solar panel hooked directly to battery

[rexlion]

I've heard of some folks hooking a small solar panel directly to the battery to charge or maintain it. But some panels (my 70W, for instance) can peak its output as high as 17V. My question is, how does a 17V input at low amps play out when hooked to a 12V battery? Is it healthy or unhealthy for the battery, and if the latter, what can it do?

(Side note: I do use a properly sized controller with my panel. My interest is academic.)

(EDIT: ) 🙂 Let me rephrase/refine my question. For a short time of charging a non-full (depleted) battery, would the direct connection at 16-17V charge the battery harmlessly, and would it charge more quickly than a panel with a controller that knocks the voltage down to about 14V?

[rexlion]

MrWizard wrote:
'knock down' is not the right term

PWM means Pulse Width Modulation

the power is pulse on and off hundreds of time a second to maintain a set point voltage aka 14.?

the higher the battery voltage (state of charge) the shorter the on time of the pulse and the longer the OFF time between pulses

thats the simple basic straight forward explanation of the operation

Thanks for that explanation. So if I'm understanding correctly, the PWM may not be pulsing off much (or at all) when the battery voltage (and charge level) is very low. Interesting! I can see now why a direct charge (without controller) wouldn't be appreciably faster.

[MrWizard]

'knock down' is not the right term

PWM means Pulse Width Modulation

the power is pulse on and off hundreds of time a second to maintain a set point voltage aka 14.?

the higher the battery voltage (state of charge) the shorter the on time of the pulse and the longer the OFF time between pulses

thats the simple basic straight forward explanation of the operation

[rexlion]

pianotuna wrote:
Hi,

The controller doesn't "knock down" the voltage when the battery is empty.

Well, I realize there are different controllers out there and they may work in different ways. But my Morningstar SG-4 has a stated PWM setpoint of 14.1V, so doesn't this mean that if the panel is sending higher voltage than 14.1, the SG-4 is reducing the voltage to 14.1? I'd read somewhere that MPPT controllers also reduce the voltage, but they somehow manage to send the same wattage and convert that extra voltage into more current (V*A=W so less volts means more Amps). And I thought I'd read a comment at one time that said a PWM controller loses (wastes?) this extra voltage when it steps the voltage down. This is partly why I was wondering if 17V can somehow push the charge into the battery faster than 14.1V would. Not that I'm planning to do it; like I said, it's academic... I'm curious to know how it works.

[rexlion]

eb145 wrote:
If the battery is not fully charged, then connecting the panel directly to the battery will be fine for a while - until the battery voltage rises too high as stated earlier.

Ed

Ok, thanks.

[eb145]

If the battery is not fully charged, then connecting the panel directly to the battery will be fine for a while - until the battery voltage rises too high as stated earlier.

Ed

[pianotuna]

Hi,

The controller doesn't "knock down" the voltage when the battery is empty.

[LittleBill]

rexlion wrote:
Let me rephrase/refine my question. For a short time of charging a non-full (depleted) battery, would the direct connection at 16-17V charge the battery harmlessly, and would it charge more quickly than a panel with a controller that knocks the voltage down to about 14V?

yes,no

[rexlion]

Let me rephrase/refine my question. For a short time of charging a non-full (depleted) battery, would the direct connection at 16-17V charge the battery harmlessly, and would it charge more quickly than a panel with a controller that knocks the voltage down to about 14V?

[Lwiddis]

And we are going to keep testing you...you, rjxj!

[azrving]

LOL I read your post. Yes you could connect a panel directly to your battery IF you were monitoring the voltage and didn't go above about 14.8 for a wet or 14.4 for AGM. Was this a test? 🙂

[2oldman]

The above answer seems to be on point.

[time2roll]

Plays out same as any 17v charger on a 12v battery. Voltage will steadily rise to 17 volts. Your panel is maybe 4 amps so the effect will be slow. So slow you may never get above 13 volts while in use. Monitor the voltage rise closely the first few times. Best to have a controller in storage.

Unless it has a diode, best to disconnect as the sun drops.

[Lwiddis]

Controllers are inexpensive. I use one on my solar battery tender as well as big old WindyNation system. What others do....hmmm

[MrWizard]

if the battery is fully charged, sitting in storage
then that 17v could damage the battery by drying it out, same way the old magtec converters

you can buy a $10 controller for that 70w panel and be safe

which is more important to you, the $10 and a little time involved to install
or that battery ?

if your going to use solar then you should use a charge controller