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

Question

Talking about a 12v deep cycle battery, as would be used on a trolling motor. Am trying to figure out how large a solar charger I need.

I have seen as large as 6 watts. For whatever reason they do not list the amps. Calculating it, 6 watts at 12v, is .5 amps.

Answer 1

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.

Answer 2

Marine Solar Battery Charger

Answer 3

Solar chargers are very good for keeping the charge up. They will not overcharge. You need to determine what space you have to set out the solar charger, they take space. A solar charger may not be able to recharge a trolling motor battery. Ask someone at West Marine. 1 800 boating.

Answer 4

Small solar chargers are for adding a very small trickle charge in Milliamps to keep the battery fresh, if you are looking for a fast charge or "Float" charge then solar will get very expensive. If the battery is dead, it will take you all summer to charge it up on a solar charger. I've played with these solar chargers on Sail Boats with small amounts of success. A marine battery is $60, so economically, it makes more sense just to keep an extra battery handy. You can secure one in the trunk of your car using a common plastic boat battery box. A charge can be kept on it by putting two small alligator clips on small battery leads wired to the battery teminals in the trunk. A simple cigarette lighter power plug is connected, and every now and then just plugged into the cigarette lighter recepticle for charging up, or keeping a "Float" charge on the extra battery. "Thinking man's way" ! here are some helpful places with solar equipt.
Harbor Freight
Northern tool co.
www.sciplus.com--> very helpful, and inexpensive parts for the "MAD Scientist" in YOU ! Factory over runs, cancelled government contracts, abandon Projects that you can canniblize, In short, it's a junk yard of helpful items when trying to "FABRICATE" your own invention, a really fun place ! I Hope this helped you !

Answer 5

If I wanted to run a water pump continuously and it is 5.8 amps to run it how much solar to keep the 12 volt battery charged? It wont run all the time for long lengths of time. I want to put a hot tub jet on the back of my kayak and run a car electric water pump to use as propulsion. Might not work but I think it will be fun to try anyway.

Answer 6

The Amp hour rating is a long-term measurement, amps are an instantaneous value. So it's probable that your batteries can put out more than that for a short time, but won't sustain more than 7 amp-hours. So don't expect to run your moped at full speed for very long. (Actually accelerating and uphills take the full output, sustaining a certain speed, as long as it's not the top speed, uses a lot less power).

Answer 7

Most chargers put out 14 to 16 amps for deep cell charging ...Some of your older chargers which took several days put out a lower charge rate to keep the batteries from boiling over .

Answer 8

if you put 14 amps to a battery i think it would boil dry at about hour 16, i think this person meant volts . anyways, try not to put more than 4 to 6 amps as a trickle charge, keep an eye on the water level . have fun

Answer 9

I know if you have a 110volt battery charger with 8amps it take 12 hours so if you have a sloar powered charger see what the amps are then it will have a diagram that comes with it and it will tell you. good luck

Answer 10

You can find solar panel kits for beginners at COMPAREFROGGY.

http://www.comparefroggy.com/affordable-solar-panels.html