Letter to Twike Owners from Ralph Schnyder, TWIKE AG
29 April 1998

[Provided by Walter Breitinger, translated by Katie Trumpener, translation revised by Victor Muñoz.]

Dear Twike drivers,

The first Twikes have been in service two years now. A few of them have already surpassed 40,000 km and many over 20,000 km. The battery system which was specially designed for the Twike has held up extraordinarily well. Apart from a few software problems with the temperature and voltage measurements, we don’t have any serious problems to report.

Nonetheless, the life expectancy of the battery cells is limited, and the first Twikes have already reached the expected life of a 1000 cycles, which corresponds to around 2400 Ahs per module or 35,000 km per two battery modules or 50,000 km per three modules. This is the life expectancy which was given by the manufacturer and was confirmed by us in laboratory tests.

The figures means that the battery amortization costs are around 0.13 to 0.17 Swiss francs/km. The cost of each battery module replacement can run to 2300 sf, not including the dismantling and rebuilding or reinstallation. The old batteries can be turned in at all official Twike Centers where they can be recycled. An anticipatory recycling fee was factored in when the batteries were sold.

Despite the positive balance sheet, the following points should be heeded in order to maximize their cost-effectiveness:

Ni-Cd batteries are known to have a memory effect which results in diminished capacity. But with proper conditioning the effect can be largely erased. The effect becomes evident when the batteries are not discharged below 315 V or are always used at less than 50% of capacity. In order to avoid this effect it is important not to recharge the batteries until they are completely empty or until the red battery warning lamp lights up during the first acceleration stage or cruise control.

Batteries affected can be restored to capacity through repeated complete emptying and recharging. This should happen one to two times per month, especially when the vehicle is being used for short stretches. The final stages of a deep discharging (below 300 V) can be accomplished by turning on lights, but this procedure should only be carried out when

• A complete recharging – including the symmetry stage – is to be carried out immediately afterwards. (See section 2 below on the charging procedure.) If the computer display reads "Unsymmetrical Battery Voltage" break off the discharging and proceed through a complete charging including the symmetry stage.
• Only use the low beams and tail lights for deep discharging, never the brake lights, blinkers, and fog lights in combination for long periods as this could overheat the DC/DC converter.
• As soon as the voltage dips below 280V or when the lights begin to blink, immediately shutoff all lights and begin normal charging. If you wait too long before charging you may reach a state of discharge so deep the battery software may be lost. Although software version 5.10 is identified by defective temperature indicators in the EPROM, in rare cases a software loss can lead to complications.

The Twike’s charging procedure can be carried out using different parameters. The following settings have shown themselves to be optimal.

Ni-Cd batteries can be charged without damage or disadvantage in 1-2 hours. However, for this to work, the outlet should be properly rated and fused. Charging with more than 10A is not allowed with Swiss plugs and sockets; they are not designed for this amount of current and can heat up considerably and pose a risk of fire.

Even when plugs are used which are rated for 16A (for instance, the German Schuko or blue IEC 309 plug) current should be limited to a maximum of 12A. In the last two years there have been a number of cases of meltdown of 800 plugs at the outlet which could only in part be explained by bad crimpings or manufacturing flaws. Although these plugs are rated up to 16A, apparently the rather small contact surfaces lead to heightened contact resistance and, consequently, overheating and a fire hazard. The vehicles are delivered with a default work setting (component setting) of 10A or 5A. Charging the Twike at a higher setting will occur at the sole risk of the client. We cannot make promises or offer guarantees or compensate for damage that may occur as a result.

With the new software version 5.10, the intermediate charging mode is only meaningful when you intend to drive on immediately and the batteries will not be heated too much. In temperatures under 30C we recommend the Normal Charge mode. Even then the current can be raised to 14A. After a Quick Charge the new software will no longer complete a symmetrical charging phase. Therefore, you should either drive on immediately or turn everything off so that the inverter/controller does not partially re-discharge the batteries.

