OT: solar battery charger

A neighbor's car battery ran down from parasitic loads in about a month. I think they are expected to start the engine after at least two weeks in an airport parking garage.

In general, no I do not see how. That was in respect to their question. If there is no extreme situation I do not see how not remembering to disconnect the little charger is going to damage anything. My comments about no real benefit were that there is no real benefit to not disconnecting it.

I should have given more info. It is on my father in laws lobster boat and they have a PC on it which they use with GPS, sounder etc... They would like to never shut it off but it kills the battery when the boat is not used for a day or out tuna fishing for hours on end. This is run off of a 1000 watt inverter. I think the panel is 80 watts. Would the inverter and computer (monitor off) kill the battery over night. How many watts does a computer use?

If it's a laptop, just look at the current rating of the power supply. If you use that as a guideline it will tell you. It probably has at least 1/3 safety margin built in.

Also, does the computer go into sleep or power saver mode or is it left in always on mode. You might also be able to tweak your power saver mode.

If a desktop, the fans and drives do use some power, but the biggest power sucker is the monitor. Turn the monitor off.

80 watts will charge most 12V batteries fully in a single clear summer day. On a cloudy day with a drain or during the winter maybe not.

Your 80 watt panel will give you 5-6 amps of charging current during most daylight hours.

Figure 30 AMP Hours for the average full size lead acid battery. Maybe a lot more depending on the group size, and battery type. You would definitely want a deep cycle or better yet an AGM deep cycle for this application.

My sons laptop power supply is labeled 3.5 amps at 18 volts. Figure a 1/3 safety margin, and the fact that even then it will not be pulling max at all times... monitor sleep... drive sleep etc... So average of 2 amps at 18 volts or about 3 amps at 12 volts. Add in the loss and parasitic drain of the inverter.

It could kill a single group 24 battery in 10 hours. If it's a good AGM deep cycle the solar panel would charge it back up fully in a clear sunny summer day. Go to a group 30 or group 31 battery it would probably be ok, but marginal some days. Put two of them in parallel and you should have no issues.

This is all just wild guesswork based on my own presumptions. To know you really need to look at your own equipment and do your own math.

If you use { amps X volts = watts } and then de-rate your available power by

33% you should be fine for 99% of applications.

This Dell desktop uses about 100W total, the newer one 120. The CPU units are ~30W, the rest is for the monitors, speakers and printers on standby. My 500MHz laptop's charger draws less that 30W. With Hibernation enabled they shut off and recover quickly without having to reboot. Turning down the LCD monitor's backlight cuts consumption substantially, the 22" on the other PC drops from ~50W to ~27W.

Jim Wilkins

First of all, I have three solar panels connected to my "house" batteries on a 40 ft. boat, and have had for 10 years.

My experience has been that if the solar panel/panels is capable of charging the batteries to a point where the battery voltage increases to a point higher then, say 14.4 - 14.5 volts then yes the solar panels can cause the battery to lose water, over time, until the plates are uncovered and cause battery damage.

If (as in my case) the battery voltage never gets over, say 13.5 volts, then the panels will cause no problems.

Cheers,

Bruce (bruceinbangkokatgmaildotcom)

??? Don't you have a charge controller?

Get a "Kill-A-Watt" meter for about $15

see how much power you are using. Or, you could wire an ammeter in series with the battery and get a pretty good idea.

p.

Nope. The battery voltage doesn't get above 13.5 in direct sunlight and that isn't going to bother the batteries any.

If the battery voltage reached say, 14+ volts, for any length of time, or if I left the boat for any appreciable length of time with the electrical system switched off (the panels connect directly to the battery), I would add a regulator of some sort.

Cheers,

Bruce (bruceinbangkokatgmaildotcom)

Does a killawatt work with 12 VDC?

Cheers,

Bruce (bruceinbangkokatgmaildotcom)