Hi!
I want to extend the battery life of a robotic mower with solar panels.
The robot draws a minimun 0.4 amps at a range of 23v to 28v even while doing "nothing" and the solar panel would in theory only supply a maximun of 0.3 amps at 30v.
I dont want to charge the battery.
My question is, can I run the solar panel directly in pararel with the battery with just a diode?
There is already two diodes ORing for the station charger and the battery, the idea would be to add another one for the solar panel, my undestanding is that if the solar panel voltage goes above the battery voltage power would start to be drawn from it untill the voltage equalizes with the battery, is this correct?
The project is not rated for input voltages above 32v.
I don't see a problem with your proposed method of connection, but good luck finding a solar panel of a suitable size to fit on a mower and provide the level of output you need.
Make sure the solar panel output voltage cannot exceed the maximum allowed input voltage to the mower.
I guess this is the last unkown to me.
Do I have to worry about the panel going over 32v even if it should always be a under load?
If so, what can I do to clamp the panel voltage? Here is where I start to get lost.
This is to some extent unknown territory for me and others might give you a better answer. I suspect that a panel that will fit on your mower will not produce anything like enough output to do what you want, but you need to check this with an actual panel or at least a specification for one. If you draw current the voltage will drop. If you can guarantee a minimum current draw then you can see what the maximum voltage would be at that current, under best case sunshine conditions. That's about as good an answer as I can give, you need to experiment.
For more helpful suggestions you could tell us what your battery chemistry is and what its nominal voltage is as well as what maximum dimensions your solar panel can be.. As @PerryBebbington has advised, finding a panel that can charge at 30V and meets your size constraints might be a problem. Additionally, you will almost certainly need some sort of charge controller to keep from damaging your battery,
Get a solar panel capable of delivering 10%-20% more power than that and use a suitable boost converter such as an MT3608. And place diodes to prevent charging your battery
I have a 29.4v Lithium Ion battery pack.
I dont want to charge the battery though, the idea is just to extend the time the mower can go around without recharging.
The panels are supposed to be 10v-3w x3, the idea is to offset the panel output from those 11+ watts.
It does have a ON OFF button and a latching circuit.
Its an arduino based mower.
Yeah... Arduino + PCB + GPS draw around 0.2 amps, the motor drivers another 0.2 while doing nothing, I dont think they are meant for battery powered applications.
I think I'm beginning to get a glimmer of understanding. From your circuit in post #9, it looks like you want the solar panel to power your circuit sort of in parallel with the battery, not supply any charge to the battery.
If the open circuit voltage of the solar panel under the present illumination is greater than the battery voltage, then the panel will provide as much current as it can at the battery voltage. The problem would be in finding a ~30V panel, although you might connect several in series to get that.
This will not harm, or charge, the battery because of the battery diode. You just have to be sure that the absolute maximum voltage the panel(s) can produce at high noon on a clear day on the summer solstice is not more than the mower circuits can tolerate.
I don't know what happens if you use a boost circuit on the panel output. It might work, but I'm not sure.
Adding to that: if under those conditions the panel cannot supply the lowest current the mower ever draws then the panel voltage won't exceed the maximum input voltage to the mower. I would want to know what margin of safety there was, which would mean measuring how much of the current came from the panel and how much from the battery: if the panel can supply 99% of the current I'd consider that very risky, if it can only supply, say, 75% I'd consider that safe.
I have a boost converter in a smaller panel with smaller voltages and it works. I pumps even with very low voltage in the panel.
But in this case, what would happen if the panel is providing the power because it's voltage is above the battery voltage, but the current is not enough to activate the mower?
So, the mower would need current from both the panel and battery. Will the panel voltage go down until the battery diode gets forward biased and start to supply current? Even with the boost converter in the panel?
Without the boost converter, could be that most of the time the voltage of the panel is below the battery voltage and it's blocked by the diode. Depending on the most common voltage of the panel with normal light and with the load, not the open circuit voltage.
Maybe there could be some rippling, when the diode gets activated the load will drag the voltage down, deactivate it, and so on.
I'm not sure.
I think what everyone is questioning is whether there will be enough panel surface area to make any real difference. So it kinda depends on what you mean by "feasible".
Because it doesnt bother me, once its charged it just takes power from the wall.
Its usually recharging at night.
Right, well, the idea of running a solar panel that cant meet the current draw in parallel with a battery sounds feasible.
I might be being a bit optimistic in terms of how much current it will actually provide.
