The AC adapter power supply which i plugged in to the modem is rated(output) as 12V--1.5A
I was wondering that how i can able to produce constant 12 V with a 12V volts battery even if the battery is providing 10 or 9 volts
You can make 12V from 12V as FvM say, but my opinion about this is bad. Wasting of power, and lowering efficiency.
1,3A-1,5A is not small current, to pump that efficiency is bad, you will shorten battery working time.
My advice :
With 3 x 6V = 18V (lead acid gell/glass)
Regulator from 18V give constant 12V
At 17,82V batteries are empty and at 19,08V batteries are full charged.
For Lead Acid batteries lowest voltage is 11,9V at that level battery is considered to be complete empty. At 12,73V battery is fully charged. This type of battery can be float charged at 13,5-13,8V. (temperature have influence on charging voltage). 10V or 9V what You mention is very bad, at that level there is no 1,5A from battery, battery is damaged!!!
The voltage depends on the load and the capacity of the battery. Depending on the load applied and capacity of the battery, even 11.0V may be good enough and indicative that battery is okay. Most DC-AC inverters here use 10.5V as the low-voltage level.
Here, regular car batteries (starter) are used for inverters (sometimes even > 4-5 hours a day) and usually last more than 2 years, even 3 on better inverters. I personally have had my battery running for ~2.5 years before it became completely damaged.
13.8 volt batt will do lm7812 -.
your modem will work even 10volts.
check out this link
lm7812 -
. the lm7812 is good ..wrong about loss.
use zener and transistor
batt link
13.8 v power supply and battery index
what he needs is a 13.8v cheap ready made 2-3amps charger that he will use after every session , is using his batt for modem.
all he needs is 1 resistor: 1 3055 plastic :1 resistor: 1 zener. that all...cheap ... cheap... cheap...
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