Battery Advice Needed For Middle School Competition

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gcholman
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Battery Advice Needed For Middle School Competition

Post by gcholman » Jan 06, 2015 9:44 pm

Hello all,
Hope this is posted in the correct spot. I am a teacher and my class is participating in our 4th solar boat regatta here in Northern California. The contest has really evolved over the years, and this year they have really made some changes. One of the three races is run under battery power (in case the event has rain or cloudy weather.) This year, here are the specs for the battery:

The battery bank must consist of a sealed,
marine quality battery/batteries having
a total rating of not more than 180 Watt-
hours. Example: 15 amp-hours @ 12 volts.
If a charge controller is used in the solar
charging system of the boat, a visible
means to disconnect the battery from the
charging system must be installed. No shore
charging of batteries will be permitted using
a generator or any other means. Once the
battery bank has been inspected, approved,
and tagged, it may not be exchanged.
Participants must bring a copy of the
battery specifications in a clear, plastic
sleeve envelope


We need a 24v system, so 2 x 12v batteries at 7.5 aH max. (Or a 24v battery if you have a source, even a source that can build them.)
I would love to hear suggestions for lithium batteries that would fit this spec. Since we run a 200w motor (the used solar panels we have can only give us that much useable power and you must use the same motor in all three races), I was hoping to get away from SLA in order to shed some pounds. Here is the link to the entire 38 page race packet if you are interested:

https://www.smud.org/en/about-smud/envi ... Packet.pdf

A link to the race page:
https://www.smud.org/en/about-smud/env ... egatta.htm


Our boat last year:
http://teacherweb.com/CA/Evergreen6thGr ... /apt4.aspx

Thanks again for any suggestions!

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Jan 06, 2015 9:46 pm

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Re: Battery Advice Needed For Middle School Competition

Post by liveforphysics » Jan 07, 2015 3:09 am

http://www.hobbyking.com/hobbyking/stor ... _Pack.html

In parallel with this one should just duck 180Wh.

http://www.hobbyking.com/hobbyking/stor ... _Pack.html

Store at ~3.8VDC in a cool dry non-flammable place when not using/competing.

This would be the ideal choice if your primary interest is winning the event. If the primary interest is to make a boat for daily use, perhaps the LG 2.8Ah 18650 or VTC5 if you're interested in running a healthy drive motor.
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Re: Battery Advice Needed For Middle School Competition

Post by teklektik » Jan 07, 2015 1:09 pm

gcholman wrote:This year, here are the specs for the battery:

The battery bank must consist of a sealed,
marine quality battery/batteries having
a total rating of not more than 180 Watt-
hours. Example: 15 amp-hours @ 12 volts.
The primary difficult is the vague specification of "sealed marine quality". This deserves considerable elaboration by the officials.
  • If this means "manufactured for watercraft", then you can likely pick a lithium 12V SLA-replacement battery intended for a SeaDoo or similar.
  • If it means "sealed and manufactured", then you can pick up any of a number of motorcycle or other 12V lithium batteries intended as small SLA-replacements that claim to be 'sealed'.
  • If it allows DIY batteries demonstrated to be sealed and waterproof, then you have many options using a small watertight case (e.g. pelican case or similar) and a variety of lithium batteries.
The issue of calculation of the 180Whr rating may be an issue as well and could use a little elaboration. This seems like a small point, but battery equality across the competitors is certainly an issue. The difference between Lithium and SLA battery voltages could be raised by unhappy losers if the Ah rating is not suitably adjusted.

FWIW - It's also interesting that with regard to a 'Kill Switch' the race requirements state:
Northern California Solar Regatta wrote:This device shuts power to the motor if the Pilot loses control of the craft. This device should be wired to disconnect power to the motor when the switch is shorted.
This is exactly the opposite of what is needed.... Pulling the lanyard should (most simply) open the circuit to kill the power. This is a problem of specifying how the switch is implemented instead of its function. Similar kill switch requirements for other EV racing are often met simply by tying a lanyard to a shorted connector pair that when inserted complete the main power circuit. Pulling the lanyard pulls the plug.
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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Jan 08, 2015 3:00 am

Thanks so much to everyone for their thoughts! Let me clarify a few things:

-Yes, we are interested in winning and I really like the weight of the Hobby King packs. In the interest of safety and the fact that we take them out and practice a bit, we would be willing to trade a little weight for safety. Would you consider the lipo packs properly wired safe if stored in a metal container?

