36v LiFePO4 prismatic battery for boat trolling motor

[speederx7]

I was hoping you guys could lend me some expertise on building this battery.

Background: I am using a 3 D31M Optima Bluetop batteries in my bass boat to run a 36v troller. 75AH. They cost about $700 for all and weigh about 180#

I am looking to replace with a 36v 60AMP lifepo4 system, similar to this:
http://www.batteryspace.com/lifepo4-prismatic-battery-package-36v-10c-rate-with-led-balancing.aspx

Of course, being as frugal as I am, i am hoping to cut some costs from this kit.

I have found these prismatic batteries:
http://www.electriccarpartscompany.com/60Ah-32V-2C-br-LiFePo4-Lithium-Prismatic-Batteries-br-HiPower-LiFePO4-EV-Battery-br-USA-Stock-br-45L-25W-77H-in-br-114-63-195-mm-br-46-lbs-21-kg_p_51.html

which allows me to put together a battery for around $720

My questions:

1) since i am only looking to run a max of 42 amp draw (maximum of my trolling motor). Do i really need a battery balancer or BMS? the reading i suggest is that it is only really useful for high loads.

2) can i use a 12v circuit of the 36v to power my other electronics and act as a starter? With my current setup, i use one of the optima batteries as a starter battery for the big motor, which means it also gets alternator charging from my outboard

3) with an assembled battery system hold up in a marine environment? I know companies like Lithium pros build them and i assume they are just running prismatic cells... http://lithiumpros.com/product-category/marine/36v-batteries-marine/ I do plan to rubber isolate. However, they could get wet or submerged under extreme circumstances

Any other help or advice you could give would be great. Go easy on me, since i am just a mechanical engineer and electrons are a bit foreign to me

 

[dogman dan]

You can run them without a balancer or bms, but then you must perform those functions yourself, manually. I would advise wiring up each cell to a row of Jst plugs like hobby RC packs have. Then you can quickly and easily check each cells state of charge with a Cellog 8 or similar device. The same plug can be used to charge or discharge one cell at a time for easy manual balancing. Use a voltmeter display, or better still a watt meter, so you never overdischarge the entire pack.

45 amps from that size pack will be less than 1c discharge rate, so that should be no problem for the cells.

Marine environment, well, you know already. You can't let salt water or spray get on the wiring, terminals, etc. So the whole thing will need a well sealed box.

 

[speederx7]

thanks for the response! Looks like i am on the correct path.

Now i just need to source a good 60-100A prismatic cell!

 

[dnmun]

without a BMSs to protect the pack from short circuits on the output or from over discharging, you are gonna put a big investment in cells at risk for no reason imo.

a superior BMS would cost about 3-4% of the total, and it will balance the pack for you better than you can yourself.

 

[speederx7]

I am trying to keep things simple and cheap, hence the reason i was looking to not have to run a BMS.

I was thinking that a simple volt meter should be enough of an indicator to make sure i dont overdischarge. However, if one cell has an issue, i might not see that in overall voltage.

Can you suggest a good BMS that is reasonably priced? The ones i am seeing are $150-200 USD

Here's my next question. Can i run a 12v and 36v off the same battery pack and not hurt cells? In my current deep cycle setup, i am running 36v for the troller, but using one of the 12v deep cycle batteries as my starting battery...

 

[dnmun]

use a DC/DC converter to charge your starting 12V battery so you don't unbalance the pack even more than it will be since this is gonna be such an expensive battery that losing 4 or 5 cells would cost more than the DC converter and starting battery. but you could save one of the old optimas and use it for the starting battery.