Running an MPPT Charger from Solar-DC & Shoreline AC-DC Sources

[CaneyJ]

I currently have a CTEK M200 15-Amp marine-charger hooked up to a pair of sealed 85-Amp batteries in parallel. I want to upgrade these to a pair of Trojan T105 batteries as well as installing around 400-Watts of solar panels.

 

The current CTEK shoreline charger cannot charge these future Trojans to a decent voltage nor do proper equalisation so it will also need replacing.

 

I’ve done some research and I’m looking at getting one of the newer [and cheaper] MPPT solar chargers, the Tracer A-series rated to 100VoC. Complete with an external MT-50 digital controller; the 30-Amp version only costs £127.

 

I was also eying up a Victron Bluepower IP22 30-Amp mains charger but then had an alternative thought. Why not combine the two devices to work as one by providing an alternative AC-DC converted source and feed it as an alternative DC-Input into the MPPT controller via a 3-way isolator switch to swap between either the solar input or a generator’s input in winter?

 

A 600-Watt rated 24V switching AC-DC power supply can be had for quite cheap. An example on the amazon http://www.amazon.co.uk/AC110V-DC24V-Switch-Supply-Driver/dp/B019RNKV5E/ref=sr_1_1?ie=UTF8&qid=1456146216&sr=8-1&keywords=600w+24v+power+supply is only around £34. Switching power supplies usually aren’t too picky about the quality of the AC input so it can be run from a basic 700-Watt rated generator like the Honda EX1000 or smaller Screwfix Impaxx 800.

 

This would cut down on wall-mounting two devices as space is limited and will only require connecting the switching supply via an Anderson plug only when needed and save on costs as the Tracer charger offers much more in terms of custom charge times and voltage adjustment than most budget shoreline chargers.

 

The specs state that the MPPT controller can handle panel ratings up to 390-Watts so I assume there is a point where there is current limitation of some sort so it shouldn’t overload a 24V, 600W supply?

 

1-2 hours of basic bulk charging will be done first by the engine’s 65-Amp alternator by which point; the input current will have dropped to around the 30-Amp mark and the MPPT charger can then be left take over. This setup should turn-out to be cheaper, have less cabling and be more compact having only the MPPT unit being permanently wall-mounted. Thoughts?

[Paul C]

 

 

Why not combine the two devices to work as one by providing an alternative AC-DC converted source and feed it as an alternative DC-Input into the MPPT controller via a 3-way isolator switch to swap between either the solar input or a generator’s input in winter?

 

 

 

....Because it won't work.

 

As I understand it, an MPPT controller is specifically for solar panels (which might be ~12V, but are much more likely to be in a series string of 60-100V or so). Its specifically designed to take the input of a solar panel and produce the best output based on the physical characteristics of a solar panel, ie MPPT = Multiple Power Point Tracking. Put simply, it "loads" the panels in a varying way to determine its most efficient point of operation (voltage and resulting current). I don't think it will respond appropriately with an input fed from eg a power supply run from a generator.

 

Also bear in mind that MPPT controllers aren't that great as battery chargers, in that they don't really "do" bulk - absorption - float that well, compared to a good quality battery charger. They tend to be very conservative - ie gentle on the batteries. BUT because solar power lasts all day, they can afford to be.

[CaneyJ]

Ok; my crude understanding was that a solar charge controller was basically a DC-DC step-down converter with a multi-stage output behaving like a charger that just so happens to run on a wide-VDC input.

 

The Tracer-A manual says that so long as the input voltage is +2V above the battery charging voltage it should work. In the case of charging traction batteries in cold weather, that means it needs a bare minimum of 18-19V.

 

The manual for the controller also shows efficiency curves that show whilst an MPPT can take higher voltage; like a PWM; its peak efficiency is actually best at lower input voltages closer to the battery which makes sense as the less-extreme a DC-DC conversion, the more efficient it is though there is only a tiny % efficiency difference between 17V and 68V input.

