0
\$\begingroup\$

What I'm trying to do is to make an emergency power supply for a fishbowl (In my country, the power goes out very often). When the power goes out, a relay will connect the 8 batteries in parallel to the compressor. When the power comes back again, the chargers will re-charge the batteries. I need more than 4 batteries in parallel to make the compressor work for a longer time (4 batteries only last about 4 hours).

Edit (see comments):
I'd like to connect the batteries of the chargers in parallel (one cable from battery (+) on charger 1 to battery (+) on charger 2 ,and the other from (-) to (-). I think the question is also valid for many chargers, but I only have two. The chargers have to remain plugged, so when the power comes back, the batteries are charged again. I would add a wireframe if I only know how.

\$\endgroup\$
6
  • 3
    \$\begingroup\$ Question is not clear... Are you connecting chargers in parallel, or batteries? Also, if you are connecting batteries in parallel, as the question implies, then where is the problem connecting 30 or 100 of them in parallel, instead of just 4 or 8? \$\endgroup\$ Commented Jan 25, 2013 at 18:49
  • 3
    \$\begingroup\$ Are you aware there are many commercial products that do exactly this? See example. \$\endgroup\$
    – Phil Frost
    Commented Jan 25, 2013 at 19:27
  • \$\begingroup\$ I'd like to connect the batteries of the chargers in parallel (one cable from battery (+) on charger 1 to battery (+) on charger 2 ,and the other from (-) to (-). I think the question is also valid for many chargers, but I only have two. :). The chargers have to remain plugged, so when the power comes back, the batteries are charged again.I would add a wireframe if I only know how. \$\endgroup\$
    – Daniel
    Commented Jan 25, 2013 at 19:41
  • \$\begingroup\$ Thank you Phil. I'm aware of that. The problem is that I live in Argentina, and right now, it's very hard to get these gadgets here. May be with more time (I'm about to go on vacation) I would try to buy one at ebay. \$\endgroup\$
    – Daniel
    Commented Jan 25, 2013 at 20:10
  • 2
    \$\begingroup\$ Side note. Rather than building a battery pack with a larger number of AAA cells, use fewer AA cells. AA (double-A) cells have significantly better energy density than AAA (triple-A) cells. AA are usually more available as well. Consider the fat D cells too. \$\endgroup\$ Commented Jan 25, 2013 at 21:23

1 Answer 1

1
\$\begingroup\$

The optimal solution would be to use rechargeable battery cells that have adequate capacity to run the fish pump for the duration required. Assuming that you've already got the batteries and chargers and are trying to make this work.

You haven't told us, but I'm also assuming from your post that the battery chargers you have are designed to charge 4 cells in series, rather than individually. I'm also assuming that this is a backup DC-powered pump and that you have an AC pump that aerates the fishbowl when the power is on.

You need a line-powered DPDT relay with contacts that are rated for the larger of the charger current or discharge current. Also note that you may want to design some form of low voltage cutoff, as your pump will fully deplete your cells, likely causing cell reversal.

Apologies for the chicken scratch schematic:

enter image description here

When the AC power is on, the relay is energized and the batteries are connected to the chargers as independent series (1P4S x2) strings. When the power fails, both packs are connected to the pump in parallel (2P4S), increasing your runtime. Note that there are a few aspects of this approach that will lead to shortened battery life, but hopefully its enough to get you up and running.

\$\endgroup\$
2
  • \$\begingroup\$ I really appreciate your help. My pump works at 1,5v and the chargers seem to be independent for each cell. But I've caught the idea and I will try to implement it. Could you recommend me a low voltage cutoff circuit so I can take a look?. Unfortunatelly I'm new in the forum and I am not allowed to vote. Thanks again. \$\endgroup\$
    – Daniel
    Commented Jan 30, 2013 at 17:26
  • \$\begingroup\$ You can still accept answers. Post a new question about the low voltage cutoff--it's related but is a separate issue. \$\endgroup\$
    – HikeOnPast
    Commented Jan 30, 2013 at 17:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.