Running hot water heater from batteries

sailaboutvic

sailaboutvic

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I posting this else where on a battery site but haven't really got the answers I was looking for .
So given it a go here .
Question .
Trying to see if I can run my hot water immersion heat using the excess power from my solar panels .

Batteries back 480AH solar panels 400W not sure the watts of the Immersion heater I guess 1000w

Basically once the batteries are full charge and on float the V around 13.42v current in 3A not taken iin account for the fridge coming on and off the the current rising slowly as it does,
Switching on the water heater through the inverter I'm see an 80A draw the panels push in 20A ( 1000A roughly being used ) and the volts start to drop after some mins there are down to 12V that's with the solar panels still working .
Around 17 mins of non stop power use the batteries are showing 11.8v ( underload) 28 Ah used
my question is how low can I let the Volts drop before it become unsafe to do so .
Is there some kind of table where I can work this out .
Thanks
 
There are 12v immersion heater elements. Was an option as a dump load for some wind generators. Much lower wattage available putting less strain on the system
 
Basically, your solar panels will provide UP TO 400w. IF used entirely for water heating, assuming only small losses, you have 400w available to heat water. That's not bad , and a DC immersion heater should use that efficiently. If I've got my numbers right, 400w will raise the temperature of a litre of water by 1 degree C in ten seconds. So, to raise the temperature of the water by (say) 20 degrees C will take about 4000 seconds for a 20 litre tank - that's just over an hour. As you're in the Med, I'm assuming that the water being heated is at a temperature of 15-20 degrees C already. At 12V, the current will be about 33 A.

The amount of energy used to do that is 400 watt hours, which at a voltage of 12V equates to a battery capacity of 33 Ah, which seems too low - I may have made a mistake somewhere along the line!
 
The immersion heat is a 230.Ac element.
I looking at way of useing power from my solar panels which normally just going to waste .
Come 11am my batteries are normally on float .

I could just as easy run the engine at the same time turn on the Gen to run the immersion heat and have Hot water in less the 20 mins , but that would mean using fuel with two engine running , no point in that at all.
On the other hand I don't want to mess up £1000 of batteries just to make hot water while at anchor .
So back to my question , under that type of load ( 1000A) how low is it safe to,let my batteries voltage drop ?
 
sailaboutvic said:
The immersion heat is a 230.Ac element.
I looking at way of useing power from my solar panels which normally just going to waste .
Come 11am my batteries are normally on float .

I could just as easy run the engine at the same time turn on the Gen to run the immersion heat and have Hot water in less the 20 mins , but that would mean using fuel with two engine running , no point in that at all.
On the other hand I don't want to mess up £1000 of batteries just to make hot water while at anchor .
So back to my question , under that type of load ( 1000A) how low is it safe to,let my batteries voltage drop ?
Click to expand...
Does your solar regulator have what I call an "overflow"? Mine does but I haven't bothered to wire it up. It would in theory chuck "spare" power straight at the fridge once the batteries are almost charged. If you used such a power source for your water heater, it wouldn't draw from the battery at all (although you could rig it to do so if /when desired?)
 
The figure given in the OP of 100A is what being drawn from the batteries while the immersion working through the inverter .
So that taken In account any lost .

I suppose what I am looking for is the voltage reading of the batteries when under that load to turn off the the immersion heater and let the batteries recover .
 
AntarcticPilot said:
It won't work with a 230VAC immersion heater, unless you go through an inverter, which will impose substantial losses. You need a dedicated 12V immersion heater for this to work.
Click to expand...
My plan is a 110v invertor wired just to the immersion with a VSR so that will send a quarter of the watts to the immersion heater and cut off the batteries at 12.5v under load. and only switch itself on at 13.8v. I accept the invertor loss but it makes use much simpler and means I won’t have to mind it. My main need is finding a cheap 110v invertor with the tiniest current draw when not under load.
 
I took this from an article that I read some time ago, unfortunately, I can't now find reference to it. Can't vouch for its accuracy but I recall several sources saying largely the same thing.

It seems that there are many variables and opinions, hence you can draw parallels with "best anchor" advice.

Common advice seems to be to not let lead acid batteries fall below 50% state of charge. It seems to be ok to let them go further occasionally but only if they are recharged immediately.

