Which Schottky for solar panel?

[tab a]

I've not had any luck looking for a Schottky which would work in this application--which suggests to me that either there is no such thing, or more likely, I'm looking for the wrong thing.

Solar panel is 12v, 3.5A. As far as I know there is no diode on its output. (How would I know, or test for that?) I'd like to leave it hooked up to 2 different battery banks rather than manually moving it from one to the other. Both are 12v deep cycle marine batteries, though different capacities.

I'd thought a Schottky between the panel and each battery would be appropriate, and was looking for something in the 15-20v, 5A range. Found nothing so am suspicious I'm not thinking about this correctly?

Thanks.

 

[colin55]

Just use an ordinary 3 amp diode. 1N5404 etc

 

[crutschow]

Here's a circuit which simulates an ideal 5A diode **broken link removed**.

Or here's a 5A Schottky 50SQ100 Datasheet pdf - 100V 5A Schottky Discrete Diode in a DO-204AR package - International Rectifier. Just Google 5A schottky diode.

 

[smanches]

If you want to test to see if it already has a diode, just hook it up to the battery and see if you get negative current flow when it's dark. Or just reverse the meter and check for positive current flow

 

[colin55]

Why go to the expense of a special diode. 12v solar panels have plenty of voltage "up-their-sleeve" (needs to be at least 16v to 18v under bright sunlight) and any diode will do.

 

[crutschow]

Why go to the expense of a special diode. 12v solar panels have plenty of voltage "up-their-sleeve" (needs to be at least 16v to 18v under bright sunlight) and any diode will do.

A low drop diode would increase the charge rate under cloudy conditions or when the sun is at a low angle and the voltage is lower.

 

[Ubergeek63]

put them all in an array and use a Maximum Power Point Tracking charger

 

[HarveyH42]

I've not had any luck looking for a Schottky which would work in this application--which suggests to me that either there is no such thing, or more likely, I'm looking for the wrong thing.

Solar panel is 12v, 3.5A. As far as I know there is no diode on its output. (How would I know, or test for that?) I'd like to leave it hooked up to 2 different battery banks rather than manually moving it from one to the other. Both are 12v deep cycle marine batteries, though different capacities.

I'd thought a Schottky between the panel and each battery would be appropriate, and was looking for something in the 15-20v, 5A range. Found nothing so am suspicious I'm not thinking about this correctly?

Thanks.

For panels of that size (3.5 A), you really should consider a charge controller, which includes a blocking diode and overcharge protection. 3.5 A is enough to kill a battery, if you don't check it often.

Not sure about batteries of different capacities hooked up in parallel, something I need to look into myself soon.

 

[colin55]

As I said before, you have so much voltage-overhead that a Schottkey Diode is not going to make any difference to the charging current.
Now you mention the current will be too much for a small battery. Which way do you want it?
The 3.5A will be an absolute maximum value for a sunny day and will only be for a short duration. Most of the time it will be 1.5 to 2 amps.
Work out the amp-hrs you are drawing from the battery and see what the charging current is, plus the number of hours.
If these two equate, the battery will not be overcharged.

 

[Ubergeek63]

lead acid cells take 10-14V to charge and solar cells put out 12-22V depending on illumination and temperature. While the rated value is full load under ideal conditions (full sun at the equator without being hot), they can actually put out 30-50% more power using MPPT.

At a given current the output voltage goes down temperature and clouds. I forget how the current responds.

 

[ericgibbs]

hi.
Reducing the forward voltage of the blocking diode will increase the charging current available to the battery.

I would use a schottky diode.
Ref over charging, allow for the fact the charge efficiency can be as low as 60% on some battery types.

If you work on the assumption that the charge/time current equates to load/time current your battery will be well undercharged.

 

[colin55]

If you have an overhead of 4v, how much extra current do you think the solar panel will deliver if the overhead is 4.3v? Minuscule.

 

[ericgibbs]

If you have an overhead of 4v, how much extra current do you think the solar panel will deliver if the overhead is 4.3v? Minuscule.

When dealing with solar chargers, every 'miniscule' of current important.

 

[colin55]

First of all you are say the 3.5 amp panel may be too much for the battery and now you are saying every bit of current is important.

I don’t want to be involved with stupid discussions like this.

 

[ericgibbs]

First of all you are say the 3.5 amp panel may be too much for the battery and now you are saying every bit of current is important.

I don’t want to be involved with stupid discussions like this.

I have never said 3.5A would be too much, you must be confusing my posts with some one elses.??

Ref the miniscule current, if you do the calculations for a battery charge state over a voltage range of 10.8V [discharges] to 13.8V[charged] you see that the minsicule current can be as much as 9%... hardly miniscule.

If you dont want to be involved in IYO 'stupid' discussions, dont make stupid ambiguous technical posts.

 

[tab a]

Appreciate all the replies--lots for me to think about.

Maybe a bit more info will get this out of the realm of the hypothetical and back into the real world. The batteries are 90 and 225 amp, and are used in a very sporadic manner, on a boat. Neither has been, as far as I can tell, damaged by being connected to the panel continuously, so I would expect if both are connected at once, even less of a problem. We're really just talking about essentially a trickle charger. Yes?

The idea behind the Schottky was to minimize voltage drop and maximize current. Maybe in this particular instance it really won't make much of a difference in practice?

Thanks for the link to that specific diode--was exactly what I thought I needed.

Google 5A Schottky??? Doh!

Thanks!

 

[ericgibbs]

Schottky was to minimize voltage drop and maximize current. Maybe in this particular instance it really won't make much of a difference in practice?
Thanks!

It could be a few percent increase, which obviously is desirable, why 'waste' the energy.

Lets know how it goes.

 

[colin55]

How did you work out 9%? You can't simply apply Ohms Law.

 

[ericgibbs]

How did you work out 9%? You can't simply apply Ohms Law.

For goodness sake, is Ohms law the only formula you know....

 

[colin55]

How did you work out 9%?