-Most definitely we are going to run any battery the students choose by the judges before we buy. At an earlier meeting for the event, the organizers are very accessible and willing to give advice and direction throughout the process.

-The definition of "marine" is primarily there to mean sealed. Again, we can get approval before we buy as well as be certain on race day that if the boat does capsize, the battery will be securely attached and will not be a risk to the pilot or the ecosystem.

-Our kill-switch the last 3 years has been this and has been accepted:
killswitch.jpg
killswitch.jpg (37.5 KiB) Viewed 1065 times
-One possible battery students have found is this:
http://www.lithiumion-batteries.com/pro ... n-battery/
More expensive, but thoughts?

Again, I am so thankful for all of the input!

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 01, 2016 2:10 am

Hello again!

We are gearing up for another shot at the California Solar Regatta. Our past batteries have served us well, but we are always not able to quite get the full 180 watt-hours of capacity from the lipo. I was hoping to move towards a safer chemistry and get a battery to our exact specs. Here are the specs from race rules:

"Battery Specs
The battery bank must consist of a sealed, marine quality battery/batteries having a
total rating of not more than 180 Watt-hours. Example: 15 amp-hours @ 12 volts. If a charge controller is used in the solar charging system
of the boat, a visible means to disconnect the battery from the charging system must be installed. No shore charging of batteries will be permitted using a generator or any other means. Once the battery bank has been inspected, approved, and tagged, it may not be exchanged. Participants must bring a copy of the battery specifications in a clear, plastic sleeve envelope."

Now, we were approved to use lipo if we put it in a container that was water resistant and we had the batteries in a fire-resistant bag. So no worry about the "marine quality."

Any suggestions where we could get one made? Would like a BMS and if my math is correct: 24v 7.5 aH?

Here is a picture of the boat at the start of the endurance last year. I was allowed to give the boat a "push."
DSCN1000.jpg
DSCN1000.jpg (34.84 KiB) Viewed 863 times
For reference, that is a 200w 24v brushed scooter motor hooked to our outdrive. We have plans to experiment with some other motors this year, but we will be using 24v. This battery and motor are only used for one race - the endurance. The race lasts 25 minutes. Last year, our battery died with about 20 seconds left in the race. We were able to log the most laps and won, but we know the competition will be after us this year. Any suggestions or battery sources would be appreciated!

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Re: Battery Advice Needed For Middle School Competition

Post by redilast » Dec 01, 2016 6:16 am

I would do 8S LiFePo4 cells.

I wouldn't worry about a BMS. Use a fuse for safety reasons, and attach an 8S cell logger with LCD so you can see all the voltages of each cell (these cost like $15-20online). Then use a balancing plug and low voltage alarm. BMS's just complicate things and are not necessary to a once off event. Cell balancing for just a few uses on LiFePo4's that are well matched probably isn't even needed at all.

You could do like 8S2P using K2 energy 3300mAH LifePo4 26650's. That would be 164 watt hours.

They also have these: http://www.newegg.com/Product/Product.a ... lsrc=aw.ds

2 of those would be 180 Watt hours.

Also what LiPo did you use last time? And what part of NorCal are you in?

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Re: Battery Advice Needed For Middle School Competition

Post by dogman dan » Dec 01, 2016 7:08 am

Stick with the lipo, but heres the deal,, you need to figure out which type pack sold by hobby king does the best at delivering the 180wh.

Their sometimes optimistic specs don't help much. NO lvc at 3.5v. run them down to 2.5v per cell to get the full measure.

Don't know what you ran last year, but if you picked a 20c pack, try a higher c rate pack this year. The higher the c rate, the less of your 180w will be turned into heat. Price can mean diminishing results for the money though, once past 30-40c. Turnigy 40c would be one choice I'd test.

Race pack. DO NOT run the same pack you used for practice in the race. Use new stuff, with just a few test cycles on it.

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 01, 2016 11:44 am

Thank you both for the fast replies!

We are in Paradise, near Chico.

Our race batteries were a pair of these:

http://www.store.revolectrix.com/Produc ... bel-30C-HV

We were very happy with how they performed, I just know that taking them down so far and so much time passing, we want new ones. Also, we storage charged them and keep them in a fire-resistant bag.

The watt-hours are close, but by running out, we would like to carry just a bit more power to finish the race. Thanks again everyone!

