lead acid battery backup for in car computer

Discussion in 'The Projects Forum' started by photonavenger, Jul 2, 2010.

  1. photonavenger

    Thread Starter New Member

    Jul 2, 2010
    hello folks,

    I am new here and have a project i am a little perplexed about.

    I have a Acer Revo in my car that i use with a 12 inch touch screen, the computer is powered by 12 volt inverter that puts out the required 19 volts for the computer.

    my problem is that unlike a lap top this computer doesnt have its own battery to rely on.

    When I start the car from time to time the resulting temporary drop in battery voltage exceeds the inverters treshold and causes the inverter to clip thus i get a hard power loss at the computer. this is not good.

    a plan to rectify this has been forming in my brain for a few weeks now, and has visited ULTRA Caps, a large electrolytic diode isolated Cap bank, but has finally landed on a 12 volt 7 amp sealed lead acid battery, cost and simplicity seem to wind back to this solution consistantly.

    the battery works great, its isolated from the rest of the autos powersystem with i large diode and with this fully charged backup battery in the car, i dont have shut down problems any longer.

    this is where i run into the snag that spawns this post, I have visited many options for charging my backup battery but nothing seems to be able to charge a 12volt battery from a 12 volt battery safely while the car is not running.

    currently i use a large 2 ohm 5 watt resistor and i am content that I am not going to damage the battery and that it is safe but...the battery eventually dies because i have not calulated the chargeing requirements of the battery or the total load of the computer, the 2 ohm resistor was more of a safety precaution then anything else.

    no one wants a lead acid electron containment device sealed or not to be overcharging spuing hot acidic steam while your driving down the road.

    while a new battery may not do this right away I am thinking a few years down the road when battery is not much good anymore and is forgotten in the dash.

    I need some kind of regulation circuit that will charge the AUX battery, power down when its fully charged and also supply the amperage for the computer when its running. the battery is basically to fill in the starting load and to keep the computer in sleep mode when im not using the car.

    i guess this amounts to some kind of a slight "boost" circuit to keep the charge voltage slightly above the at rest car battery voltage of 12 volts and a current limiting stage that will throttle back the amperage when the load is nominal but prevent it from frying the battery should it be completely discharged for some unforseen reason, while STILL providing the computers inverter with happy power.

    the computer inverter is not isolated from the AUX battery.

    I have already considered augmenting the capacitors in the inverter as a last resort.

    the Aux battery is isolated from back feeding the car battery voltage coming from the automotive fused circuits.

    any thoughts or ideas how i can simply do this, would be very appriciated.

    I have a large wealth of surplus power parts to choose from so i am open to any circuit design as long as it is resonably simply, robust and SAFE.
    I dont want my car to leave a black sticky spot on the side of I95

    I really want to KNOW why any system i may choose works because current regulation has always been a dynamic In which i sorely lack experiance, and i have a bunch of laser diodes i want to play with when i finally understand it enough to be dangerious with them. :)

    Thank you again for your attention.

  2. sceadwian

    New Member

    Jun 1, 2009
    The secondary battery if it's just for holding up power to the laptop may be of the wrong type, I don't believe Sealed lead acid or absorbed glass matt lead acids will be happy getting straight output from the car.
    One simply solution to this problem could be getting pair of paralelled lead acid cells to replace the cars existing single battery, or more sanely getting a larger capacity high quality battery that can provide enough cranking amps while maintaining voltage. Car PCs always have problems, they're just not meant to be in there.
  3. photonavenger

    Thread Starter New Member

    Jul 2, 2010
    I have seen several automotive alarm systems with backup batteries in the siren itself. so i have hope that since i am using the same type of battery I am on the right track.

    also, since i have begun using this battery, i have no problems with my mobile office system. it works nicely and goes to sleep like a good puppy when the stereo power cuts off. its only after a few days that the battery gets too low,all i need is a good charger :) that fits the bill.

    the aux battery is not directly connected to the auto supply, I would never want to rely on such a dangerious situation. it is isolated with a large diode and a 5 watt 2 ohm resistor.

    thanks for your comment sir.:)
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    There are devices that are made to do what you want. For the life of me I cant remember what they are called, but look around RV stores and sites. It is an auto-battery-relay.

    You can use 1 alternator to charge 2 separate batteries, completely isolated from each other.

    They check battery condition and smart charge them as needed. So, as your secondary battery dies, the next time you drive, (or start the car) once the starter battery is topped off, it will charge your secondary.

    AH-ha, I found it.. They are called split-charge-relays. And/or isolators

    here is some info:
    Last edited: Jul 2, 2010
  5. SgtWookie


    Jul 17, 2007
    You don't want to charge the SLA battery from the vehicle battery when the engine isn't running. Automotive batteries do not like being "deep cycled"; they need to be kept charged >12.5v.

    A car we used to have, had an electrical problem that caused a 70mA load on the battery. It drained a brand-new $150 Optima battery low enough so that the engine wouldn't turn over in the span of about a week.

    Your SLA cell won't like being discharged past about 30% either. If you routinely discharge it to 50%, you will cut it's service life by 2/3.

    SLA's need to be charged at a maximum of 1/5 their AH rating. It'll take about 6 hours to fully charge them at that rate. It is possible to charge some AGM batteries at a much faster rate, but they'll need a higher voltage.

    Basically, you need a charge controller to run only when your vehicle's engine is running. A good charge controller is a UC2906/3906 or UC2909/3909 from Texas Instruments. Here is a link to an Application Note on using the UC3909 to charge a battery from 120VAC:
  6. photonavenger

    Thread Starter New Member

    Jul 2, 2010
    Thanks everyone for the advice, this search has led me to SEPIC circuits.... and its a whole new world. I love magnetics !

    so... I will be winding some small coils and creating a boost,buck,charger for the SLA battery that will keep it perfectly charged from the battery 12 volt source at 12.5 volts and provide the constant variable current adjustment for the computer.

    in fact, I am told, if i use the right parts i may not even need the SLA becasue the SEPIC can work down to around 6 to 7.5 volts before it quits. so if it is clean enough to run the inverter.... I may even kick the 19 volt boost device for the computer out of the car and just build my own with an improved drop out spec... this has been the orginal problem all along, and i get to learn the math of SEPIC which is pretty darn cool !

    Thanks again.


  7. SgtWookie


    Jul 17, 2007
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