help me find cheap charge controller for 270watt panel and 12v battery

[RShackleford]

Pretty basic ...

I installed a 4kW grid-tied system, and bought a spare panel in case something happened to one of them (installation accident, falling limb, etc). Just for fun, I'd like to build a tiny off-grid system to power a few landscape lights on my other undeveloped lot.

I managed to pickup a used Optima Yellowtop battery (12v, AGM) for $20. Now I need a charge controller. I'm trying to keep it cheap, so PWM. I believe I have to go with a 30amp unit, since 270watts/12v is greater than 20amps. Very few of them specify the maximum voltage of the connected solar panels (my panel specs 38.7 open-circuit voltage). The ones that do, don't seem to allow that high an input voltage if using a 12v battery. Am I going to have to source a 24v battery ? Or should I be able to locate a suitable charge controller ? Bonus points if the output voltage (to the auxiliary load) is 24v or more (my landscape lights use higher than 12v, but I can always use a cheap DC-to-DC boost converter). Also, is trying to pump 270 watts into this battery going to kill it ? TIA !

[bcroe]

Running a 24V battery might work out. PWM will not be a terrible loss,
and you can run really small wire around to the loads. Use a tiny, cheap
buck converter at the lights to get the optimum voltage, no worry about
wiring drop.

I have been considering something like that here, to back up communication
stuff (ant amps, cat5 E amp, server, a small TV). Bruce Roe

[RShackleford]

Running a 24V battery might work out.

Yeah, but I'm trying to do this on the cheap, and I already scored that 12v AGM battery fro $20. Do you think it's unlikely I'll find a charge controller that works with a 12v battery but can handle a panel voltage of 40v or so ?

[Mike90250]

I think many PWM controllers should handle up to 50V.
Since you are not using MPPT, you will only have the amps listed on the sticker on the back of the panel Imp:____

I'm guessing about 7 or 8 amps. So you just need to look for something that can handle 40V of PV

[RShackleford]

I think many PWM controllers should handle up to 50V.
I'm guessing about 7 or 8 amps. So you just need to look for something that can handle 40V of PV

Yeah, that's the rub. Most of the cheap ones I see either don't say what the maximum panel voltage is, or it's less than 40v unless the thing is in 24v mode. I did find one, that seems to allow 50v without specifying 24v mode, but it has terrible reviews: https://www.amazon.com/dp/B07F7RJ6BM...2s9dHJ1ZQ&th=1.

There's a zillion of these things, and you have to look pretty hard to find the maximum allowable panel voltage (assuming it's spec'd at all.

I can't think of any technical reason that a 12v charge controller shouldn't be able to handle a 40v panel.

[bcroe]

With a 12V battery you either need to use an MPPT controller, or throw
away half the energy the panel generates. And be a lot more concerned
about limiting wire voltage drop. Bruce Roe

[RShackleford]

With a 12V battery you either need to use an MPPT controller, or throw
away half the energy the panel generates. And be a lot more concerned
about limiting wire voltage drop. Bruce Roe

You saying I can connect my 40v panels to one of these PWM controllers in 12v mode, as long as it's one of the rare ones that allows 40v input, such as:

https://www.amazon.com/BlueFire-Digital-Display-Regulator-Controller/dp/B073STGTHT/ref=pd_di_sccai_5/137-7250755-0415062?pd_rd_w=fbpXS&pf_rd_p=c9443270-b914-4430-a90b-72e3e7e784e0&pf_rd_r=ENX4QS8HP3129JC0T8B9&pd_rd_r=c07ecf4b-1d32-42db-8a2e-856c30b05c7f&pd_rd_wg=R8FSL&pd_rd_i=B073STGTHT&psc=1

... and it'll work, it just won't fully utilize the panel's output (about half of it) ? IOW, it doesn't do any kind of DC-to-DC conversion on the panel's voltage, it just dumps whatever isn't used to charge the 12v battery or power the 12v load ?

Not too worried about wiring loss with my system. I envision a total load well under 100 watts; and I'll boost the 12v to 30v immediately (with a DC-to-DC converter) before going into my lights.

