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Battery backup to a battery supply.

  1. Mar 29, 2013 #1
    Hi all, I hope you're all well :)

    I am shortly going to be fitting a battery operated device outdoors. Due to the power consumption, I will get 2.5 days out of each battery (max - if I'm lucky).

    What I would like to do is be able to connect another battery to it before disconnecting the old one for recharging - IE so the system doesn't loose power and have to be rebooted etc.

    Can I do this quickly and easily just by using a diode on each positive supply wire?

    Many thanks,

  2. jcsd
  3. Mar 29, 2013 #2
    Yes, if you can tolerate the diode drop.
  4. Mar 29, 2013 #3
    0.6V right? Should be ok.

    Is there a better way?
  5. Mar 29, 2013 #4
    If you could find a single pole double throw switch or relay with make before break contacts. Connect each battery to separate switch contacts with the load connected to the armature or center contact. This would avoid the diode drop. If you can't find a make before break switch or relay, a large capacitor may be able to sustain the circuit during the fraction of a second it takes to switch from one battery to the other.
  6. Mar 29, 2013 #5


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    I'd be inclined to use a break-before-make switch, and use the output capacitor to give you the hold-up during the brief switching time. Using a make-before-break switch could give you pretty big spike currents during the switchover, and maybe eventually cause the switch to fail (weld -- yikes).

    I'd also recommend that you be sure to include a fuse at the output of each battery, before connecting it to any switches or other things.
  7. Mar 29, 2013 #6


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    Are you able to switch the device to an idle or low-power state before performing the swap? The current demands during the swap-over may be significantly lower, if you can.
  8. Mar 29, 2013 #7
    Unfortunately not. It's a cctv camera with inbuilt processor and OS etc. It needs to remain powered up at all times hence the question,

    I think I'll just go with a double light switch setup so I can switch the new battery on and then the old one off to take it away for a recharge..
  9. Mar 29, 2013 #8


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    Can you estimate the current spike out of the new battery and into the old battery due to their difference in output voltage and their respective internal resistances? You need to be sure that connecting the two different voltage sources won't generate a damaging current...
  10. Mar 30, 2013 #9
    Put a diode in parallel with each switch. When you're ready to swap batteries turn both switches off. When the swap is complete turn them back on.

    Also, it might be more convenient to use just one double pole switch.
    Last edited: Mar 30, 2013
  11. Mar 30, 2013 #10
    What kind of batteries are you using?
  12. Mar 30, 2013 #11
    Erm, the whole idea of this is that I don't need to turn off the device. I simply connect one battery before disconnecting the other.

    If I use a double light switch to disconnect each battery, what would be the point of turning them both off and powering down the device?

    No offence but you don't seem to understand the issue.
  13. Mar 30, 2013 #12
    Car batteries. Before you say anything, I'm aware that they're not perfect but they're all I have access to and will have to do.
  14. Mar 30, 2013 #13


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    Last edited by a moderator: May 6, 2017
  15. Mar 30, 2013 #14
    I think you misunderstood my post.


    This is the same as your original idea of using two diodes except the switch will eliminate the .6v drop during normal operation. And when swapping batteries it will eliminate the possible large current flow from one battery to the other. Since you said in a previous post:

    Why not just stick the diodes in there?
    Last edited: Mar 30, 2013
  16. Mar 30, 2013 #15


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    If these are small batteries that might not be a problem but anything above a few Ah of capacity can have huge currents flowing from a fully charged battery to the discharged one and can arc a AC power switch easily. You need to use a DC rated switch.

    http://www.eaa.org/sportaviationmag/2005/0502_switches.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  17. Apr 3, 2013 #16
    Do you mean that when the first battery pack is empty you want to connect the new pack and take the empty pack AWAY to charge it?
    You can't put a charger on the battery?

    Or if you just want to increase the time why you don't put the 2 battery packs in parrallel?
  18. Apr 3, 2013 #17
    Turtles idea will work well, you can also look for a changeover switch (SPSO) and have the diodes in parallel with each side of the switch - this will prevent both batteries from being hard connected at any time.
  19. Apr 3, 2013 #18
    Actually thats a very good idea now you've explained it that way. I like it. I apologise for being abrupt towards you previously, I didn't think you understood the issue. Clearly you had a better understanding than I realised and you were one step ahead of me. Please accept my sincere apologies.

    This is what I'd originally been thinking:

    I like your method and now I'm thinking I should go with it. As you can see from mine, it would have dropped about 0.7V from either battery (although using a car battery which is at 13.5v when full that may not be such a bad thing).

    Yes that is correct. When swapping batteries I want to leave the device powered up while I attach the new battery and THEN remove the old battery.

    No. Re-read my post above. It's an outdoor device and using batteries. This is because I can't use mains electricity in that location so how would I use a charger?

    Because then I would need two more batteries to put there while recharging the first parallel set. Oh and I don't have a charger suitable for parallel car battery charging.
  20. Apr 3, 2013 #19
    No problem. I should have explained myself better in the original post.

    I noticed a safety issue with my method that you should be aware of. When connecting the battery, there will be a current flow of up to one amp. That is not much, but it may be enough to cause a spark at the battery terminal. If the battery is being brought straight from the charger, there is a chance that the spark could cause a battery explosion due to hydrogen gas build up. The best solution would be to add a series switch in addition to the one in parallel with the diodes (combining my idea with yours). In your diagram, just add a switch in parallel with each diode. That way you can connect the battery, then flip the series switch, causing the spark to occur at the switch instead of the battery terminal.
  21. Apr 4, 2013 #20
    So by that do you mean like this?:


    Just out of interest, in a car do all the switches spark internally then?
  22. Apr 4, 2013 #21


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    The quick and easy thing to do IMO is to use a make-before-break switch as berkeman suggest. I wouldn't be concerned about the switch failing as there are those made for these kinds of conditions. e.g.

    Current Transformers

    Here's what a double pole one looks like; which I think you should use.

  23. Apr 4, 2013 #22
    Yes, that's it.

    Yes. Switch and relay contacts will spark on make and break. The degree of sparking depends on the voltage, current, and switch design. Explosion-proof switches are available for hazardous locations. These are basically normal switches housed in sealed flameproof blocks.
  24. Apr 4, 2013 #23


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    And they make and break more than once when you flip the switch.


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