When do you need a 12v tender controller?

[PHeller]

I recently left my truck in Phoenix for a week and came back to a dead battery. I've noticed in these parts some people like using little solar panels to tend the battery while it sits.

I see a lot of options for little 5w panels with alligator clips and USB outputs, as well as the Goal Zero Nomad 7 (no alligator clips), but do these panels include, or for that matter even need a controller for battery tending?

I don't need a huge setup, $60 max, but would like to charge the 12v battery safely and still have USB output at 2.1a (.5a sucks).

 

[dwh]

Not needed with only 5w. Someone, Toyota maybe, sticks little 5w PVs in new cars to keep the electronics alive during shipping. Seen quite a few of those for sale.

If you think .5a sucks for a phone...

5w ÷ 13v = .38a for your truck battery.

And of course, when it gets hot it'll put out less...

 

[mtnkid85]

At what wattage does a charge controller become necessary? Or are there any other factors to consider when running a solar maintainer?

We have tried running various 15-5w panels to maintain different batteries on our tractors, trailers, ect. but have had real mixed results.

 

[whitenoise]

The battery tender ones are $60 and have a built-in charge controller with temp compensation, which you almost certainly need in Phoenix.

http://a.co/46sA0Hv

You can use their USB adapter if you need 5V / 2.1A output: http://a.co/fQxsdEq

 

[dwh]

At what wattage does a charge controller become necessary? Or are there any other factors to consider when running a solar maintainer?

There are a few factors to consider...but the answer to the question is actually pretty simple. I'll start with the factors to consider.


First is the rated Vmp of the PV. If it has a STC Vmp of 15v and when the PV gets hot it will have a lower Vmp, so it probably won't ever overcharge a decent size lead-acid battery. Except when the temp is really cold and the PV has a Vmp above 15v it could overcharge the battery - but if the battery is really cold too, it'll need a higher voltage anyway, so probably a wash.


Next would be the battery's self-discharge rate. Some self-discharge faster than others, so if the wattage/voltage of the PV is low enough (5w/15v) then pushing the battery up to 14.something volts for a couple hours a day might really just be compensating for the self-discharge and keeping the battery topped up. But if the battery has a higher self-discharge rate and/or phantom loads, you might need more watts to keep up.


Then there's the PV/battery size ratio. As I showed above:

5w ÷ 13v = .38a.
But
5w ÷ 14v = .36a
5w ÷ 15v = .33a

So then the question is whether the PV can even overcome the battery's internal resistance and charge the battery at all. A 5w/15v PV might achieve something pushing against the resistance of a 55ah battery, but might make no headway at all pushing against a 105ah battery.

And the PV puts out less as it gets hot, so for estimating we usually figure we might get 80% of the PV's rated output on average.

So
.38a x .8 = .30a
.36a x .8 = .29a
.33a x .8 = .26a

Even the smallest battery "maintainers" have a potential to put out at least 1a. Anything less is just pretty much irrelevant to a lead-acid battery.


And all of that is based on the PV facing full sunlight dead-on. If facing the sun at even a somewhat bad angle, enough photons will ricochet off to make the output of the solar drop, and any shade at all on the PV - even half the size of a dime - will make a small PV's output drop to nothing.




But the answer to the question of when you must have a charge controller: You must have a charge controller when you have enough PV potential to overcharge the battery.

The primary purpose of any charge controller is to first protect the battery from overcharging by limiting how high the voltage of the battery is allowed to rise.

But if you don't have enough PV to overcharge the battery, then you probably don't have enough to charge it in the first place. Adding a charge controller isn't needed to protect the battery - and if you don't have enough PV to start with, adding a charge controller won't change that and make an undersize PV magically start charging the battery.