Recently in Batteries / Power Systems Category

I'm thinking of getting a new battery system for my EV conversion. Right now I'm using a Prius Gen 2 NiMH pack in a 6S2P setup (43.2 volts nominal @ 13 Ah), which cost me $144 in December of 2009. I can conservatively pull about 330 watt-Hours, so it works out to 43 cents per Watt-hour.

(Nominally it is 561 Wh for the pack, but there is considerable voltage sag at the end which reduces the amount of energy I can extract). The nominal price/performance is 26 c/Wh….

Let's price out some LiPo solutions….

18650 cell (single)LiPo$42.4 Ah3.7V 8.8 Wh 45c / Wh
18650 cell (single)LiPo$72.8 Ah3.7V 10.36 Wh 68c / Wh
Zippy Flightmax 8000 mAh 6S1P 30CLiPo$888 Ah22V178Wh 49c / Wh
Zippy Flightmax 8000 mAh 5S1P 30CLiPo$748 Ah18.5V148Wh 50c / Wh
Zippy Flightmax 8000 mAh 4S1P 30CLiPo$588 Ah14.8V118Wh 49c / Wh
Zippy Flightmax 8000 mAh 2S1P 30CLiPo$288 Ah7.4V59Wh 48c / Wh
Turnigy 5800 mAh 8S 25CLiPo$1005.8 Ah29.6V172 Wh 58c / Wh
Zippy 5000 mAh 10S 25CLiPo$1065 Ah37V185 Wh 57c / Wh

I checked the prices in May (2013.05.20) and there was no change in prices. But there are some bigger cells coming on the market which might change things. This is from

26650 cell (single)LiPo$7.606.0 Ah3.7V 22.2 Wh 34c / Wh

I had hooked up one of my larger SLA batteries to a winch to move some logs around. I thought that the 30 amp PowerPole connectors on the battery leads were maybe a bit undersized for the winch, but was lazy and went with them anyway.


I originally thought one of the PowerPoles wasn't properly seated and it melted down. That's perhaps one of the problems with the small PowerPoles, there isn't a clean "click" confirmation of seating.


Closer examination of one of the melted PowerPoles showed however that the tongue that grips the connector had arcing on it; obviously it had shorted and overheated, causing a melting of the connector. Very strange.


Closer examination of the crimped connector showed that the connector itself was bent upwards. What I now think happened was that the main body of the plug was bent and not seated in the plastic case, and this caused the mating connector's plug to wedge itself between the tongue and the plug (instead of on top of the plug), causing sparking and overheating.

DealExtreme is one of my favorite online stores. It's a distributor of inexpensive electronic gadgets based in China. I'm always finding something new there. The latest treasure is this little-but-very-bright bare LED: 10WattLEDsku_5876_1.jpg

DealExtreme lists it as a 10 watt LED (SKU 5876). Unbelievably it's just under $12 with shipping included!

Looking at the die shows that it is 9 discrete high-powered white LEDs in a single package. DealExtreme is bad about specs, but the comments in the DX forum seem to suggest that 700 mA at 12 volts is a reasonable spec for this LED. This would yield 8.4 watts.

(I'm wondering though if it isn't 3 x 350 mA @ 3.5 serial LEDs in a 3 parallel strings, which would be 1050 mA @ 10.5 volts. But for now, I'll run it at 700 mA).

DealExtreme lists it as 500-600 lumens @ 6500K color temperature.

As with most LEDs, you need a good current regulated driver circuit since you can't just run these things off a resistor. I decided that the easiest and simplest driver would be one based off the amazingly versatile LM317 chip.

As before, these sites have good javascript based circuit diagrams for calculating LED driver circuitry:

Plugging my values (700 mA) into them yielded the need for a 1.8 ohm resistor with my LM317. Here's the schematic that I designed around those figures (courtesy of ExpressPCH):


Bodged together and plugged into a li-ion pack from my model helicopter and voila, an amazing amount of light. I'm thinking of using it on the headlight of my Piaggio (which currently uses a 3-watt LED) or to replace the bulb on my old 15-watt Niterider headlight, which has seen happier days.


(More photos and photometric testing after the jump)

The weather was finally nice enough to commute to work this week on my Piaggio Boxer EV with Prius NiMH batteries.

Here's the data from my CycleAnalyst:

Run #1
Run #2
Run #3
Run #5
Distance5.2 km4.85 km4.21 km4.19 km
Efficiency45.1 Wh/km39.2 Wh/km42.2 Wh/km47.6 Wh/km
Energy used234.23 Wh189.87 Wh176.92 Wh199.03 Wh
Charge Used5.90 Ah4.28 Ah4.01 Ah5.01 Ah
Max Amps106 A101 A91 A101 A
Average Speed20.6 km/h23.3 km/h24.8 km/h24.1 km/h
Max Speed36.6 km/h40.3 km/h39.0 km/h36.6 km/h
Starting voltage---49.950.247.2
Ending voltage43.8v45.9v---44.2v
Run time15 min12:3010:1010:24

My commute is slightly uphill on the way to work and downhill on the way back, which accounts for the difference in energy efficiency going to and from work.

