Best Way to Convert 4S Lipo voltage to 13.8v SLA power?

[sikeston34m]

Hi Guys,

I've been researching various ways to convert 4S Lipo Power (16.8v) to 13.8v. 13.8v is typical for 12v sealed lead acid batteries.

Purpose:

I'm tired of replacing the deep cycle sealed lead acid batteries that run the trolling motor in my boat. You might get 2 years out of a regular deep cycle battery.

So 3 years ago, I replaced the tired deep cycle battery with 2 - 6 volt golf kart batteries. They are rated for 5 years of service, but I can already tell, they are getting tired.

Solution:

Build a 4S pack from 18650 cells. (or other cost effective cells)

Problem:

16.8v is not compatible with the system.

I have to come up with a way to step down the voltage to the 13.8v level and maintain this level of amp draw.

My trolling motor is a 12 volt 55lb thrust Minnekota. A general rule of thumb is 1 amp for each pound of thrust.

So, we have to figure on 55 amps at full speed, plus some overhead for comfort.

I've been thinking about paralleling several of the Vicor MegaMod converters for a total of 800 watts of converted power, BUT the units I'm looking at, have an input range of 18 to 36 volts. This would mean a 6S pack would be required to power this unit.

https://docs.google.com/fileview?id=...NTkwYjJm&hl=en

Question: If we are drawing 55 amps from the 13.6v low side, will 55 amps be required on the 6S high voltage side? Probably not, but I still wonder, since voltage conversion is being done with high frequency switching.

Is there a better way to go about this?

I've already checked into 10ah Lifepo4 cells, and just don't like the discharge curve.

By using a DC to DC converter, a steady 13.8 volts can be fed to the motor allowing that fresh, full power feel, all the way to the end of the charge. Thus, improving performance, even over that of a brand new deep cycle SLA battery.

Any ideas? Suggestions?

Let's talk electrical. Feel free to post here guys.

Thanks.

[lincpimp]

A123 cells sound like exactly what you want. 3.3v nominal 3.6 fully charged. 1000 cycle life or so. Since it appears you will be charging them with a 12v style charger they will not reach full charge in a 4s config, but will likely last longer.

I had planned to make a 4s3p pack for my motorcycle, since the charging system should not exceed the max charge voltage.

Plus a123 are much more robust and can handle lower discharge better than lipo. Only downfall is weight and the round cell dimension. Neither should be an issue for your purpose.

[sikeston34m]

Originally Posted by lincpimp

A123 cells sound like exactly what you want. 3.3v nominal 3.6 fully charged. 1000 cycle life or so. Since it appears you will be charging them with a 12v style charger they will not reach full charge in a 4s config, but will likely last longer.

I had planned to make a 4s3p pack for my motorcycle, since the charging system should not exceed the max charge voltage.

Plus a123 are much more robust and can handle lower discharge better than lipo. Only downfall is weight and the round cell dimension. Neither should be an issue for your purpose.

I have considered the A123 26650 cell and even the 38120 10ah Headway Lifepo4 cell.

My problem is: the discharge curve for 4S Lifepo4.

When the 4S voltage sags below 12 volts, the motor is going to begin to feel tired.

It would probably stop operating altogether at 10 volts. So, despite the decreased performance, how much capacity is above 3 volts per cell?

A Lifepo4 pack would also be very costly, considering it needs to be a 40ah plus pack.

For a Lithium Ion pack, the hobby charger will be required. This is ok though.

With Lithium Ion, the biggest problem is the voltage converter. Hmmmmm...........