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I have a bunch of plants on my balcony and I would like to automate the watering.
I don't want to drill holes in my window frames so I can't provide power from within my apartment.
I'd like to have a self-contained system, a 12v battery (for the pump) and a solar panel to charge it.
Safety is my number one priority so I'd like to chose a battery chemistry which can be safe to operate on the balcony:

  • temperatures go between -6C in winter to +45C in summer. The balcony is east-facing and gets a lot of sun in the afternoon
  • battery size is not an issue
  • capacity also is not a big deal, it would have to run the pump for a few minutes every few days and then have a few days to charge back
  • it can rain a lot, but presumably I can protect the battery from water

I don't want to use lithium batteries because of the fire risk.
What other battery types can I use here?
Would a small lead-acid battery work? Given that they are used in cars and can withstand all sorts of temperatures. Is fire a risk here?
What about NiMh or NiCd, are those safer?

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    \$\begingroup\$ I'm not even slightly close to a safety expert on battery chemistries and I don't have a chemistry degree with specializations that would allow me to quickly develop needed theory, either. But there are relatively safe battery technologies. NiCd seems quite safe from past experience. But so has been a small motorcycle battery -- both flooded and sealed (though I prefer flooded because of decades of use and familiarity with hydrometers for them.) Chemical leakage would be an issue I'd want to watch for, plus your temp range. Lead acid should be fine though. Lots of safety experience with them. \$\endgroup\$
    – jonk
    Nov 1, 2022 at 5:04
  • \$\begingroup\$ The OP's spec is summarized below: (1) Safe (number one priority) , won't explode, (2) 12V self contained, (3) Applicable for automated plant watering pump, (4) rechargeable by solar panel, (5) operating temperature: -6C in winter to +45C in summer, outdoor balcony rains often, (6) battery size not issue, (7) Would run the pump for a few minutes every few days and then have a few days to charge back, ... \$\endgroup\$
    – tlfong01
    Nov 1, 2022 at 5:24
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    \$\begingroup\$ I have a similar application where I use a sealed lead-acid AGM battery in conjunction with a small solar panel and charge controller to pump standing water out from under a deck. We don't often get temperatures much below 10 C in winter, but it has been working reliably for several years now. You might want to check the battery's minimum operating temperature to be sure it's OK. \$\endgroup\$
    – John D
    Nov 1, 2022 at 22:36

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Sealed lead acid batteries are pretty robust. Get sealed ones so they won't leak acid if knocked over. NiMH is also quite safe. NiCd is now obsolete, as cadmium is toxic, and they offer few advantages over NiMH.

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All lead-acid batteries have the problem that the reaction is between a plate and the acid. The acid's chemical composition changes from full to empty. And as it gets emptier, its freezing point raises.

This is a risk that needs to be managed. But it's unlikely to freeze at -6C, and you shouldn't be dipping a lead-acid battery that low in any case.

That is another risk; it damages the battery to dip it below about 65% charge. (that is, you only have about 1/3 the battery's paper range to actually play with reliably). This is one way in which they are pretty terrible.

Another is their short overall service life. They only last 3-5 years at best. Meanwhile I have a set of nickel-cadmium wet cells that were old when we got them in 1986, were horribly neglected from 1990 to 2013, and they still work. (probably need an electrolyte change). I'm not saying "use nickel-cadmium", but many chemistries - any chemistry - will perform better than lead-acid, both in terms of service life and tolerance.

However, lead-acid has more readily available "charge controllers".

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    \$\begingroup\$ I have had SLA batteries, and flooded deep cycle batteries, last at least 10-15 years with infrequent use and only occasional charging or continuous trickle charging. \$\endgroup\$
    – PStechPaul
    Nov 2, 2022 at 2:54

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