T-Pain Is On A Boat: [Electric Boats] modifying electric garden tool battery packs for trolling motor

Tuesday, June 2, 2015

[Electric Boats] modifying electric garden tool battery packs for trolling motor

Hi group,

I recently acquired a 12V trolling motor (Canadian Tire Duramaxx Drifter
II) for my inflatable dinghy.
Normally, I would prefer to build a 12V pack out of the same cells as my
main boat's 48V propulsion pack, the Sinopoly SP-LFP100AHA 100Ah cells.
However, at $145ea, plus shipping and BMS, that makes for a pretty
expensive battery pack for the dinghy. I did recently notice the
Greenworks 40V lithium ion battery
(http://www.canadiantire.ca/en/pdp/greenworks-40v-lithium-ion-battery-0601758p.html#.VW3O_MVZB8M) which is a 40V, 4AHr lithium ion pack for garden tools, etc.
Now, that is only 160WHrs of power. Not a lot by any stretch. But at
$119CDN with free i-store pickup, it certainly will give me enough for
messing about and it doesn't weigh much at all -- about 2.5lb shipping
weight. This works out to about $0.75/wh, which is a very respectable
price for a complete battery pack, including BMS.

Reviews of these battery packs seem to indicate that users get about 45
minutes of operation in their snow blowers. That means that the pack is
delivering ~200W of power for 45 minutes. This works out to about 5.3A
draw, or about 1.3C I am going to assume that the cells can be worked
at at least 2C safely. This gives me a load max of 8A per cell.

Charging:
With an adequate BMS attached, charging is simply a matter of connecting
a CC/CV power supply set to the proper voltage and current limits, and
allowing the BMS to cut it off at the proper time. I can get such a
supply to step down my 48V pack to 16.8V with a 3A limit for about $18.
This should completely recharge a pack in about 1.5Hrs, which is more
than acceptable.

My trolling motor is 12V so some repackaging of the cells will be
required.
At full power, 38lbs of thrust, my motor will draw 40A@12V or 480W.
I found a 30A capable, 4-series lithium ion BMS board for about $8.
3.7V cells in a 4-series string produces 14.8V.
To produce 480W at this voltage will require 32.4A of current.
In order to fit in the 8A max draw per cell, I would need to double up
on my battery packs.
I could limit my use of the throttle to 15A in order to get by with a
single pack. Good enough for simple ship-shore commuting and a runtime
of 30-45 minutes.

The motor was given to me. So it came at zero cost, but I actually
really like the fact that there are more and more lithium battery packs
of decent size and low cost coming on the market. I even saw an 80V
pack, albeit only 2AHr capacity, for the same 160WHr. The price was
much higher, however. All the manufacturers are probably using the same
4000mah cells internally. The packaging looks to be 3 rows of 4 cells,
but at 3.7V/cell, the math doesn't work out. 11 cells would be a closer
fit. So perhaps it 4-3-4, and the missing space is used by BMS
electronics.

In an ideal world, I would have a trolling motor that could handle the
40V directly, but I am trying to use what I have on hand. If ever I
have to replace my trolling motor, it will certainly be for one that
runs at higher voltage. Especially now that these battery packs are
readily available. At the very least, it'll be a fun side-project.

/Jason

--
Jason Taylor
--
S/V Fugu
1978 Beneteau First 30
Electroprop PM-20