Here is a short video that shows some footage under battery power:

https://youtu.be/Bu8Y71woezY

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 01, 2016 12:01 pm

One last thing. After the race, battery-pack was barely even warm. We wired a CA inline and our draw ranged between 350-600 watts. There is a brief break in the middle where we change pilots.

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Re: Battery Advice Needed For Middle School Competition

Post by redilast » Dec 01, 2016 9:45 pm

gcholman wrote:One last thing. After the race, battery-pack was barely even warm. We wired a CA inline and our draw ranged between 350-600 watts. There is a brief break in the middle where we change pilots.
Wow, really 350-600W? From 22v thats a lot of power, I'm surprised your 200W motor pulled 2-3x its rated power. If you are drawing that kind of power probably best to stick with LiPo. Or if you really want to go with LifePo4 then A123 systems cells, although it will be hard to get right up to the 180Whr mark with those due to their size.

Also, you probably already know this but brushless motors are about 10% more efficient, so if you really want to maximize efficiency then use no BMS + brushless motor and get as close to 180Whrs as you can. :)

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Re: Battery Advice Needed For Middle School Competition

Post by dogman dan » Dec 02, 2016 6:25 am

Re the capacity,, what you need is to find a pack that is labeled 180wh,, but actually in fact puts out 200. :wink:

The 30-50c turnigy packs might be the best one to try. Not anything touted as lighter, or smaller.

Overcharge them on race day.. this will pack in some more wh. Do they test voltage? What ever they say the absolute max is,, charge to that. This may be 4.3v per cell or more,, go for it!

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 02, 2016 11:52 am

Redilast: it is crazy the watts we are pulling - they peak as you would predict on acceleration and back off once at speed. We have a large box of burned up motors over the last 6 years.... We did not know exactly what we were using until we installed a CA from Grin last year.

We are exploring brushless. And, in case you are wondering, the quandry of the race is that 2 other races are run under solar power only. The panels are rated a total of 260 watts, however they are used and more than 25 years old. For all races you must use the same motor.

Dogman Dan: They have never checked voltage, until they read this. :D

Thanks for the ideas, and again thank you all for chiming in!

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Re: Battery Advice Needed For Middle School Competition

Post by redilast » Dec 02, 2016 6:16 pm

One thing you might consider is attaching some heatsinks to the motor. It can make a pretty big difference. Thermally epoxy them onto the motor housing or something.

Also wow, 25 year old solar panels. Can you do more than 260W? Or is it just not in the budget for newer panels? I'm sure you could get 350-400W in the same surface area that you have now with newer panels. Maybe a local solar company has some slightly damaged ones they would sell cheap or donate? If the 260W worth of panels are 25 years then they have likely lost ~20% of their max power.

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 02, 2016 6:27 pm

Good points - the competition gives every team the same panels. (I kind of like that since we all have the same disadvantage.) It forces us to make improvements in other areas. We have talked about heat sink, but never made it that far. Great idea about epoxy - that way we can mount several and not need to have it custom made. Thanks!

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Re: Battery Advice Needed For Middle School Competition

Post by gcholman » Dec 02, 2016 10:17 pm

eTrike:

Thanks for the reply, I love that picture too! That happens to be the same pilot a year earlier. SMUD (the utility putting on the event) took that photograph from the water. What a great contradiction: solar powered boat and a decommissioned nuclear power plant!

I love the cover idea - I will pass it on to our build team!

Since we live so far away from the race, they have given us several panels to test. An electrician from a local solar company helped us test each panel under load to determine the best. Students even tried several combinations to "pair them" with the most consistent voltage.

We are going to try some radical ideas this year with voltage as last year's sprint winner overvolted their motor and won the (solar only) sprint competition at around 26 seconds. (It is about 100 yards.) Previous best times including ours were in the high 50 second range and up. The idea is that a motor can take the overvolting for a brief period.

Finally, I really appreciate the donation offer. I will check with our battery group of students and parents. It is above my skill level, but if we have someone with some experience, I will message you. Thanks!

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Re: Battery Advice Needed For Middle School Competition

Post by dogman dan » Dec 03, 2016 7:31 am

Hmm, you are leaving some efficiency on the table going brushed. Charge those suckers till they swell on race day. 4.4v or so.

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Re: Battery Advice Needed For Middle School Competition

Post by BlueBell » Sep 28, 2017 3:01 am

Wondering how it turned out last year gcholman?

