Alternate DC input into solar regulator other than panels?

[steve.lorimer]

Hi folks

I've got an overland campervan which I'm currently driving around Africa. I recently found my electrical system has some severe shortcomings, and since I'm in the middle of deepest darkest Africa, I'm having to try sort things out myself.

I have 3 x 100Ah deep-cycle batteries, 2 x 60W panels and a Steca PR 2020 solar regulator. The Steca solar regulator uses PWM control to achieve multi-stage charging, resulting in optimum battery charging. However, at peak solation, I'm only getting about 7.5A from the panels, and often I'm taking more than that out.

Now I've also got a 15A DC/DC converter which runs off my truck's alternator, converts 24V to 12V, and connects the 12V output straight to the deep-cycle batteries. It works, up to a point, but won't be able to charge the batteries fully (apparently it will only achieve about 50% charging). I'd like to take advantage of the solar regulator's 'smart' charging system to fully charge my batteries when driving.

My solar regulator is rated up to 20A.
My panels produce maximum 8A.
My DC/DC converter produces maximum 15A.

What I've tried to do is connect the solar panels to the solar regulator when I'm parked up, and then when the truck is running, disconnect the panels and connect the DC/DC converter in their place.

I've done this successfully with a relay, but when I look at the solar regulator's display for the amount of current being produced by the "panels" (aka: the DC/DC converter), and the amount of current being sent to the batteries, both read 0.0.

Now the only determinable difference I can see is that the panels produce in the order of 16V, while the DC/DC converter is producing about 13.5V. From what I can see in the regulator's specs it says Open circuit voltage solar module: <47V. The operating instructions say [I]Voltage range 12V: 6.9 V

[Mike90250]

OK, I take it that your truck is a 24V system, and your 3 deep cycle batteries are wired together for 12V ?

12V deep cycle batteries need about 15V dc, to get fully charged.

Not knowing what sort of protections you have, on your DC-DC downconverter, I cannot recommend connecting it directly to your batteries.

The Steca Controller, is a standard PWM controller, and should be able to to charge the batteries from the PV panels.

First task is to get the batteries charged. Your 120W of panels, is undersized for charging your 3 big batteries. You should have a charging source of about 30A to recharge with.
You should have panels with Max Power Volt of about 17 or 18 V, open circuit of about 21V.

If you have a meter, verify your panels are both producing voltage when not connected to anything, and then place the meter in AMPS mode, and measure the AMPS from each panel. Should be about 3 amps each panel. Then set meter back to VOLTS.

Connect the functional panels directly to the batteries, and measure every 10 minutes, you should see a slow rise in battery voltage as they charge. You will be charging with < 6 amps, and it will take a long long time.

Since your charge controller is PWM , you cannot connect it to your 24V battery system, it would burn out instantly. If you had a 2 ohm, 500watt resistor, you could wire that to limit current, and charge the 12V via the Steca from the 24V truck.

Other choice is a 24V inverter, powering a 40A battery charger about 900 Watts.

Not knowing your resources and skills, it's kind of limiting. It's a shame the system is not working for you.

[steve.lorimer]

Thanks for the reply.

What do you mean by protections on my DC/DC converter? It's just a standard switch-mode 15A 24/12V converter. Input is from the 24V batteries, output is +/-14V. It's turned on with a relay from the ignition, so only runs when the truck is running, and therefore the alternator is running (to prevent the truck's batteries from being drained when we're not running the truck)

I have tested the panels with my multimeter, they are working as expected.

I also ran a test with a light bulb and my multimeter. Panels by themselves can light the bulb, and I get a reading of 2A (not peak solation, so can't get the full 7.5A that is theoretically achievable).
When I swap to the DC/DC converter the bulb burns brighter and I get a reading of 3.5A. ie: the DC/DC converter was producing more current.

Then I swap the light bulb with my Steca regulator. Multimeter for the panels connected reads the same - 2A. When I swap the panels for the DC/DC converter, current reads 0A.

