PV Gap

Frances Said:

How many watts does it take to fully charge a 12 volt battery in 24 hours?

We Answered:

It depends on what size of battery you have.

First, a little background. To charge a battery properly in a way that prolongs battery life, you charge in 3 phases:

bulk phase - "heavy lifting" phase. Most of the charging is done here. This gets it to 75% recharged. Happens at 14.4 volts.

acceptance phase - keep charging at 14.4 volts, but reduce the amperage. This charges the last 25%.

float phase - not actually charging. This is continuous voltage pressure on the battery that is intended to prevent discharge. It happens with about 13.3 volts and very low amperage.

A smart charge regulator will automatically charge the batteries this way, and doing it this way will extend the life of your battery. The regulator will pay for itself after a while in terms of battery life, so look into getting one.

Now to figure out how many amps you need, you have to multiply the amp-hour capacity of the battery by about 30%. Check out the writing on the side of the battery and see how many amp-hours it holds.

Example: 200 amp-hour battery * 30% = about 60 amps to charge.

The calculation of wattage is:

amps x volts = watts

Now, to charge that 200 amp-hour battery in the bulk phase, you need:

14.4 volts x 60 amps = 864 watts

That's quite a large solar panel!

But, we're not quite done. Solar panels only get their rated output in ideal conditions. ie. direct sun and when the panel is facing right into it. Most of the time, this doesn't happen, especially on a boat, where things are moving around all the time and things like the boom or sails sometimes cover up the panel.

So, the rule of thumb is that you should assume the panel will only operate at about 40% efficiency during the day. Put another way, that means you need to get a solar panel that is 2.5 times as much as you need. The charge regulator will make sure that not more than the 864 watts will go to the battery if you happen to be in ideal conditions and the panel is generating more than you need.

So, calculating that out:

864 watts x factor of 2.5 = 2160 watts

That's a HUGE panel.

Now the part about the 24 hours charge. The problem is that batteries will only accept charge at a certain rate. You cannot put more volts or amps into it and expect it to charge faster. Actually, you can do it to a certain extent, but you will ruin your battery really, really quickly in only a few charges. This of course is counter productive.

Look below for the link to the west marine advisor article about battery charging, which has a lot of good information.

Scott Said:

can a 12V 8A spa pump be powered form a 12V car battery?

We Answered:

The short answer is Yes. Other considerations:

It depends on how long you intend to run the pump. Look at the watt-hour rating on your battery. Every hour the pump runs you draw 12*8 or 96 watt-hours out of the battery.

The next consideration is whether you need a regular car battery or a deep cycle battery such as is used in marine applications running say a trolling motor. A car battery is made to supply high currents for a short period of time and then be recharged after the engine starts. 8A is rather low and you'd be drawing it for more than 15 seconds at a time, so the car battery would wear out. Deep cycle would be more appropriate. they're designed to run lower currents over a longer period of time.

Either can be trickle charged.

Jennie Said:

How do you charge a 12v AGM maintenance-free lead-acid battery?

We Answered:

A fully charged 12 volt AGM battery's voltage should read about 12.7 volts. You should charge it with about 1 - 2 amps of charge current at charge voltage 14.2 - 14.4 Vdc then drop the current to a trickle and float the voltage at about 13.4 after full charge.
here is a nice charger in your price range, if you do a search you may find a better deal, but Northern is a well known company and from the description this charger will address all your concerns.
http://www.northerntool.com/webapp/wcs/s…

I would not use a charger that does not have current limiting, i.e. a normal flooded-cell car battery charger or any other method without limiting current. connect + to + and - to -.
have fun!