High side switch

[ronv]

Yes, I got to thinking about the cap directly to the solar panels, but the open circuit voltage is probably higher than the capacitor rating. But if you have a buck boost you can set an upper limit. If you could fit a larger battery - say 12 AH the high charge current wouldn't be so hard on it. Let me think about your other ideas. In the meantime here is the circuit mod for 7 to 9 volts.

 

[mmmalmberg]

Fantastic, thanks Ron. I've now got a board designed for this l.v.cutoff circuit so I could possibly put two of them on a small board easily and set one for the battery and one for the cap.

The 1.2 AH SLA is as large as I can fit on these things. I've been charging it with a little solar battery charge controller but it's not limiting the charge current at all relative to this little battery. I guess I could current-limit the battery charge, as long as the caps were picking up the quick recovery charge.

 

[mmmalmberg]

p.s. yes open circuit on the panels, which are nominally 15V panels, is something like 23V.

 

[ronv]

The problem with the cap is that it really doesn't store much energy (compared to what you need). If battery life is the issue it might be easy to limit the charge current to c/10 and run everything off the panels when there is enough light. You could then set the buck boost to the maximum desired battery voltage (+ some for the current regulator). The downside is that it will take longer to charge the battery back up than it does now.

 

[mmmalmberg]

Yeah I guess that's the essence of what I'm after is the benefits of both. Most of all the somewhat infinite lifespan of the caps, but also the capacity of the battery, to whatever degree possible. If I could tune things up to get 10 years out of the battery instead of 6 months then that would be fine... Since I'll be thousands of miles away from these pieces most of the time, I want them to be as maintenance free and reliable as possible.

I do think though that the caps are plenty to work with through the day, and that if I could just have the battery slowly charging during the day to extend activity into the late afternoon, that would be great. I'm thinking maybe as simple as a current-limiting resistor between the cells and the battery charge controller so that most of the current goes to the capacitor. I'll have to do a sketch of some sort but this seems like it could be relatively simple.

The solar cells would feed both the buck/boost regulator to the supercap, and also a resistor-limited charge controller for the battery. That would be the basic charge circuit.
Discharging, there'd be a low voltage cutoff on both the supercap and the battery, with the supercap having a lower cutoff. This would let me cut the battery at a conservative voltage (I've been cutting it off pretty low) and have the caps still available to do some low-current stuff like a few led lights and some sounds at the end of the day.

 

[ChrisP58]

I haven't digested this whole thread, so please forgive me if I am wrong, but as I understand it, the problem you have with the SLA is not so much the daily runtime, but the service life you get from one battery.

If so, adding the supercap in parallel, even if it is sequential parallel, will not fix that problem. You probably need to focus on correcting what I expect is an abusive over charge issue with the SLA.

Also, capacitors are much less forgiving than SLA batteries to over voltage conditions. Doing so often triggers their rapid self dis-assembly feature (boom)

 

[mmmalmberg]

Thanks for chiming in Chris. I have a switching buck/boost regulator for the supercaps so they should be fine in that regard. I have been planning on getting rid of the batteries altogether, as I've successfully run smaller pieces using caps only and they've lasted for years. But this piece has much larger motors so I went initially with the SLA's. I completely agree, I'm abusing them, but I can't go larger with them.

During the day, when there's plenty of sun, I just need rapid charge/discharge capabilities and I'm pretty confident the supercap will be fine; the solar panels' output will be exceeding the average demand. What I'm wondering is whether I can use the batteries in different way, augmenting the caps by accumulating overflow energy during the day and then extending the time things run when the sun gets low.

With the capacitor only, I have a pretty much permanent power supply, even if the performance envelope is dimished, and that longevity is a very high-value aspect for this application.

 

[ChrisP58]

What are the specs of your buck/boost regulator? Could it be used to more gently charge the SLA?

 

[mmmalmberg]

Sure, it's 80W, I think it has current limiting too. I need, however, to be able to soak up power quickly, more quickly than a 1.2AH battery should be charged. I have three motors, the largest being a gearmotor current-limited to 4 or 5A. They only run maybe 10% of the time tops but if I were only charging the battery at a tenth of an amp, it won't keep up with the demand and I'll be wasting the 1.8A my panels put out. I need to be able to jam current in and out quickly which is why the caps are perfect other than their ultimate capacity - which mainly comes into play late in the day when the sun's waning.

