Solar car battery charger

You can cheap out and skip the control module if you keep an eye on the voltage. A little overcharging probably won't hurt not the greatest batteries. Just check the electrolyte levels and keep them up. Put a blocking diode in series with the panel.

Do you think this 10 Watt (claimed) solar panel can charge a car battery?

The goal is a DIY science project but it has to work to do the job.

I doubt it. It says it has a switch for 6, 9, or 12 volts which makes me think it has adjustable voltage regulator similar to

Now for the bad news. How many watt-hours is your science project going to require, In other words how many watts does it require, and how long are you going to run it per day? Then consider the solar panel. The 10W rating assumes full sunlight. Even on a sunny day in southern AZ, unless you have a tracker, if you graphed the output you'd get a nice curve maxxing out when the panel is perpendicular to the sun, building up in the morning and declining in the afternoon. You won't see full output on overcast days and obviously nothing at night.

They work well in the summer but with shorter, darker days they struggle. That's solar power in a nutshell. Without knowing further details of your project my guess is you won't be happy.

I've seen those simple potentiometer diagrams which show how to use that LM137 as an adjustable power supply. My goal is to build a solar-powered spa heater that slowly (or quickly - it doesn't matter the speed) pumps water in the summer from the spa (which is roughly about four feet cubed) through a few hundred feet of black hose back to the (now warmer) spa.

I'm thinking of three major components might be needed to accomplish that.

1. 12VDC water-transfer pump (of some sort - I have an RV pump now)
2. 12VDC car battery (I have three old ones - which will work for now)
3. Black garden hose (I have a few hundred feet of Continental Rubber hose)
4. Some way of charging the battery using sunlight as the source

My first problem is how to figure out what solar wattage I'll need.

Oooh. I like it! The Amazon description says it's a SUNER POWER 10W 12V Solar Battery Charger (Bulk, Absorption, Float) Maintainer, Built-in Smart MPPT Max Power Point Tracking Charge Controller, Adjustable Mount Bracket, Waterproof 10 Watt Solar Panel Kits for Car Automotive Boat Marine RV Trailer with a floating charge voltage of 13.8VDC & an optimal-charge current of 0.56Amps with a trickle-charge current of 0.2Amps & an overcharge-protection voltage of 14.4VDC.

The connector type they show is what my RV pump uses. What's it called? They called it an "SAE harness" in the picture on the Amazon listing.

In their diagram, the red is the female & the black is the male. But on my pump, the red is the male and the black is the female.

Do you know what name to look up to get it with alligator clamps? And what is the overcharge-protection voltage anyway?

Is that the highest that it will go before it limits its cutoff voltage?

Dunno. My friend dropped off a second RV pump for me to try.

That makes the 1st pump about 55 Watts & the 2nd about double that. The old "Interstate Batteries, All Seasons" battery sticker on the top says MT26, 625CCA, 655CA & RC85.

Do you think that car battery (let's assume if it was new) could run the smaller pump for about five hours during the hottest part of the day?

Using 50 Watts and 100 Watts for simpler math, I think that means the battery needs five times that in watt hours, or 250 to 500 Watt hours.

Does my quick calculation above sound about right?

I'm trying to size the solar panel, where you're helping a lot, as I just realized only when I did the math above that I may need something like a 50 Watt panel for the smaller pump to run it real time.

But don't I need a smaller panel if it's running off the battery?

Unfortunately both battery & panel have to work together during the day but the battery can be charged all day while the pump only needs to run during the late morning and early afternoon to get the hottest sunlight, right?

Thanks for helping me run the calculations.

Do you think the car battery can run the motor long enough for the hottest parts of the day? And do you think maybe a similar-sized 30 Watt solar panel can charge it back if that solar panel charges for the entire day?

There is no magic. The battery is a storage device. If you take 250 watt- hours out, you need to put 250 watt-hours in, or more like 300 for inefficiencies. You have to figure out how many hours of dependable sun light you have, allowing for overcast days.

Think of a battery like a checking account. It doesn't matter when you deposit money or when the checks clear as long as there is as much or more going in than out. Erring on the high side is better. Say you have three or four overcast days where you're not putting much in. That will discharge the battery.

Car batteries are designed mostly for cranking amps and do not like being deeply discharged and recharged. RV or 'deep-cycle' batteries are designed for that use.

The gold standard for solar systems are golf cart or forklift batteries. They are heavy, expensive, and what you find locally may be 6V so you need two.

I'm leaving out lithium ion batteries, given your shoestring budget.

You don't need a battery: just run the pump directly from the solar panel. If the solar panel doesn't generate electricity, it means there is no sun, so no heating the water either.

You may need a voltage limiter, perhaps.

Sure. The question is how fast it can charge.

Figure a cell that gives you 10W in full Arixona sun gives you about 1W on the average in upstate New York given cloudy conditions and the sun often being lower on the horizon. Figure nine hours a day of sun in December in New York. How many amp-hours do you need?

--scott

You might not need a pump. If the heat collection coil is placed low relative to the top of the spa with the water from the spa taken out at the bottom and the water going back to the spa going in near the top, the hot water from the heat coil will rise to flow back just from the heated water rising into the spa. The more heat it collects, the more water flow it will have.

Black polyethylene water pipe could make an excellent water heating coil.

It MIGHT work as a maintainer to keep a battery charged on an idle vehicle. Charging a DEAD car battery????? Maybe in a month or two.

The first 2500-ish Model Ts had a water pump (1909)

The Model T system is known as a *thermo-siphon*. The *fill point* is at the top of the radiator which is also the highest point in the system. All points lower in the system are sealed.

A low point in a spa, the water surface in the spa itself, will not be sealed. That means, in order for it to work as a siphon, thermo or otherwise, you will need a means of bleeding the air from the pipes on the roof. Then you will likely have another problem, the circulation will be slow if not pump assisted so the water in the roof pipes is likely to boil creating a gas barrier (steam) which will prevent the thermo-siphon effect from working.

I have a solar hot water system on my roof which takes water from the tank at ground level and sends it around the solar collector on the roof then back to the tank. It has a small pump at the tank just for circulation, the thermo-siphon effect being insufficient to provide adequate water flow. The circulation pump only consumes a few watts of power @ 230 Volts.

Obviously a convection circulation system requires the heated water to go up from the heat source. Hot water weighs less than cold water /volume so it rises.