Universal Battery Module for Solar, Lawn, Recreational EVs

Batteries, Chargers, and Battery Management Systems.
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AmpEater   10 W

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Joined: Mar 19 2008 11:09pm

Universal Battery Module for Solar, Lawn, Recreational EVs

Post by AmpEater » Mar 03 2015 3:56pm

Every tool/vehicle I own has been converted over to electric operation. I don't need to explain to you guys why, the smooth silent power of an electric motor speaks for itself.

I have a zero turn mower with twin motors (one cut, one drive), a vintage wheelhorse garden tractor, a small narrow-deck riding mower, a snow blower, a wood chipper, and plans this summer for an e-UTV and electric boat. Converting something from an engine to a motor is easy, the hard part is the battery. And when you've got multiple tools the cost of an individual battery for each can grow unreasonable quickly.

My solution to this has been to use a single battery pack for everything. In the past I've had a 2.5kWh pack made of Lipo (~35 pounds?), a 4 kWh pack made of Enerdel blocks (~100 pounds), and most recently I've been using a pack made of used Leaf cells 2.5kWh (~55 pounds) or 5kWh (~110 pounds). None of them have been ideal, but all provided the ability to share a single battery between multiple tools.

Recently I've been thinking about Tesla's approach to building a battery. All of my attempts have been constrained by the physical dimensions of the cells. And since the pack has to be manipulated by a human the total energy is constrained by the pack weight. By adopting Tesla's model the size/shape of the pack is highly configurable and the energy content of a managable pack goes up by almost a factor of 2.

My current design involves 420 18650 cells mechanically configured as sheet 20x21. The dimensions would be approx 14"x15"x3" thick. This should package well into the "engine bay" of most small equipment. Electrically they would be 30P14S for a 48v pack. If 3400mah cells were used the total capacity would fall right around 5kWh which is, IMHO, amazing for a 45 pound package that is just 1/3 of a cubic foot.

By adopting such a compact package it would open up the possibility of using multiple packs to customize run time based on need. A homeowner with a small yard could use just 1 while a commercial customer with a multi-hour use case could fit 4 or more. If longer operation is needed the packs could be quickly swapped out rather than waiting for a recharge.

At this point I'm going to move forward with the creation of at least one prototype for personal use. I'll be modifying all my existing tools to share this common pack. I'd also like to modify my 10kw solar setup to accept a number of these modules in place of the 16kwh CALB pack I'm currently using. That way I can charge the modules directly from my solar panels rather than the currently inefficient process of solar - lifepo - inverter - charger - tool battery. In this way I can achieve a greater return on investment from owned batteries, solar battery can be used for mobile tasks, and mobile batteries can become part of the solar pack when not in use.

I'm got a few things to work out before I can build my prototype;
Packaging. I'd like to use a metal case for durability but how do I insulate the cells from the metal without insulating them thermally? I'd also like to minimize unnecessary bulk so that I can keep energy density as high as possible. An extra 3/4" of casing could be the difference between fitting 2 modules or 3 in a given space.
Sourcing. I'm surprised at how expensive small quantities of 18650 cells are despite their popularity. Most prices I see are $6/cell+ when my understanding is that Tesla gets them for ~200/kwh which would be closer to $2/cell. I understand they purchase in huge volume but I'd still like to get a better price. I've been on alibaba looking at suppliers and some have prices closer to what I'd expect but that leads me to my next problem
Cell Quality. How much quality/performance would be sacrificed going with a non-Panasonic cell? Because I'd be looking at such a large number of cells per module the consistency of cells becomes important from a reliability standpoint. I'd love to source the same cells that Tesla uses but it's seemingly pretty difficult.

I think having a standardized module would be revolutionary for all sorts of industries. It could bring the price of solar down by allowing a user-customized capacity, it would open up a new market for seasonal-use electric vehicles like snowmobiles or boats where the fixed battery costs are currently prohibitive. And it would decouple the battery from the equipment allowing the manufacturer of something like a lawn mower or golf cart to focus on improving the reliability or cost and not designing another custom battery pack.

I'd love to hear thoughts or suggestions on the concept.

Hillhater   100 GW

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Re: Universal Battery Module for Solar, Lawn, Recreational E

Post by Hillhater » Mar 03 2015 5:20pm

Have you seen the post in the sale section for genuine Tesla ModS pack modules ?
440 cells in a module for ~ $1900
http://endless-sphere.com/forums/viewto ... =9&t=66805
This forum owes its existence to Justin of ebikes.ca

AmpEater   10 W

10 W
Posts: 91
Joined: Mar 19 2008 11:09pm

Re: Universal Battery Module for Solar, Lawn, Recreational E

Post by AmpEater » Mar 03 2015 5:31pm

[quote="Hillhater"]Have you seen the post in the sale section for genuine Tesla ModS pack modules ?
Interesting! That would save me the trouble of finding cells as good as Tesla's, as least for the first prototype. I'm not sure about the bare metal cans or lack of per cell PTC protection, I was hoping to figure out a means of assembly less complex than tesla's laser cut conductors and ultrasonic-welded fuses. Either way I contacted the seller. Thanks for the tip

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