If you have 7 to 10 hours to spare, the charging current can be reduced to 3A by setting the charge mode to Normal. This will insure the batteries will be charged more evenly and completely by morning or the end of the period. During this charge mode the JOG light will blink once each second.

After an overnight charging the cells in each module should be equalized. Slight differences of charge among individual blocks is nonetheless possible because of temperature differences, caliber variances, and individual defective cells.

Symmetrical charging equalizes the capacity differences among the 280 cells connected in series in each module. A small amount of current is all that is required or safe to fully bring even the last cell to a complete state of charge. Cells that are already full will convert the excess charge current and no longer storable energy to heat. The battery may then warm up as much as 2-5 C. When the cells are new and their capacity differences are slight, symmetrical charging should take up no more than an hour. As the cells get older and the capacity differences become more pronounced, this time may extend to 11/2 or 2 hours. Under the following conditions we recommend increasing the symmetry charging time:

• At each 10,000 km service.
• If during the symmetry phase the temperature does not rise.
• If you often get the message "Unsymmetrical Battery Voltage" with almost empty batteries.

For situations in which the Twike is to be charged unattended or when it cannot be manually unplugged within at most 10 hours, we have developed a timer supplement. The supplement permits you to insert a week-long timer switch between the charge cord plug and the wall socket. This lets you, for instance, while on vacation, charge the Twike once a week for two hours or program the charge period to end at a certain time. The supplement can be built-in at all Twike Centers. Without this supplement, it can be detrimental to the batteries (memory effect) to spend more than 10 hours charging, as well as being a waste of electricity.

Our experience shows that, per module, about 1 cell stops functioning every 10,000 km. To some extent this is not serious since the cells will then make an internal short, and thus the battery remains fully functional. A voltage difference of 1.2 V, given an overall voltage of 336, is hardly noticeable.

The causes of cell death lie, on the one hand, with the manufacturing finishing tolerances and, on the other, the fact that the inherently weaker cells are more susceptible to cell reversal under deep discharge because they become empty before the others. The tolerances become increasingly more evident as the cells age. Hence, symmetrical charging and charging with a lower voltage becomes also increasingly more important.

When two or more cells drop out per module the following can occur: The difference of voltage leads to an imbalance between battery modules during partial discharging and the cycles become smaller as a consequence, which increases memory effect and results in the module becoming even weaker. The lower total voltage leads to a smaller transfer charge rate between modules, 0.05 to 0.01A. The relay does not open properly between the batteries. A loss incurred through a self-discharge rate of 15% or more a day leads to a condition in which the batteries discharge one another within a few days. The result, of course, is a greater necessity to recharge and a strengthening of the memory effect.

To minimize cell death the following measures should be taken:

• Do not discharge to a voltage below 280 V if immediately before the batteries have not been fully charged overnight.
• After a deep discharge immediately recharge again.
• Regularly drive until the batteries are completely empty, that is, until the voltage has fallen under 300V.

Naturally, we are fully engaged in insuring that the Twike retains its reliability and therefore have instituted these measures through our Twike Service Partners.

At every 5,000 km service a complete charging must be carried out overnight. After that the battery is run completely empty and then completely discharged. On the second night it is completely recharged again. This means, of course, that the Twike has to stay in service at least two days.

Every 10,000 km the batteries are tested with a PC after a full charge and defective cells are replaced. Special training for carrying this out is provided to the Twike Centers. The costs for cell replacement run from 200 to 400 sf in addition to the normal service charge. It is, however, worth doing because since the life expectancy of the entire battery is significantly extended.

4.2 Service Stations

The development of a special Twike battery service station will be started as soon as the means are available for it. It will permit a battery module to be cycled automatically and under controlled conditions 2-5 times, thus erasing the memory effect and evening out cell disparities.

We hope through this set of instructions, though somewhat complicated, to have contributed to the Twike’s overall performance and reliability – and thus to its continuing to lead the way to the future.

©TWIKE AG
http://www.twike.ch

Translation Copyright © Victor Muñoz