# 10 Watt Solar To Car Battery With Diode

1. Dec 8, 2013

### John1397

I have an 800 amp car battery no load connected at anytime to a 10 watt solar panel with just a regular diode in between after 4 or 5 hours in the sun terminal voltage reads 16 volts is this good? Would a battery connected this way last longer than one that sits idle uncharged for 6 months?

John

2. Dec 8, 2013

### TurtleMeister

Assuming you want to maintain the battery rather than charge it, I think 16 volts is a little high. I think 13.8 volts is the magic number. However, that will vary a little depending on temperature. I would use a voltage regulator instead of just a diode. And for safety I would use a fuse between the regulator and the battery.

3. Dec 8, 2013

### phinds

I agree w/ turtlemeiser, and the fuse should be a very low value since you really only want a trickle charge.

4. Dec 8, 2013

### Averagesupernova

A 10 watt solar panel connected to a car battery is able to raise the voltage to 16 volts? I find that very hard to believe unless the battery is complete junk.

5. Dec 8, 2013

### AlephZero

A 10W solar panel will deliver less than 1A as 12V, and that is trickle charging for a car battery.

Agreed. Anything over 14.5V is doing to damage the battery if it isn't damaged already. The absolute limit for equalizing the charge between cells in an attempt to get a damaged battery back to life is 15V, and you need to monitor the battery temperature while doing that.

A 10W panel doesn't have the oomph to reach those voltages, unless the battery has a high internal resistance (a.k.a "complete junk").

6. Dec 8, 2013

### OmCheeto

There is another explanation. 12 volt solar panels usually run above 16 volts at no load. Mine run between 18 and 20 vdc. My guess is that the connection to the battery became corroded, or loose, and the 16 volts was from the panel alone.

If the panel was able to raise the batteries voltage to 16 volts, then I agree, something is seriously wrong. And it would also be extremely dangerous, as hydrogen gas evolution starts at voltages above 14.4.

7. Dec 8, 2013

### John1397

I make my own panels so I always use 42 cells, but that being said it still figures out to 10 watt at 12 volts using ohms law or about .80 amp . Battery is not new but still cranks a car motor. It was -10 degrees F when took reading.

8. Dec 8, 2013

### dlgoff

It would depend on the where in it's lifetime (number of deep cycles) the battery is.

And how it's been charged in the past.

Quote and Images compliments of http://batteryuniversity.com/learn/

9. Dec 8, 2013

### John1397

So this would be 14.7 at most if I install two diodes in line this would be a crude way of lowering voltage enough. Summer months it was putting out 15 volts must be cold factor more voltage and or amps. I know LM317 would work but have more diodes laying around.

10. Dec 8, 2013

### TurtleMeister

At that temperature the maintenance float charge should be 14.8 volts. The 13.8 volt value is for a temperature of 68F. If the battery is being stored in a variable temperature environment then you should use the higher temperature value to prevent over charging. However, if your panel can only output 800ma then over charging may not even be a concern, considering it will only be charging when the sun is out.

11. Dec 8, 2013

### John1397

Looks like amps figure into the formula to so I will have to measure amps to not just volts. I know when you take off the covers there is no bubbles coming up out of the water, but you do hear a whizzing sound when the covers are on so you know they are charging some. In twenty years when the plates have all turned to powder then we can say it was to much voltage.

12. Dec 11, 2013

### sophiecentaur

Have you actually measured the current into the battery? As others have said, it looks as if it really isn't charging significantly.

13. Dec 11, 2013

### John1397

I added a few more diodes I now have 15.25 volts and 300 ma this might be ok as the previous posts have stated to much no good to little no good. This should be better I think then leaving battery sit for 6 months uncharged.

14. Dec 11, 2013

### sophiecentaur

So you have measured the current in this condition. What is the current with direct connection? I can't find it in your posts.

15. Dec 11, 2013

### John1397

15 volts with output of 300 ma is with the battery hooked up I have 60 feet of 16 gauge wire so probably lose some.

16. Dec 11, 2013

### sophiecentaur

I'm confused now. You just said that was with diodes added.

17. Dec 11, 2013

### John1397

I was getting over 16 volts without those extra diodes. The panel puts out with no load about 22volts and 500 ma. With battery connected it puts out 15.25 volts and 300 ma the rest must be lost in the wire from solar panel to battery.

18. Dec 11, 2013

### RocketSci5KN

Note that diodes don't have a fixed voltage drop. They tend to have zero drop as current gets near zero. This will raise the voltage beyond what you expect. Check the diodes datasheet.