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Battery backup — Parallax Forums

Battery backup

max72max72 Posts: 1,155
edited 2011-07-06 00:21 in General Discussion
I'm looking for circuit ideas for a battery backup system.
I'm planning to use a propeller and a couple of 5v modules, and I would like to be able to have the system running for a couple of minutes, or maybe less, in case of power failure. It should be able to send an SMS or something similar in case of power failure. I caould also accept to shut down everything else but the micro and the GSM module.

The main supply would be a wall wart.
I think a 9V rechargeable as a backup and a 12V wall wart would be more than enough.

With google I found the following examples:
http://electroschematics.com/6279/battery-backup-circuit/
http://www.mcminnarc.com/helps/charge/charge.html

Are there other, better options? Is this approach viable, or is there a better solution (super cap or anything else)?

Thanks in advance,
Massimo

Comments

  • FranklinFranklin Posts: 4,747
    edited 2011-06-30 10:45
    Have you looked at the Parallax li-on battery chargerhttp://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/charger/List/0/SortField/4/ProductID/758/Default.aspx
  • max72max72 Posts: 1,155
    edited 2011-06-30 14:27
    Franklin wrote: »
    Have you looked at the Parallax li-on battery chargerhttp://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/charger/List/0/SortField/4/ProductID/758/Default.aspx

    Thanks for the suggestion.
    I dismissed it as a non viable solution but reading the docs it looks pretty good, even if maybe overkill.

    Funny thing is I searched the forum for similar discussions, with no luck. Now the thread is open in the bottom area there are a lot of suggested threads spot on.
    I guess I have to improve my search skills..

    Thanks,
    Massimo
  • kwinnkwinn Posts: 8,697
    edited 2011-06-30 17:52
    I would think 3 or 4 rechargeable AA cells, a diode, and a resistor for trickle charging the batteries would be enough.
  • max72max72 Posts: 1,155
    edited 2011-07-01 06:17
    kwinn wrote: »
    I would think 3 or 4 rechargeable AA cells, a diode, and a resistor for trickle charging the batteries would be enough.

    This looks as the easiest solution, and the cheapest. I don't need to run for a long time from the battery, after all.
    I'll make a small setup to see if it fits my needs.
    Thanks,
    Massimo
  • ercoerco Posts: 20,261
    edited 2011-07-01 09:04
    If you want cheap AA batteries and don't need a long run time, see http://forums.parallax.com/showthread.php?132461-3000-mAH-AA-NiMH-56-cents-each
  • max72max72 Posts: 1,155
    edited 2011-07-01 14:22
    erco wrote: »
    If you want cheap AA batteries and don't need a long run time, see http://forums.parallax.com/showthread.php?132461-3000-mAH-AA-NiMH-56-cents-each

    Another seller shipping worldwide with the exclusion of Italy.... sometimes contacting the seller and saying I knowingly take the risk works, sometimes not...
    Anyway for this use it would be a nice fit...
    I assembled a test veroboard, I'll verify if it works and post the results.
    Massimo
  • TorTor Posts: 2,010
    edited 2011-07-01 14:27
    Is it common that Italy is excluded from worldwide shipping? I haven't seen that before. I looked at the list.. I can understand why Indonesia is excluded, I know about a luthier who was scammed $5000 on a sale to Indonesia (apparently buyers there can simply withdraw the money after payment has arrived in the seller's bank). But Italy?

    -Tor
  • kwinnkwinn Posts: 8,697
    edited 2011-07-01 16:25
    Massimo, here is a simple circuit I have used in the past. If you have an LDO regulator for the 3.3V it works well with 4.8V (4 NiCd or NiMh batteries). With the LM317 you need at least 6V in.
  • max72max72 Posts: 1,155
    edited 2011-07-02 13:49
    Thanks kwinn for the circuit. Test circuit I made is similar, it only has another diode after the voltage supply. I'm away for a couple of days, then I'll test it.

    Tor, Italy is considered the country with the worst postal service, so many sellers prefer to avoid shipping to Italy. I live in a quite area, and my delivery rate is 100%, but I agree it is not the same in all places.
    Even Parallax in a discussion about USPS unsigned uninsured delivery expressed this kind of reserve about Italy. Other sellers simply state that estimated delivery times do not apply to Italy (Olimex with signed for delivery, just to cite one..).
    Usually saying that I now it is risky and accept it unconditionally works, but not always.

    Massimo
  • ercoerco Posts: 20,261
    edited 2011-07-02 14:05
    La mia Italia, my favorite place of all. Pity about their postal service, but look on the bright side... you're in Italy! Whereabouts are you? I've bicycled a lot through Italy. Amalfi Drive and around Sicilia are two of my favorite routes.
  • max72max72 Posts: 1,155
    edited 2011-07-03 05:49
    erco wrote: »
    La mia Italia, my favorite place of all. Pity about their postal service, but look on the bright side... you're in Italy! Whereabouts are you? I've bicycled a lot through Italy. Amalfi Drive and around Sicilia are two of my favorite routes.

    I live in Mantova, near Verona. I'm really close to Venezia, Milano, the Garda lake and the Alps. Nice thing about Italy is it is so small that almost everything is close.. anyway I'm far away from both Amalfi and Sicilia...

    Massimo
  • max72max72 Posts: 1,155
    edited 2011-07-05 04:40
    I tested the diode solution.
    I used two diodes, a resitor and a big fat capacitor.
    Voltage supply is 9V, and the backup is 4 AA NiMh.
    I have to tune the limiting resistance, but it works perfectly.
    Thanks,
    Massimo
  • kwinnkwinn Posts: 8,697
    edited 2011-07-05 16:56
    I have used a resistor calculated using R = (Vin - Vbat) / (0.01 x AHbat) where Vin is the supply voltage, Vbat is the nominal battery voltage, and AHbat is the amp hour rating of the battery. This has worked very well for SLA and NiCd batteries. Typically they last for several years before needing to be replaced.
  • max72max72 Posts: 1,155
    edited 2011-07-06 00:21
    I started with a higher current value, but at the moment I setlled to a 0.01C trickle charge current, like the one you suggested.
    Thanks for the feedback.
    Massimo
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