Now the fact that the regulator's specs say it can handle an input voltage range of 6.9 V

[Mike90250]

Please can you give me the math behind the 2 ohm, 500watt resistor being able to charge directly from the 24V system (by the way, the alternator produces 55A at +/-26V when running)

Thanks
Steve

NOT DIRECTLY, but as a limiter to the Steca

I think you can safely try the Steca from the DC-DC converter, but you won't end up with full batteries, except after several hours. 15V - 1.5V loss in the Steca = 13.5V, barely enough to charge a very low battery.

What VOLTAGE are your panels putting out? Got to be at least 17V, or else the Steca's overhead losses will keep you from charging.

math:
The 2 ohm resistor was a quick approximation. Maybe a spare headlight (use both beams) can substitute for a resister.
25V (engine running) charging 12V = delta of 13V
limit 13V to <20A (dont burn Steca ) 13v / 20a = 0.65 ohm @ 260W
because of internal resistance in the battery, you could likely get away with a half ohm resistor (0.5), but still, at least 300W capacity.
It's unlikely you can find that in the bush, so if you had a spare headlight, each beam (filament) in a sealed beam headlamp is good for about 3 amps, which is a couple of ohms. Don't handle a halagon/quartz bulb inner capsule, the finger oils will etch the quartz and kill the bulb.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FVE300-.6-ND

You could also come up with a scheme, to disconnect 2 of the 12V batteries, and connect them in SERIES (giving 24V) and jumper them to the truck, while engine is running, swap batteries around so you get the 3 of them charged somewhat. But that's really messy.

What VOLTAGE did you measure from the panels when you got the light bulb lit? It may be the panels are not correct for your application, or damaged and not providing enough voltage.
Regardless, your panels are too small for charging 3 large batteries, they are barely more than a trickle charge. 6a ,compared to the 30a you should be charging with.

[Mike90250]

What do you mean by protections on my DC/DC converter? It's just a standard switch-mode 15A 24/12V converter.

There is a chance you could connect it directly to the batteries, but I expect the batteries would try to pull a lot more than 15A from it, so if it

internally limits itself to a safe operating range, great.
If it uses a fuse, you will have to replace a burnt fuse.
If it cooks itself - it has no protection.

There are 3 ways it could work - do you feel lucky? I would not try till I have a spare one.

[steve.lorimer]

Thanks for the response Mike. Sorry it's taken me so long to respond - I've been on the road with limited access to internet. I'm now in Uganda (luckily Idi Amin is no longer around!)

re: DC/DC converter:

There is a chance you could connect it directly to the batteries

Actually that was the way it was originally. The DC/DC converter charged the batteries directly. The DC/DC converter produces ~15.8V (sorry, made a mistake on the previous post about the voltage produced) Now from what I understand, if the DC/DC converter just shoves 15.8V at 15A into the battery, it will charge it up to a point (roughly 50%), but is not capable of charging to full capacity. It could also cause over-charging and damage the batteries. To achieve 100% charging of deep-cycle batteries you need to use a 'smart' charging algorithm, (ie: my regulator has PWM charging)

What VOLTAGE are your panels putting out?

Getting about ~16-17V from the panels. Getting ~15.8V from the DC/DC converter.
I keep on going back to this quote from the Steca regulator's specs:

input voltage range of 6.9 V – 17.2 V when operating in 12V

This leads me to believe the regulator will use a DC transformer to alter the input voltage to the batteries to match it's needs. Ergo, 15.8V from DC/DC converter should work.

The panels are working, albeit they are doing a trickle charge (7.5A at peak solation here at the Equator) rather than a nice big 30A. Again, this is a contributing reason to why I'd rather use the 15A produced from the DC/DC converter when driving.

[Mike90250]

A good battery charging voltage, is 15V. That will fully charge deep cycle batteries. Without regulation, that will overcharge them in time. Depending on your loads, and engine run time, you may have trouble, or not. Best is to get a good voltmeter, and measure your deep cycle batteries several times a day while engine is running. After your batteries read 14V, they are getting pretty full (brush measurements, not for a long term household system)

You cannot get 30A from your 120W of PV, most you could possible get is 9A, and that is even doubtful.

There are complex charging formulas, but the 15V brute force method should work for a month or two till you can get local help troubleshooting it. Also, be sure you use DISTILLED water to top the batteries up. Nothing else. There are no magic additives, aspirin, or anything that "fixes" batteries, only proper charging and watering.

