Greetings to the readers of the site 2Schemes . A couple of years ago, the motorcycle battery failed and had to be replaced, it was decided to buy a lithium battery, expecting it to be light, reliable and durable. But despite the fact that it cost twice as much as a lead-acid battery, the battery did not turn the starter very confidently from the very beginning, and after the winter it completely weakened. So it’s time to take matters into your own hands and solve the problem once and for all. This is how the lithium motorcycle battery shown here was created.
Let’s start by measuring the inrush current. The current was measured with a Uni-t 203 clamp meter. The ammeter showed about 80 A when starting a cold engine. By the way, when starting a car (gasoline), it shows 200 A. Now, knowing about more or less real power consumption at startup, LiFePO4 A123 batteries were chosen, which have a series resistance of 6 mΩ and a pulsed current of 120 A.
The balancer is based on the classic TL431 + LM385 assembly. The circuit works as follows: as long as the voltage on the cell does not exceed 2.495 V (ref TL431) + about 1.24 V (ref LM385), the balancer is inactive. When the voltage gets higher, the TL431 starts to open up and allows current to flow between the cathode and anode. Indeed, at a voltage below 3.7 V, the balancer takes a zero-sequence current, at 3.72 V, the consumption current is 15 mA, and at 3.75 V, the current increases to 55 mA. At 4 V, the current is approximately 70 mA. The maximum current can be controlled by changing the value of the resistors R2, but you need to remember that the maximum current of the TL431 is 100 mA and this value cannot be exceeded.
The disadvantage of this balancer is the inactive current leakage. For 3.3 V it is 180 µA, for 3.6 V it is about 400 µA. The phenomenon can be combated with the LMV431/TLV431 (current consumption below 55 µA), but remember that its V-ref is 1.25 V and to maintain the performance of the balancer, you must use the LM385 in the 2.5 V version. In addition, choose resistors: R1 must pass through the LM385 at least 10 μA, and the resistance R2, for the same balancing current as above, should be increased by about 2 times.
With the current balancer configuration (200 µA leakage) and the nominal battery capacity of 2500 mA, the battery will run out in about a year and a half, which is generally not a big problem. Theoretically, instead of the LM385, you can simply use a divider, but since the REF TL431 output consumes 2-4 μA of current, in order to avoid voltage drops on the divider, rather small resistance values \u200b\u200bof 10 kOhm would have to be used.
The case is printed from PET-G on a 3D printer. The PET-G material must withstand the temperature of the motorcycle. If it turns out that the battery is being overexposed to heat from the engine and the case melts, then what’s left is an ABS print or some powerful PLA. Conventional PLA is ruled out due to its low softening point.
Power connectors – M5 screws. It would be nice to use those connectors that are usually used in the battery, but nothing like this is available. Nothing similar was found on Chinese portals, except that you can buy an empty battery case with terminals there.
The screw contact is connected to the body with a nut and fixed to the body with two M3 screws preventing it from turning. Thanks to this, the housing does not interfere with the tightening of the contact screw and it was possible to tighten everything with maximum effort.
An electrical connection is made with 4 pieces of steel tape 8 mm / 0.15 mm, which corresponds to 16 mm / 0.3 mm. The welder managed to weld such a combined tape without problems.
After turning the key without starting the engine (turning on the light, ECU and fuel pump), in the case of the previous battery, the voltage drops by about 900 mV. In turn, for this homemade battery, the voltage drops by about 300 mV under the same conditions. The motorcycle starts reliably and without problems. but if there was not enough current during the tests, the 4S1P configuration would change to 4S2P.
The battery has a height of 95 mm and a length of 150 mm, which corresponds to the factory battery, but the width is 2 times smaller (40 mm instead of 85 mm). So it became easier, and the place remained.
