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I do not know much, but I have a portable shower (http://help.venditiogroup.com/support/solutions/articles/6000178636-zenvida-portable-tankless-water-heater-outdoor-shower-260-pcwh01) that I would like to set up to be powered by the 12V car battery that runs the water pump which is a necessary but separate part of the shower. All the 2 D batteries do is spark to light LP to heat the water. The lock on the battery compartment broke and the MFG wants $10 to send me a replacement. This is a mostly permanent set up where the car battery is on a solar charger and I just don't see the logic in maintaining the D cell batteries but I am clueless to what would be necessary to convert and would appreciate any assistance that can be provided.

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  • \$\begingroup\$ This would cost far more than $10. If you still think it's worth it, give us more information on what's inside the machine and maybe we can help. How is the high voltage for the spark generated? \$\endgroup\$
    – Hearth
    Oct 9, 2018 at 16:02
  • \$\begingroup\$ You are not going to end up in less than $10 in any scenario. Since you don't really know what you are doing I would suggest to go with the simple replacement. \$\endgroup\$
    – Eugene Sh.
    Oct 9, 2018 at 16:02
  • \$\begingroup\$ All that I know is that the 2 D cells are in series and the battery compartment plugs into wires going deeper into the heater. I have not found any specks online (I have to drive to town for cell service so no internet at heater) so additional info is not readily available. \$\endgroup\$ Oct 9, 2018 at 17:10
  • \$\begingroup\$ Welcome to the site. \$\endgroup\$
    – mike65535
    Oct 9, 2018 at 17:12

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Two D batteries provide 3V. You can use a 3V regulator (LM317 wired properly) to decrease the car battery voltage to 3V.

However, since this is a gas heater, I'd recommend paying the $10 and getting the replacement... for peace of mind and preventing a fire (explosion) hazard.

To explain about the possibility of explosion hazard... If it is possible to supply the 3V using long wires (i.e. the battery and voltage converter is somewhere else, at least a few meters away) this should be safe. Considering also that if the heater is waterproof, it may also be outdoors, not in an enclosed space, which reduces the risk. If this was a manually operated device, i.e. you turn a knob for the spark, and gas line shuts off if flame goes out, there would not be any worry. However, the supply is 12V, so 9V is dropped across the regulator. To create sparks a substantial current is drawn by the boost converter (which creates a high voltage), hence the D cells (instead of AA cells). Being automatic, if there is some intermittent fault in the gas line, the boost converter will operate for a longer time and draw high current. At 9V drop, this will heat up the voltage regulator, maybe a lot. If located near the gas line, could lead to an explosion. Risk can be minimized by using a 5V or 6V supply for the LM317, and a large heatsink for it. In any case, it is best to place it far away from the gas lines. Other than that, it should all be fine. Use thicker wires and make sure they are connected properly (crimped).

One could potentially use a step down buck converter to decrease 12V to 5V, and then regulate it down to 3V (or 3.2V to account for voltage drop on long wires). This is probably safest, and most energy efficient (considering it may be running from solar), but may just be too much work. With a step down buck converter to 3V, the regulator may not even be necessary, but that may be a bit risky, without knowing what's in the boost circuit inside the heater. If the gas does actually turn off, if the flame igniter fails, then it may be safe, otherwise not.

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  • \$\begingroup\$ Sounds to me like the broken battery door and its replacement cost is beside the point - the OP wants to be able to run the shower off the big battery and not use any D cells. \$\endgroup\$
    – mike65535
    Oct 9, 2018 at 17:14
  • \$\begingroup\$ Thanks, I will try to obtain more info about the unit. Again, this current only operates to create a spark to light the gas to heat the water. So when the power is turned on, it is not on unless the water is flowing, and the water is cold. Its kinda like the electric igniter in a gas grill, just that it keeps sparking until the gas lights and and sparks when needed if the gas shuts off because of water presser and/or temperature. \$\endgroup\$ Oct 9, 2018 at 17:18
  • \$\begingroup\$ Mike65535 is mostly correct. That would be the preference, but I am also looking at the cost benefit consideration of the conversion. \$\endgroup\$ Oct 9, 2018 at 17:22

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