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Unread 08-07-2009, 11:03
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Re: Solar Battery Plant Question

Ok....hmmmm...

Keep in mind that solar panel power ratings are a bit funny, they are rated at some voltage and at some current, but the voltage is at no load, and the current is into a short circuit, so you do NOT get the max rated voltage and current at the same time! A 75w panel might put out only 5 amps when charging at 12v in full sun.

Think carefully about how long you will want to run the device, and how long you can have it charging. Usually it's more cost effective to buy more batteries and leave it charging as long as possible, than to buy more solar panels.

You probably want a charge controller that is rated for a bit more current than your panels can generate.
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Unread 08-07-2009, 11:26
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Re: Solar Battery Plant Question

I'm thinking we want to implement this in phases.

1st we install 6 batteries, with or'ing circuit.

2nd install a big enough charge controller to handle maybe 400 or 500 watts.

3rd install a single panel on the craft, maybe 200 watts.

4th install another panel on the craft, or 'apu' style on the ground to get us to 400+ watts.

So we have to do some planning.

It is a solar powered lake rake....
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Unread 08-07-2009, 11:35
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Re: Solar Battery Plant Question

Have you measured the current draw while at cruising speed? or loaded down (I'm not sure what a "lake rake" is)
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Unread 08-07-2009, 11:44
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Re: Solar Battery Plant Question

we have to do some instrumentation work and collect some numbers. we had gotten some quick lab numbers with stalled motors and such but it isn't really realistic.

so back to the lake.

a 'lake rake' is a term we invented. sort of like a robotic lawn mower, it skims the lake for 'flotsam' - floating debris.

eventually it should be able to run autonomously we might think......

In Melbourne Australia at Parks Victoria there is a static litter trap shown here

This is a robotic mobile trap.
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Last edited by ebarker : 08-07-2009 at 11:46.
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Unread 08-07-2009, 11:46
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Re: Solar Battery Plant Question

neat!

I suggest putting a motor in a big sink/bathtub and measure current draw at full speed. Motors and propellers do interesting things....you might want to set up a bollard pull tester if you can get up the motivation....
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Unread 09-07-2009, 08:45
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Re: Solar Battery Plant Question

Ed,
Your power output on PV arrays is limited by size and sunlight. In Georgia during the summer, you should be able to get plenty of sunlight provided you can keep the craft in the sun and not under trees. In order to get an array capable of 500 watts, you may have to increase the size of your craft. A quick check found 125 watt units are 25 x 60 inches but the cost is in the $300-$400 range. Current output would be around max charge current for one battery, 6 amps@16 volts in full sunlight. I am guessing you would mount the array flat to provide less wind load so you have to fudge the current and voltage output a little. Without some form of switching, it is difficult to prevent the array from providing some load current which takes away from the charge current. Even at 6 amps, our batteries will take 2-4 hours to fully charge, each.
As to parallel operation, there are varying opinions on this subject. Many people will tell you it is OK to do it, some will tell you never do it. The Battery Institute of America gave me this advice...If the batteries are of the same type, age, temperature, and charge/discharge cycle life then it can be assumed that the only difference lies in manufacturing tolerances. If any of these factors are different, then the individual battery chemistry is sufficiently different that terminal voltage will vary from battery to battery. Millivolt differences will cause current flow from the higher voltage to the lower voltage battery until the difference is equalized. However, the charge state, internal temperature and internal resistance of the higher voltage battery will have changed to the point that it's terminal voltage has now changed. The result is that both batteries will self discharge over some period of time as one feeds current to the other. It is expected that used batteries with our type of load history will have greater than a few millivolts terminal voltage differences due to changes internal to the battery, and the experts expect self discharge to occur in perhaps a day with just two batteries. (Diodes prevent this circulating current.) AGM batteries are not very tolerant of deep discharge and may be damaged by this kind of use. Different battery types (chemistry and construction) differ in terminal voltage variance so blanket statements cannot be applied to all battery types used in parallel. One application that used batteries in series/parallel operation was electric submarines. Remember that these were large, wet cell batteries where the specific gravity of the cell electrolyte was checked several times a day.
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Unread 09-07-2009, 10:25
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Re: Solar Battery Plant Question

I think the application may make a difference as to whether or not you'd want to run the batteries in parallel or not. I think it's likely that this particular application is something that would not be left alone for long...it will be turned off and have the batteries all disconnected at the end of each work day. Also, it's working time will be limited.
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Unread 09-07-2009, 12:32
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Re: Solar Battery Plant Question

Quote:
Originally Posted by Al Skierkiewicz View Post
One application that used batteries in series/parallel operation was electric submarines.
Hopefully this machine will not convert itself into a submarine.. haha..

We are starting to think at this phase of the project to simplify it and use a single deep cycle marine battery. One battery, no or'ing....

One panel about 200 watts or a couple at 125 watts, and some variety of charge controller, not sure what kind. We are not looking to generate 500 watts but at most half that. So now we need to pick panels and charge controllers.

The craft is large, just over 9 feet long and about 6 feet wide but we would like to not cover more than 2/3 of the top.
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Unread 09-07-2009, 12:45
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Re: Solar Battery Plant Question

You really need to figure out how much current the thing draws....then you can actually design a power system for it.
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Unread 09-07-2009, 17:24
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Re: Solar Battery Plant Question

Ed,
That sounds like a better solution. There are many places where you can pick up PV cast offs, surplus and smaller sizes which can be rewired (series/parallel) to fit your needs for power and size. There are also many suppliers of charge controllers designed for RV and agricultural use that would be perfect as well.
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Unread 09-07-2009, 17:38
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Re: Solar Battery Plant Question

Our battery expert had the following to add:

Quote:
You can put Pb-acid batteries in parallel. You can do this with other chemistries that are charged in the same way (current limit to voltage limit to taper current cutoff), such as Li-ion and Ni-Zn. All you need to do is charge them fully before connecting. Ni-Cd and Ni-MH are very difficult.



Two important issues with putting cells and/or batteries in parallel are:

Safety. If one cell or battery is shorted internally, the energy from the other flows to the shorted one, thus increasing fire safety hazard significantly. Thermal fuses may be required.
Balance. If the parallel group is exposed to frequent thermal cycles or large temperature gradients, such as under the hood of a car, then charge will be transferred between the components in parallel. Under favorable conditions, such as indoors, this is not significant. A symmetrical layout can help reduce thermal gradients caused by self-heating.
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Unread 09-07-2009, 21:56
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Re: Solar Battery Plant Question

Thanks for the comments.

I just got in at 9:30 pm. This is as bad as build season. 4 and 5 days a week.

We are thinking about building a simulation in Labview to model the power management to help get a feel for how this is gonna roll. We have to get on the lake in a few days. We can do a bollard test and profile the power consumption at various throttle settings.

Any recommendations on charge controllers - simple type - manage the battery without disconnecting the load.

Thanks again.

Ed
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Unread 09-07-2009, 22:51
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Re: Solar Battery Plant Question

We used one of these

http://www.affordable-solar.com/morn...controller.htm

on our underwater ROV power system.

What kind of motors/props are you using? trolling motors or something else?
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