Inadequate Charging Voltage?

drnatus · February 2019

History: Once upon a time I ran a ham radio station from a Series 27 deep cycle marine battery and a 20v 1a panel. (20 watts.) The system worked really well for 10 years before the battery had to be replaced. No controller, just a 2 amp diode in the B+ line for reverse bias protection. Technically, the diode dropped the voltage by 0.7 volts, but when I put the battery into play, it's internal resistance dropped actual charging voltage so much that I was floating only 14.7 volts across the battery in full sunlight. The consumption load was a 50 watt radio, a 10 watt radio, an antenna rotor, a Commodore computer, a small B&W TV, and a 25 watt light bulb, all 12 volt. My solar panel was a cast-off from an AM broadcast station in Ohio, and had been in service already for two decades before the station swapped in new panels and sold the old ones. Eventually, the old panel started developing open traces. When I got tired of fixing the bad connections by shoving pins through the silicon gel into the failed joints, I replaced the panel.

Current Status: The new panel came with a controller which includes reverse bias protection. Output of the combination is rated at 15.6v 1300mA. (20 watts.) I'm still using a single Series 27 deep cycle marine battery for storage -- this one brand new -- but am unable to keep it properly charged with the panel alone. I've found that the panel and controller float only 12.4v across a battery, whether the big one or pocket-sized gel cell, whereas the "smart" automotive battery charger I have for testing purposes provides -- you guessed it -- 14.7 volts and easily recharges the battery.

My question is simple. What's the deal with products that cannot maintain batteries for which they advertise as being specifically built?

My son ran into the same problem. He wanted simply to be able to use solar power to recharge his laptop, so he purchased the recommended panel, gel cell, and inverter. His panel is rated at 17.9 volts, yet fails to float more that 12.4v across his gel cell, and his controller uses 10mA/hr just to be in the line. Consequently, his gel cell discharges to 11.7v within 48 hours, a level which will not operate his inverter.

Suggestions?

Raj174 · February 2019

Is this group 27 battery a 100 amp hour AGM battery? What ever size the battery is, you will need a panel that will produce amperage equivalent to 5% to 13% of the capacity of the battery, depending on the load and location of the system. Take into account 20% to 25% panel power lose. The CC will need to be able to handle charging amperage.

Estragon · February 2019

At 12.4v, the controller isn't floating, as that would be the voltage of a partly discharged battery. Fully charged and "resting" ( no charging or loads), voltage should be ~12.7-12.8v. Float is typically 13.x, depending on battery type setting.

If the panel and controller are working properly (and there are no loads), this voltage should slowly rise until the controller absorb voltage setting is reached. This would generally be 14.x volts, and may be adjustable, by way of dip switches or otherwise in the controller.

I would expect a nominal 20w panel properly tilted to full sun to produce ~15w, and in most locations maybe something like 60-100watt-hours over the course of a day. Obviously, if loads exceed watt-hours produced, the battery will run down over time.

It may help if you could post a link to the "recommended" package.

drnatus · February 2019

Thanks for the quick responses! Raj174, if I understand you correctly, you're telling me that my highly successful system of 10 years duration could not possibly have worked. My flooded lead-acid battery both then and now was/is spec'd at roughly 115 amps. For 10 years I charged said battery to capacity quite well with a very old 20v 1a panel which, even when brand new, was rated at under 1% of the ah capacity of the battery. Hmmm.

Estragon, I'll find out from my son what link/info he used and let you know. His panel is rated at 1.14A and allegedly is charging a 7Ah gel cell, for roughly a 16%-of-capacity source. Yet his system fails in the same way as mine, under no load other than the constant 10mAh consumption of the controller. I can put in any of three fully charged batteries and find, through the LED readout on the controller AND with spot-checks on the battery itself with a VOM, the system and battery voltage gradually dropping from 12.7 to 11.7 over two days' time. I can even watch it drop from 12.7v to 12.4v in the middle of a sunny day while the controller shows the battery charging. A quick check of the panel, briefly disconnected on that same day, shows ~16v. All this with the inverter disconnected. But it looks as if I'm using the term "floating" incorrectly. With it I mean to indicate a particular voltage being present across the battery terminals when the battery is connected to the panel in full sunlight. I've slept since then, but when observing the old system, that voltage figure was pretty consistent no matter how much reserve capacity I had used.

Would still love to get back to the simplicity and functionality of my original system......

drnatus · February 2019

My son says https://www.suntactics.com/laptop-solar-charger

tabbycat · February 2019

Go back to your original system and see if it still works. I wouldn't trust the output of an off brand controller. Stick with the name brands such as those sold at NAWS.

Estragon · February 2019

Did you get the charge controller from them?

If so, note that their controllers appear to be designed to charge USB (5v nominal) device batteries. If they see ~12v on output, they would (at best) have a pwm duty cycle of zero (no charge current), or at worst damage the controller and/or discharge the battery.

If not, can you link to the controller used?

Raj174 · February 2019

The fact that your original system work so well, for so long is a bit of a puzzle, but the math on this is pretty simple. A 20 watt panel will output about 1 amp at charging voltage and with a 100 watt load for example, (about 8 amps at 12 volts) on 100 amp hour battery, there would be a 7 amp per hour deficit. Under that load after 8 hours, the battery will be close to a 50% state of charge. Not to mention that flooded lead acid batteries are only about 85% efficient, so that will also add to the charging amps required. So you can see that far more power is need to charge the battery at a reasonable rate, and 12 to 13 amps sounds like a reasonable rate, considering this load, for a balanced system based on a 100 amp hour battery. This is assuming 4 to 5 average solar hours (insolation) for your location.

So what would the panel requirement be? I would estimate two 100 watt panels minimum, but better, two 120 watt panels in parallel considering the previously described example load.

Estragon · February 2019

If the old system worked, it follows the duty cycle of the listed gear was short enough for a 20w panel to charge. Maybe they go to the radio shed 1x daily, run stuff for a few minutes, then shut down?

I can run my electric toaster for a few minutes/day, but obviously couldn't be running 24x7.

Inadequate Charging Voltage?

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