Which is the most efficient and cheapest battery available?

[mitchellleary]

Hi guys and gals,

A contact of mine pointed this forum out as a great place to ask for advice so I'm hoping some of you may be able to help.

I am currently working on a project to construct an autonomous, solar powered boat on a budget. I have a few questions which I'm hoping you can shed some light on but firstly I want to point out that I am a complete novice when it comes to electronics (hence the need to seek advice from more experienced engineers).

Questions;

1) Which Battery would be best? - originally I was working on the plan to use normal car batteries in a bank of 8 x 100Ah 12V but I have recently been told that there are more efficient batteries out there. Which batteries would you advice that last a long time and are relatively energy efficient?
Some that I thought of are; Forklift battery and Golf Buggy battery.

2) Now here is an area that I am having real problems with. I am following the formula; (kWhr x 1000 / V) / minimum discharge % to work out how many A/hr will be consumed by my 2.5kW motor. So a 2.5kW motor running non-stop for 3-4 hours: (7kW/hr x 1000 / 48) / (0.5) = 292 A/hrs. If I give a 15% safety margin, that will give 335 A/hrs.
So my battery bank will be 8 x 100Ah = 800Ah. Does that mean that my battery bank could easily run the motor for at least 3-4 hours if not double that (as 800Ah is more than double the 335 A/hr required) Am I right or as I suspect; wrong? :?

Please feel free to advise me on your recommendations for the best battery to use in your opinion and I would really appreciate your input on my second question.

If you need more info, please don't hesitate to ask.

I look forward to hearing from some of you.

Thanks in advance,

Mitch

 

[Chalo]

Does the design of your autonomous boat imply that the preponderance of its weight will be batteries? If that's true, then using a battery with higher specific energy, like a lithium battery, will allow your boat to be much lighter and its power requirements much lower. In that case, you could get away with less battery capacity. It could be possible to have less total expense on a higher performing battery that costs more per watt-hour, simply because you wouldn't need as many watt-hours.

I'm most cases, you'll find the same principle applies to using a higher voltage system versus a 12v system. At a higher voltage, you may need much less motor and cable mass to get the same job done.

 

[mitchellleary]

Hi Chalo,

Thanks for your quick response.

Yes, the major part of the weight will be the batteries. The maximum weight of the boat will only be 500kg with the potential for it to be much less.

I have found through speaking to different people that I will get a much more efficient system by not using 12V but increasing to 24V or 48V. I even have the option of a 72V motor. Would this also mean I would need a 72V battery?

Do you have any recommendations or examples of a battery that you think would have the higher specific energy? Or Ah that I should be aiming for? I found some batteries with a higher energy but the weight was much much more and even if I cut back on the number of batteries, the weight would still be more(this is why I ask for a kind recommendation from you).

Thanks again! Your advice is much appreciated!

Does the design of your autonomous boat imply that the preponderance of its weight will be batteries? If that's true, then using a battery with higher specific energy, like a lithium battery, will allow your boat to be much lighter and its power requirements much lower. In that case, you could get away with less battery capacity. It could be possible to have less total expense on a higher performing battery that costs more per watt-hour, simply because you wouldn't need as many watt-hours.

I'm most cases, you'll find the same principle applies to using a higher voltage system versus a 12v system. At a higher voltage, you may need much less motor and cable mass to get the same job done.

 

[Voltron]

Just fyi... the motor shouldn't care if its fed 72 volts exactly unless it has an internal controller... it cares about how many watts ( the volts times the amps) before it overheats. Its the controller that is going to run at a certain voltage, and then a battery that matches that. And in general, higher voltage means less of the electricity is wasted as heat from resistance in the windings of the motor. Higher than 72v starts getting pretty sparky, and could be at dangerous levels in a boat situation if you're not careful when working on it. Just make a miniature Planetsolar and you should be good to go...

mmmmm... semi submerged carbon props, twin motors, extendable side wings, wave piercing bows...

 

[999zip999]

Lot's of stuff in 36v and 48v.

 

[mitchellleary]

Hi Voltron,

Thanks for the note. I looked into it a bit more and you're right that 72V upwards are seen as potentially dangerous. I'll probably stick with 48V. If I could guarantee safety, then I would consider higher voltage but I'm certainly not ready for that.

I'd love to make a miniature Planetsolar! haha It is a sexy little beast!

Just fyi... the motor shouldn't care if its fed 72 volts exactly unless it has an internal controller... it cares about how many watts ( the volts times the amps) before it overheats. Its the controller that is going to run at a certain voltage, and then a battery that matches that. And in general, higher voltage means less of the electricity is wasted as heat from resistance in the windings of the motor. Higher than 72v starts getting pretty sparky, and could be at dangerous levels in a boat situation if you're not careful when working on it. Just make a miniature Planetsolar and you should be good to go...

mmmmm... semi submerged carbon props, twin motors, extendable side wings, wave piercing bows...

 

[mitchellleary]

Cheers 999zip999. I've had a look and I'll probably go with 48V. I just wondered if there were any specific batteries that are out there already used for other continuous use purposes?

Lot's of stuff in 36v and 48v.

 

[dogman dan]

It's a bit hard to figure out what to say in response. I have to say I'm confused about what the size of this boat is, and what you want it to do with it's 2500 watt motor.

Solar autonomous? Does that mean you don't have to plug it in? Or that it drives around doing something without a human aboard?

A big boat can use the lead ballast, if going someplace fast is not the plan. If you want to ride along, and get there fast, then you want to lighten up. That means lithium for sure.

Lots of mid price level lithium batteries out there in China now. Not made from the best cells, but if you pile up that much capacity it will run at 2500w just fine. But in the size you are looking for, lifepo4 is also an option. I'd look at 48v, because lots of 48v stuff is out there for golf carts, or the higher powered scooters and bikes.

This thread discusses one vendor of a dirt cheap lithium cell battery I just bought. You should be able to get something like it, but in a larger size, and then connect several of them parallel.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=71119

 

[LockH]

Hehe... It was battery-electric traction for small ferryboats that first got me into EVs. So for a "larger" small boat, traveling only one nautical mile between plugging in to recharge, at harbour speed limits (`round here ten km/h max in the middle, slower then that approaching the docks etc.) and given the long, skinny "needle shaped" hull design with weight supported by water... I see Siemens in Norway list went lithium-ion for a much "larger" ferry:
http://www.siemens.com/innovation/e...-motors/electromobility-electric-ferries.html

... but for a smaller boat I'd still be dabbling with glass tanks holding sheets of lead in water-diluted acids. :|

So far your mission statement seems a bit vague: "maximum weight of the boat will only be 500kg" (with the "potential for it to be much less")

and "solar". ("Solar" only? No opps to plugin to recharge?) My favorite "solar" means of propulsion is the wind. :wink:

 

[anon0973923]

"Most efficient and cheapest"

RC LiPo.

 

[jaunty]

Hi guys and gals,


2) Now here is an area that I am having real problems with. I am following the formula; (kWhr x 1000 / V) / minimum discharge % to work out how many A/hr will be consumed by my 2.5kW motor. So a 2.5kW motor running non-stop for 3-4 hours: (7kW/hr x 1000 / 48) / (0.5) = 292 A/hrs. If I give a 15% safety margin, that will give 335 A/hrs.
So my battery bank will be 8 x 100Ah = 800Ah. Does that mean that my battery bank could easily run the motor for at least 3-4 hours if not double that (as 800Ah is more than double the 335 A/hr required) Am I right or as I suspect; wrong? :?



Mitch

Lets say you were going with the 48V motor...

using Power=Volts*Amps
2500 W = 48V * I at this this rating you'll be pulling 52 amps continuous

When you add those 12V 100AH batteries in series, you cant have your cake and eat it too . In other words, The AH ratings remain at 100AH per series bank. So, with your 48V system using the 8 12V lead acid batteries you get 200AH. shorthand for this battery configuration would be 4S2P: 4 in series, 2 in parallel. This is two 48V-100AH batteries in parallel which equates to 200AH. So you could theoretically run for about 4 hours at 50 amps. Unfortunately lead acid amp-hr ratings are much less because they can't really tolerate 100% discharge and if you do, they don't like it.
So yeah, you will want lithium batteries. You will need money though! lithium-ion, lipo will have the most energy density, but lifepo4 (iron phosphate) will probably last through more cycles.

i'm just going throw some numbers out there:
calculate the surface area of your solar panels, on a small boat i'd guess you'd be lucky to fit 500 watts worth of panels.
so in the course of a day you'd generate 500*8hr = 4000W-hr but you're using around 1250w-hr (running around half throttle probably moving a few mph). I use a minnkota 12V 30lb thrust battery. It draws anywhere from 10-30 amps which is 30*12= 360 watts and it moves my 16ft aluminum 1-2 mph. Thats about 0.5 HP.
my 50HP 2-stroke equates to about 37000 watts! Some times we take for granted the energy density of gasoline

 

[999zip999]

Is a lithium battery with 200 cells 400 connections in a saltwater environment . How would it do with electricity on the connections would collect salt water so fast the battery might not last as long as a sealed lead acid battery in the marine environment might last longer.
It's hard enough just to keep the terminals clean.

 

[LockH]

Still looking for speed, distance between recharges (if can be done) and how "slippery" is shape of hull. Both hydrodynamo and above water aerodynamic. Generally re "hull" shape see velomobiles like this:

Fishies know how to slip through the ambient (water, in their case) with a minimum of energy expended. I would paper upper/air foil/hull with thin-film solar panels. AND still add a foil standing up (think "sail") for yet more "solar".

 

[dogman dan]

If you did buy packs built of 18650 cells, it would be a good idea to pot the whole thing in something very waterproof. Just the salt in the air will be very hard on a pack with hundreds of cells.

For that reason alone, large 50- 100 ah lifepo4 cells could be a good approach.

 

[999zip999]

The electricity running thru the battery would attract the salts and that green corrosion stuff to the connections.