How to use a BigBlue 28w 5v USB solar panel to charge my car battery

jimster99

New Member
Hello all! This is my first ever post on this (awesome) site :)

Experience level: I have a reasonable grasp of the theory of electricity (no problem comprehending volts/amp/watts) but am fairly lacking in real-world electronics skills.

I have the following questions:

Question 1: What gadgets do I need to charge my 12v car battery with a 28w (nominal) BigBlue solar charger that puts out 5v through a USB type A socket?

Questtion 2: Is this actually going to work? In real world conditions the charger puts out about 0.5A - 2A (or 3A in fierce sunlight) so really it's a 2.5w - 15w charger. I think that although it will be very slow to charge the car battery, it should be capable of doing it. I really only intend to use it as a trickle charger for when the car is unused (sometimes for a few months).

I already have the big blue charger so I think I will need the following:

- a voltage step up converter or DC boost converter (to adjust from 5v to whatever the car battery optimum charging voltage should be). Is there something out there that can do this? And what voltage should I step up to? 15v? 13.8v? 12v?
- a charge controller unit that can shut the charging off when the battery is full, prevents the car battery discharging into the solar panel and ideally does clever things to change the charging rate depending on how full the battery is.

I don't have a soldering kit with me so I'd prefer to get something that is plug and play (i.e. a USB connector at one end to connect into the solar panel & a dual cigarette lighter socket / direct battery connection at the other).

I've searched ebay and amazon and am struggling to find the right units to do this job.

Can anyone help out by telling me if this idea is a waste of time, and if not, by pointing me to the right bits to purchase?

Thanks very much!!!!
 
Last edited:

snoobler

Solar Honey Badger
Moderator
1) https://www.amazon.com/DZS-Elec-Converter-Adjustable-Regulator/dp/B07L9V4S39

2) yes but with limitations. Since you can't provide a proper lead-acid charging profile, you're limited to floating a presumably fully charged battery. For flooded 13.2-13.6V should be your output voltage. For AGM, 13.8V OR whatever your battery manufacturer states for either "standby" or "float" operation. Operating in float mode means you never have to worry about shutting it off. The downside is that if the battery doesn't start fully charged, it likely won't fully charge it, but it will charge it as much as possible and is better than not charging at all.

I believe you could simply cut a USB cable and attach the wires. No soldering necessary.
 

jimster99

New Member
Thanks Snoobler!! Appreciate the detailed and helpful reply :)

Your comment about not doing a normal profile charge makes a lot of sense, although presumably I could manually alter the voltage going into the battery (with caution) to do a full charge if that ever became necessary (although hopefully it won't).

I'll look into the unit you posted and will let the forum know how I get on.
 

jimster99

New Member
PS the schematic for the unit you linked says it needs 5.5v+ input, whereas my solar panel only puts out 5v. I assume that's a deal breaker for this particular unit?
 

jimster99

New Member
Thanks again for the link! Some of the comments to these step-up devices suggest they spike the output voltage to 30v+ when the input drops below the minimum 4v which - I assume - happens quite often for a solar panel. Is that a concern for a car battery? Obviously sustained high voltage would be a disaster but momentary spikes with low current might not be so bad ... or is it?

There's more to this than I initially thought!
 

jimster99

New Member
I've found some cheap solar power charge controllers on ebay (one example: https://www.ebay.com/itm/30A-Solar-...938612?hash=item342dd080f4:g:~8kAAOSwQotfKwdY).

Would it be better to link the following chain to offer better voltage & current protection and potentially allow even for a (slow!) full charge from flat if that should ever be necessary?

(1) Solar panel (output @ 5v) -> (2) Step up device such as the one posted helpfully by snoobler (input @ 5v and capable of output @ 12 - 20v) -> (3) solar charge panel (capable of "smart charging" the car battery)?

I guess this would entail significant conversion losses but I don't really care as the main purpose is a float charge.

I also found some step up devices similar to the one you posted with USB input and terminal output, allowing me to directly plug into the solar panel - this is one example, although it gets some bad reviews in the comments so I won't get this one: https://www.amazon.com/PEMENOL-Adju...66HV88ES376&psc=1&refRID=KWDATGM0466HV88ES376
 

snoobler

Solar Honey Badger
Moderator
The battery probably won't notice. It effectively acts as a brake on anything the output does. The only way you could get the battery to actually spike its voltage is to supply a massive amount of current to it - way beyond anything this thing can do.
 

jimster99

New Member
Thanks :) That makes sense.

