20w solar panel -- what size battery can I get for long lasting performance?

[mad*sparrow]

I have an excellent charge controller (overkill mstar mppt) and 20 watt panel and hope to install a battery somewhere in my engine compartment on my nissan frontier. And Is it best to use a dual bat hookup still to keep my engine battery topped off or am I limited to charging just the one bat because of the small wattage solar panel?

 

[akrvbob]

Hi Mad Sparrow, no problem hooking them both up at the same time. The controller will protect the battery from over charging by the solar panel and the trucks voltage regulator will protect it from overcharging the battery. <br><br>20 Watts is a very small panel, but combined with the alternator may work well. Just minimize your draw by being frugal with your use. And keep a close eye on the voltage of the battery and do not draw it down below 12.2 volts. <br><br>Bob

 

[mad*sparrow]

Hi akrvbob, It would charge my motorcycle battery to full in a few minutes from nearly dead. Anyway its a 3 ft x 2 ft panel. My worry is having too much battery for the charge and having to replace it often but I want as much charge as I can get from the panel if I can.

My sources of need are simple with just electronics.



Im thinking roof mounting it on my shell and using an auxiliary battery for my daily needs boondocking. I think if I put a gel or agm battery in the shell or a liquid battery in the engine compartment and tie in the engine battery I can keep both topped up without risking engine start problems. Im finding some threads on this site though suggesting that the cabling needs to be as short as possible so now Im thinking inside the shell is best.

Thanks for replying.

 

[wrcsixeight]

A 3'x2' panel is probably closer to 80 watts. &nbsp;Any solar is better than no solar, but too little solar spread across a large battery bank is not ideal either. &nbsp;In general, for best battery performance and longevity, the solar panel should be capable of at least 5% and not more than 13% of the capacity of a battery. &nbsp;So a 100 amp hour battery 'should' get a panel rated at at least 5 amps, &nbsp;and this requires about an 85 to 100 watt panel to actually get that much current to the battery for the few hours around solar noon on a good sunny day.<br><br><br>&nbsp; YOu do not want to hook this controller directly to both house and engine batteries if you want them isolated without the engine running, as they will then be paralleled through the charge controller and might release its magic blue smoke. &nbsp;<br><br>Some charge controllers have a load diversion feature. When the main battery is fully charged, it will then send extra current to another source, in this case being the engine battery. &nbsp;I do not know of your morningstar has this capability.<br><br>If you can secure another battery under the hood, then this is the preferred location. &nbsp;Just use thicker wire/ cable to make up for the extra distance. &nbsp;Copper is $$$, but &nbsp;there is a good chance you will find 80 watts to be insufficient someday, and installing oversize solar wire now allows you to not have to redo it later when you decide to add more solar.<br><br>I View my engine battery as for starting purposes only. &nbsp;I have every load, like stereo and lights and all ciggy plug receptacles, everything 'camp' related, wired directly to the house batteries with a single fuse block to distribute the power. So the only load my engine battery sees with the engine off is the minimal parasitic drain of the engine computer memory. &nbsp;This way I have no need to divert solar amps to the engine battery and send them all to the house batteries in my attempt to meet that 5% minimum recommendation.<br><br>There are all sorts of different techniques and products for allowing the alternator to charge a secondary battery and isolate the engine battery from house loads with the engine off. &nbsp;<br><br>A &nbsp;quality Dumb continuous duty solenoid wired to activate on the HVAC fan blower motor circuit is the easiest and cheapest in my opinion.<br><br>Voltage sensing or time delay solenoid/relays exist too. Some of these parallel the batteries whenever charging voltages, on either battery are sensed. &nbsp;AKA dual sense relays. &nbsp;I am not keen on these as I don't want the relay/ solenoid sending solar current to the engine battery. &nbsp;Some solenoids will eat up an amp of current when activated and holding the batteries in parallel. &nbsp;Not a big deal when the engine is running, but taking a significant portion of a solar panels output to do it.<br><br>There are also battery isolators which act like a one way valve, allowing alternator current into the house battery but not allowing the house battery to pull anything from the engine battery. &nbsp;These generally drop charging voltages and reduce current making it into the battery from the alternator, increasing the likelyhood of chronic undercharging and premature battery death.&nbsp;<br><br>I use a manual switch and can start and run the van on either set of batteries or both, and use it also to distribute alternator current, and solar current.<br><br>These are not the only methods and each has their benefits/detractions.&nbsp;<br><br><br>

 

[Willy]

Here:&nbsp; http://www.amazon.com/HQRP-Battery-...keywords=dual+battery+charge+controller&nbsp; I have the one rated for 20 amps and Morningstar makes a 20 amp model too. ..Willy.

 

[mad*sparrow]

Thanks everyone. Ive got agood idea now and a lot of homework to do to understand this better. Engine compartment it is and heavy wire since eventually I will add more solar. Plus heavy guage means less resistance.

If it makes a difference I remember that the panel runs at 18v so when I purchased the controller I made sure that it would handle the load to a 12v battery. I bought the panel a couple of years ago and it is the high efficiency type for a house and expected to last for 25 years. The controller I got from amazon and the solar blogs rated it well. It cost just under 100 dollars if I remember correctly.

If I have anything left after taxes this year I hope to add in that 71v xunlight from amazon. Then when the zombies attack I can use my deathray.

 

[mad*sparrow]

Hi Willy, great minds think alike! I bought that one when I got my panel but somehow misplaced it in all my moves/junk and couldnt find it again.

I replaced it with this one I think

http://www.amazon.com/Morningstar-S...20898&sr=8-25&keywords=morningstar+controller

they are in storage at the moment a 100 miles away so I cant verify for a little while.

 

[Willy]

I want to get one of those Morningstar units, but have to get my sister to gt it for me since I don't have a credit card. I always want to have a spare, especially for my solar, cuz if the charge controller goes.. I'm hooped.

 

[VanLifeCrisis]

20w?&nbsp; I wonder if im overestimating my solar needs, i plan to buy these myself <img rel="lightbox" src="/images/boards/smilies/rofl.gif" class="bbc_img"><br><br>http://www.amazon.com/145w-Polycrystalline-Solar-Panel-Pack/dp/B007VPQP5Q/ref=cm_cr_pr_product_top

 

[Willy]

20w is just a bit under 1.5 amps of charge.. very little. That's about what's put out by those trickle chargers they use to maintain batteries when in storage. You gotta remember that it's 20w under IDEAL conditions. Clouds, dirt, birdshit, and whatever else works to give you less juice. Remember too, at night time you're getting ZERO power and will be chewing through what you've stored up pretty quickly. Just running a laptop draws 45-90w of power (peak), and that's BEFORE you factor in various inefficiencies (inverter draw, copper losses, power brick, etc.), so compare that to 20w.. hmmmm.. &nbsp;

 

[mad*sparrow]

Luckily my only needs are my tablet and my phone. My tablet can be charged from usb as well as my phone. Both once charged last at least 4 hours of which I shouldn't require extensively off grid.

Plus my wind, steam, water power generation once perfected should suffice in the zombie apocalypse

There is now a small wood stove for backpacking that produces power for usb charging although it doesnt advertise how long you have to keep the fire burning to fully charge a device like a phone.