Size of solar maintainer for car battery?

[Almot]

Had a 2.4W panel for $30 or so, on my truck while in storage. Seemed to be adequate at latitude 27 with plenty of sun and almost no clouds. Flat on the roof. Didn't worry much because the battery was dying.

Now getting a new battery. As much as I hate spending $45 on slightly bigger 5W gizmo from Amazon, I'm willing to do this for $100 battery. These small chargers are all overpriced, but it comes with all the cables, cig lighter plug and night diode. Not just a bare panel.

Wouldn't want the panel to boil it at 17V on a nice and warm 105 degree day when I'm not there to take care of it.

Q: What self-discharge current has a truck battery with 600-800 CCA? Or how many AH it can be compared (I know there is no such thing)?

[D_E_Bishop]

I received a 15 watt DelTran battery tender for Christmas a couple years ago to use in the MH while in storage. It is spendy but has a built in controller and will maintain my to CG2 batteries. It is not a charger but a tender for charged batteries and it works great.

[Almot]

MEXICOWANDERER wrote:
Picky-picky? Yes. But this is coming from a hate-to-buy-batteries cheapskate.

You telling me. 1,700 pesos a pop in the only auto-shop in puebla. A good tip on "dusk overcharging" and mini-cycling, thanks. I might still get 4.8W panel, looks like no potential to harm anything and battery would feel better. Begging around for a jump-start is never a fun.

Ambient temps 100F (make it 110 under the hood) is likely in daytime during my absence in summer, dropping to 85 in the night. 85 is still pretty high, so 0.8-1.0 AH self-discharge over 24 hours is possible.

[Matt_Colie]

I have been using a tiny (2~3 watt) panel on the dash to keep my seldom used truck alive for years at 43°N. This is Michigan, things get moldy if they don't freeze, but they never get sun bleached.

Matt.

[MEXICOWANDERER]

Battery should go beddie-bye holding a solid 13.2 volts. I'd prefer to see 12.75 at dawn configuring a formula with your estimated ambient temperature. The object is to nursemaid an afternoon/dusk slight surface charge to help feed the red-headed stepchild parasitic loads at night. Minimizes mini cycling, which takes it toll on battery lifespan. Don't be afraid to gain to 13.5 volts max during the peak "harvest". Even 14.0 is lower voltage than most alternator voltage regulator "cold" settings.

Yeah, this is baton-grade orchestrating. It is meant to minimize hard sulfating. Picky-picky? Yes. But this is coming from a hate-to-buy-batteries cheapskate.

[Almot]

Zero parasitic loads in truck. No LED/LCD display when off, no nothing. No engine computer - this is 1993 truck.

MEXICOWANDERER wrote:
... connect the panel you have now, then two weeks later check the voltage at the battery terminals. Do this before the sun has a chance to recharge...

Was watching it for a week. 2.4W panel keeps battery at 12.6 at dawn, can't raise voltage any higher, but battery is almost dead. Borrowed another used battery for a week, in a better shape - panel brought it up from 12.67 to 12.71 at dawn after 2 days, and was keeping it at 12.71.

2.4W/18Voc = 0.13A Isc. 0.13A*6hrs = 0.8 AH a day. This is what I'm getting most of the days. Clear desert skies, Lat 27, plenty of sun. Hot weather in summer should cause high self-discharge, and this is what I was trying to find out. Ballpark figure for self-discharge of NEW battery of this size is 0.4 AH a day at 77F. So... say, 0.8-1.0 AH at 100F.

From experiments it looks like 2.4W was doing its job. Or almost doing. Will give it a try with new battery - regular wet. $200 AGM is too much to swallow.

[MEXICOWANDERER]

Q. Divide an unknown into a mystery and what have you got?

A. Pure unmitigated guesswork.

From my perspective connect the panel you have now, then two weeks later check the voltage at the battery terminals. Do this before the sun has a chance to recharge - meaning in the morning. Either it's going to work or it isn't and so far you haven't spent a dime more...

Some model cars snack on the battery more than others.

[RAS43]

I have the exact charger (5 watt) and have used it for 4 winters to date. The vehicle is a Ford with 2 batteries. The truck may sit for 7-10 days outside between use. Prior to using the charger the batteries would be low and the engine would start but turn over slower then it should. With the charger I connect it immediately after shutdown from a drive. I use the lighter plug and lay the panel on the dash. When I start the truck again the engine turns over and starts normally. The charger works for me. :C

[RoyB]

I do the same thing here for my F150 Truck. I moved a small solar panel from my jeep days to lay in the windshield area of the truck. This unit has the 12VDC plug on it and plugs into an always hot DC Socket. It is around 5WATTS output when in high sun.

Looks similar to this - google PHOTO.. Mine just lays on the dash - no sucktion cups..


For me this is right on the borderline of being large enough to keep my truck start battery topped off. My battery stays topped off for about a week and then I start seeing the DC VOLTAGE start dropping down to the 12.3-12.4 level when I remove the charge and measure the settled reading. I wish I had the 15WATT version.

I suspect I have two problems with my setup - The windshield is probably killing the high sun and of course the newer trucks have all kinds of parasitic drains to over come... Having the solar panel laying on the dash behind the windsheild is not the best place to use one of these type chargers. Probably never catches maximum charge from the sun...

Roy Ken

[Sam_Spade]

smkettner wrote:
New battery should need less power. Stick with 2.4 if that is working.

This.
If your new battery is AGM, it will be just fine over 3-4 months if first fully changed and then disconnected....or connected to the small solar charger.

The self-discharge in an AGM is almost negligible.

[MNtundraRet]

I have been using a Coleman CL-600 solar panel, 600ma, 15V, 9Watt, for about 5 years now. I have 2 panels: one for V10 starting-battery, and the other for the pair of 6V Trojan T105's.

MH is stored for 5 to 6 months with only the solar-panels. I check monthly and run engine, and generator (if temperature is around 20F or higher). Done to exercise both as needed. I always check batteries with volt-meter before starting engines. They are always found at full charge.

It should be noted that I have no currents drawing down batteries. If you have a radio with digital displays drawing current continuously you will need 5 or more watts.

[dclark1946]

I suspect that your current panel may be marginal but as noted it depends on your parasitic loads. You can monitor your battery volatge in the morning before charging starts for a week or so to determine if it is staying at 12.6 to 12.7 or decreasing.

You can also measure your parasitic load and do the calculation of how many AH you will need to maintain your battery.

I have maintained a single RV 12V battery in the past here in TX with a 10W panel with some parasitic loads eliminated by pulling a fuse. I think if you go above 5W you should have a controller to prevent overcharging.

Dick

[time2roll]

New battery should need less power. Stick with 2.4 if that is working.

[Ed_Gee]

Not sure I'd bother to upgrade from a 2.4W panel to a measly 5W panel. According to my calculations, the 5W panel would trickle charge your 12.8V battery at ~ 400mA. By my estimate/guess that is about 0.4% of that new battery's AH capacity. The 2.4W panel probably charges at about < 200mA . Neither one of them are in any danger of overcharging your battery, and quite frankly I am dubious they could keep up with just a maintenance charge given they only work during the day, and your 'truck' probably has some parasitic draws on the battery like a clock, red LED security light?, engine computer, etc.

I'd probably be looking at a 10W panel for serious battery maintenance under your conditions - Latitude of 27 and probably cold .....