Philzter888
New Member
I did research on this with regards to lithium cells but to some extent it applies to lifepo4.I also did some further research to determine the discharge efficiency of my planned setup with a LiFePo4 battery. As you know, you might get a large battery rated for so many amp hours, but that will depend on the type of battery, the load, and the efficiency. I found information that says this difference in efficiency is due to the Peukert Effect, which describes the inefficiencies in the charge and discharge of batteries. To quote this web site, Battery University.
Peukert Law
The Peukert Law expresses the efficiency factor of a battery on discharge. W. Peukert, a German scientist (1855–1932), was aware that the available capacity of a battery decreases with increasing discharge rate and he devised a formula to calculate the losses in numbers. The law is applied mostly to lead acid and help estimate the runtime under different discharge loads.
The Peukert Law takes into account the internal resistance and recovery rate of a battery. A value close to one (1) indicates a well-performing battery with good efficiency and minimal loss; a higher number reflects a less efficient battery. Peukert’s law is exponential; the readings for lead acid are between 1.3 and 1.5 and increase with age. Temperature also affects the readings. Figure 1 illustrates the available capacity as a function of amperes drawn with different Peukert ratings.
As example, a 120Ah lead acid battery being discharged at 15A should last 8 hours (120Ah divided by 15A). Inefficiency caused by the Peukert effect reduces the discharge time. To calculate the actual discharge duration, divide the time with the Peukert exponent that in our example is 1.3. Dividing the discharge time by 1.3 reduces the duration from 8h to 6.15h.
As for the Peukert Exponent that applies to LiFePo4 batteries, I found this web site which suggest it to be between 1.01 an 1.03.
Bottom line is that you'll get close to the rated capacity of the battery in discharge. About 97%.
Edit: So I did some battery life calculations using an online battery calculator.
Camera power use during parking mode. Using the front and rear camera. Measured on the 5v input: 2.23W.
2.23W into amps at 12V = 0.185 amps.
12V to 5V conversion overhead: 10% to 15%.
12V power draw including overheads 0.185*1.15=0.21275 amps.
100 amp hour battery
1.03 Peukert exponent
Here's the results
View attachment 56211
Considering that the safe acceptable depth of discharge for a LiFePo4 battery is around 99%, we can get close to the full discharge time of 491.778 hours or 20.5 days. Is this wrong?
Its best to discharge to 15% of capacity and charge to 70-85% of capacity if your objective is longevity. Most cells are really rated to 80% of the capacity, and the remaining is a kind of overcharged state with higher heat. Your cycle life may increase up to 200% if you charge only up to these levels and it’s well ventilated. From memory cycle life may go from 300-500 all the way to 800+.
The good thing about running a dash cam is that the discharge is very gentle and so starting the constant recharging is not at all hard on the battery. I am running only a small 38A pack (144whr) with around 8-10 hrs driving with a lot of short trips and the battery has never gotten flat in 9 months with 3-4w constant load. Your battery has 8x more capacity since it’s rated at 12v and mine is rated at 3.7v.
One benefit with your setup is the dash cam is always powered even after an accident. I had a rear end recently and I had to keep the car running to make sure to capture the actions of the other drivers. Some cars just shutdown completely if it’s a big crash just in case oil and coolant is leaking or the car is turned over and ignition is not left on. Night use for parked recording is going to be of limited use - it’s pretty much impossible to get plates at night so you may not need as much recording as u think.
With such an old car and a huge battery with small load, there is no need for really any aggressive charging. So make sure the controller has some adjustment in it. Your car wiring is old and fragile so as Indian said this is the only way for you to go with such a large battery. Guys have melted wiring just from installing aftermarket LED bulbs, so they don’t over engineer the wires...
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