Ebike upgrade

I just ordered 30, LiON poly cells from Eric Powers to replace the lead-acid battery in this puppy:



Shortly after I got it, I had to build my own charger:





The original charger had a restriction on being plugged-in and connected to the battery. My version has no such restrictions!

I tried to get some technical data . . . no luck!



It uses a heavy, 48V, lead-acid pack rated at 10 miles with a standard, 170 lb rider. I went 7 miles a couple of months ago when I was at 270 lbs. It is also speed limited to 25 mph BUT I'll look at the drive chain and maybe do something ... interesting.

So what I'll do is build two strings:

• 15 cell, 3.3 V, 49.5v
• Operate on first string until it is exhausted
• Switch to second string and head home

There are excellent battery management chips available so I'll configure the battery with a pair of built-in, cell balancing, circuits. Once I get an idea of the actual energy use, I may look at solar charging options.

Bob Wilson

 

I am planning a dual-stack design with an integrated battery management system (BMS):

The dual stack, A123 poly batteries, will significantly improve performance:

• 10 miles one-way - existing lead-acid battery assumes the battery level indicator is accurate (HAHAHAHA!)
• ~13-15 miles one-way on one stack - switch to the second and return home. Expected charge 250-300W in about 4 hours for one stack. So I can ride to work on one stack; plug-in to recharge that stack; run other errands as needed, and; if first stack goes dead, switch to second stack and 'go home' to get the Prius.

I just realize this eBike will have nearly the same, straight-line, electric vehicle range as today's Volt.

Bob Wilson

 

Hi Evan,

I remember a smaller number, 10 miles but you thin folks probably can get 20 miles. <grins>

In April I rode a little over 3 miles to Airport Road to pickup some Chinese and back . . . into a power outage. A little over 6 miles, at the end I was pedaling but I had to use the lights more than expected.

These are Eric Power's cells running just under $30 each. My 30 cells are coming in at $810, delivered. I expect to spend another $200 for the battery management system. Here are the rough, expected characteristics:

	Column 1	Column 2
0	[th]length[th]width[th]depth[th]lbs[th]Wh[tr][td]17"[td]4.125"[td]7.5"[td]38 lbs[td]~700 Wh[tr][td]17.9"[td]4.26"[td]6.3"[td]32 lbs[td]1	850 Wh

NOTE: packaging not included in LiON cell calculations. Estimated energy based upon weight of assembled batteries and this source, Battery energy storage in various battery types.

There are still packaging challenges but saving 5 lbs and gaining about 2.6 greater range, a 13 mile radius, makes it well worth it. More importantly, being able to run half the pack to full exhaustion makes range anxiety moot.

We won't know the thermal issues until after the cells arrive (shipping delayed), packaging, and testing. I'm expecting a 4-8F increase in temperature during a 15A discharge which is ~1C for the measured 17-18Ahr capacity over at Ecomodder (Daox picked some up at GreenCarExpo.) I'm intentionally designing the charge circuit to run at 0.33C (~5A) to further reduce the heat load. Having two strings means another option that a single string pack lacks. <wink>

Bob Wilson

 

The eBike is more of an electric moped with pedal assist. The A123 cells will hopefully provide range to work on one string with the second one provides 'an out' if I run out. The maximum current draw should be 15A so we're looking at less than 1C discharge and a 0.33C maximum charge rate. In balance mode, 1A, we're looking at less than 0.07C.

I'm having difficultly finding detailed engineering data on these cells from A123. I'll try again on Monday but I'm not holding my breath. So one thing the eBike does is provide field metrics about how well these cells work. I'll also learn more about battery management systems. Dumping the data records will give metrics not otherwise available.

Bob Wilson

 

That had been my experience, only twice this year for useful, short errands:

• April - rode to Chinese restaurant about 3 miles away at Airport Road. Airport Road is the nearest 'restaurant row' where I can get take-out to bring home.
• July - my Fluke DC probe was not working so I dragged the battery to the eBike and ran to Radio Shack to get a replacement. The eBike made it a quick errand and I got back to work on the emergency inverter. Later I brought the battery back in the house for the charger.

If I had to do this solo, I would have passed but Daox at Ecomodder had bought some and started analysis.

Bob Wilson

 

I appreciate the simple solution but it is also driven by the available eBike:

Per 'specs,' each A123 cell is rated 65Wh and the flat, ploy format means maximum density. Five cells, just under 18V, should give 325Wh. My 30 cell, two-string pack might have 1,950Wh if the specs are correct and everything else works. (Somewhere Murphy is preparing a 'clue by four' and God is laughing.) But sometimes you have to 'look through the telescope.'

Bob Wilson

 

Happy DAY!

I was planning to spot weld the battery tabs to make a quality, high-current, interface. Then I read about a capacitive discharge system and another problem got solved.

I need to fabricate an engine mount for my Dragonfly (airplane) and the traditional approach is to cut tube; spot weld, and; professional welder complete the welds. But it looks like a capacitive discharge welder will be the perfect spot welder for this task.

A capacitive discharge, spot welder makes a small, localize, but mechanically welded joint. It only has to be strong enough to overcome gravity. If there is a problem, a hammer tap breaks the weld, reposition, and do it again. So I can easily assemble the two-part, engine mount, and get the dimensions exactly right.

Synergy, when one solution solves more than one problem!

Bob Wilson

 

Sad to say, the Chinese suppy at the original price dried up. There were some other, secondary grade cells but my eBike has a lot of vibration and temperature changes. I would never be able to tell if it were the lower grade cells or the eBike that might cause a problem. However, I have my original plan . . . some NHW11 modules.

Bob Wilson

 

Going with the older, NHW11 modules changes the charger design:

• Zener driven power transistors - at the upper limit, 8V, the 4W/module, 0.5A, would burn out ordinary zeners. Instead, it will turn on a power transistor and dump the shunt current into a power resistor, 5W.
• two-rate, constant current, 2.5A and 0.5A - a discharged pack will take the higher current until one module reaches 8V. Then the shunt will kick in and the power supply will scale back to 0.5A. This will let each module reach 8V safely, reducing the temperature gain and heat. As each module reaches 8V, the shunt regulators will by-pass that module preventing overcharge. I will need one shunt stage per module unless I decide to treat them as pairs or triplets.
• MSP430 control - this microprocessor will manage everything from sensing the current; pack voltage; and driving the switching, buck regulator. Software controls everything giving flexibility to optimize the charge and balance.

As long as we start with balanced modules and the shunt regulator works as expected, I may be able to get by with just module pairs or triplet monitoring on the discharge. The nice thing is I have 38 perfectly fine . . . and free . . . NHW11 modules. This allows me to be a little rougher than ordinary, warranty supported batteries. <grins>

Bob Wilson