# Small solar panel charges big battery

#### pyroartist

Joined Oct 9, 2015
98
Hello, I have a 1 foot square solar panel that is flexible. It is supposed to be capable of 100 ma. output and the open circuit voltage in bright sun
is about 16 Volts. (I am limited to this panel size for this application.) It is connected to a small charge controller circuit (Amazon) that charges a
12 Amp hour SLA battery.The battery is only used for engine starting so it usually is not drained much. I have connected the charger output to the battery through a Schottky diode (low voltage drop) as I don't know if the charge controller would tend to drain the battery overnight without it. Yesterday in late afternoon sun I measured the panel output at 13.1 Volts, the charger output at 12.88 Volts and the battery voltage at 12.65 Volts (indicating the diode drop is 0.23 Volts). The system had been sitting in bright sun all day and I was hopeful the battery would be closer to 13 Volts.
Because the panel output is only 100 mA. would it be better to connect the solar panel directly to the battery? Should I keep the diode between them? My goal would be to keep the battery around 13 Volts.

#### BobTPH

Joined Jun 5, 2013
3,315
Charging a 12 Ah battery at 100mA will take 120 hours. Wait a month and you might see results.

Bob

#### pyroartist

Joined Oct 9, 2015
98
Even if it is already charged up to 12.5 Volts??

#### WBahn

Joined Mar 31, 2012
26,398
It is capable of 100 mA at what voltage? I wouldn't be surprised if that's the short-circuit current. You need to measure the actual current when it is connected to your battery.

But to put some rough numbers on things, let's say that you have a fully charged battery. You then pull 200 A cranking the starter for 5 s. That's 1000 A·s of charge. At 100 mA (assuming you can actually get that) it would take 10,000 s to replace that charge (something approaching three hours).

#### nsaspook

Joined Aug 27, 2009
8,382
It might float maintain a already charged battery at the needed voltage of about ~13.6V but it's unlikely to fully recharge a battery due to the internal resistance losses loading down the solar voltage required to maintain the required absorption/acceptance voltage of a proper cycle recharge.

At 12.5V open voltage your Sealed Gel or AGM battery is likely not at 100% SOC.

#### oz93666

Joined Sep 7, 2010
737
...would it be better to connect the solar panel directly to the battery?.
Yes ... charge controllers are trouble ... damp always getting in and destroying them , better spend the money on more panels ... your panel is designed to be directly connected to a 12V battery .

#### mvas

Joined Jun 19, 2017
538
Hello, I have a 1 foot square solar panel that is flexible. It is supposed to be capable of 100 ma. output and the open circuit voltage in bright sun
is about 16 Volts. (I am limited to this panel size for this application.) It is connected to a small charge controller circuit (Amazon) that charges a
12 Amp hour SLA battery.The battery is only used for engine starting so it usually is not drained much. I have connected the charger output to the battery through a Schottky diode (low voltage drop) as I don't know if the charge controller would tend to drain the battery overnight without it. Yesterday in late afternoon sun I measured the panel output at 13.1 Volts, the charger output at 12.88 Volts and the battery voltage at 12.65 Volts (indicating the diode drop is 0.23 Volts). The system had been sitting in bright sun all day and I was hopeful the battery would be closer to 13 Volts.
Because the panel output is only 100 mA. would it be better to connect the solar panel directly to the battery? Should I keep the diode between them? My goal would be to keep the battery around 13 Volts.
You need to supply the URL to the exact model Solar Panel that you are using.
You need to supply the URL to the exact model Battery Charger that you are using.
You have not provided us with enough details to answer your questions.
You have not stated the temperature of battery.

A 1.5 Watt ( 100ma x 15 v ) Solar Panel is not designed to recharge a discharged battery.
It is designed to maintain a fully charged battery.

By adding a schottky diode between the charger and the battery, you reduced the charging voltage,
which is not something you want to do.

#### mvas

Joined Jun 19, 2017
538
Yes ... charge controllers are trouble ... damp always getting in and destroying them , better spend the money on more panels ... your panel is designed to be directly connected to a 12V battery .
Given that this Solar Panel has an unregulated 16 Volt Open Circuit Voltage and
the battery is a small 12 AH AGM, I would not leave the Solar Panel connected day, after day.
Every day that the voltage is too high, will cause the AGM battery to vent gas, causing more battery capacity to be permanently lost,

#### BobTPH

Joined Jun 5, 2013
3,315
Given that this Solar Panel has an unregulated 16 Volt Open Circuit Voltage and
the battery is a small 12 AH AGM, I would not leave the Solar Panel connected day, after day.
Every day that the voltage is too high, will cause the AGM battery to vent gas, causing more battery capacity to be permanently lost,
A panel with a max output of 100 mA is not going to destroy a 12AH lead acid battery. When connected to the battery, the voltage is going to drop to nearly the battery terminal voltage.

On the other hand, it may maintain a float charge, but that is about it.

Bob

#### nsaspook

Joined Aug 27, 2009
8,382
A panel with a max output of 100 mA is not going to destroy a 12AH lead acid battery. When connected to the battery, the voltage is going to drop to nearly the battery terminal voltage.