This table is based upon 25 deg C. 100% is a rested voltage, say 12 hours (no load) after a full charge. SG is said to be the most reliable indicator.

SoC 100%, Voltage 12.7, SG 1.255-1.275
SoC 75%, Voltage 12.4, SG 1.215-1.235
SoC 50%, Voltage 12.2, SG 1.180-1.200
SoC 25%, Voltage 12.0, SG 1.155-1.165
SoC 0%, Voltage 11.9, SG 1.110-1.130

Hope this helps.
 
sailaboutvic said:
I posting this else where on a battery site but haven't really got the answers I was looking for .
So given it a go here .
Question .
Trying to see if I can run my hot water immersion heat using the excess power from my solar panels .

Batteries back 480AH solar panels 400W not sure the watts of the Immersion heater I guess 1000w

Basically once the batteries are full charge and on float the V around 13.42v current in 3A not taken iin account for the fridge coming on and off the the current rising slowly as it does,
Switching on the water heater through the inverter I'm see an 80A draw the panels push in 20A ( 1000A roughly being used ) and the volts start to drop after some mins there are down to 12V that's with the solar panels still working .
Around 17 mins of non stop power use the batteries are showing 11.8v ( underload) 28 Ah used
my question is how low can I let the Volts drop before it become unsafe to do so .
Is there some kind of table where I can work this out .
Thanks
Click to expand...
Pinching Antartic Pilots figure and assuming you tank is 20L then the temp should rise from 20C to 80C after 15mins and your batteries at 11.8 ( but will bounce back quick?)
would first of all see if your solar regulator can be easily made to dump excess power. If not the a simple control to cut off supply to water heater set at, say 12 volts?
Either way you should safely get hot water within half an hour?
 
As I recall, the OP has 4 x Trojan T125 batteries in series/parallel to give a 480Ah 12v bank. With a 100A load, each battery will be providing 50A. If you put the figures into a Peukert calculator, this means that the effective capacity of each battery is reduced from 240Ah to about 170Ah, and so the battery will be totally flat in about 3 hours. Assuming you want the batteries to have a decent life, they shouldn't be discharged more than 50%, so say 1.5 hours max running time on the inverter.
 
If you must use an invertor how about putting it on a cheap AC timer to go on 10 mins, off 20 or 30 mins, on 10 mins etc and repeat ? On 5 then 15 off would be better but most cheap mechanical clock times don't offer that. Obviously a 250 watt or 500 watt 12 volt immersion element would be the best option of all.
 
sailaboutvic said:
I posting this else where on a battery site but haven't really got the answers I was looking for .
So given it a go here .
Question .
Trying to see if I can run my hot water immersion heat using the excess power from my solar panels .

Batteries back 480AH solar panels 400W not sure the watts of the Immersion heater I guess 1000w

Basically once the batteries are full charge and on float the V around 13.42v current in 3A not taken iin account for the fridge coming on and off the the current rising slowly as it does,
Switching on the water heater through the inverter I'm see an 80A draw the panels push in 20A ( 1000A roughly being used ) and the volts start to drop after some mins there are down to 12V that's with the solar panels still working .
Around 17 mins of non stop power use the batteries are showing 11.8v ( underload) 28 Ah used
my question is how low can I let the Volts drop before it become unsafe to do so .
Is there some kind of table where I can work this out .
Thanks
Click to expand...
Geem explained to me that a 12v to 110v inverter is what to use. By halving the voltage of the inverter, less power is used? Some one else can explain it better than me I am sure!
 
pvb said:
As I recall, the OP has 4 x Trojan T125 batteries in series/parallel to give a 480Ah 12v bank. With a 100A load, each battery will be providing 50A. If you put the figures into a Peukert calculator, this means that the effective capacity of each battery is reduced from 240Ah to about 170Ah, and so the battery will be totally flat in about 3 hours. Assuming you want the batteries to have a decent life, they shouldn't be discharged more than 50%, so say 1.5 hours max running time on the inverter.
Click to expand...
Your calculations of my bank is correct , ofcourse I haven't any intention of running it for 3 hours or anything like that . But 45 mins early in the day would be good leaving enough day light to being the batteries back up to float again .
100Ah for 45 mins that about 75A well about the 50% ,
MY concern is the voltage , will the voltage at this point be at a point where it may damage the batteries.
I only tested this for 17 MINS , voltage then drop to 11.8 v but recovered to 12.5 v very quickly , the recovery time was with the panels turned off ,

This is Why I keep asking what the safe reading of voltage under a load of 100A I can go down too.
 
