1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Linking battery packs to change voltages? and how to charge?

  1. Jul 14, 2011 #1
    Hi Everyone

    I have a query, We have some low voltage (24v) led light strings that run off a 24v transformer (down from 230/240v - UK).

    We wanted to run them off battery packs so we bought 2 x 12v (in series) batteries to make 24v.

    When we wired up the batteries the lights were duller! So we put a volt meter on the transformer a realised the output was actually around 29v!

    Am I correct in thinking this will be why the lights are duller?

    If so would the following work? Can I wire up 2 x 12v and 1 x 6v batteries in series to make 30v?
    Would it matter that the 6v battery would have different Ah
    Would the extra 1v damage the LED lights? If so is there anyway to reduce 1v using resistors etc?

    Finally how would we go about recharging the 30v battery as one unit?As we do not want to split the batteries to charge them. Could we use a 24v charger?

    The other option someone suggested is to use an inverter with the 24v battery packs and run the lights using the transformers that they come with but it seems daft to run and inverter off 24v to make 230v then to bring back down to 29/30v. Do these inverters drain the batteries at the same rate as whatever is plugged into the inverter or does the inverter use battery power itself when nothing is plugged into it?

    Any help would be brill.

    Thanks Phill
    Last edited: Jul 14, 2011
  2. jcsd
  3. Jul 14, 2011 #2


    User Avatar
    Homework Helper

    The inverter would consume additional power, an additional drain on the batteries, but if you're using two motorcyle or larger type lead acid batteries, you've probably got plenty of spare capacity.

    I've seen LED strings powered by batteries in many store displays and there's no issue. The main difference is that the battery powered ones don't flicker. Newer LED strings don't flicker even with a AC power source. Note that the LED string involve some bit of circutry at each bulb, and perhaps your LED string is having issues with a voltage source that doesn't have any AC component as it would from a transformer's DC output.

    Have you measured the voltage from that 24v transformer when it's connected to the LED string? I wouldn't try using a higher battery voltage until you measured the transformer output. It's possible that you need to use a lower voltage to keep the LED circuitry "happy".

    As far as charging the batteries in series, it depends on the batteries. It shouldn't be an issue for nicad, nimh, or lead-acid batteries, but for li-poly batteries, you'll want to use some type of balancer that equalizes the voltage between the two batteries.
    Last edited: Jul 14, 2011
  4. Jul 14, 2011 #3
    Thank you for your quick reply.
    I am using lead-acid batteries.
    I forgot to mention that the transformer has an ac to dc convertor attached on a wire then the led lights attach using a connector into this wire. When i measured the 29v output from the transformer i measured after the ac-dc convertor. So i was measuring dc 29v. does this affect the advice you have just given me?
  5. Jul 14, 2011 #4


    User Avatar
    Homework Helper

    The issue here is that lead acid batteries output a bit more voltage than their ratings, especially under a very light load like the LED string, so 12 volts, is more like 12.6 volts or a bit more on a fully charged battery. If the LED string can be replaced cheaply, you could try five 6 volt batteries. In the case of two 12 volt and one 6 volt battery, charging them in series could be an issue since it's unlikely they'll have the same capacitance.

    The other option is to use a single 12 volt battery and a cheap inverter (100 watts), since it won't need to output a lot of watts to drive some LED strings. The issue here is almost all the cheap ones use a car type dc plug, so you'd have to get a socket with cables to connect to the battery. It could be cheaper overall to get the lowest wattage inverter that uses cables instead of a plug for the dc input.
  6. Jul 14, 2011 #5
    Am I the only one here totally lost?
    If you use a 24V transformer, it means the transformer output is 24AC. Unless they already make AC LEDs, there must be other circuits to convert 24AC into DC.

    Battery voltages are DC (I may be living in caves if they now have AC battery) How can you get 29V, AC or DC, from a transformer when the input is DC from battery?

    Something is seriously missing here.
  7. Jul 15, 2011 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    From Post #3:
  8. Jul 22, 2011 #7
    Hi Everyone. Going to try something else.
    Sorry for the confusion.
    We have a cheap transformer that is plugged into the mains. It is labelled as a 24v transformer. There is a wire coming out the transformer with an AC to DC convertor on it. However i think the transformer is rubbish as when we measure the voltage after the AC-DC convertor it measures closer to 29v.

    We wanted to use two 12v batteries in series to power the lights to the same brightness(as the mains sets will be next to the battery set) but this only gave us 24v. So the lights were duller.(Compared to the mains transformer)

    We have tried using the batteries with an invertor and the transformer plugged into the invertor. It worked but the batteries didn't last long enough.

    I've thought of something that may be a simple solution? Replace the 29v transfomer(labeled 24V) with a transformer that actually is outputing 24v.

    Would this work or am i missing something?

    I'm recharging the batteries and sourcing another 24v transformer as i type.

    P.S. thank for all your help.
  9. Jul 22, 2011 #8
    Invertor worked perfectly but batteries didn't last long enough. :( thanks for the advice.
    Please look at my last post - do you think this sounds sensible.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook