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What current and voltage output do i need to charge this battery?

  1. Dec 7, 2013 #1
    For a uni project one of my concepts is to use thermo couple generators like these http://www.thermoelectricgenerator.co.uk/TEGinstallationandspecifications01.pdf on the exhaust to generate electricity to replace the alternator. The problem is i cant figure out how much current and voltage is required. The system will be in use on a huqurvana 610 e powered formula student car with a varley redtop 8 battery that is 12v 8Ah. The mains charger designed for the battery is 12v 2.7A so do i just match that or will it need more as its being drained at the same time? If that is all it needs itd only take 2 of those modules connected in series assuming the temp difference was 300C wouldnt it? Sorry im useless at electronics.
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  3. Dec 7, 2013 #2


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    You will need to provide 12v 2.7A like the charger. It may be slightly different to what is listed and it may also have temperature compensation. You must also provide all the current needed by the vehicle.

    Since the open circuit voltage is 14.4 and falls to half that with a matched load, (i.e. maximum power transfer), you will need to use two in series to get the charge voltage required.
    Last edited: Dec 7, 2013
  4. Dec 7, 2013 #3
    Thanks mate. How do i work out how much current the car needs? All i can think of that uses electricity on it is the ignition system the dash and a small light on the rear. Do you think if i connect 2 in series and parallel it would be more than enough?
  5. Dec 7, 2013 #4
    You can measure what the car uses. Do you have an inductive clamp ammeter in your uni lab? It's one of those meters that you can clamp on a wire to measure current flow. Most meters with that inductive clamp can only measure AC but I've got a nice Klein model that can do AC and DC. Fluke and Ideal make them too.
  6. Dec 7, 2013 #5
    Unfortunately someone has decided to disassemble the car so its not currently running so i can take the measurements. Is there anyway i could estimate? The maths doesn't have to be perfect for now. Its a small one cylinder bike engine, a digital dashboard and a rear light, no head lights or anything so it can be too higher current can it?
  7. Dec 7, 2013 #6
  8. Dec 7, 2013 #7


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    There is no correlation between the output from a battery charger, and the output from the alternator on the car.

    For example your 2.7A charger would take over 3 hours to fully change an 8Ah battery. That might not be enough to handle repeated engine starts and short test running periods.

    The minimum requirement is to provide all the current the car needs when running, i.e. engine management or ignition, plus lights etc. You should be able to estimate that from the electrical equipment actually on the car (e.g. you know the rating of the light bulbs, in watts).

    For example two "standard" halogen car headlight bulbs will use about 100W, or about 8A current. A 2.7A charge current that reduced the battery drain from about 8A to about 5A would still result in a flat battery.
  9. Dec 7, 2013 #8


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    The first step should be to reduce the worst case current requirement by replacing all inefficient light globes with LEDs or similar sources. It is often more economic to reduce the need for power than to generate power.

    In the time it takes to generate the temperature difference required to support the entire load, it must be supported by the battery and replenished shortly afterwards.

    It is too much to hope that the system will always have only the 300° needed to operate safely, within specification. You will need to use some form of physical temperature regulator, such as a bimetallic strip, that conditionally dilutes the exhaust gasses to prevent overtemperature damage of the generator.
  10. Dec 7, 2013 #9
    What would be the current required for the ignition system of a single cylinder motorbike engine do you think? I cant find anywhere that says it. Thanks baluncore, i understand that it wont always be 300c at the but wouldnt the battery hold enough charge to run the car while it heats up? Hiw would you use a bimetalic strip to regulate the temprature?
  11. Dec 7, 2013 #10


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    An ignition system would require maybe one amp. You must measure it, or convert to magneto ignition, (which is really an alternator in disguise).

    A battery usually only has to start the engine, then the alternator takes over immediately. Your situation is quite different and so may require a larger capacity battery. If the mass of an alternator capable of filling in the cold start phase was less than the extra mass of battery needed, then an alternator could be a solution. It will still be more economic to use exhaust gas heat than to turn an alternator once the engine is operating with higher temperature exhaust.

    When the exhaust gas is hotter than 300° a bimetallic strip could divert the flow of hot exhaust gas while replacing it with colder air. That would protect the transducer from damage by high temperatures.
  12. Dec 7, 2013 #11
    Yh maybe a reduced size alternator would be best maybe that could be disengaged when the exhaust is up to temperature. Cant really use a larger battery for packaging reasons. Ohhhh i see, yh thats a good idea thanks.
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