Would adding a charge controller into the equation help at all (to allow a full charge, if ever needed), or is that adding unecessarily complexity with no benefit?

Also, my solar panel has 3 x USB 5v outputs. In theory, could I combine the three ports together in series to up the voltage to 15v to avoid the need for a step up device at all? I'm guessing the answer is no or that even if it was possible, it would be inadvisable since (i) you'd have to wire the sockets together somehow, and I doubt that's safe to do (plus it would prevent using the panel for other purposes), plus the solar panel internals might get fried or confused by unexpected voltage increases coming in the wrong direction. And I'd still need a step-down device to bring the ultimate output back down to 12.7v or whatever it needs to be (haven't located the specs for my car battery yet).
 

schmism

Solar Addict
So when you get this rigged up give us an update with some photos of your clamp meter (or DMM if you want to put it inline) and let us know what kind of real world current you can push to the battery.
 

jimster99

New Member
Thanks @schism, I definitely will!

As background context, I originally bought the 28w solar panel to use for camping trips but have found it impractical (primarily, it needs the angle adjusting too often, is floppy and has no kick stand so very hard to position correctly, is a bit too bulky to attach to a rucksack and it's tricky to avoid overheating the phone and it does need a fairly sunny day to work at reasonable speeds, and ultimately a USB battery pack is just easier and lasts for several days or, if car camping, you can easily charge the phone in the car, so that's what I do). If I was buying again today, I'd just get a similar panel with a 12.7v output to avoid all the fiddling but now I have the panel, I want to make it work :)
 

jimster99

New Member
One other problem - apparently the float voltage for a lead acid battery varies significantly depending on air temperature . The car is usually stored in the UK, where temperatures range probably from -4c to 30c (and very ocasionally outside those ranges) but most commonly in the 5c to 20c range. The wikipedia page ominously says "Not compensating for temperature will shorten battery life by over- or undercharging."
(https://en.wikipedia.org/wiki/Float_voltage).

I assume it's safer to assume a high summer temperature to avoid overcharging (I don't like unexpected car fires) but that will mean selecting a lower voltage and in winter it'll be set at too low a voltage, and the battery will run a bit flat. Is this not really a concern in practice (better to have a low float voltage than none at all?) Or am I going to have to manually adjust the voltage every season? Or does a charge controller do this for me?
 

jimster99

New Member
Thinking a bit deeper, isn't it possible to connect the three USB outputs of my solar panel together the way the dude in this video has done? I could just decrease the voltage further with a bigger resister, to hit the target float voltage? Then I wouldn't need any fancy electronics at all! Would that work or am I deluded?

 

jimster99

New Member
As a starter for 10, here is what I have assembled thus far:

My solar power panel & the three 5v USB outputs

IMG_20210113_203717863.jpgIMG_20210113_203744325.jpg

PortaPow USB amp/volt/amp hour meter that I will use to measure the draw once installed (very useful for testing USB cables and charging equipment - it retains memory even when power is lost and cumulatively measures amp hour output, and if you worry about the parasitic draw - I don't - you can separately power the meter through a micro usb input)

IMG_20210113_203726965.jpg

Car Battery that needs topping up (I haven't found the manufacturer spec sheet yet so I don't know what float charge it needs or even what type of lead acid battery it is, other than "maintenance free"):

IMG_20210113_203635453.jpg
 

snoobler

Solar Honey Badger
Moderator
Thinking a bit deeper, isn't it possible to connect the three USB outputs of my solar panel together the way the dude in this video has done? I could just decrease the voltage further with a bigger resister, to hit the target float voltage? Then I wouldn't need any fancy electronics at all! Would that work or am I deluded?


No. He has 3 separate sources in series. Your USB ports are connected to the same source, thus their voltages won't stack.

Voltage compensation is good, but most if not all vehicle systems I've seen conduct temp compensation. Just float it at 13.6 and call it good. Check fluid levels periodically.
 
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