12v solar panel/battery setup project- NOT lead/acid


I have a project that I'm trying to design/build and could use some help/advice with some of the electrical theory. I feel like this should be really simple but I'm having a lot of issues. I have done a lot of googling but can't seem to find applicable answers so I started my own thread. Any help will be greatly appreciated! If I can solve this, I will be happy to write up a detailed instructable so that others can use this information as well. 

The short version: Trying to build a lightweight solar panel>battery pack>UHF radio charging setup. Nominal voltage is all 12v. I'm having difficulty regulating amperage between the units.

The long version: My friend is in charge of a campsite which needs to have a 2-way radio on at all times. They need to be able to charge these radios on-premise because they are out there for a little over a week at a time. A solar panel seems ideal for this application. The whole charging setup needs to be lightweight since it is a >10 mile hike into the campsite. Ideally, I would like to use an array of NiMH AA batteries because they are small, relatively cheap, and can be replaced easily if there is an issue. I am also under the impression that they handle repetitive charge/discharge cycles well. I am not married to that though so if there is a compelling reason to use lipo or something else, I am open to that.

The radio: is a Motorola Radius CP200. The wall charger that comes with it outputs 14v at 1.5 amps. The battery in the radio (according to the sticker on it) is 7.4v 16.21Wh lithium ion.

Setup 1 that I've been trying: I have a 12v solar panel charging a pack of AA NiMH batteries. I started with an 8-pack holder but moved up to a 10-pack to increase voltage and total mAH a little. I've been using little voltage regulators in between the solar panel and the batteries as well as between the batteries and the radio. These function as diodes and help me get the correct charging voltage. My problem is that I can't regulate the amperage. The battery pack will pull as much as it can from the solar panel, and the radio will pull as much as it can from the battery pack. What ends up happening is when I first plug the radio into the battery pack, it works fine. But after a couple minutes, the amperage skyrockets to like 3 or 4 or more amps. The voltage regulator hits some sort of thermal protection limit and the whole thing shuts down. I tried throwing this amperage regulator in there as well to keep it below 1.5 amps, naively thinking that it would just affect the amperage and not the voltage. But what happens is when it's in constant amps mode, it just cuts the voltage to compensate and keeps the total power the same.

Setup 2 that I've been trying: I thought maybe since I was having issues with the 'smarts' of these batteries charging and discharging, I would switch over to USB battery packs since they have a lot of built-in protection circuitry. I got this solar charger, an older version of this battery pack, and used the same voltage regulators to step up the voltage from 5v to 14v to charge the radio. I get the same issue of high-amperage discharging of the battery pack. I tried using this step-up converter between the battery pack and the radio but it doesn't seem to work or maybe I'm not using it correctly (I can't seem to get the adjustment pots to actually do anything...).

It seems like I'm thinking about current wrong or something. Any suggestions or insight into what's wrong with my approaches would be super helpful. If anyone has any suggestions for radically changing my approach, I'm open to those as well. Thanks for reading!

If you need to go solar then the best and only way is to match all voltages to the solar system.
So instead of a fancy, battery powered radio take a standard car version and make sure you can adjust the output power - comes in handy if you really need to conserve energy and the distance is not too far.
Like that you only need a 12V solar panel, 12V solar charge controller and a 12V deep cycle battery.
If the normal drain voltage of the battery is too low for the UHF to operate properly than you can still add step up converter set to 14V and a few nice sized capacitors to prevent unwanted interference.
The size of the panel depends on the battery size and how much real sunlight you get in a single day.
You want the battery big enough to make it through 3 days without charge and the panel(s) big enough so they can fully charge the battery on a day with light clouds.

Cking0987 (author)  Downunder35m1 month ago

Thanks for the reply!

So as I understand it, your comment about matching all voltages is directed towards the second setup, in which I have been using a USB battery pack. That's fine. I can scrap that idea.

The radio he uses is not fancy, and it's standardized across many campsites so unfortunately changing that is not in the cards. But in terms of it being a 'car version', charging at 14v is standard for car appliances so I would classify it as being well-suited for a car environment.

Would you put the capacitors in series in the positive side of the circuit to reduce 'ripple' in the current?

My biggest problem right now is the charging rate between the battery and the radio. When the radio is plugged into the original mains charger, it charges at around .74A (half of the 1.5A rating printed on the power supply). That's at a consistent 14v. When I plug the radio into a battery and hold the voltage at 14v, it draws over 3A. What am I missing??

Yes, the capacitors are for the ripple, these step up converters produce quite a bit of it, sometimes you also need an inductivity to eliminate them fully.
You will find out for sure if there is unwanted noise during RX/TX.
The mains charger is most likely designed to charge at this rate.
In a car the drive time is often short so they try to charge as fast as possible.
Also possible that it hs two inputs for charging that are different.