Real Life Example of a Really Dumb Design

Recently, a long time Robobiker was involved in an electric vehicle project (NEV) where the selection of the battery was not given enough of a priority. It was decided that six 12 volt batteries, each weighing 62 pounds, would fit nicely into the available space and would likewise fit well within the budget. So far, so good. However, things soon went wrong. Those of us with battery power experience did a mental calculation and concluded that this battery pack of six 12 volt batteries wired in series was capable of delivering 30 amps for 90 minutes and if charged properly, a reasonable life of 500 charge/discharge cycles could be expected. Nothing but good news. The ball park estimate is 72 volts x 30 amps x 1.5 hours = 3240 watt-hours. Two hundred watt-hrs per mile @ 25 mph = 16 miles range. These are good numbers for a vehicle such as this running on level ground at a reasonable speed.

As the design evolved, it was further decided that it would be nice to use a boost controller (72 volts in 300 volts out) to deliver 300 volts to the motor which, because of its inductive properties, would operate more efficiently at a higher voltage. Boost control impacts the energy budget with a 10% loss in efficiency. The gain in motor efficiency probably doesn’t offset the loss created by boosting but OK, let it go. Then it was decided that the controller should deliver 300 amps at the boosted voltage. That is 90 kw plus the already mentioned loss for a total energy demand of 99 kw. Peukert, here we come.

Why the original plan was changed is difficult to rationalize except to say that it was changed by a design engineer with no experience with batteries. Google, while a good source of information, is a poor source of knowledge. Google played a large part in the series of events just described. Apparently, one of the design team found a link to a battery powered drag racer and used that as evidence of fantastic performance. Needless to say, the NEV project was not successful.

 Let’s go through a reasonable selection process as it applies to a typical ebike application.

The design constraints are a bike with 20+ mile range at 30 mph, easy to control, good acceleration, long lived batteries and moderate hill climbing performance. By actual measurement, we budget 30 watt-hrs per mile at this speed. Included in this estimate is a generous reserve to ensure long battery life. Thirty mph requires 500 watts of power. This is a real fast bike and is not street legal in most states. Other inputs Cd = 1.2, FPEA= 1.5 ft², rolling friction coefficient .005. These inputs are all worse case.

 

 

The above illustration shows a schematic representation of the energy budget of our high speed electric bike application. Keep in mind the chemistry under discussion is lead acid. We will discuss other chemistries later. The battery under test weighs 31.5 pounds. Sounds like a lot but it’s nothing for the bike we built for the test. At this discharge rate (15 amps), a new battery this big will deliver 15 amps for less than a half hour, will travel 10 miles on a full charge and move along at a brisk pace at first. Battery life poor. Triple the battery to 95 pounds (again, not a problem for the electric limo), then the 15 amps can be supplied for 2.3 hours, or 4.5 times as long. A 20 mile range gets you to less than 50% depth of discharge guaranteeing good life and most important where I live, will operate in cold weather. Triple the battery and get four and a half times the range. These results are from actual test data done at Robobike. They approximately equal the published data from individuals, battery manufacturers and other users. It all depends on things such as temperature, condition, state of charge, etc etc. Remember, the Electrathoners go fifty miles for an hour on 64 lbs (30 kg) of lead acid. In spite of what you think, SLA is still a valid choice for energy storage.

How can a threefold increase in battery mass result in a nearly fivefold increase in range? The answer is Peukert factor, an exponent added to the discharge calculation. We won’t try to explain that here but ask anyone with experience in the field and you will get similar answers. That is why, in the opening example regarding specifying the battery for the NEV, those of us with battery experience were sitting in disbelief when those without experience drove the design to ridiculous expectations.

The following illustration represents a good model for a discussion about the Peukert factor as applied to a high discharge application such as an electric vehicle. When the test is begun with a fully charged battery, the meters read high. As long as the coiled resistance wire controls the current to a reasonable value (high resistance), the battery is able to maintain a high rate for a long period of time. If the resistance is reduced, that is, the discharge rate increases, the current starts even higher but begins to drop almost immediately. The voltage drops too. Multiply the voltage by the amps to get watts. Sample the results until you get to the low voltage cutoff point, say 1.8 volts per cell. Do this for both discharge rates. When you are finished, it is easy to see that the battery delivers more watt-hrs at the lower discharge rate. That’s your Peukert factor.  Most batteries don’t die a natural death of old age, they are killed by negligence. A proper load is a load that allows for a smooth continuous discharge to 50% depth of discharge over a period of one hour. Any faster than that and you will kill your batteries in no time.

 

 

Peukert really kills batteries and battery performance. Damn Peukert. Time to switch to Lithium. We have not conducted tests and are unable to report on the effects of discharge rate on battery life. We can Google the subject and fill our pockets with information and try to pass ourselves off as experts, but we’ll leave that to the know-it-alls. Robobike has equipped our monster bike with 600 watt-hrs of lithium cells. Since no reserve is required, we expect to get 40 miles range at 20 mph. A measurement is worth a whole bucket full of opinions. We also expect to add a BMS to the system, a processor based total bike management system, and an updated charger. Stay tuned for updates on building a lithium system from scratch.