Solar Power Battery Chargers

Alex Said:

could solar battery chargers help fuel economy?

We Answered:

I’m not sure if that much fuel goes into recharging a car’s battery. I believe that once a car is running, you could remove the battery and it would still keep running. That suggests to me that the only energy someone needs to replace into the battery is what was used when first starting the car.

When you see solar battery chargers on the market, those are usually for keeping the battery alive in a car that isn’t driven much. For example, if I park a car up at a cabin, and don’t go back for 2 months.

Shelly Said:

More about solar power charged battery project?

We Answered:

1. You probably will need to use parallel-wired batteries, because stepping-up 12V to 120V will draw 10 amps from the batteries for every amp drawn by the load (plus a bit more to account for inefficiency in the system). If you want to pull 13A from the 120V output, then you will be drawing 130A from the batteries. Multiply that by time, to find the amp-hour capacity you need — when batteries of the same voltage are connected in parallel, the amp-hour capacities are added.

2. The mains is almost always the cheapest source of electricity, simply because the electricity companies have economy of scale on their side. Solar isn’t free; there is a cost per kWh which can be worked out as:
initial cost of panels / (power output of panels in W * 0.001 * MTBF in hours)
and don’t forget that the MTBF in practice may be shorter than you think.

Since you will inevitably lose some of the energy coming from the battery charger, charging batteries from the mains and running appliances from said batteries by means of an inverter will always cost more than running your appliances straight from the mains would have done.

How about modifying an old computer UPS? These can often be picked up dirt-cheap second-hand, and usually the only thing wrong with them is that the batteries can no longer hold a full charge. And they are specially designed so as to be able to switch from mains to inverter within a half-cycle. Most of the ones I have seen use four 6V sealed lead-acid packs in series to give 24V. So you could use (series-wired pairs of 12V batteries, each giving 24V) in parallel. Disconnect the charging circuit altogether, because it’s not going to be doing much (it was designed for a much wimpier pack than you are using) and have the batteries charged directly from your solar array (which will now need series pairs of 12V panels in parallel). But leave a mains supply to the UPS. Then add a circuit which will measure the voltage coming from the batteries and, when it exceeds a certain threshhold (say about 11V), triggers a relay which disconnects the mains supply to the UPS and so trips it into action, powering your appliances from the solar-charged battery. When the batteries drop below 11V, the mains supply will be reconnected and your appliances will now work from the mains. This is the opposite of how it was supposed to work (appliances normally supplied from the batteries, with backup supply from the mains!) but should work in your case.

Peggy Said:

Could solar battery chargers help fuel economy?

We Answered:

Yes it would but it would be by an insignificant amount. A car alternator running at it’s maximum output puts out about one kilowatt which is approximately equal to one horsepower so it takes about one horsepower of energy from the engine to run the alternator. Actually most of the time it puts out much less. If you had a 100 horsepower car to generate the one kilowatt the best you could hope to do is a 1% improvement in mileage. since the alternator is rarely running at full power the improvement would actually be much less.

A typical solar panel puts out about 10-20 watts so your mileage improvement would about 0.01%