My first question is related to charging and discharging: I don't have much knowledge of LiFePo batteries, but I've heard a lot of horror stories about LiPo batteries, and have even seen first hand many fires caused by them (mostly remote controlled car batteries or motorcycle batteries that people are charging unobserved in their garages, which ultimately overcharge and start on fire). But a quick Google search seems to indicate that these are not the same technology and not nearly as prone to fires, but they still suffer from some of the same disadvantages as other lithium batteries. My question is: is there some kind of protection system in place to prevent the cells from over-charging or discharging too much? From what I am reading, they seem to be pretty sensitive to discharging below 2.5v, or charging above 3.5v, which seems like a very small range to work within. I would imagine the DC to DC charger handles the over-charging piece, but what about discharging too low? Does that damage the cells and make then unusable?
Joe, I'm surprised you have not done more research, this is not like you
LifePo4 batteries are the safest of the lithium batteries but also the least energy dense(still far better than lead acid), the safety factor along with the lower voltage than most other lithium technologies make them perfect for automotive 12v systems in a 4s(4 cells in series) configuration, this leads to a usable voltage between 14.4-14.6 to 10v. Normal lead acid batteries(Flooded, AGM, Gel, Sealed) actually have a smaller usable voltage range, normally between 12.6-12.8v when full to 10.5v when completely discharged, and they are 6 cells in series. They are safer in the fact that they boil off water when overcharged but still have the risk of exploding if the hydrogen gas generated cannot vent. normal cutoff for LifePo4 and lead acid is around 12v, this is 3v per cell (80% DoD)for lifePo4 and 2v per cell (50% DoD) lead acid.
As for charging/discharging, yes lithium does not like to go outside it's designed voltage limits, but really no battery technology does, over/under discharging any battery is bad. A BMS takes care of Over/Under voltage Over/Under discharge Amp and balances the cells Lithium has the advantage that the voltage curve is very flat so current stays lower, you can also regularly discharge them down to 80% vs lead acid not liking less than 50% DoD. This drastically improves the usable Ah of lithium over a lead acid battery. Your Yellow top (37ah) has a usable capacity of ~19ah, while a lithium of the same size would have ~30ah of usable capacity, not to mention be much lighter and more compact
Lithium cells can be reconditioned just like lead acid if the voltage is allowed to go too low, but just like lead acid, they will not have the same capacity after such an event.
Which leads me to the second question: how is the power distributed from this aux setup to your accessories? I presume, since you are running the DC to DC charger, that the batteries themselves are completely isolated from the vehicle's main 12V circuit? If so, does that mean you can't ground any of the accessories connected to this circuit to the vehicle's body? Sorry if that sounds like a dumb question, but I've never really understood how a completely isolated aux power circuit works within a vehicle setup, as it's different than simply adding a separate battery and using a solenoid to make sure they run in parallel while the vehicle is on (and therefore both charge from the alternator), but are isolated when the car is off. I have a lot of lights and other accessories set up to run while the vehicle is either on or off, and I'm trying to determine if that means I'd have to re-run a lot of my wiring to this new setup if I go with something like this.
Isolated 12v systems only need to be separate on the positive side, ground is ground and can be shared. You probably already have an isolated system and don't even realize it. DC-DC charging is no different than if you had a 2 batteries with a solenoid connecting them in parallel which is honestly never a good idea if those batteries are not exactly the same age, chemistry and location. I'm glad DC-DC chargers are becoming more popular on this side of the pacific. A DC-DC charger is basically the same thing as the solenoid, it has the advantage that it can properly charge the aux battery with appropriate voltage and maintenance charge. If you have lights that are connected to your aux battery through an aux fuse panel, then you already have 2 isolated systems. The solenoid just joins those 2 isolated systems when activated to charge the lower voltage battery somewhat.
Final question: how durable are the LiFePo batteries? Can they handle the abuse of off-roading (especially washboard roads) like the AGM stuff can? If so, these seem like an amazing solution for many automotive applications! I've gotten very lucky and gotten great service out of my little 37 amp hour Optima yellow top that I use as my second battery, but it's definitely not going to last forever, and I would LOVE to upgrade to something with this kind of capacity.
The biggest downside to lithium is the price, but that is becoming less and less of an issue. The size, weight and cycle life are all big advantages over lead acid, not to mention depending on what type of lithium cell you use, you can make them fit different spaces. LifePo4 batteries come in prismatic cells like above or they come in cylinder form like the popular 18650 or 21700, they also have larger 40150 cells that are 1.5" in diameter and over 6" long!
I see this topic getting some discussion so I may split it into another thread.
