Easy PV water heating

[BruceM]

Hey Stef,
Glad you got good service from your direct PV to heating element setup. Bravo!
I see there are some Chinesium MPPT charge controllers that will do up to 100A at 12V, with 200 max open PV string,  which is a good match for your setup.  I would not go less than 200V max rated so you some headroom for spikes and such.  Your 4 panels in series open circuit voltage will likely be as high as 4 x 44V or 176V.

So the two figures to look for are the rated current for 12V nominal battery, and the max PV voltage.

Most of the ones that match your needs seem to be out of stock. 
Chinesium is a bargain price but the likelyhood of failure is significantly higher.  This is a concern for this type of controller as when it fails, cheap or expensive,  it is likely that it will then apply the full PV voltage to your battery. This has caused the destruction of very expensive lithium battery banks, as the over voltage safety protection can't handle that high of a voltage so fail closed also. This cascade of failures then destroys the entire battery bank.  There has been much discussion of this issue and there are lots of ways to add some safety hardware.   One way is to use a cheap 20A or better rated solid state 230VAC relay to short the PV panels if the regulated output goes above your equalization voltage.  A 12V, 1/8 or better Watt zener diode to the SSR control will do that.  Once its triggered, the relay will not turn off unless you open the PV breaker.

I'm not up on the high end charge controllers.  Midnight solar does have a model that is rated for 200V PV. $650 US
https://www.thesolarbiz.com/midnite-solar-classic-200-charge-controller.html?gclid=CjwKCAjwp9qZBhBkEiwAsYFsb557Er92paX7G92NCg8ApiMg-kZTMpCHzpbCiCwv6jPxHiBZNSQdhxoCAa4QAvD_BwE

Makes me think more fondly of the Chinesium units with an added safety SSR and zener.

[dieselspanner]

Hi Bruce

The direct pv worked well, thanks to yourself and all the others that weighed in, this is a great forum to be a part of!

I've just ordered a 200v, 50amp version of this.....

EPEVER®MPPT Solar Laderegler Tracer 5420AN 6420AN 8420AN 10420AN 12V/24V/36V/48V

The Midnight Solar one is well sexy but with import and customs duties it'll be about twice the cost of the whole system so far, including two days of Juliette the roofer helping me with the bracketry.......

At least I'm in France and away from the plummeting pound!

The above controller is coming directly from Germany so it's an inter European deal.

I like the Zenner diode / SSR cutout, the most I've seen when charging is 14.7v at the battery bank, so would a 16v Zenner diode be appropriate?

Thanks again,
Cheers
Stef

[BruceM]

Hey Steff,
Good choice on the MPPT charge controller you picked.

The AC solid state relays (SSR) often need 3 volts to trigger.  Some may trigger at 1.5V.  So 16-3= 13V zener,  16-1.5= 14.5V for a lower SSR trigger.  You can add zeners and even forward diodes (roughly 0.3 V each for such a small current) to get the safety trigger point you want. The beauty of this battery over-voltage protection method is the latching of the AC SSR when used for DC, and their cheap prices.  I would test by adding alkaline battery cells or similar in series to the +12V batter terminal, and running that to the zener-SSR.  Or an adjustable bench supply.  Many ways to do it with what you've got on hand.  If you get false triggering from spikes on the 12V, you can add 100 ohm resistor then roughly 1uF capacitor across the 12V to smooth out those glitches.  This will not cause ANY drain of the battery normally, and only a about 4 ma of current when its triggered and shorting the PV to save your batteries.
 

Best Wishes,
Bruce