How far can you push a 48 volt immersion heater?

[Lord Dust]

Hi all, sorry if this question has already come up - I tried searching for an answer but didn't find it

I have a 300 litre / 80 gallon thermal store with two immersion heater slots which are currently filled with 2 x 240v AC 3Kw heaters
I'd like to replace one with a 48 volt 1500 watt heater
I can power this from a 48v 440ah Lead Acid battery bank for a couple of hours per day

Can I ask the wise Gurus of this Forum ?‍? ?

How much I can push that 48v immersion heater given that my battery bank sits with an open circuit voltage of 55v?

As I understand it it will cope with +5% so only 50.4v

Thank you

Lord Dust

 

[time2roll]

Assuming those FLA batteries will be discharging I doubt they will be above 50.4 long enough to matter. I don't see an issue.

 

[400bird]

How much I can push that 48v immersion heater given that my battery bank sits with an open circuit voltage of 55v?

As I understand it it will cope with +5% so only 50.4v

My understanding of resistive heating elements is they'll take anything for voltage as long as they don't get too hot. Luckily you've got some water around to keep the element from melting ?

I think it will be fine, the element might not last as long due to the higher current and heat.
If my math is right, at 55 volts it will be putting out about 2000 watts, vs 1500 @ 48 volts.

 

[MurphyGuy]

Depends if they're high density or low density elements. The high density element won't last long unless its in a current (flowing water). If you're mixing the water with a pump, it will last a lot longer, but in a static tank like a water heater where someone isn't using any water, it will probably do the job for a while, but will most likely have a short lifespan.

Low density elements will do much better, and as time2roll mentioned, those batteries won't be up at 55 volts for very long.

You could always use a mercury contactor and switch it on and off on a short cycle timer, that would effectively lower its duty cycle.

 

[Lord Dust]

Amazing, thank you all for taking the time to reply. I figured the batts would drop their voltage under load and over time as they grow old
Points noted about the low density elements - I'll source that in future, thank you
For now I guess I might just have to drain it down and replace the heater more regularly ?

This 48v circuit is currently fed via a 100amp 12v relay that is actuated by a voltage detection relay
So I wonder what 'cut out' voltage I should set the detection relay to so as to protect my 48v Lead Acid battery bank?
Is this acceptable at 48.24 volts or should it be higher?

Again thanks in advance for any advice

best wishes

LD

 

[time2roll]

Is this acceptable at 48.24 volts or should it be higher?

How deep are the cycles to be?

I figure 35 amps the battery can go 12 hours. Or 3 hours to use 25%. I would measure the voltage at 3 hours and set it there. Can be difficult to judge capacity discharged while under load.

Of course if voltage sags way more than expected it could be the battery has lost some capacity. Adjust accordingly.

Assuming they are wet cells, could dip them with a hydrometer once to verify.

 

[Lord Dust]

Thank you Time2Roll, funny enough I've set the timer for a burn from 07:00 - 10:00 so I will see what they look like at 10:00 and go from there.

The batts are all new and not yet challenged, they're charged from a 48v wind turbine or Victron MPPT. Annoyingly they're sealed so you can't dip them!

It's pretty grey and overcast here in Dorset UK until later in the year, but might even be able to get a second burn in mid afternoon if the UV holds up - or it's windy.

Many thanks

LD

 

[Lord Dust]

Update on this question:-
I've fitted the 48v immersion heater and was actually able to power up the circuit for the first time and it drew power off the battery bank as expected
Interestingly the voltage drop on the 6mm cable feeding the immersion over 4 metres is quite significant! So I was reading 46.7 at the immersion

More interesting was the 12v 100amp relay that is meant to switch it on/off. It burnt the contact out almost immediately!?
Surely a 100amp rated relay shouldn't fail at 28amps?

Anybody have any suggestions for a better 12v switching option?

Many thanks all

LD

 

[cyd]

Yes ditch the DC setup and install an AC element. Use a 4000va inverter. Then use a 240Vac SSR to switch the element. No moving parts, zero crossing switching, no arcing. Also cable power losses will much smaller.

You seem comfortable draining down the tank so it shouldn't take long.

 

[JHovel]

That relay looks like it didn't extinguish a switch-off arc! Are you certain it was meant for DC switching? They need a MUCH larger gap between contacts.

 

[time2roll]

Have you looked at solid state relays?

https://www.amazon.com/SSR-40DD-Solid-State-Relay-Output/dp/B08FQL4H8R

(random search, shop around)