Tuesday, October 30, 2012

Batteries and Chargers

Yesterday I drove home from work at midnight and took the long way so I could do a top speed run. The Electric Booger achieved a top speed of 85 km/h (indicated). After the top speed run came the uphill. After the uphill I decided to drive around a bit more to see how the batteries performed. The result: they SUCK. BAD. My total drive was 12.0 km and for argument's sake we'll say 15.0 km if there was no top speed run and no uphill. By 3 km from home the batteries were sagging so bad that my top speed (trying not to let the voltage drop too low) was 50 km/h. Not fun.

Battery Math

I cannot find an amp-hr rating for these batteries but going by weight/size and comparing them to other batteries, they are each approx 100AH. That means I have 14,400 watt-hrs of theoretical energy. Factoring Peukert effect at 50%, that should give 7200 watt-hrs to 100% DOD. Assuming the eBooger consumes 300 watt-hr/mile (186 watt-hr/km), a 15 km trip will use 2796 watt-hrs of energy. Assuming I was down to 80% DOD last night, this means that my pack is only delivering 48% of the power I expected. Unfortunately I do not have an amp counter so I do not know how much power the Electric Booger is actually consuming. Maybe on today's commute I will install the remote inductive amp meter and take notes.

Battery Alternatives

I can get 100AH marine batteries from Interstate for cheap (~$100 each). But I have a sneaking suspicion that they will not be much better than the batteries I already have because like mine, they are not really designed for repeated deep cycles. What I would really like is AGM (absorbed glass matt) batteries, a la Optima Yellow or Blue tops because they do not sag like flooded batteries. Unfortunately these are expensive (~$275 each) and only last 200 cycles before they start crapping out. Lithiums would be the answer, but for a 77V 100AH pack (24 cells) with BMS (battery management system) and a proper charger we are talking $4000. Considering the eBooger is a beater to begin with, I don't really like that option. For now, at least.

Does anyone have any suggestions?

My Highly Inefficient Chargers

Yesterday I charged with my new Kill A Watt power consumption meter plugged in. My 2796 watt-hr drive home took 5070 watt-hrs out of the wall to charge. That's 55% efficient. And that's pathetic. They use 122 watts even after my batteries are fully charged and the chargers are doing nothing but making a humming noise. My round trip commute to work ended up being 627 watt-hr/mile or 54 mpge. Yikes. That's horrible, but still much better than it did with an ICE doing the job. Keep in mind these calculations are based on the assumption that the car averages 300 watt-hr/mile. I may know actual numbers after today's commute.

So the Electric Booger is doing the job, but I am a little bit unhappy with the way the car runs. That being said, I am only into the project for $2250, so should I complain?

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