Need advice for solar charging car acid 75AH battery

[krivis]

Hi guys,

as the title says, i need advice on how powerful solar system should i buy for charging 75AH car acid batt. In the moment i have 150W 220v AC inverter connected to the 75AH batt, and i change that battery every 2~3days. I measured the power draw between the inverter and batt. and its 12v dc 1.3 amps. The inverter works 24/7.

Thanks in advance for any advice!

[Laspher]

While not being actively involved with solar, or battery yet, I would suggest changing to a Deep cycle battery. Car batteries are meant for a rapid release of energy, for just a momentary shot. They are not meant for long drains, nor are they meant to be cycled. It will probably die quickly.

As for charging, rough guess is a 200 watt 12v panel with capable charge controller.

Again, I am still learning myself, so take all my advice with a grain of salt.

[Sunking]

Forget the car battery.

[NEOH]

Let's just say, your Solar Panel receives 4 Hours (avg) of strong sunshine and
it must replace all of the Watts that your Inverter uses on a daily basis.

The math goes something like this ...
12 Volts DC x 1.3 Amps x 24 Hours = 374 Watt-Hours used per day by the inverter.
Size of the Solar Panel is approx 100 Watts = 374 Watt-Hours / 4 Hours Strong Sunshine

If you have 4 hours of strong sunshine every day, your car battery may last 30 - 90 days ( maybe longer ??? )

[PNjunction]

With a vehicular SLI, or shallow-dischage starting battery, do not draw more than 10% of it's rated capacity per cycle if you want to get a rough vehicular 3-4 year equivalent out of it.

That means for your 75ah batt, then each cycle should draw no more than 7.5a TOTAL before recharging. That's a lot of lead to be humping around for only 7.5Ah available per cycle, but it can be done. Discharge further than that, and your life cycle degrades rapidly.

Stop using the ac inverter, as many are not designed to be chargers, and may not have tight ac-ripple specs. Get yourself a charger *before* you get solar. Something like the Tecmate-Optimate 6 or 7 would be fine - especially their voltage-selectable "select" models. (14.4 / 14.7 switchable).

Since you are using an SLI shallow-discharge battery, we'll assume that you are not depending on it for many day's worth of autonomy. Therefore, a simple 100-150w panel and charge controller would do fine, until you get super-serious.