Charging voltage: Solar controller vs car alternator

[bjorn]

I have a 30w solar panel connected to a 75ah AGM batteri. The solar controller (PWM) has a aborbsion voltage of 14.4V (+- 0.2V.)and a float of 13.8V. So far so good. When I connect the battery to the controller and then the solar panel and the charging begins the voltage on the battery is just above the voltage measured on the battery before connecting.This voltage will slowly increase as the battery is charged. This is supposed to be like this, The PWM controller will reduce the input from the panel (which is about 18-22v when not connected to the controller) to a voltage just above the voltage on the battery and slowly increase when charging

However, when the alternator in my car start to charge the battery, the charging voltage is constant around 14.4-14.6V until the alternator has fully charged the car battery, then the voltage wil drop to 12.7-12.8V (ie. the alternator wil stop charging the battery) Why does a car alternator and a solar panel controller charges so differently? Since the input from the solar panel always is above 18 V, why don't the PWM controller just put on a 14.4V charging just from the start?

[ButchDeal]

In your case the 30w PV module is barely able to do anything to your 75ah battery.

[Sunking]

The issue is you do not understand how a battery works.

Battery Under Charge Voltage = Battery OCV + (Charge Current x Battery Resistance)

So you have a battery with an OCV of 12.0 volts, Resistance is .01 Ohms, and Charge Current = 2.5 amps. So you set your 30 watt PWM controller to 14.4 volts, connect the controller, and start charging. What is the voltage?

12.0 volts + (2.5 amps x .01 Ohms) = 12.025 volts. It will take several hours to charge a 75 AH battery with 2.5 amps. At 12 volts OCV indicates the SOC is roughly 40% and roughly 45 Amp Hours are needed to fully charge up. Simple math.

Amp Hours = Amps x Hours
Amps = Amp Hours / Hours
Hours - Amp Hours / Amps

So how long wil it take.

45 AH / 2.5 Amps = 18 Hours.

Being on grossly undersized solar with 4 Sun Hours per day will take 4-1/2 days to recharge.18 hours / 4 hour-day = 4.5 days.

Now your engine alternator if it is a rice burning toy car will have a 750 watt Alternator and at 12 volts is roughly 60 amps. Take the same battery and connect it, start the engine and the voltage jumps from 12 volts to 12.0 volts + (60 amps x .01 Ohms) = 12.6 volts. How long until it is recharged.

45 AH / 60 amps = 0.75 Hours or 45 minutes until you see 14.4 volts.

Think of it this way. You have a car and 45 miles to drive. You can go 2.5 mph or 60 mph. How long will it take?

Bottom Line your 30 watt panel and PWM charger on its best day can only generate 1.6 amps or 20 watts of power. Your alternator can generate 700 to 1000 watts or 60 to 80 amps of current. 35 to 50 times more power, thus 35 to 50 times faster. So in effect your 30 watt panel and PWM controller is a Trickle Charger on a 75 AH battery. It is not capable of charging the battery in any meaningful amount of time. All it can do is keep a fully charged battery charged up. You bought a 19-inch push mower to mow a golf course.