[steve.lorimer]

Got this response from the manufacturer:

From what I understand, with the DC/DC converter connected straight to the battery it just shoves 15.8V at 15A into the battery, and will either over-charge and damage the battery if we leave it on for a long time (ie: drive for a long time), or if for a short period, will not charge efficiently. To achieve efficient charging of deep-cycle batteries you need to use a smart charging algorithm that alters the current and voltage sent to the battery according to battery state (ie: Steca's PWM charging system)
Yes, if the DC/DC converter is connected directly to the battery, then someone will have to make sure "manually" that the battery is not overcharged.


As you know from my last email I have already connected the DC/DC converter to the Steca regulator, and (luckily) neither were damaged! Can you tell me why they could be damaged? Can you also tell me why the Steca receives 0A from the DC/DC converter when I know the DC/DC converter is capable of producing current?
The Steca charge controllers use a regulation mode which short-circuits regularly the energy source. This is fine for solar modules, but can certainly damage any other energy source. Additionally, the mosfets inside the charge controller can heat up and get damaged irreversably, if the IV characteristic of the solar module is not recognised.


I have a solar system that is working correctly - it is the alternator charging system that I have the problem with. What I need to find is a PWM or MPPT battery-to-battery charger than can take my 24V alternator output and charge my 12V deep-cycle batteries, except this is not easy since I'm in Africa. Hence the reason I'm trying to use the Steca as an ad-hoc battery-to-battery charger. What would happen if I connected my alternator directly to the regulator? (it produces about 26V, and can produce 55A)
This will not work, because again, we can only use solar modules at the input, and secondly, the current is way too high for the PR controllers.

[steve.lorimer]

Got this response from the manufacturer:

From what I understand, with the DC/DC converter connected straight to the battery it just shoves 15.8V at 15A into the battery, and will either over-charge and damage the battery if we leave it on for a long time (ie: drive for a long time), or if for a short period, will not charge efficiently. To achieve efficient charging of deep-cycle batteries you need to use a smart charging algorithm that alters the current and voltage sent to the battery according to battery state (ie: Steca's PWM charging system)
Yes, if the DC/DC converter is connected directly to the battery, then someone will have to make sure "manually" that the battery is not overcharged.


As you know from my last email I have already connected the DC/DC converter to the Steca regulator, and (luckily) neither were damaged! Can you tell me why they could be damaged? Can you also tell me why the Steca receives 0A from the DC/DC converter when I know the DC/DC converter is capable of producing current?
The Steca charge controllers use a regulation mode which short-circuits regularly the energy source. This is fine for solar modules, but can certainly damage any other energy source. Additionally, the mosfets inside the charge controller can heat up and get damaged irreversably, if the IV characteristic of the solar module is not recognised.


I have a solar system that is working correctly - it is the alternator charging system that I have the problem with. What I need to find is a PWM or MPPT battery-to-battery charger than can take my 24V alternator output and charge my 12V deep-cycle batteries, except this is not easy since I'm in Africa. Hence the reason I'm trying to use the Steca as an ad-hoc battery-to-battery charger. What would happen if I connected my alternator directly to the regulator? (it produces about 26V, and can produce 55A)
This will not work, because again, we can only use solar modules at the input, and secondly, the current is way too high for the PR controllers.

[Mike90250]

You CANNOT connect any PWM (your Steca) controller input, to a higher voltage battery. You will pop the fuses or burn the unit up.

A MPPT type controller can accomplish this, but the only reliable small one, is too small to be of any benifit to you - Morningstar 15A MPPT.
Outback has a 60 & an 80A MPPT, Xantrex has a 60A MPPT. But they are pricey. I still think the best you can do in the bush, if you can't get one of the MPPT's is get a 24V inverter, and a switching type 40A 12v battery charger to run off it. Use the DC-DC converter till then, just watch the voltage at the battery.

Can you also tell me why the Steca receives 0A from the DC/DC converter when I know the DC/DC converter is capable of producing current?

The voltage output of 15.5V is not high enough to overcome the internal 2-3V drop a charge controller has, they want to see at least 17V input