• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Capacitor To Kick-start Solar-powered Motor?

Status
Not open for further replies.

primroselondon

New Member
Hi there,

First-time poster here, and an electronics amateur with what is probably a very basic question, so I'm not even sure I'm in the right place...!
I have a small device that uses power from a solar panal to run a small motor for a pump. The device also has a single-cell battery to store power to run the motor while the sun's gone. I've had a look at the wiring inside and it's very simple - I've knocked up a circuit diagram which is attached.

There is one problem, however. Because the load on the motor is a reciprocating pump it has uneven rotational load on the motor, making it hard for the motor to start. Frequently, therefore, the device will take some time start as the motor will be stalled trying to overcome the initial resistance. However, even in dim light, if you manually start the motor (overcoming the initial resistance) it will run fine.

So, my question is this: is there a simple way to modify this circuit so as to charge up a capacitor from the solar panel that would be able to release a burst of charge to 'kick-start' the motor? I know capacitors can do this sort of thing, but I'm racking my brains back to school days and not getting very far!

The solar panel is about 6"x4" and is probably a 3-5v rated panel. The cell in a single (1.2v) NI-MH D size. The motor is a small (<1" diameter) thing like you used to use at school. Not sure what voltage rating it is, though.

Many thanks,

Andrew
 

Attachments

Sceadwian

Banned
Out of curiosity, what kind of pump runs on a single NiMH battery?

You'll be hard pressed to find a capacitor that doesn't meet a voltage rating of 5 volts. Just pick the largest electrolytic capacitor you can find, attach it in parallel with the battery make sure you get the polarity right (it's marked on the case) the cap will supplement the batteries power during starting, how much depends on the exact current draw and size of the capacitor. Experiment with different value capacitors. If one isn't enough try two in parallel. Wait to a count of at least 10 after connecting the capacitors to test it depending on how weak the battery is it may take a few seconds to full charge them if the value is high.
 
Last edited:

MikeMl

Well-Known Member
Most Helpful Member
Solar panels put out an open-circuit voltage which is much higher than their loaded voltage. In your application, the loaded voltage on the solar panel is determined by the battery being charged. If you removed the battery, and replaced it with a capacitor (~1000-10000uF), the solar panel would charge the capacitor to ~3 to 5V (much higher than the battery ever gets to), and when the switch is closed, the extra voltage in the capacitor might be sufficient to kick the motor into starting.

Once the motor is running, you would reconnect the battery so that it can accumulate charge from the solar panel, and then run the motor as the panel looses sunlight.

It would take a fairly complex circuit consisting of CMOS OpAmps and some FETs to automate the switching of the capacitor and battery. Are you up to this?
 

Sceadwian

Banned
Mike, why bother with something so complicated? Simply putting a large capacitor in parallel will provide an instant high current pulse to the motor when it's turned on, WAY more than the battery can provide, if the capacitance is high enough the motor will start and it doesn't have to be removed ever.
 

MikeMl

Well-Known Member
Most Helpful Member
Mike, why bother with something so complicated? Simply putting a large capacitor in parallel will provide an instant high current pulse to the motor when it's turned on, WAY more than the battery can provide, if the capacitance is high enough the motor will start and it doesn't have to be removed ever.
Because the battery is permanently tied across the solar panel, the highest voltage that the battery will reach is determined by the chemistry of the battery; not the open-circuit voltage of the solar panel that would be captured in the capacitor if the battery were temporarily disconnected. If it were not for the battery, the capacitor would charge to about double the battery voltage...
 

Sceadwian

Banned
What does that have to do with anything Mike?
For something that small running off a single battery it's the starting current not the voltage that's the problem, the large capacitor (or better yet a few moderatly sized ones) tied in parallel permanently will provide an extremely low resistance current source for the initial starting current of the motor, the batteries internal resistance from the starting current isn't enough to kickstart the motor with an initial load. The capactior for short bursts will reduce the effective impedeance of the power source by half or better. Using a few mid sized electrolytics in parallel will drop it even more as in parallel capacitance increases and series resistance decreases. Especially when the battery gets weak (it's resistance is highest) the capacitors will allow it to have the same starting current as a freshly charged cell, IF it can still provide the running current required.

The OP stated two things, he's an amateur and he wants simple. You don't get much simpler than "Attach a large capacitor or several mid sized capacitors in parallel with the battery"
 
Last edited:

MikeMl

Well-Known Member
Most Helpful Member
Try it an see if it helps. I predict it won't, because the problem isn't the internal resistance of the battery, which is what paralleling a capacitor across the battery would help.

I think that the OP needs a voltage higher than the battery voltage to start the motor. The way to get that is to mechanize the following sequence:
1. disconnect the motor.
2. disconnect the battery
3. let the solar panel charge the capaitor.
4. when the capacitor voltage reaches X Volts, where X > Vbat+delta, connect the motor.
5. if the panel voltage remains > Vbat (i.e. panel is producing enough output to run the motor), then connect the battery (excess output now charging the battery).
 
Last edited:

Sceadwian

Banned
Could always just use two batteries in series.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
Adding a capacitor will not help.
The motor simply does not have enough power at the low voltage to start running the pump.
 

Sceadwian

Banned
How can you say that audioguru? A capacitor in parallel will approximately HALF the series impedance of the power source, more will do better than that, especially if the cell is low on capacity.

You can't tell me that might not be enough to get that pump moving past it's peak compression point? Honestly I don't now the forces acting against the pump so it really depends on how stiff the stall force actually is for how much of the cycle to get the pump actually going. To rule it out without trying it is wrong.

He wanted simple. Capacitor(s) in parallel is\are simple. It will work, or not. If it doesn't no biggie but he should at least try it with a couple mid range electrolytics. The only thing simpler is using two batteries in series.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
Energizer's AAA Ni-MH battery cell has an internal resistance of 0.1 ohms.
Their AA cell has an internal resistance of only 0.03 ohms.

If the motor has an extremely high starting current of 2A then adding a capacitor in parallel with the battery cell will slightly help start the motor. I bet it won't.

The motor is probably rated for 3V or 5V, not just 1.2V.
 

Sceadwian

Banned
If the parallel cap doesn't help, use another cell in series, problem solved.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top