I know that the higher voltages are great for long cable runs and that you can use the cheaper high-voltage 250W house panels over the expensive 100-Watt 18V caravan ones.

 

Indeed some MPPT controllers can only operate up to 50V VoC max like the EcoWorthy 20-Amp controller. Switching power supplies can also be bought cheaply in the 600-Watt ratings which have 36V & 48V outputs should there be a need to supply a 24V battery bank.

 

I’ve seen controllers that are rated upto 100-Amps. If they are supplied with enough watts input, how are they gentler on the batteries when supplying the initial full bulk charge?

[WotEver]

How are they gentler on the batteries?

Most (none?) of them can be configured for a 14.8V absorption charge.

 

Tony

 

ETA, and they go into float MUCH too soon for generator charging (which, ideally, shouldn't have a float stage at all).

Edited February 22, 2016 by WotEver

[CaneyJ]

Which is why I specified the Tracer with the MT-50 input controller. It allows seperate adjustable absorption, equalize & float ratings between 9V to 17V in 0.1V increments using a custom charge profile. It also allows adjustable timing on how long it remains in absorption phase from 10-mins up to 3 hours. I'm guessing with deep cycle you need to be 7+ hours before float? There is no shut-off once it reaches float but that is usually when you shut-off the generator anyway or simply set the float to 13.2V for Trojans to leave in storage.

[WotEver]

You're proposing doing two things, which require different setups...

 

1. Charge with a genny. This needs to be set up (for T105s) somewhere along the lines of "14.8V at all times". When the tail current reads low enough you will stop the genny so that you can...

 

2. Charge with the panels. This needs to be set up much more gently with a good long float charge to gradually get the batteries really close to 100%.

 

Tony

[Paul C]

By all means try it - you've obviously given it a lot of thought. I don't think the MPPT controller will behave as you predict though. Another aspect is that with the generator/PSU connected, the solar panels will be disconnected and its not 12V you're messing with, it could be 100V. That's a dangerous level of voltage so I'd use the same precautions as you would with mains wiring, ie Andersen connector is not suitable because it leaves pins exposed (I know it shields them slightly, but not enough IMHO). Its easy to solve by using (for example) a continental mains plug and socket (you'd not want to muddle it up with 230VAC!!).

[swift1894]

You're proposing doing two things, which require different setups...

 

1. Charge with a genny. This needs to be set up (for T105s) somewhere along the lines of "14.8V at all times". When the tail current reads low enough you will stop the genny so that you can...

 

2. Charge with the panels. This needs to be set up much more gently with a good long float charge to gradually get the batteries really close to 100%.

 

Tony

This is why I think.... genny on in the morning for bulk charge and then let the panels take over when (if!) the sun comes out for the rest of the day.

[Loafer]

 

....Because it won't work.

 

As I understand it, an MPPT controller is specifically for solar panels (which might be ~12V, but are much more likely to be in a series string of 60-100V or so). Its specifically designed to take the input of a solar panel and produce the best output based on the physical characteristics of a solar panel, ie MPPT = Multiple Power Point Tracking. Put simply, it "loads" the panels in a varying way to determine its most efficient point of operation (voltage and resulting current). I don't think it will respond appropriately with an input fed from eg a power supply run from a generator.

 

Also bear in mind that MPPT controllers aren't that great as battery chargers, in that they don't really "do" bulk - absorption - float that well, compared to a good quality battery charger. They tend to be very conservative - ie gentle on the batteries. BUT because solar power lasts all day, they can afford to be.

 

Point of order, M'Lud.

 

MPPT = MAXIMUM power point tracking, not 'multiple'.

[WotEver]

This is why I think.... genny on in the morning for bulk charge and then let the panels take over when (if!) the sun comes out for the rest of the day.

Absolutely. Nothing wrong with the principle, it is exactly what many do.

 

What is unknown is how well the solar controller will work for you as you propose.

 

Please let us know how you get on. If it works it'll be a different piece of advice we could offer.

 

Tony