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Need new batteries? Going from 12V to 18V or 6V to 12V? Wiring Questions?
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By Quada
Good Day everyone :)

I'm new to electronics and it'll be my 2 week practicing it as my hobby though i've done a lot of DIYs and stuffs just by following tutorials but when my friend ask me regarding the maths of my projects, i can't retaliate at all since even i researched for 2 days, i haven't gone far to understanding the math specially on batteries.

I have made a DIY regarding USB charger and its block sequence is like this:

12v Sealed Lead-Acid Battery -> 5v Voltage Regulator Circuit -> 2x (Li-Ion TP4056 Circuit Module) -> 2x Smartphone Battery

I want to know how would i compute this entire project with regards how much Ah, Volts, power, battery consumption, etc. if there will be 1 smartphone that will be charged and if it will be 2 smartphone also.

Some specs of this entire project are as follows:

a) 12v SLA Battery 10Ah

b) 5v Voltage Regulator 1A output

(i don't know the efficiency of voltage regulator, i just used LM7805:

My 5v voltage regulator circuit is like this:
beL4b.png (8.87 KiB) Viewed 845 times
  • (Addition:
    100uF 25v cap before 0.33uF cap
    10uF 10v cap before 0.1uF cap
    some 1n4007 diodes from its ground and output
    feedback 1n4007 diode from its output to its input as protection they said)
c) Li-Ion TP4056 charger 1A output

d) Smartphone batteries 4.2v 3000mAh

It's ok for me if only formulas will be given with respect to discharge, power, etc for me alone to do the math, i just need some guidelines because to be honest, i don't know how would i explain if my projects are efficient or later will have a drawback, doesn't last long, not practical, etc. though i really wanted to know how would i compute the battery consumption/discharge on this project.

I hope you can help me because I'm overwhelmed right now, i have a lot of tabs on my browser to search that don't have guide where to start. There's always a miss or a variable or process i don't know that applies on the answers i see on web

I don't think this forum is your best bet.

I can tell you that using a 7805 for this is not going to be a good idea, since it's a linear regulator. This means that it merely drops the voltage without touching/adjusting the current. So if you are trying to charge at a rate of 1amp, it will convert your 12V input @ 1A into 5V @ 1A, wasting 7 watts in the process (~40% efficiency).

A more efficient design would employ a buck regulator, which would convert 12V @ ~0.5A into 5V @ 1A (80-90% efficiency).

Something like this: ... Sw711ZepLG

Generally speaking, you can look at the source battery capacity in W*h (12V * 10Ah = 120Wh), multiply by approximate efficiencies (80% for converter, 80% for battery charging itself = 80% * 80% = 64%) -> 77Wh usable, and then compare to your smartphone (4.2v * 3ah = 14.6Wh), to estimate you could charge 4-5 phones on one SLA battery. In reality there are a lot of assumptions here about the actual SLA battery capacity (the # of Wh usable is dependent on your drain rate; here it is fairly low so you'd get close to the 10Ah rated). The efficiency for the battery charger is not known.

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