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Upgrading an AC immersion heater to 48V DC

Most thermostats made for AC will not be reliable with DC, as the points can arc with each opening, causing all sorts of problems, including welding the points together. Generally you can swap out elements if the threads fit. I have considered this but I would get a simple temperature control that could switch a DC relay or build a relay driver circuit for the standard thermostat.
 
You're on the right track. Just don't forget that the "T&P relief valve" has to be correctly rated for the heating element and that it is properly discharged to the exterior. The discharge line can ONLY run down hill from the T&P, reason being is that a trap/water setting in the line could freeze and create a plug in the line thus a potential bomb. Eg:
 
It looks like Missouri Wind and Solar do a 48V DC compatible thermostat... The product page says:

can handle 25 amps at 12 volts! No relay needed! (Can handle AC voltage up to 220VAC and DC voltages up to 50VDC)

... but doesn't mention what wattage it can handle at 48V... I've emailed them to ask this... What does everyone think?

I guess ideally I'd want a second safety cut-off because if the thermostat failed and the immersion heater stuck on, I'd have a very bad situation...
 
If the original thermostat is a mechanical points type, it should work fine switching the 48 volts DC at under 1 amp to drive a 48 volt DC contactor to switch the current to the heating element. You will see many relays have a 120 volt AC rating and 48 volt DC ratings at a lower current. Be sure to have a clamp diode on the contactor coil as well. That will also keep the opening voltage spike down.

If the factory element is 120 volt, you can use it on 48 volt DC, but it will be about 1/4 the watts heating the water. It will take longer to recover, but it will still get hot.

50 VDC switch rating is probably fine at low current, but most 48 volt battery systems are over 55 volts most of the time. Mine charges to 57. I like to see 60 volt ratings.
 
Hi, I'd like to use one of these Isotherm Water Heaters, but change the electric heating element to a 1500W, 48V DC element. Can I just swap the heating element out with something like this? Would the thermostat also need to be modified? Some people have warned that AC thermostats might not enjoy switching DC...
Keep in mind that a normal household electric water heater is between 4.5kw and 5kw so with a water heater with a 1.5kw element i would expect to perform about 3x worse than a normal household version.

Your BMS better be top tier. As your likely thinking that your going to tie your 48v heating element directly to the battery pack so the only thing to keep it from over discharging (bad) is going to be the BMS. Unless you setup some sort of external voltage monitoring and disconnect for the water heater load.
 
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