[bcroe]

You saying I can connect my 40v panels to one of these PWM controllers in 12v mode, as long as it's one of the rare ones that allows 40v input, such as:

... and it'll work, it just won't fully utilize the panel's output (about half of it) ? IOW, it doesn't do any kind of DC-to-DC conversion on the panel's voltage, it just dumps whatever isn't used to charge the 12v battery or power the 12v load ?

Not too worried about wiring loss with my system. I envision a total load well under 100 watts; and I'll boost the 12v to 30v immediately (with a DC-to-DC converter) before going into my lights.

Right, the red line shows typical panel performance, you will collect the
maximum energy by adjusting the load current to pull the voltage down
to the Maximum Power Point. Running your panel into a 24V bat will
come fairly close to that even with a PWM controller. But sliding down
the curve to 12V bat voltage will cut the operating voltage in half, the
current does not increase a lot, so you are getting just over half power.

If you use an MPPT controller, it will adjust and transform V-I from the
best panel voltage, to whatever battery you use, with minimum power
loss. Certainly the simplest and most efficient system will match up
panel, battery, and loads without voltage conversions. Bruce Roe

CellScan.png

[RShackleford]

Maybe I should start a new thread for this, but ...

Is it possible to go into the little black plastic box on the back of a solar panel - that's on the back of the panel and the cables come out of it - and rewire the panel to have lower voltage and higher current ? It looks like a pretty common way for a 60-cell panel to be wired is three sets of 20 cells in series, with a bypass diode for each set (so 60 cells in series if none of of the bypass diodes are are conducting). So if the diodes are in that little box, it means all 6 wires (the two ends of each group of 20) are available at that box. So they could be re-configured so that the three groups of 20 are in parallel with each other. So my 40volt/9amp panel would become a 13v/27amp panel, which might work nicely with a 12v battery. Just a thought.

[SunEagle]

Maybe I should start a new thread for this, but ...

Is it possible to go into the little black plastic box on the back of a solar panel - that's on the back of the panel and the cables come out of it - and rewire the panel to have lower voltage and higher current ? It looks like a pretty common way for a 60-cell panel to be wired is three sets of 20 cells in series, with a bypass diode for each set (so 60 cells in series if none of of the bypass diodes are are conducting). So if the diodes are in that little box, it means all 6 wires (the two ends of each group of 20) are available at that box. So they could be re-configured so that the three groups of 20 are in parallel with each other. So my 40volt/9amp panel would become a 13v/27amp panel, which might work nicely with a 12v battery. Just a thought.

Why fustz with a good thing. Just spend the money on a quality MPPT CC. It will save you in the long run and not mess up the life of a solar panel by rewiring it.

[bcroe]

Is it possible to go into the little black plastic box on the back of a solar panel - that's on the back of the panel and the cables come out of it - and rewire the panel to have lower voltage and higher current ?

In a word, NO. The intermediate connections are for bypass diode connection,
but do not separate the string sections to allow rewiring. Bruce Roe

[RShackleford]

The cheapest MPPT charge controller that I can find, is $89 (almost as much as the $100 I paid for this extra panel ):

https://www.amazon.com/gp/product/B08B7QS5XG/ref=ox_sc_act_title_1?smid=A3GRIWLFOYL9K2&psc=1

I'm happy to suffer the inefficiency of a PWM unit - the battery I have will only hold maybe 200-300 watt-hours, and my project should use only a small fraction of that per day (LED landscape lights on for a few hours after sunset). So a 270watt panel shouldn't have much trouble charging it, even with the PWM inefficiency.

What isn't ok is if running a panel with Voc = 40v into a 12v battery cooks either the charge controller or the battery itself. How likely is this ?

[SunEagle]

The cheapest MPPT charge controller that I can find, is $89 (almost as much as the $100 I paid for this extra panel ):

https://www.amazon.com/gp/product/B08B7QS5XG/ref=ox_sc_act_title_1?smid=A3GRIWLFOYL9K2&psc=1

I'm happy to suffer the inefficiency of a PWM unit - the battery I have will only hold maybe 200-300 watt-hours, and my project should use only a small fraction of that per day (LED landscape lights on for a few hours after sunset). So a 270watt panel shouldn't have much trouble charging it, even with the PWM inefficiency.

What isn't ok is if running a panel with Voc = 40v into a 12v battery cooks either the charge controller or the battery itself. How likely is this ?

With a cheap CC anything is possible.