# 36V solar panel with a 12V battery, results in 12 watts a day?

So I bought a retired 175W solar panel 36.8V rated voltage, 4.9A rated amps. I hooked it up to a solar charge controller and a 12V battery. Normally the controller doesn't work cos the PV voltage is too high, but if it's connected in the morning before any sunlight, then the voltage is low enough for everything to function, then later in the day, the battery drags down the voltage into acceptable range.

I guess I'm starting to answer my own question, if the PV is only 5 amps, then it's 5 amps at 12V or 60W, or 5 amps at 36V 180W. But anyway the question is,

How and why is it possible that after leaving the system as is for 24 hour intervals, I'm only getting 11 watts of work on the output of the whole thing. On the load section I have a 70 amp relay that connects the inverter to the battery to avoid parasitic drain.

Even if it was changing at 60W, it would be 60W for hours, so I end up having one-twentieth of the power than I should even under the worst conditions, where does it go?

I'd the PV makes no amps, and it's connected, does the battery push current the other direction and discharge itself? Do I need to put a diode or something?

Yeah I'll put more 12V batteries in series, I'm just testing things for fun.

• Notes: "12 watts a day" isn't a proper measurement. "12 watts for a day" would make more sense and would equate to an energy of 12 W x 24 h = 288 Wh. "11 watts of work" isn't right either. Work would have units of joules (= watt-seconds) or Wh. Commented Aug 4, 2022 at 9:40
• Sorry for the confusion. After many different days of having the solar controller connected to the battery and the pv, each day, with a kw meter, I'm only able to get 0.011 kw. So the system is only making 0.011 kw a day, despite it being a 175w panel being hooked up all day. Commented Aug 4, 2022 at 10:31
• Just to clarify again, 'kW' (note capital 'W') is the power it is producing at an instant of time. 'kWh' is the energy it produces in a day or some other time period. Energy = Power x Time. Commented Aug 4, 2022 at 11:46
• "where does the [power] go? It's not necessarily going anywhere, the controller may not be taking it. Just because your panel has a maximum output of ~5 A doesn't mean the panel produces 5 A just because there's sunshine. The controller needs to actively extract the current, and hence power. If the controller isn't designed to connect a 36 V panel to a 12 V battery then it may not take the available power. If the controller is designed to connect a 36 V panel to a 36 V battery then you need to provide a 36 V battery to get the controller working properly. Commented Aug 4, 2022 at 22:35
• How are you determining that you are getting "11 watts of work" or "powers it for 11 wh"? As Transistor noted, you're confusing power and energy, but if you can explain how you're measuring the 11 whatevers, it may help with answering the question.
– Mark
Commented Aug 5, 2022 at 4:37

5A rated means someone did some rating math at 5A.

Check the charge controller specs. It may shut off if the panel voltage is too high ie would result in too much current to battery.

Check panel Open Loop Voltage and make sure it is in spec for charge controller (in the 12V config)

Finally panels also behave like resistors. They will get hot- more if the charge is not flowing to the batteries. Overheating panels get damaged... and retired. You may be able to find which cell(s) is/are defective and bypass those.

Stressing the resistor behavior, when you attach panels directly to batteries (and there isn't much sun available), you are putting a resistor across them and draining them (and putting unnecessary wear on already retired panels)!

• A resistor does not dissipate heat with no current flowing and neither does a Solar Panel. It is not like a resistor, it is a voltage-limited current source that peaks around 83% of Voc for Vmpt. Commented Aug 7, 2022 at 23:51
• And how do you propose to stop the unfavorable flow of current in the panel itself at the location where voltage is generated while still allowing the flow you desire? People have been mucking with the cell composition for years to improve it and the panels we have are the result. Voltage limited, light dependent energy source with a resistor in parallel.
– Abel
Commented Aug 8, 2022 at 12:06
• The PV generates voltage with some impedance while the load generates current in a loop. With open cct, Voc , there is no current nor heat dissipated. While the solarity is proportional to current, the MPT impedance rises with lower current while the absolute current remains relatively constant when shorted below Vmpt as you expet from a high impedance current source. Commented Aug 8, 2022 at 15:32
• @Tony people like you seem to only believe something they're told by a "reputable" source so look at Rsh in the image at pvpmc.sandia.gov/modeling-steps/2-dc-module-iv/…
– Abel
Commented Aug 8, 2022 at 17:04
• It is not just a belief. The shunt resistance does not dissipate any significant heat or draw significant current during Voc. Your answer contradicts this fact. (-1 for that ) Commented Aug 8, 2022 at 17:17

A proper Solar controller will keep the PV voltage high where Maximum Power can be delivered with a matched impedance, while the DCDC converter reduces the voltage to manage the charge voltage with the power available.

Your controller does not appear to do this. A photo or model with a link can confirm it.

This is the standard PV characteristic at max solarity. With reduced sun power the curves shink in current and power while voltage only drops slightly due to the losses from the series resistance. For example Vmpt~ 82% Voc at 100% solarity and 72% Voc and 10% solarity while Zmpt rises 9x or so.

Meanwhile, a car battery with very low ESR in tens of milliohms or less with hundreds of Amps would be seen as a short circuit to a PV thus the green load line would be vertical. A 12V battery on a 36V PV array reduces the maximum power to 1/3 rated power unless a tracking or impedance matching switching DCDC converter is used for MPT tracking to raise/match the PV voltage to Vmpt.

If however there is no reverse blocking diode added the internal diode characteristics go from being reverse biased supplying current to forward biased and will self-destruct from heat supplied by a battery in darkness.

• You're assuming its an MPPT controller, PWM is still widely used. Commented Jan 5, 2023 at 19:55
• @GregLocock and some expect it to work without any controller... Commented Jan 5, 2023 at 20:10
• BTDT when debugging a ratty old system Commented Jan 5, 2023 at 20:20
• @GregLocock you are assuming what? I show “how to control PV” for max efficiency and power transfer using % Voc . Even open loop would work. PWM is what efficiency? Commented Jan 10, 2023 at 17:43

I may be wrong but from memory back in the day solar controllers had quite tight max panel voltage limitations, these days now that MPPT for 12/24v are ubiquitous not so much. For instance this is running mine, and the max continuous voltage (100V) is less than double the max battery voltage (60V for 3 minutes, 48V nominal)

https://www.rpc.com.au/pdf/plasmatronics_PL_reference_manual.pdf

So a nasty cheap old 12V system might struggle with a 36 OCV panel, but you are right, that doesn't affect your explanation.

It will charge at 5 amps until the battery reaches float voltage, then the charge controller will turn off. After it turns off, it won't charge any more.

If it stays on all day, charging at 5A for 10 hours, then you've got 5 * 13 * 10 = 650 Watt Hours. If your battery is in good condition, you should be able to get most of that out again.

If you aren't getting ~500 Watt Hour (~400 Amp Hour) into your load (12W for 50 hours, or 2.5A for 20 hour or whatever), then either you aren't getting 5A for 10 hours, or your load measurement is wrong, or your battery has failed.

(The panel will run coolest when operating at Maximum Power Point: short circuit or open circuit it will run warmer. Warmer or colder, the 12-14V current will mostly depend on the sunlight, and for the same sun will be slightly greater than the specified operating current.)