility of failing one day eliminates all the advantages. You have also to add relays for load control as another element susceptible to failure.
3. Battery degradation: At least we place a circuit and relay to disable charging 13.6 V (A123 recommends charging 3.4V), our car will always stop carrying our li-ion battery by supplying 14.5V (charging voltage of lead acid), situation in which its service life can be halved (from 10 to 5 years).
4. The alternator stops consuming once the battery is full: not worth cutting the current with a relay to disable the alternator, because the use disappears once the battery reaches full charge at 14.5V. It is much more advisable to reduce electricity consumption of the car, like changing all the LED lamps, instead of reinvent the wheel; we will save in maintenance and fuel (generating electricity with oil is very expensive).

For all these reasons, the possible lifetime and lightness advantages of placing a LiFePo4 battery are fully dissipated, being recommended only to do this in smaller engines; if we need a 12V battery for a motorbike, paramotor or the like we can use LIFEPO4 batteries, with the advantage that lasts longer and is not discharged whether it is not used as the lead-acid ones. Probably if we build one for a motorbike next time; I will log it here.

Then, I will use the acquired LiFePo4 battery pack for a new electric bike , since in the area are steep slopes and cobalt ones suffer very much; after a year of use and about 20 recharges I have already noticed a reduction in capacity of 30%. 
For the car homemade one I thought in using a BMS (loading control) available in Aliexpress 6 N-channel power transistors AOD4132  for each direction (upload / download), each with a maximum capacity of 85A 30V, so it could withstand peaks 500A, perfect for point 300A demand in the car boot. Now I will use it for another more useful thing I hope.

To accommodate lithium batteries I prepared an emptied car battery, because it preserves size and same connection (with the content of lead and sulfuric acid properly managed by authorized manager, and precautions). Sometimes it is better to rectify than being sorry for a failed experiment...

Basic care of a LiFePo4 battery