i want to charge a 12 v car battery using a solar panel
i was thinking a transistor resistor and a power transistor or a relay
i was thinking just conect a transistor circuit to the battery figure out a value for the resistor and conact that to the other baterie terminal then when it got to 12V it would shut off. by trigering a power tansistor or a relay. would this work if so any ideas for the resitor value? would a latching relay be better?

 

I'll post my standard response for anything lead acid related. Texas Instruments makes a really great little IC called a BQ2031 that controls all aspects of the charging process. Here is as app note for implementing MPPT with it.

http://focus.ti.com/lit/an/slva378/slva378.pdf

And here is the TI page for it.

http://focus.ti.com/docs/prod/folders/print/bq2031.html

I built a charger using this appnote that came out really great but since you are using a solar panel the MPPT arrangement is probably better. It is almost the same circuit with a minor change.

http://focus.ti.com/lit/ug/sluu023b/sluu023b.pdf

One of these days I plan to put everything together to post in the completed projects forum.

 

Just for your information; electromechanical relays are "power hungry"; they would consume a good bit of power just to keep the coil energized. Similarly, standard transistors and Darlingtons are also power hungry; they need current through the base to pass current via the collector and emitter.

 

i want to charge a 12 v car battery using a solar panel
i was thinking a transistor resistor and a power transistor or a relay
i was thinking just conect a transistor circuit to the battery figure out a value for the resistor and conact that to the other baterie terminal then when it got to 12V it would shut off. by trigering a power tansistor or a relay. would this work if so any ideas for the resitor value? would a latching relay be better?

F.Y.I. When the battery terminal voltage is at 12V It's time to start charging it rather than ending the charge. The battery needs a higher voltage than it's current discharged state to initiate charging. For example, if your battery voltage reads 12V and you place a charging voltage of 12.3V, the battery will only charge to 12.3V and would be considered somewhat discharged. It is generally recommended to charge it to near 14V with a higher current before switching to a trickle charge or 12.7V at a lower current.

 

The "float charge" voltage for lead-acid batteries is in the 13.6v to 13.8v region when the battery internal temperature is 25°C/77°F. For 12v batteries, there is a negative temperature coefficient of 18mV per 1°C offset from 25°C; the higher the internal temperature, the lower the float charge voltage.

Most chargers on the market don't use temperature compensation, so battery life suffers.

 

The "float charge" voltage for lead-acid batteries is in the 13.6v to 13.8v region when the battery internal temperature is 25°C/77°F. For 12v batteries, there is a negative temperature coefficient of 18mV per 1°C offset from 25°C; the higher the internal temperature, the lower the float charge voltage.

Most chargers on the market don't use temperature compensation, so battery life suffers.

You are correct. Silly me, I was quoting the voltage from a very cheep battery float charger! The better chargers I use float @ about 13.54V.

 

thanks this helps me a little. does anybody have any shcmatics or any info on how the ones that you can buy work? i have a small about 6X8 in panel that was ment for batteries that has a blocking diode in it it puts out about 15v ish in full sun would this be ok just to hookup and let sit it has a output i thnk is about 15 MA. i also have a giant panel that makes 20V at between 11/4 amp. i know the big one would need a controler. but what about the little one? all this needs to do is maintain a boat batery so the bilge pump won't kill it. would this small one be sufecient?
i saw some solar panels that you can buy that say somthing about the charge palsates this causes a minimization in the stuff that builds up on the plates i don't know that much about these LA bateries so i don't know if this is that much better.

 

15mA is hardly worth the effort to make a regulator circuit for; it certainly won't keep up with a bilge pump's demands.

Having an automatic bilge pump installed in a boat will give one a false sense of security; if there is a leak, sooner or later the pump will fail or the battery will die or shore power will go out, and the boat will sink anyway. If the leak starts off at one rate and gradually increases, sooner or later it will probably exceed the bilge pump systems' capacity, whether electrical or mechanical.

You would be better off to have some kind of an alarm go off if the water in the bilge rose to a certain point, or the water level level started increasing at some predetermined rate - even then, you still can wind up with an inoperative alarm system due to a dead battery, corrosion, etc.

But if you want to try using the larger panel, see jerseyguy's post #2 for the best solution. It's not really something for a beginner to build; you'd need to have an intermediate level of experience.

 

ok thanks this helps. the leak is not that major if it is not on a hoist the pump wil run 1-2 times per week. and there is a backup pump also should i conect this backup to the troling motor bateries incase the engine batery dies?

 

i just mesured the output of my small solar panel it will put out aprox 18V and 45-75MA is this sufecent for a automatic bilge a few times a week?

 

How did you measure the panel output?

How much current does your bilge pump require while it is pumping water?

Once the water rises to the level that the pump turns on, how long does the pump run?

What is "a few times a week"?

Without details like this, we can't even begin to venture a guess.

But, since the output of the small solar panel is so anemic, I will have to guess "no". You will wind up with a dead battery and a boat full of water hanging from its' mooring lines.

 

I would have to agree with Sgt. Wookie. I'm not that would even compensate for the batteries self-discharging rate.

 

i used a analog multi meter maxed out 50 ma range in direct sun so went to 500 range sowed aprox 70MA
when i have heared it run it is very quick 15-20 sec.
it's a small pump 2-3 A draw.
2-3times a week. unless it rains then maybey a nother time or 2

 

By the time you build a regulator and add that to the circuit loop you will have even less current to trickle charge the battery.

You didn't mention the Amp-Hour rating and chemistry of the battery you have.

I'd almost guess that just leaving the 15V - 18V @ ?mA - 70mA connected directly to the battery with a blocking diode would probably not hurt things much, but this is just a wild guess. On one hand, if the pump never turns on you could run into an over-charge
situation while on the other hand you still might have a battery that is still loosing it's charge. And if there is no assurance that the setup is reliable, you might just as well just go to the boat a couple of times a week and manually turn the pump on.