 

[ronv]

What is the temperature environment. It could be we could switch to Nicd and terminate their charge on temperature. It would be crude (and hard to life test) but a pack of 10 AA cells would take all the panel could put out and have 2X the capacity.

 

[mmmalmberg]

Outdoors, next installation is Albuquerque NM through the winter.

 

[ronv]

Well, that wasn't a very good idea anyway. The more I think about it I'm coming around to a LiPo pack since the charge can be terminated on voltage. ( Come to think about it do you ever stop charging your battery now?) The downside is that it would be a 3 cell pack so would need to be balanced. In this case you could use a battery pack like is used for RC cars and "stuff". Can you build a circuit from a schematic for the balancer?

 

[ChrisP58]

Sure, it's 80W, I think it has current limiting too. I need, however, to be able to soak up power quickly, more quickly than a 1.2AH battery should be charged. I have three motors, the largest being a gearmotor current-limited to 4 or 5A. They only run maybe 10% of the time tops but if I were only charging the battery at a tenth of an amp, it won't keep up with the demand and I'll be wasting the 1.8A my panels put out. I need to be able to jam current in and out quickly which is why the caps are perfect other than their ultimate capacity - which mainly comes into play late in the day when the sun's waning.

Yes, super capacitors are good for accepting large currents quickly. But only until they're full.

At 1.8 amps, your 58F capacitor will charge from zero volts to 16V in a little less than 9 minutes. After that, it cannot take any more current without overcharging.

 

[mmmalmberg]

Well, that wasn't a very good idea anyway. The more I think about it I'm coming around to a LiPo pack since the charge can be terminated on voltage. ( Come to think about it do you ever stop charging your battery now?) The downside is that it would be a 3 cell pack so would need to be balanced. In this case you could use a battery pack like is used for RC cars and "stuff". Can you build a circuit from a schematic for the balancer?

The sla's are currently charged with a controller that's made for charging lead acid batteries with solar power. So yes it's voltage-controlled charging but at a higher current than a 1.2AH battery should get. I can build a balancer of some sort from a circuit, surely. But my understanding is that LiPo's have a pretty limited lifespan as well in terms of number of cycles. No direct experience though.

 

[mmmalmberg]

Yes, super capacitors are good for accepting large currents quickly. But only until they're full.

At 1.8 amps, your 58F capacitor will charge from zero volts to 16V in a little less than 9 minutes. After that, it cannot take any more current without overcharging.

I'll be charging the caps with a switching regulator; the caps are rated 16.5V and I'll be charging them to 15. Thanks for that calculation btw! These caps supposedly have pretty low leakage so I'm going to cut them off with a low voltage disconnect at around 7-8V so that hopefully in the morning they'll charge up to 16V more quickly...

 

[mmmalmberg]

For those who missed it earlier in the thread, here http://www.markmalmberg.com is a link to my website that shows the existing piece "Albireo" in a prior installation in Philly. Two Arduino-controlled sculptures communicate wirelessly by XBee, interacting in motions and sounds. I monitor the voltage state with the Arduino so I can use the motors more when there's more power available, and less when the voltage is low. In the middle of the day, there's lots of activity, the motors are much more involved and the battery gets charged with 1.8A and discharged with bursts of up to 5A or more. The cap will handle this part of the day really well and last just about forever doing it.

For the next installation in Albuquerque, there will be a third similar piece, and I'm rebuilding kind of everything to eliminate some weak links and to formalize the electronics much of which is currently wired point-to-point. I have some things also to get working that didn't get finished last time - encoder reading, some Hall sensors, amplification of the audio etc.

 

[ChrisP58]

The equation for calculating the voltage on a capacitor is fairly simple. It's all ones.

The voltage on a one farad capacitor will change one volt per second per amp of current. This is true both for charging and discharging.

 

[mmmalmberg]

The equation for calculating the voltage on a capacitor is fairly simple. It's all ones.

The voltage on a one farad capacitor will change one volt per second per amp of current. This is true both for charging and discharging.

Sounds simple anyway - I'll work on it thx.

 

[mmmalmberg]

Here's what I'm thinking... View attachment 64517

 

[ronv]

I think this is what you are looking for. During normal times the power is supplied by the buck boost and the cap is charged to 15 volts. When the load is on the boost and cap are used down to the battery voltage, then the battery and cap are in parallel. When the battery is dead the cap is used until it is discharged. It needs some more work but you can think about the concept.