Everything you need to know about batteries, power and heating water for free!
In most installations, there will be three main sources of power. These will be power from the sun, from the engine or from a charger. Any of these can be used to heat water.
A 12v electrical system works on balance. If the power coming into the system is higher than the power being consumed, the excess power will flow into the battery raising its state of charge. If the power being consumed is higher than the supply power, the battery will discharge and supply energy. Note that with lithium batteries power will flow to the loads before the battery gets charged- with lead acids the charge rate is usually unaffected as it is slower than with lithium.
Examples of how the controller works
In this graph, we can see the voltage climbing to the heater switch on voltage of 13.2. The voltage is climbing because the sun is out, and the charge controller is raising the voltage in order to charge the batteries. We can assume that in this scenario, when the heater is switched on there is around 100 watts coming from the solar panels.
As soon as the heater is switched on, the voltage drops because there isn't enough solar to sustain the heater. The battery is having to supply 200 watts of power, which it cannot do without a voltage drop. This shows the importance of having a time delay in the controller as if there was no time delay the controller would quickly oscillate between the two states, cycling the element every few seconds which is to be avoided.
In this example, we have 300 watts of solar being generated. When the voltage crosses the threshold and the heater turns on, the voltage is maintained.
In our final example, the power coming in exceeds 300watts. The voltage isn't affected by the load as the load is smaller than the available power.
As mentioned earlier, a given load applied to the batteries will drop the voltage. The amount that this drops depends on the size, type and age of your battery bank, as well as the state of charge of the battery. A lead acid battery may drop something like 0.8v per 1000 watts, with a lithium battery dropping around 0.2v per 1000w. Therefore we need to calculate our switch off thresholds depending on how much of the battery bank we want to use. If we simply assume that a battery has a linear scale between 12v and 12.8v and this is proportional to state of charge, we will make incorrect calculations because if a full battery has a load on it, we would expect to see a lower voltage, especially with a lead acid battery.
With lead acid batteries you want to only turn on your element either when driving or there is enough solar. However, you want to turn off threshold to be relatively high as lead acid batteries have a limited number of cycles and you don't want to use battery cycles heating water.
With lithium batteries you can control the turn off threshold more easily and can use some of the battery energy for water heating. You will need to test different voltages and monitor the battery perentage to see how much energy was used by the battery for each value. For our setup of one 150aH lithium battery the values were as follows:
13.0V: 99%
12.9V: 79%
12.8V 66%
12.7V 40%
12.6V 33%
12.5% 12%
These values were read from the controller with the water heater switched on- this will then account for the voltage drop on the battery when heating water.
In summer we are happy to use up to 67% of the capacity so will set the voltage cutoff to 12.6%. However, towards the end of September until the beginning of April we will modify this to 12.8V as there is less solar replenishment and in winter we will turn the system off completely. Note that this is for our useage and our battery, your system will be different; if you have a smaller battery you may want to increase the threshold whereas if you have a large battery then a lower threshold allows you to use more of it.
It is also recommended that the controller time delay be set to 5 minutes as the controller will effectively 'test' the input by switching the heater on 5 minutes after the voltage threshold has been reached, which may happen when there is the slightest bit of sun.
Here are suggested values for different battery banks:
| Turn on threshold | Turn off threshold |
Lead Acid | 14.0V | 12.8V |
Lithium Summer | 13.5V | 12.6V |
Lithium Spring/Autumn | 13.8V | 12.8V |
Note about seasons- at times when incoming power is at a premium (an English winter with no hookup, no sun and limited driving) it is better to switch any lithium based system off completely. To heat the tank from cold will consume around 60-70 aH of battery capacity so diesel is the best method for heating in these periods. It is much better to use this power to charge the batteries than heat water. For lead acid batteries the power will be supplied by the alternator directly rather than through a B2B charger so the system can be left on.
To set your controller watch our youtube video here: