Battery+BMS solution for electric boat with solar panel

[Vasily]

Hi, guys!

I'm trying to equip my catamaran with sun-electric motor setup. Motor will drain about 600 watts at maximum load (24V, 25 Amp), and my solar panel will produce around the same or even more power at full sun (will be about 4 square meters, 64s7p of 3*6 inch cells, 32V 25 Amps). Surely, I need battery to move when it's cloudy. I plan to charge it directly from solar panel.

I'd like to pay for battery not more than $400 including BMS. Endless-sphere online market offers different types of relatively cheap used batteries, and I need an advise - which choice will be cheaper?

What I need from battery:
1) 24V (or slightly above)
2) not less than 20 AH (the more the better)
3) discharge current 20-25 Amps, charge current 20-25 Amps

What I need from BMS:
1) Overcharge and overdischarge protection.
2) Overload protection (say, disconnecting at 50 Amps).
3) Charging control for CC-CV charging (PWM-based?)
3) Balancing - same voltage for cells in series.

What kind of cells are available:
1) Prismatic A123 3.3v 20Ah. I need 8 cells in series. The simplest case, the minimum weight, but not very cheap. Cells are $250.
2) Cylindrical 18650V, 3.7v 1.5Ah. I need 7 cells in series and, for example, 20 chains in parallel. It will give 24v 30Ah for about $200. But in this case I will need more complicated and more expensive BMS. Or not? I have read that there is no need for balancing this kind of cells. Is it true?

So, I need advice.
And another thing that I don't know - where should I search for appropriate BMS? May be somebody on E-S could make it for reasonable price?

 

[wesnewell]

If the solar panel puts out 32-33V I'd just use 8s lipo hooked directly into the line to the motor. I wouldn't use a bms at all, just monitor battery voltage and don't let it fall below 28-29V. a 20ah 8s lipo pack would run about $180. If your panel has a voltage regulator for 24v output, then you could use 6s lipo instead and run it down to 21v. Or you could use the output of the solar panel to power a lipo balance charger to keep them charged when not in use.

 

[John in CR]

Wesnewell, are you trying to sink the guy's boat with a lipo fire?

 

[jonescg]

My advice is to go for one of these solar regulators:

http://www.plasmatronics.com.au/common/pl204060.html

And a 20 Ah sealed lead acid battery(s), and don't bother with BMS since SLAs need little maintenance. Last I checked these units were under $200 but I could be wrong.

 

[John in CR]

You've got a displacement hull, so weight doesn't matter all that much. At your price range you'd have to stick with lead batteries, because the batteries and charge controllers are cheap, so you'd have half of your panels feeding the charge controller of one battery and half to the other. You still may not hit your target though, because higher current charge controllers might get a bit spendy.

 

[Vasily]

Thanks you all for your answers.

Why I'd like to use lithium instead of cheaper SLA? My catamaran is collapsible, and sometimes it goes on my shoulders, when I move from a small river to another one over a swamp. That is why weight matters.

Hooking 8s lipo directly to the solar panel sounds good, but I'm afraid that one of these eight cells sooner or later will burn my cat. 33V/8 = 4.12V, and if one cell will loose its capacity faster than others, it will be overcharged. My catamaran is inflatable, and in case of fire it will sink with me and my family

Little weight and safety - LiFePO4 and LiMn are good compromise for me. But where to get BMS and how much will it cost? I think I could do it by myself if only I had circuit scheme and components.

 

[wesnewell]

I wouldn't hesitate using lipo encased in fireproof container, but you could do the same with lifepo4 with 10s cell count. I wouldn't use a bms for it either.

 

[Vasily]

Wesnewell, may be you are right and there is no need to think about charge control. I just should to get more cells in series to never overcharge them.

But charge control is not the only thing I need. I have to make a system, that will automatically protect my battery pack from overdischarge, short circuit and overheating. Have any ideas?

 

[E-racer]

Use a motor controller that allows you to set a low voltage cut off or keep an eagle eye on your pack voltage. Hobbyking also sells a "Cell log" that monitors up to 8 cells independently. The cell log has an alarm output that can warn you when a cell gets to low. Never let the lipo cells drop to below 3.7 volts each. Never charge them over 4.2v each. Drawing 600w on Turnigy lipo will have no threat of ever over heating as you can discharge 20+ C or Charge up to 5C. In a 20Ah pack that is 400A max discharge and 100A max charge. Your setup will never see these numbers unless something really went wrong. Install a circuit breaker.

 

[dogman dan]

Lipo to run. Charging solar is where it gets complicated. Maybe a big lead storage battery charging off the solar, then run your lipo charger from that. Not perfect, but at least it won't overcook your lipo.

Multiple lipo packs, one charging while you run the other of course.

 

[Vasily]

Thank you all for you answers! Now it's time to buy batteries

 

[BikeFanatic]

And please update us when you get it all together, with pics too

 

[999zip999]

Looking after lipo is a hobby in it's self.