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Constant 12 V 2A power Supply from battery

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yampire

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I want to build a power supply from a battery source for my internet modem. My modem is rated as DC 12V-2A. Suggest me the way i can do that. I read about the LM7812 regulator but they say that it is a loss of energy but it can provide constant 12 V. So help me to find solution of my problem I want to power on the modem at least for one hour on battery. What type of battery will i be needed?
 

- What's the actual modem supply current? You need to measure it.
- Does the modem actually require exactly regulated 12V, or would it be satisfied with 10 - 14V unregulated battery voltage? I guess, it will tolerate the battery voltage. If you don't want to analyze the modem circuit, you should check if the regular power supply is regulated at all.
If exact 12V is a requirement, you'll need a buck-boost converter, or (preferably) a higher battery voltage and a buck converter.
 
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    tpetar

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Use three small gelled 7Ah 6V batteries in serie to get 18V, or two 12V 7Ah to get 24. (better 3x6V 7Ah).
Battery capacity can be higher no problem, you can put 14Ah or higher.

Use LM338K TO3 to regulate voltage as this circuit :

LM338-circuits.jpg

Put LM338K on heatsink. With 5K pot or trimmer pot set voltage on exact 12V and thats it, this can provide current up to 5A. LM338 have internal protection about over current above 5A and over heating.

This is nice and easy, just with few components. To go with switcher you will have less efficiency.

You can use small gell/glass batteries, car batteries, .... you can even use small NiMh or alkaline battery bank D type.
If you run that modem few times, not often, maybe alkaline is good, alkaline also can be recharged with no problems (crazy but works).

My recommendation is to use 3x6V 7Ah (or bigger Ah - recommended) in serie to form 18V, and regulate with LM338K.

Look manufacturer datasheet about particular battery, discharging curve, time and current, to see how long will the battery last on this job.
 
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The AC adapter power supply which i plugged in to the modem is rated(output) as 12V--1.5A
 

The AC adapter power supply which i plugged in to the modem is rated(output) as 12V--1.5A

Its declared current and voltage (power), no problem with that. But if you whant to see exact current you should ampermeter instrument for monitoring current.
 

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
 

With a buck-boost converter, as previously mentioned.
 

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.

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!!!

If you whant to go 10V use other battery chemistry!

See this circuit example:
12V Regulated Inverter Supply circuit diagram and instructions



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.
 
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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.

The linear regulator you showed before, using the LM338 linear regulator, is likely to have a lower efficiency than a buck-boost switching regulator. The buck-boost switching regulator, in fact, is likely to have longer battery working time than the LM338 linear regulator, especially if the current is 1.3A - 1.5A.

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.

Hope this helps.
Tahmid.
 
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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.

I agree, but without load 11,99V lead acid is considered to be empty!

Most commercial inverter with 10,5V kills starter batteries, when people use it on that.

Starter or Deep Cycle? People dont know that starte bat cant be loaded with high amperage long time, may but :???: not good for battery. Its used in car for starter to start engine, that starter pull about 100A and more from bat but on few sec not long time, in that period voltage can go down to 9V.
 

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.
 

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.

I dont know for that but I know that starter car batteries and small lead acid gell/glass have life about 3-5 year declared from manufacturers on regular usage in car. But I beleive You, all is possible.

I have one oldtimer Bosh S3 now almost 13 year. I use desulfatisator with that oldtimer often. I have special thretman for that Bosch battery. I meassure internal resistance and other things even capacity with AC smart diag tool,.... every year at winter I try this baby in my car to cold crank on very cold wheater (bellow -10C, often -18 -20C with 24h pre-cooling on that temp).

Starter bat should be used as float bat not cycling bat. For that purposes is deep cycle bat. But deep cycle bat allow lower amperage, not higher as starter. And price of deep cycle bat is :roll:



And I apologize to FvM posts #2 #7 and Tahmid post #9 buck-boost converter can be more efficient then LM338K, i make mistake about that!
 
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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

The 7812 is very inefficient if the difference between input voltage and output voltage is large, since it is a linear regulator. So, there are large losses. With a small difference, losses are smaller. However, the modem is rated for 2A current, whereas 7812 is rated for 1A continuous current.
 

There aare variations for the basic 7812 see this link
**broken link removed**

about efficiency what i meant is for a cheap i component solution its not logical to use any other solution.
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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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...

How did you deduce this?

We don't know his battery capacity, so battery voltage may dip below 12v when using. He needs a power supply to maintain 12V output from 12V battery. The only solution I see to that is a buck-boost regulator.
 

Ok i don't know his batt, i sent him link with scores of batts.
next buck boost is good for saving power of the regulator and its expensive .the regulator : coils :capacitors other components.
i work a lot with buc.. they are troublesome. batts can be trickle charged with 5-10 $ adapter chargers , all the components may cost him 15$. myself i am using a old car batt cost me nothing , for my emergency 12v lighting all over my house about 10 1 watts leds .
About u i can see u r good with pics and micro, so i will ask u if u know how to build a cheap voice recorder and store into ram without any software, just speak pressing a button and my voice will be stored into eeprom . wen needed i will press talk button and ill hear myself.
remember no soft ware no programming.
 
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Ok. There are quite a lot of ICs available for this. I've got scores of APlus voice ICs with me that have upto 1 Mbyte of memory. They can be programmed with a PC or have a "recording" mode that does what you need. They are also pretty cheap. I got them from India a few years back. I think they cost less than 100 Rupees back then (not sure though).
There are also a lot of ICs in the ISD series of chips. They're pretty expensive but can be found in the local market. I still don't get the point of this discussion in this thread as it is off-topic.

Back to the original topic. Buck-boost regulators aren't ALL THAT expensive especially since he's making this for his own use and if it costs max $1-$3 more in lieu of improved efficiency, lower heat and better battery backup, I think it's worth it. Anyways, it's upto the OP and what he thinks most suitable.

Hope this helps.
Tahmid.
 
I observed in a similar case, when opened the unit to figure out the minimum voltage that can support, there was 7805 regulator inside for the main circuit although supply requirement labeled was 12V. So it could operate down to 8V.
 

well done
use 7808 (not 7805) there maybe some hidden copper routes going to leds or other parts in the circuit u cant see .
boost the output up to 9.4-10v by adding two diodes in series to grd pin of regulator. and work with 12v batt.
good luck
 

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