The bike feels much lighter than with the SLAs and faster too (even with the gear reduction) so I have to say it's an unqualified success. I just hope I can get good life out of these batteries.

After Run #4, I was in a rush and so I put the charger on and went to a talk and then came back. About 3 hours had passed and the charger had over charged the batteries. It actually wedged the battery holder apart. So I'm worried now that my batteries will be weakened -- even though most of the bulging has subsided.

I reinforced the battery holder this morning and we'll see how it holds up.

LED Voltmeter

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Found a great article that describes how to make a LED voltmeter using a chip designed specifically for that, the LM3914.

I'm working on a bicycle assist motor project. I thought about using a hub motor or chain drive but given the economy, I'm going with a friction drive. Friction drive is cheap, has few little moving parts to go wrong, and is cheap. I think I can make it using parts almost entirely found around the shop -- with the exception of the friction roller.

Now what I like about friction drive is if you use a roller with a one-way bearing and take advantage of some physics, the motor can release from the wheel entirely when freewheeling, so the bicycle remains entirely pedal-able on its own.

I decided to go with rollers from the defunct EV Warrior project. They're available on the surplus market, have one way bearings, and are nicely knurled. Other people are making their own friction rollers from BMX wheel hub extensions, but they don't have one-way bearings.

Here are some dimensions almost entirely for my own benefit.

Shaft OD0.500"12.70mm
Shaft ID0.315"
(a tad over 5/16")
Shaft Width3.016"76.61
Roller Width2.375"
2 3/8"
Roller OD1.275"
~1 1/4"
Key Notch Width0.130"3.32
Key Notch Depth0.411"10.4

In my search for info on calculating EV efficiency, came across this interesting article with java calculators:

The articles mention that standard EV car efficiency is around 4.4 kilometers (2.7 miles) per kilowatt-hour.

My little scooter gets around 40 watt-hours per kilometer on SLA (I need to get new figures for the 43.2 volt NiMH packs). Flipping that around, that's 25 km per kWh. That's around 16 miles per kWh.

PriusGenIINiMH.pngI just won a bid for twelve Prius packs! They should hopefully be arriving later this week or early next.

Update: Batteries arrived and ready to be installed!

Each pack is a 7.2 volts 6.5 Ah prismatic NiMH battery with six cells. I'll be configuring them as 6S2P for 43.2 volts 13 Ah. I'm hoping that I can get at least 60% of the SOC from them (80% to 20% as on the Prius). That would be 43 volts, 7.8 Ah or 335 watt-hours.

This is perfect as I'm currently consuming 200 watts-hours each way with the heavy SLAs. The lighter NiMH packs should get my power consumption down even lower.

SLATempest TR35-12 (rated 35Ah)
-- 35 Ah @ 36 volts
-- I don't get 35Ah, closer to 10 Ah due to Peukerts

Specific Energy 35 Wh/kg nominal
Specific Energy 10 Wh/kg derated
360 Wh 1 @ 11.8 kg
3 @ 35.4 kg (78 lb)
NiMHPrius Gen II Battery (rated 6.5Ah) @ 1040 grams
-- 13 Ah @ 43.2 volts
-- derated 60% for a conservative Ah of 7.8 Ah

Module Weight 1040 g
Specific Power 1300 W/kg

Specific Energy 46 Wh/kg nominal
Specific Energy 27 Wh/kg derated
335 Wh 1 @ 1.04 kg
12 @ 12.48 kg (27.5 lb)

It looks like I'll be saving a good 20 kilograms or 50 pounds with only a slight loss in Watt-hours! I will have to bodge up a new battery carrier as the cells need to be kept under compression when being charged.

Revised bill of materials (BOM) (dated 2009.09):

[moved to after the jump]

This is the post-flameout, rebuilt Piaggio Boxer EV. I changed the battery carrier from a top-mount system to saddle bags. This lowers the center of gravity and makes it easier to handle. I'm going to replace the seat with something more classic looking.


I welded the saddlebag carriers myself from steel tubing with my MIG welder.


The Electronic Speed Controller (ESC) is sitting on top. It's the replacement HV-110 that I received from Castle Creations with additional capacitors soldered in parallel on the input lines.


It's sitting in a Lexan enclosure that I bodged together. Unlike the old metal enclosure with a single temperature controlled fan, the new enclosure uses two fixed speed fans that pull the air across the ESC and the voltage converters (sitting below). They also are assisted by draft air when the vehicle is at speed.


This photo was taken a few minutes after a short run, the temp of the caps is 34 centigrade which is nominal. Anything less than 60 centigrade I think will be ok. The caps are rated at 85C.

Although the angle of the photo above makes it look like the ESC could short out on the voltage converters, it's actually held a centimeter or so above and everything is well insulated.

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