On the other hand, it may maintain a float charge, but that is about it.

Bob
Exactly.

A battery is a electro-chemical device not a linear electrical storage system. You are not pushing (or separating like a capacitor) electrons into a box for later retrieval. The battery internal electric field is continuously generated from chemical redox reactions that try to balance the redox equation with energy demands of the battery. Up to about 80% of LA battery full charge the relationship to coulomb counts to chemical energy converted is fairly linear and efficient (pushing a weight on a flat surface) but after that the work function needed to drive chemical reactions pass (pushing a weight up a hill) the plates surface charge results in resistive losses (energy from the 100mA solar panel just heats the battery slightly) that should reduce the electrical to chemical conversion process to just about nil with low charge currents.

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#### Audioguru

Joined Dec 20, 2007
11,249
The diode prevents the battery from discharging into the solar panel at night.
With a low voltage drop Schottky diode and a solar panel that does not "follow" the sun then the charging current will be low and not for long each day. Then the battery will never fully charge after a deep discharge but it will not be damaged when charged day after day for a long time.

#### pyroartist

Joined Oct 9, 2015
98
Thanks for all the good knowledge. This system has been working OK for 3 years now. I was just wanting to bump the voltage up a bit.
Maybe I will remove the Schottky diode and see if that helps. I would think the charge controller would have some kind of back flow protection.
The panel is no longer listed but was from Jameco. The charge controller is very basic, from China and has minimal documentation.
This battery never gets deeply discharged and spends a lot of time in the sun between uses. Essentially this is a float charger system.

#### nsaspook

Joined Aug 27, 2009
8,382
Thanks for all the good knowledge. This system has been working OK for 3 years now. I was just wanting to bump the voltage up a bit.
Maybe I will remove the Schottky diode and see if that helps. I would think the charge controller would have some kind of back flow protection.
The panel is no longer listed but was from Jameco. The charge controller is very basic, from China and has minimal documentation.
This battery never gets deeply discharged and spends a lot of time in the sun between uses. Essentially this is a float charger system.
That sound about right. The battery might not get fully charged so total capacity is likely reduced but you don't need that for this application. It's slow battery death by a thousand cuts.

#### BobTPH

Joined Jun 5, 2013
3,315
Taking the charge controller out is more likely to bump up the voltage than taking the diode out.

As an aside, I once researched on the net whether a diode is needed, and the "opinions" are about 50:50 Many say that either the panels have one built in or that they naturally perform as if there were. Could not decide who was right. And do not have any panels to test.

Bob

#### crutschow

Joined Mar 14, 2008
27,213
I once researched on the net whether a diode is needed, and the "opinions" are about 50:50 Many say that either the panels have one built in or that they naturally perform as if there were. Could not decide who was right. And do not have any panels to test.
So I suggest the TS measure the back current from the battery when it's directly connected to the solar panel in the dark, and see if there's a significant discharge current through the panel.
That will determine whether a diode is needed.

If one is needed, an ideal diode circuit, using a MOSFET, would reduce the forward drop to a negligible value.

#### nsaspook

Joined Aug 27, 2009
8,382
Even the cheapest Chinese PWM charge controller eliminates the need for a blocking diode with a single panel.
All of my commercial 100W+ panels have builtin blocking and bypass diodes so they can be connected in various string configurations without external parts.

#### splud

Joined Jun 30, 2013
18
As an aside, I once researched on the net whether a diode is needed, and the "opinions" are about 50:50 Many say that either the panels have one built in or that they naturally perform as if there were. Could not decide who was right. And do not have any panels to test.
A small module as described is unlikely to have a diode built in. Ones actually kitted as a battery maintainer (usu with indicator LED, and outputs to battery clamps or an accessory plug) do, but that didn't sound like what this one was.

if you connect the ground to your battery, and set a DMM to measure amps (or in this case, 100's of mA) and connect the DMM in series with the solar module + and your battery +, you'd see the current flowing from the solar module into your battery. If you cover the solar module so that it is not getting illuminated, if the polarity changes and you have a non-zero reading, then current is flowing FROM your battery through the module, and that confirms there is no diode.

Any charge controller worth having SHOULD have diode (or mosfet) protections on the outputs so that it isn't a burden on the battery when there is no solar input. Charge controller circuitry though is likely to consume a chunk of the 100mA solar output, especially if it was designed for higher current - however a charger controller could boost the voltage to a 13.7 or 14.4 charge level (even at lower current).

I'm in agreement with those saying that a 100mA solar module is unlikely to over-charge an SLA. You'll be lucky to get a maintenance charge from it.

If the goal is to "top off" a battery to replenish regular consumption, figure your typical number of hours of irradiation of the module * current is the most power you can extract daily from the battery - if you exceed that, the solar module isn't going to magically charge it more. A bare solar module is NOT going to charge the battery if the solar module outputting less than the battery voltage - so partial irradiation (a less than full-sun day) won't get you anything - that's where a charge controller with boost is beneficial, since the output voltage of a solar module can be boosted to the point that it is useful, even if substantially less current.