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ebarker 07-07-2009 21:41

Solar Battery Plant Question
 
We want to connect 6 ES17-12 (the ones that came with the robot) in parallel to create a large capacity power plant.

Do the batteries need to be connect with or'ing diodes or just hook them in parallel ?

This plant will be recharged with a solar PV array.

Al Skierkiewicz 08-07-2009 07:30

Re: Solar Battery Plant Question
 
Ed,
I have been cautioned by the manufacturer and the Battery Institute that connecting two like batteries in parallel should pose no particular problems. Provided that the batteries are of the same age, type, and life (charge/discharge) history. However, with seven in parallel I would recommend steering diodes or switching of some sort. Depending on the current capability of the array, you might want to switch batteries for charging. i.e. have one on charge while the others are delivering load current. Most RV suppliers carry such diodes for RV dual battery systems. They use Schottky diodes for a lower forward voltage drop in most cases.
It would seem that the problem exists with circulating current between batteries of differing terminal voltage. As one battery sags, the other battery will try to deliver current until the terminal voltages are again equal. Unfortunately, this turns into a domino effect.
If size is a major concern, perhaps single cell batteries in series might better suit your needs. What is your application?

EricVanWyk 08-07-2009 08:40

Re: Solar Battery Plant Question
 
I'd recommend putting some low value high power resistors in line with each battery when you put them in parallel for the first time. This will slow down the balancing and prevent the initial spike of current. This method is slow but easy. After they have settled, take the resistors out. Be sure to run a few quick numbers to estimate the power your balancing resistors will put out, and how long it will take.

Since our batteries are charged with 2, 4 or 6 amps, I'd multimeter verify that they are all within a volt and then put half ohm or quarter ohm 1 watt resistors in there. Since we have 18 Amp hours, I'd plan on waiting a full day. If you do the math, I'm being too conservative, but why rush?

At this point, you should treat the pack as one unified battery, and avoid taking them out of their parallel configuration for anything. Otherwise, you will need to balance again.

As for diode-oring them:
From your description, I assume you are using the pack as a back up for a solar array. Charge during the day, discharge at night. If this is the case, be very careful about where you put your diodes. Frankly, I wouldn't use them in this case.

If you still want them :
I'm not a huge fan of steering diodes because of their voltage drop. Even a big schottkey will drop a third of a volt. To reduce that, you could use an ideal diode controller like the LTC4352, tps2413 or ISL6144. If your currents were lower, you could use an integrated ideal diode like the ones from picorpower. Since you have 7 of them, you could roll your own like this.

ebarker 08-07-2009 09:46

Re: Solar Battery Plant Question
 
The application is on our off season project that we are working on again for the next 4 months.

Pictures were posted here last fall.

During the next 2 or 3 weeks we need to update our power scheme with 6 (not seven) batteries, plus a solar charge plant.

We have to get some more power measurements done but rough numbers we may be charging maybe half of our full load power draw. Working from memory we may have a maximum plant draw of 50 amps at 12 volts. We will charge on the run. We may be able to upgrade the power plant to charge at the machine full power draw rate. We will see.

We are not opposed to or'ing diodes if that gives us the safest and most reliable operation. We just want to get it right.

We'd prefer to use 6 robot batteries instead of a single large deep cycle for several reasons but that may not be totally practical.

MrForbes 08-07-2009 10:00

Re: Solar Battery Plant Question
 
Lead acid batteries seem to be pretty forgiving when you connect them in parallel.

With solar power, you want to make sure you are using as little power as necessary, since power costs a lot to generate. Conservation is the most cost effective approach. How well are your pontoons designed? are they really just rectangular blocks? How fast does it really need to move? Could you reduce speed a little bit and reduce power consumption a lot?

We used solar power for our underwater ROV for the NURC this year....good stuff....

ebarker 08-07-2009 10:11

Re: Solar Battery Plant Question
 
It is what it is.

For better or worse we are moving forward with the current platform design.

Not the most efficient hulls, not the most efficient motors, etc.

So we will deal the cards that are on the table.

Our hull speeds requirement is low - about walking pace.

If we wound up with 400 watts max power draw and 200 watts off the panel on a sunny day we can live with that.

MrForbes 08-07-2009 10:15

Re: Solar Battery Plant Question
 
Do you have the solar panels already? If so, then don't worry about it....

Do you have a solar charge controller?

ebarker 08-07-2009 10:23

Re: Solar Battery Plant Question
 
we do not have any solar equipment yet, so keep talking......looking for recommendations

MrForbes 08-07-2009 11:03

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.

ebarker 08-07-2009 11:26

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....

MrForbes 08-07-2009 11:35

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)

ebarker 08-07-2009 11:44

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.

MrForbes 08-07-2009 11:46

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....

Al Skierkiewicz 09-07-2009 08:45

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.

MrForbes 09-07-2009 10:25

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.

ebarker 09-07-2009 12:32

Re: Solar Battery Plant Question
 
Quote:

Originally Posted by Al Skierkiewicz (Post 866156)
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.

MrForbes 09-07-2009 12:45

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.

Al Skierkiewicz 09-07-2009 17:24

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.

EricVanWyk 09-07-2009 17:38

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.

ebarker 09-07-2009 21:56

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

MrForbes 09-07-2009 22:51

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?

Al Skierkiewicz 10-07-2009 08:00

Re: Solar Battery Plant Question
 
Ed,
When I was checking I found this site...
http://www.solarpanelstore.com/solar...ntrollers.html and this site...
http://www.absak.com/catalog/index.php/cPath/27_137

Both have a variety of controllers, some of which have load sensing and cutoff for battery protection. Both sites have the Morningstar product Jim recommends which looks pretty good if it fits your load requirements.

ebarker 10-07-2009 19:47

Re: Solar Battery Plant Question
 
Quote:

Originally Posted by squirrel (Post 866251)
We used one of these

http://www.affordable-solar.com/morn...controller.htmWhat kind of motors/props are you using? trolling motors or something else?


Minn-Kota Endura 30 - 30 lb thrust, 12 VDC, factory props. IFI Control System - 2005 model if I remember correctly. Victor 884 speed controllers. Electronics are contained in a Pentair NEMA fixture.

The motors are 4 wire circuits. A call to their tech support reveals that we cut and cap two wires and connect the other two to the victor.

8020 automation framing attached to lightweight fiberglass, foam, aluminum pontoons.

The pontoons are closed cell polystyrene foam topped with an 0.030" aluminum cap that is shaped in a 2" tall, 10" wide beam.

The cap acts as a beam and binds the automation frame to the pontoons.
The pontoons have a fiberglass skin that is essentially in tension.

If you look at the side of the craft it looks like a bridge structure, the aluminum cap is in tension as the downward force on the legs try to spread. The cap disperses the load across the foam.

The trick was to figure out how to disperse the stress and keep the weight very low. One days we need to make some drawings and explain how the stress work.

Each pontoon is only about 25 or 30 lbs each. A design goal was to be able to carry the machine to a lake that doesn't not have a landing. So the basic machine needs to weight something like a heavy FRC robot. That is asking a lot considering this is 9+ feet long.

We have been working on this for two years now. Construction pictures are here The pictures haven't been updated in a year.


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