I have a backup sump pump, a 12V "Basement Watchdog".

However, immediately after installing it, I found the battery was basically dead.

Not wanting to buy a very expensive replacement battery, I got a Deep Cycle Marine battery, which works beautifully.

I've been told that the "Basement Watchdog" battery is specially designed to be safe to use indoors, as it is sealed, while my Marine battery "emits dangerous fumes".

The sump pit is in my basement, in a crawl space. While there is certainly a lack of ventilation, it does have an hvac register (which doesn't prevent the crawl space from smelling like standing ground water when there are a few inches in the pit).

There's no warnings on the battery regarding indoor use.

I thought the battery only released said "dangerous fumes" when rapidly charged, and mine is charged via a very low current charger. Is it safe to use this battery?

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    My recommendation would be to contact the manufacturer and ask them. Oct 11, 2018 at 14:03
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    "Safe" is a subjective term. You'll have to decide for yourself whether the crawl space has adequate ventilation to prevent acid fumes from making their way into your home and whether you accept the risk.
    – isherwood
    Oct 11, 2018 at 14:03
  • I believe it isn't acid fumes, but hydrogen gas that can be produced while the battery is charging. So, in theory, you could have an explosive condition under your house. That's only in theory, though; I have no idea what the risk is in practice. Oct 11, 2018 at 14:37
  • It is hydrogen sulfide gas, which, while flammable, is detectable (has the smell of rotten eggs). The manufacturer promotes their use with a sump, which is typically indoors, but I'd like to know the risks. I've reached out to the manufacturer, but maybe I'll just store it in my garage until the power goes out. Oct 11, 2018 at 14:42
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    H emission is directly related to the charge rate: the faster charge, the more hydrogen. i saw a video last night about using a marine bat as a UPS backup, and he claimed to run the numbers and that by charging it at rate to replenish in 30 hours, it would take 2 months to build up enough hydrogen in a sealed room to be flammible; nowhere near enough to be dangerous...
    – dandavis
    Oct 11, 2018 at 20:39

1 Answer 1


3% hydrogen to air is explosive. Even sealed batteries discharge some hydrogen gas when charged--they have to release the pressure or they will break the case. The seal is accomplished with a rubber (or silicone like material) over the port and usually a plastic shield welded in place to keep the seals in place. These batteries are not designed to be refilled.

The risk of explosion is very low with both battery types unless overcharged. Open cell or vented batteries require a flame arrestor (see NEC 480.11). In some systems this is as simple as a strip of mesh fabric like red scotch bright secured over the cells it needs to be acid resistant because a large component of batteries is sulfuric acid.

With the flame arrestor in place both batteries present similar hazards with the exception that marine deep cycle batteries many times end up being larger (more amp hours) so they take longer to become over charged, so there is really no difference in hazard after a flame arrestor is installed.

  • Thank you for your answer. Is explosion the primary concern with marine lead acid batteries? I will install a flame arrestor per to your recommendation, (although my battery is trickle charged with a tiny 500mA supply). Are there health concerns? Are there any other rules of thumb for safety when using battery indoors? Oct 11, 2018 at 16:05
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    A drip tray is a good idea just in case but at this low of a charge level I would not be concerned to extend battery life a float charger is what I use some trickle chargers charge all the time where a float charger stops when the voltage is at target. Other than this it sounds like you have a good setup.
    – Ed Beal
    Oct 11, 2018 at 16:25
  • Thank you again for your comments on this. I'll leave it open for a few days before accepting just in case... My biggest concerns were health-based. On another note, the "Basement Watchdog" constantly trickle charges, can you recommend any float chargers, or perhaps, something like an in-line float circuit I can use in conjunction with my setup? Oct 11, 2018 at 17:04
  • Float charge provides enough power to drive the load and also trickle charge the battery. So only trickle charge is needed because power will be out when that specific load will be called upon. This of course assumes that you have both a power sump and the battery sump. I didn't want to plumb 2 sumps so I use a power sump with an inverter which only gets connected to the sump when the power goes out. Love my power out relay!
    – user68386
    Oct 15, 2018 at 22:29
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    Thanks Ed! I stand corrected. To further eliminate the debate, please read here... batteryfloatchargers.com/how_float_charger_works .
    – user68386
    Oct 16, 2018 at 20:32

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