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I have a garden water feature powered by a 15V Photo-Voltaic array. The PV array powers a low voltage DC pump.
Obviously, the water feature operates only when it is sunny.

It starts to run when the sun barely touches the array at a low angle (I estimate that the solar cell is at 50% of its power).
At this time, the water feature produces just enough flow to run the water through it.
At about 75% of solar power the pump gives a nice looking stream for the water feature.
In full sun the pump runs like crazy splashing water around; I had to compress the hose from the pump to reduce the flow.

As you can imagine, the annoying thing is that the fountain stops operating when the sun goes down. It is fine overnight, but even a small cloud passing over the solar battery stops the fountain.

I want to add a few 14500 Li-Ion batteries that I have left over from a different little project of mine.

I do not know how to connect them.
I am thinking of putting 3S battery pack in parallel to the pump so that the batteries are charged while the pump is operating; then when the Sun goes down, the pump will continue to run until the batteries are discharged.

Will this work, or I need some extra device to make this work?

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  • \$\begingroup\$ What does the battery pack manufacturer's data sheet tell you about how these batteries can be charged? \$\endgroup\$ – Andy aka Apr 18 '17 at 7:25
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    \$\begingroup\$ If you want a fairly dumb charger, then you'd be better off with a 12v SLA, which can be float charged at 13.8v indefinitely, and more or less suits your voltages (use an LDO after the solar panel so battery float voltage does not exceed 13.8v). There are no short cuts to using lithium chemistries, if you want your batteries to last. You must not float, and you must not over-discharge. \$\endgroup\$ – Neil_UK Apr 18 '17 at 7:40
  • \$\begingroup\$ Lead-acid, NiCd, or NiMH batteries could be used how you're proposing; but a Lithium-based battery pack will require additional components to work. See my answer. \$\endgroup\$ – Robherc KV5ROB Apr 18 '17 at 7:50
  • \$\begingroup\$ There are tons of info about Li-ion battery charging on the Internet, including this site. You should read some of that before asking. \$\endgroup\$ – Dmitry Grigoryev Apr 18 '17 at 11:28
  • \$\begingroup\$ Thanks guys for the comments, there is no reason to downvote if you do not agree with what I have said, a good guidance would do, like the one from Rodherc below \$\endgroup\$ – cha Apr 18 '17 at 23:06
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  1. Yes, you can parallel a battery pack with your load to store some excess power and "cushion" power fluctuations from clouds/high-sun/dusk on a PV-powered device (if you take apart a solar path light, you'll find a NiMH/NiCd battery pack doing almost exactly that).

--but--

  1. Lithium-chemistry (Li-Ion, LiPO, LiFePo, etc.) batteries don't work the same as lead-acid, NiCd, or NiMH batteries. While the other chemistries of rechargeable batteries desulfate, or self-balance when charged past "100%," Lithium-chemistry batteries tend to self-destruct (sometimes with a fireball included) when over-charged, and take more damage from being over-discharged as well.

If you're 100% sure you really want to use the lithium batteries for your fountain; then at the very least, you'll have to add a battery protection circuit to your setup, and it would be highly recommended to also add balancing resistors.

Preferably, a separate power circuit, connected to your Vpos & GND from the PV array would be much better.

Best solution (for Lithium batteries) would be to connect the PV array to a charging circuit for the batteries, then parallel-connect a buck converter to the battery pack (and parallel several capacitors as well, to "catch" some of the transient power spikes/dropouts from the PV array without wasting either the power, or your batteries' lifespan).

This way, you could harness all of the energy collected by the PV array, and send a constant power-level to the pump, so that the water feature is always running at "the ideal flow" any time the PV array, or the batteries, can put out enough power to do so.
The rest of the time, the pump will simply be "off;" neither spraying water everywhere, nor wasting power making a trickle that doesn't do anything useful.

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