Those suggesting using any terminals on the solar controller (call them anything you want) is inapplicable here. The immersion heater is 240v, so will only run via the inverter.

Vic, if the immersion heater is drawing 100a via the inverter, it's almost certain it is, as you suspected, 1kw @ 240v

With a suitable battery monitor you could automate a safety shut-off for the inverter. The Victron BMV series battery monitors have inbuilt relays that can be configured to cut in/out at user set voltages. This could be connected to an external relay that would turn the inverter off when the battery voltage reached a pre-set voltage and turn it back on when it rose above another pre-set voltage. It could we wired into the 240v outlet side of the inverter, but my choice would be to connect it to the remote on/off switch of the inverter.
 
PaulRainbow said:
Those suggesting using any terminals on the solar controller (call them anything you want) is inapplicable here. The immersion heater is 240v, so will only run via the inverter.

Vic, if the immersion heater is drawing 100a via the inverter, it's almost certain it is, as you suspected, 1kw @ 240v

With a suitable battery monitor you could automate a safety shut-off for the inverter. The Victron BMV series battery monitors have inbuilt relays that can be configured to cut in/out at user set voltages. This could be connected to an external relay that would turn the inverter off when the battery voltage reached a pre-set voltage and turn it back on when it rose above another pre-set voltage. It could we wired into the 240v outlet side of the inverter, but my choice would be to connect it to the remote on/off switch of the inverter.
Click to expand...
Hi Paul
But what's voltage can it be set at ie what's the safest voltage at that load taken inconsiderate the spec I given .
There where I trying to work out .
Is it a case where the voltage doesn't matter and the percentage that importation ?
In which case doesn't it not matter what the voltage read underload as long as I don't go down to 50% of battery capacity.
 
sailaboutvic said:
Your calculations of my bank is correct , ofcourse I haven't any intention of running it for 3 hours or anything like that . But 45 mins early in the day would be good leaving enough day light to being the batteries back up to float again .
100Ah for 45 mins that about 75A well about the 50% ,
MY concern is the voltage , will the voltage at this point be at a point where it may damage the batteries.
I only tested this for 17 MINS , voltage then drop to 11.8 v but recovered to 12.5 v very quickly , the recovery time was with the panels turned off ,

This is Why I keep asking what the safe reading of voltage under a load of 100A I can go down too.
Click to expand...

Hi Vic,

We do this all the time without problems. We now have LiFePo4 batteries but we also did it when we had Lead Acid batteries although we did have a bigger battery bank than yours. We had 1000AH LA and now have 400AH LiFePo4. We also have more solar capacity than you so the actual battery load in our system is less because our solar produces more power.

To answer your original question - if your batteries are 100% and your solar is producing around 20A then it is perfectly acceptable to run the immersion heater for 30 - 40 mins without risking damage to your batteries. Technically it may shorten the total lifespan slightly but other factors will have more impact on battery lifespan so you will be unlikely to notice any difference. The battery voltage will get pulled quite low due to the large load but it will quickly rise again when the immersion is turned off as you have already seen. If you run the immersion for 30-40 mins the water will be plenty hot enough for whatever you need and the battery soc will still be at least 75% so no problems.

The actual battery voltage under a large load is not that important, we would regularly see 11.5V on our 1000AH bank if we were running a large load through the inverter. We would also run our watermaker through the inverter and this would mean over 200A being drawn from the batteries, but only for 15-20 mins at a time. Our LA batteries lasted over 10 years and still showed 850AH capacity when we changed to LiFePo4 so they obviously did not suffer too much from our abuse!!

The single most important factor for Lead Acid battery life is to make sure they are regularly charged to 100%. Long periods without 100% charge will reduce capacity for sure. Obviously you are getting to 100% almost daily so you have very little to worry about.
 
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