How to tell if a car battery is charged with solar cells?

[gary350]

I have 9 solar cells each one is 15 watts at 17.5 VDC. These are designed for charging a car battery.

I have them connected in parallel and connected to a car battery. I have a 300 watt DC to AC inverter connected to the battery. I also have a 400 watt inverter and a 1000 watt inverter I can use if I need to.

I can do a lot with 300 watts or 400 watts, lights, soldering gun, etc. I could use a way to know if I am pulling power out of the battery too fast or too slow so I can pace my electrical use so the battery will last all day.

Is there any easy way to know how much power is going IN or coming OUT of a car battery?

 

[JimB]

Use a couple of ammeters.

Connect one of the ammeters in the line from the charging circuits, and connect the second ammeter in the line supplying the load.

If the input ammeter reads more than the output ammeter, the battery is charging.
If the output ammeter reads more than the input ammeter, the battery is discharging.

Easy.

JimB

 

[Nigel Goodwin]

Even easier - use an old car type ammeter, these are centre zero and show charge and discharge

 

[nsaspook]

Install a +/- dial amp meter inline from the - terminal of the battery to power ground. If the dial reads positive amps you are charging, and negative reading is discharging power from the battery.
https://www.tyagiracing.com/product_images/c/amps_meter_gauge__66067_thumb.jpg

https://www.sailnet.com/forums/electrical-systems/71197-wiring-installing-battery-monitor.html

 

[Nigel Goodwin]

Install a +/- dial amp meter inline from the - terminal of the battery to power ground.

It could just as well be in the positive lead, and that's the normal way it's done (on negative earth cars).

 

[nsaspook]

I normally use the negative lead because it's safer from a voltage/power point of view. If the meter is shorted to ground directly from the + terminal you could have major arcing. A short from the - terminal to ground at the meter is a potential of a few tenths of volts max. Almost all Solar charging shunts are on the - terminal to ground.

 

[Nigel Goodwin]

I normally use the negative lead because it safer from a voltage/power point of view. If the meter is shorted to ground directly from the + terminal you could have major arcing. A short from the - terminal to ground at the meter is a potential of a few tenths of volts max.

In a car you can't do that - the starter motor current would go through the meter if you did.

 

[nsaspook]

In a car you can't do that - the starter motor current would go through the meter if you did.

Ganted, but this is for Solar and the normal convention is on the - terminal for battery shunts/amp meters.

 

[Nigel Goodwin]

Ganted, but this is for Solar and the normal convention is on the - terminal for battery shunts/amp meters.

But it really makes no worthwhile difference either way.

 

[nsaspook]

But it really makes no worthwhile difference either way.

For a simple analog meter other than having the higher voltage on a terminal that's correct but if you need send that signal to a opamp or monitoring device a high-side circuit can be more complex.

https://en.wikipedia.org/wiki/Shunt_(electrical)#Low-side_versus_high-side_current_shunt_insertion

I used Hall-effect sensors in my DIY monitor for both high and low side monitoring instead of shunts.

 

[JimB]

I agree that a center zero ammeter would be a good solution, but the average cz meter found in car accessory shops is rated at +/- 30 or +/- 60 amps and usually does not have very good resolution.

By using separate ammeters there may be more choice, especially when you consider that moving coil meters are quite expensive these days.

Having thought about this a bit more, an interesting solution would be to use something like a PIC driving an LCD display, then with the aid of a current shunt or a hall sensor, the current could be monitored, and to get a good idea of the state of charge in the battery, monitor the battery voltage at the same time.

If you want to get really enthusiastic, you could have Charge Current, Load Current and Battery Volts all on the same display.

JimB

 

[Reloadron]

Since you mention in part:

I have them connected in parallel and connected to a car battery. I have a 300 watt DC to AC inverter connected to the battery. I also have a 400 watt inverter and a 1000 watt inverter I can use if I need to.

Running a 1 KW inverter under close to full load your current draw can be as high as about 100 amps so I hope you have some AWG 2 welding cable lying around.

Since you do not mention the maximum current I will just assume 100 amps based on what you mention. That being drain. The charge current on a bright sunny day would likely be a little over 11 amps.

All things considered, I would vote for using a good current shunt along these lines. I would choose the 100 Amp / 50 mV flavor. Then signal condition the output using a good instrumentation amplifier. Power it from a dual supply so it can swing + / -.

Option 2 is just buy a good hall effect transducer with a voltage output like +/- 5 volts or + / - 10 volts **broken link removed** They run about $125 USD.

Now if you really want to get cool, do as was suggested and build a display (or buy one) as well as a data logger. There is no shortage of ways to go about what you want to do. Mostly a matter of how you want to go about it and what you want to spend.

I really like this thought from JimB:

If you want to get really enthusiastic, you could have Charge Current, Load Current and Battery Volts all on the same display.

Ron

 

[Nigel Goodwin]

Having thought about this a bit more, an interesting solution would be to use something like a PIC driving an LCD display, then with the aid of a current shunt or a hall sensor, the current could be monitored, and to get a good idea of the state of charge in the battery, monitor the battery voltage at the same time.

A very long time ago the magazine Practical Electronics ran a series using the LM3914 LED driver chip - one of the projects was a car ammeter, they used an opamp to amplify the voltage drop across the earth strap from the battery (so absolutely no loss whatsoever) - I always thought what a clever idea it was.

 

[Reloadron]

That is cleaver, using the earth strap as a shunt. Saves the cost of buying a shunt. Pretty cool.

Ron

 

[Nigel Goodwin]

That is cleaver, using the earth strap as a shunt. Saves the cost of buying a shunt. Pretty cool.

It impressed me at the time

I even threw a opamp circuit together on a piece of veroboard at one time (just feeding a multimeter) when I wanted to do current tests on a car I had (I can't remember why now though?).

 

[nsaspook]

A simple AMPLOC open-loop hall sensor is pretty cheap and can be interfaced directly to a PIC adc input for a complete charge monitoring system.

**broken link removed**
https://www.electro-tech-online.com/custompdfs/2012/01/KEY1002020AMP200202020AMP300.pdf

https://www.flickr.com//photos/nsaspook/sets/72157626291491551/show/
https://code.google.com/p/solar-monitor/

 

[gary350]

I am not finding much I can run on this tiny power source. This time of the year days are over cast and short and nights are long it might take 2 days for the battery to get a full charge.

I might be lucky to run one 60 watt light for a few hours. I don't know yet what I really have, I need to see how long it takes for the battery to charge on over cast days and full sunlight days too.

 

[More Coffee]

I know its old school.. but the most reliable way to determine battery charge state and determine to overall condition is to use a "hydrometer"..
a specific gravity of 1.277-1.300 on each cell is a fully charged battery, readings of 1.172 approx,and the battery is being killed..literally..its life will get sucked out its being discharged to far

also using the hydrometer will tell straight up if the system is wasting wattage to charge an apparently good battery that has one weak cell.

Its the only way to be absolutely sure whats being done to the battery.