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Re: Battery Advice Needed For Middle School Competition

Post by BlueBell » Sep 29, 2017 1:38 am

We're in the race this year and are considering Lipo and a brushless lawnmore motor.

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Re: Battery Advice Needed For Middle School Competition

Post by DrkAngel » Sep 29, 2017 6:46 am

.
...
Efficiency!
1. Brushless motor allows possibly 10% greater speed from same input.

2. A 200w (output) motor drawing 350-600w (input) is poorly "geared". (57-33% efficiency.)
A shallower propeller "pitch" will allow higher, more efficient rpm. (and help motor last much longer!)
Compare 479w battery power vs 369w battery power for same resultant speed! Or, more speed from same battery, or solar.
(tire size emulates propeller pitch)
Determine solar panel output watts in varied light conditions to optimize propeller pitch!

Image
..............................................................................................Note Wh/mi, range and motor temperature! ▲

3. Boat hydrodynamic efficiency also can supply more efficient movement through water.
Compare shapes ... canoe, flat bottom, boat tail, torpedo etc.
Small scale models of identical weight:
identical push with resultant distance travel
or
spring resistance scale in flowing water
?
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Re: Battery Advice Needed For Middle School Competition

Post by DrkAngel » Sep 29, 2017 7:12 am

.
...
Not Cheating?

For a slight improvement-advantage ...
Consider a large boxy backpack, and hat, covered in aluminum foil or space blanket to reflect a bit more sun to panels?
A *"space" suit?
Last edited by DrkAngel on Sep 29, 2017 7:46 am, edited 1 time in total.
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Re: Battery Advice Needed For Middle School Competition

Post by DrkAngel » Sep 29, 2017 7:31 am

Adjust propeller Pitch for optimal solar efficiency!
Vary voltage for optimal battery mode. 18.5V 9.72Ah or 22.2V 8.1Ah or 25.9V 6.94Ah or 33.3V 5.4Ah etc
or
Change prop ... if allowed?
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Re: Battery Advice Needed For Middle School Competition

Post by DrkAngel » Sep 29, 2017 8:06 am

Consider mechanism to vary angle of solar panels to collect more direct sun!
45% angle to sun = 50% energy of direct face to sun.

Hinge between panel pairs.
Foot pedal to raise front edge of front panel in each pair when sun behind.
Alternate foot pedal to raise rear edge of rear panel in each pair when sun in front.

Image

Should give a nice advantage, especially in some degree of circuitous course.
Left \ right angles more complex.
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Re: Battery Advice Needed For Middle School Competition

Post by DrkAngel » Sep 29, 2017 8:42 am

Overvolt (48V?), from solar panels, will sag voltage to fairly reasonable levels. At load, compare to normal (24V?) using watt meter (8S cell logger?) from panels.
Watt and speed advantage should vary greatly, dependent on solar input

Switch for quick 24V-48V overvolt (TURBO) change.

Image

Bypassing throttle control, for direct power might increase efficiency somewhat? - only with brush motor!
Switchable using alternate wiring of above switch.
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Re: Battery Advice Needed For Middle School Competition

Post by BlueBell » Nov 05, 2017 4:07 pm

DrkAngel,

Thank you for your thoughtful reply, you're obviously keen to help.

From your first response:
1. We've already decided on brushless.
2. Agreed, matching motor power curve to propeller properly is key.
3. We have the boat shape and design covered, thanks.

From your second response:
We are allowed reflectors and plan to sea-trial them.

From your third reply:
We are allowed to change props between heats so we will be optimizing.

From your fourth reply:
We are also able to "tilt" the boat (roll/list) to optimize solar gain.

From reply five:
We are not using brushed motors.

We have our hull speed and resistance (drag),
200 - 250 watts nominal from the four MSX-64 panels,
a battery of 180 watt-hours, and an assortment of ball-park props to test.
These are fixed and can't be changed, so the motor is our only remaining variable.

There are three heats. Two are solar panels only and one is a 25 minute battery-only endurance heat.
The endurance heat is a must finish heat.
If we don't make it through to the end of the 25 minutes we get disqualified from the heat!

Propellers work better at lower RPM, larger diameter.
But can we use a lower RPM motor?
Gearing is not an option as the prop will be on a straight shaft (one small bearing) off the motor like a long-boat in Thailand.

Thanks

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