Solar Panel Project For School

Well you could have 2 batteries for each robot and the solar panel charges one while the other is in use. Might be the simplest solution

you could 3d print a holder for that battery & charge using programmable charge controller. would save a lot of messing with ac/dc converters.

Not being negative and not wanting to pull you up on this as I'm sure you have the safety of the students in mind, but is mixing mains voltage and secondary level student projects safe? I'm in two minds about whether it is or not, but you're the best judge. Bear in mind that these inverters have poor or no safety devices and aren't inherently safe just by virtue of their dual 110v sources. They don't have RCD's and absolutely cannot be earthed (unless specifically designed to be).

4-gauge is 5.2mm diameter or 21.1mm^2 and 1000W at 12v DC = 83A. That's hefty stuff alright, but if you're really aiming to pull anything near the rated power of the inverter, then you need to go for it to reduce the losses in the wire alone. You should be able to source that spec of wire in a motor-factors, or else they'll tell you where to get it. Don't buy it off ebay or other similar vendors, it tends to be sold as copper but can be aluminium instead (AL burns over a lighter flame, CU doesn't).

Anyway, back to basics: why do you want an inverter? Bear in mind that if you're charging a battery off a battery, the worst plan is to convert that lovely stable 12v DC into 220v AC mains through an inverter just to convert it back into DC at some other low voltage. The efficiency losses for each step are approx 20% and all of the lost power just goes off as heat. Maybe that's actually a good equation for the students to measure and calculate and understand, but otherwise it can be avoided by just converting the 12v into the required battery voltage by using a DC-DC charger. Can you source a suitable 12v or 24v charger instead?

Come back with questions and ideas, happy to help.

So what type is the robot battery? Do you have a part-number and link? Maybe they have their own DC-input charger.

Ah VEX. You linked to that above, but I didn't realise that you were using it.

So that's a 7.2v 2AH battery made up of 6x 1.2v (NiMH - link to battery university) cells in series, with a positive and negative pads and what is probably a charge status (??) connection in the middle. Can you put a multimeter across the terminals and measure the voltage from (-) to the middle connector of the battery? I'm guessing that the battery might have a charge controller within it and potentially that middle pad is the status output for the LED on the charger and there may be no voltage present on it when you read it.

Li-ION batteries often have a middle pin/connector, but that's used for balancing individual or pairs of cells during charge - but with 6 cells in series in an NiMH pack like that - maybe I'm wrong but I'd hardly think that there would be much point in using balancing at the middle point.

If you have a dead pack, open the casing slightly and see whether there is a small circuit in the battery pack (I'm talking about opening the plastic casing, not touching the battery cells).

Anyway, please check what voltage is read between (-) and the small middle pad and if it's coming back as 0v then you could try get a NiMH 7.2v portable/vehicle charger and try a test-charge on a empty battery to see if it takes it (come back to us here before you take that step though).

Yeah, so I was doing a bit of digging... ;)

Turns out that the middle pin is used by the charger to identify the battery, and that's why you see no voltage from it. But - from what I can understand - the battery isn't locked to the charger, and that means that our plan to use a DC charger still holds some water.

The theory behind it is that a completely flat battery which is too flat for the charger to recognise will just give a red LED on the charger. But these flat batteries can be recovered using a manual process and these IQ brain systems. To recover the flat battery you connect it up to an IQ brain and connect a motor to one of the ports of the brain and use the motor to generate a small amount of power which is fed back into the battery. After about a min of twisting the motor to charge the battery like this on the IQ, if you then move the battery over to the charger, it should hopefully see the battery and start charging it.

Anyway, the point is that if you can do that with with a simple motor, there should be little stopping you from charging the battery from a standard 7.2v NiMH DC-input charger... that is if you can find one!

I'll search out a charger over the coming days, but they appeared elusive during my first search.

That 12v to 5v board looks like this one here:

That's a buck converter alright and it has a pretty good input range of DC 9V--36V. So yeah, ideal for your needs as long as the 100w panel sits around 15v or so.

Just in terms of wiring - don't skimp on good quality wire when you're working with higher loads later on. Consider using mains cable if you don't have 2.5mm^2 red and black (but label it up very well).

The other think that you can do is get one of these USB testers - it will allow you to watch the cumulative energy put into a device/battery. Very educational!

Finally, if you get yourself a variable USB load tester (or just a selection of big resistors), you can test the performance of the power converter.

Here are some 12v to 5v supplies which I was messing with. The worst obviously was a "Benq" branded board which was so bad that the output voltage dropped to 4.2v at 1A (it was rated for 3.5A, so clearly a failure).

The Anker PowerDrive was a common USB car-charger, quite impressive.

Anyhow, all food for thought!

That might have been the phone if the power quality wasn't good enough. You'll need to measure some of the voltages/currents while it's setup to see if it's working properly.

• Measure the panel voltage when open-circuit (no devices) and in full sun (preferably not behind glass).
• Connect up the converter module and re-measure the voltage (should be roughly the same).
• Put a load on the output of the converter - something between 33 and 80-ohm resistor (for a load of between 63mA and 150mA.
• Measure the voltage on the panel as well as on the output of the converter module. It should be 5v (+/- 5%) on the converter module and the solar panel should have dropped from the open-circuit voltage too (but still needs to be above 9v).
• Measure the current on the same ^
• Then attach a USB device, such as a partially charged portable battery pack, and re-measure all voltages while the pack hopefully pulls up to 500mA.

We might find the reason for a lack of charge then!

OP if you are making up "experiments" then these plugs for low voltage stuff might be handy for making up cables https://www.amazon.co.uk/LightingWill-Barrel-Connector-Adapter-Security-10Pair-Female-Male-DC-adaptor/dp/B077YGV3F4/ . That size connector is fairly universal and fits a lot of 12V (and other) wall wart power supplies making the connections available as two screw terminals. You'll note it's also the same size connection on the buck convertor. There are lots of other possible cable plugs you can use but sometimes its easier to get away from crocodile clips particularly if you want the polarities to be right each time.

Edit> Just a thought you could do a class request for any old unused power supplies might net you a few useful ones?