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DC UPS for my Laptop----Help???

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Full Member level 5
Sep 25, 2010
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HI Guys. (You have to read it fully to understand my Problem)

I have to use my laptop for longer period that is to say i have to download movies so i keep it on for days and thats how my laptop battery was wasted as it is recommended to remove battery if u have to used laptop for a long time. I can run it directly on charger but load shedding is a big issue here for example 10 hrs of load shedding and time for it is random ( and it sucks).

So i wana build a system which can run laptop on AC mains which is to be step down and regulated and a relay system to switch to batteries when there is no light. In short a DC UPS.

I have two 12V 24AH batteries which i connected in series for 24V. My laptop works on 19V and 4.77Amps. This is the rating on laptop charger.
For that i picked LM338 regulator which is capable of delivering 5 Amps. There should be at least 3V of difference between output and input of LM338 so i couldn't use 24V Peak from transformer since there would be ripples at capacitor end and it wont be regulated properly. So i bought a transformer 12 + 12 V i.e. center tapped (25Amps). which gives 24.4V RMS and when rectified and filtered it becomes 34V DC.

I set the regulator to 19V and connected the output to laptop. Laptop started when switched on and switched off in the middle of loading windows. I put the voltmeter to the output and switched it on again. The voltage was varying between 18 to 19v and as it became less then 17.5v laptop switched off again. It means the laptop is not being provided proper current which when i tested could not exceed above 2.5 A. off course because 5 Amps is guaranteed when input/output difference is less than 10V which wasn't in my case. So i used and external PNP (Attachment 1) transistor to full fill the requirement. The result was great on both battery and Rectified output.

Explanation (Attachment 1) : The BD242's Emitter-Base potential difference is 0.6V, When 1.2Amps passes through R1 the potential difference becomes 0.6 and transistor BD242 starts to contribute. The R4 will be fed to a Micro-controller which can turn on Q1 to shutdown the regulator if suppose voltage exceeds above 20V somehow. Now here my question is : if voltage of batteries goes to 22V the regulator will not regulate, so will the voltage goes above 19V? for this i have added Q1.

Now i want to use the same transformer to charge the battery. A good Algo to charge the battery (which i googled) is to first change the battery using Constant current and that constant current can not exceed the battery rating, e.g. a 10AH battery can be charged at constant current of 10A or less, not more.This will be the first phase. Now when the battery voltage reaches to 13.8V then we switch to constant voltage mode which can be between 14.4-15v.This is the 2nd phase. Now we have to monitor current and when current reduces to 200mA or less, the battery is fully charged.

A single 12v battery requires 15V for nominal charge, for batteries in series it becomes 30V. and for constant current voltage for a 12v battery the voltage goes upto 24V which becomes 48V when batteries are doubled which i dont have. So i have to use batteries as 24V in series to run my system and split them when i have to charge them. A splitting circuit can be build which i will discuss later. for now consider charging batteries at 12V. Almost similar circuit to Attachment 1 with some modification is provided as Attachment 2.

Explanation (Attachment 2) : Here i m using LM317 Without heat sink. As R4 is connected to it which is of 27 ohms so there will current in mili-amps through it and it wont get hot. this means a potential difference of greater than 0.7 would be developed in an instant across R4 which will turn on Q2. Q2 is a darlington pair which is capable of handling current upto 50Amps in ideal situation. It can with stand up to 300 degree centigrade but is derating factor is quite high i.e. 1.71 w/centigrade. So thermal handling should be appropriate. The R5 is creating potential difference across Q4 and two diodes D3 and D4 attached to it. which is 0.6+0.4+0.4= 1.4V. At 1.4v Q4 will turn on i.e. 1.4/R5 i.e. 1.4/0.1 = 14Amps. Alternatively at 14 Amps the Q4 will turn on, giving excessive current to LM317 to thermally shutdown it. So we are actually protecting Q2 by limiting the current so it wont get hot and get damaged, its an expensive transistor. The switch J1 when connected to upper side the R7 get ignored as R6 is quite small. so 1.25/R6 = 12.5Amps constant current. When J1 switches Down there will be a constant voltage mode, along with it when Q1 is turned on via micro controller the voltage drops to 13.8v.

I have completed the Circuit this Far. Now My problems

Problem 1: When my charging circuit voltage switches to 15V or 13.8V, the laptop voltage also drops. When i measured the voltage at the capacitor end say E1 end without battery charging,it was 32-34v and when i connected the battery in 15V mode, the voltage at E1 drops to 20V. I have two solutions for it. 1. using separate transformer for laptop. 2. connect a second rectifier and points 2-3 i.e. parallel to rectifier D1 as i have noticed that voltage at points 2-3 drops from 24vRMS to 22vRMS.
I don't wana use second transformer as it would be hell of a bulky circuit. I haven't tried solution 2 so not sure about it. wat u guys say?

Problem 2. I have found an old IBM PC case so i will fit my circuit in it. I have removed all the plastic and will reassemble it later. I am using this body as a heat sink also for transistor Q2, i dont know whats its material but its not Aluminum for sure. I have attatched the pics of the casing. i have tested the transistor on this casing in constant current mode that is 12.5 amps and its temperature raised to 150 centigrade and when switched to 15v the current was 8 Amps and temperature raised to 200 centigrade. I wonder why temperature is being raised higher in constant voltage mode compared to constant current mode even current is low!!!!!!

Problem 3: I want to used transformer to its full capability so connected more transistor like Q2 on another heat sink in parallel to inject more current but couldn't go above 13 Amps. later i found that wire gauges were low so i doubled them but even then i couldn't get more than 14 Amps. later again i found in the data sheet that transistor Q2 (MJ11033) have voltage saturation at 2.5 and worst case 3.5 V. and i measure the voltage across Collector_emitter....yes it was 2.5. So this is what actually restricting it to deliver more current. for this i have to increase transformer voltage but LM317 put me in restriction to use input voltage less than 40V. if there is another voltage regulator having wide input range and having thermally shutting down capability then let me know and if not then propose some solution.

I will proceed my project after your comments and suggestion and will let you know for the further developments. thanks.


What type of batteries are you using?
Batteries can be charged at different rates depending on the requirement.
Typical rates:
-Slow Charge = Overnight or 14-16 hours charging at 0.1C rate
-Charge = 3 to 6 Hours charging at 0.3C rate
-Fast Charge = Less than 1 hour charging at 1.0C rate
As for the 24Ah sealed lead acid batteries type, at 0.1C you need only 2.4A to charge...

Thanks for the reply Mister_rf. Yes i m using Sealed Lead Acid batteries (deep cycle). i have learned a lot about them over the net and came to know that these batteries can be fast charged till there temperature is less than 60 degrees. and it will do no harm. i have been charging small lead acid batteries of 4.5 AH and its about yea they are performing well.
The think is here load shedding happens typically after every 2-3Hrs. So i need fast charging system.

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