Dmppt for Charging only

[Franklin Brown]

Is there any problem with using the Dmppt and Bms120 with no heating? Just using them for charging. I will eventually add heating but will this be ok?

[Dacian Todea]

Franklin.

There is no problem to use the DMPPT450 and SBMS120 for charging only. That is how I use it now in summer as I do not need heating just battery charging.

If PV array is larger than what your battery can accept (what you set as charge current limit in the SBMS120 in the DMPPT menu) then only part of the PV array will be directed to charging by the DMPPT450. If it is cloudy then the entire PV array will be used for charging and this is all done automatically by the DMPPT450.

[Franklin Brown]

Next question. I have a 4.5kw solar array up. I’m heating up a metal barrel of water. I don’t want to fully use all my energy. If I were to put 2 1200w resistive heat elements in parallel on the 50amp max output would this be safe? I’m assuming the dmppt would only send a max of 50amps to the heating element, some to the battery and just open circuit the rest.

[Dacian Todea]

Say you have you have 15x 300W PV panels each capable of putting out around 6A to 9A in the afternoon depending on how hot the panels are.

So that will be 90 to 135A total.

Then say you have a 200Ah battery and you set max charge current to 50A then you will have 40A to 85A extra for the heating elements when the battery needs charging.

The DMPPT450 will prioritize the battery so if SBMS120 asks for 50A then it will redirect as many of your 15 panels as needed to get around that 50A and if less than that is available from all of them say cloudy or later in the day then all panels will be redirected to battery.

Then if there is some extra say 40 to 85A when battery is less than fully charged or 90 to 135A when battery is already fully charged and requires 0A charging that will be available for heating.

Say you have two 36V DC heating elements rated at 1200W each that means they are 1200W / 36V = 33.33A and 36V/33.33A = 1.08Ohm

So if 40A is available for heating from the PV array then that will be enough to supply to one of the two heating elements in order for the voltage to stay above 29V to be close to max power point so that one heating element will get around 30A maybe at 32V (depends on PV array temperature) so 960W will go to that heating element. DMPPT450 will try to connect both but it will see that there is no enough power from the PV array to supply both so it will just have one switched ON.

If say 85A or even 135A is available all for heating then both heating elements will be supplied maybe with 33 or 34V on the PV array and thus 33V / (1.08Ohhm/2) = 61A will be used the rest will remain unused so about 2000W will be pushed in to heating water.