Sharp ND-208U1, 208 watt solar panel

[Brianellul]

The Vpm of this panel is 28.5V. Is this high enough to directly charge a 24v battery bank?

Brian

[crewzer]

Brian,

I'm afraid the answer is "no". A "24 V" battery bank needs to be pushed to ~28.8 V at 77 F (25 C) in order to transition from bulk to absorb mode. Cold batteries require a higher voltage, and the EQ voltage for a flooded-cell types would be ~ 31 V.

The Sharp's STC Vmp of 28.5 V is based upon module cell temperature of 77 F (25 C). Since the cells can operate at ~63 F (~35 C) above ambient in full sunlight, the panel must operate in a pretty cold environment to produce rated voltage. The output voltage drops as the temperature increases.

The California Energy Commission rates the Sharp 208 at 183.3 W based on an ambient temperature of 68 F (20 C). You should therefore expect to see a mid-day Vmp in the range of 28.5 V x (183.3/20 = ~25 V, not counting losses in the array wiring and the controller.

In sum, what is in reality a "25 V" module usually won't meet the 28.8 V charging requirement for a 24 V battery.

HTH,
Jim / crewzer

[Brianellul]

Thanks crewzer

That's what I thought as well! I think that in this case I'll go for the Sharp NT-180UY1 180 watt solar panel. This has a Vpm of 35.86V, slightly higher that what you recommended but on the other hand will compensate for the array looses etc.

Regards
Brian

[niel]

i see your original question was for directly charging the 24v battery bank. do use a controller to prevent overcharging your batteries and if possible with a battery temperature sensor feature to compensate charge voltages when the batteries are colder or hotter. if you do this without one you risk the batteries' lifespan which can cost you far more than the controller.

[mike90045]

Originally Posted by Brianellul

The Vpm of this panel is 28.5V. Is this high enough to directly charge a 24v battery bank?

Brian

*maybe*
the Open Circuit voltage is 36V, but there won't be much current at that voltage. It may take a day to charge the bulk of the battery, and another day to top it off when you go past the MPV point. What are you intending to do with 1 panel? You may run into boiled batteries if you try to leave it unattended. Some of the charge controlers are MPPT style, and they should work OK with this, except for the EQUALIZE function, which may take a whole day at the low amperage. Or just get the 36V panel.
Photo is off the back of one of my panels

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