Electric bikes – battery charging

We went for a longish ride today, and the battery I had been testing with and using for the last two weeks finally gave up the ghost, with a few hundred metres of uphill tracks to go! Boy are these bikes heavy without power (31Kg). I have  been building up to a mild panic over the last week, as whenever I plugged in the charger, I kept getting the green light – indicating the battery was fully charged. As the indicator on the bike was also showing 4 lights at rest, I thought it was just using up the initial charge.

I plugged in the charger to the flat battery, and still got the green light. Was the charger or the BMS faulty? The results were the same with both chargers and both batteries, and identical faults in both items was a bit suspect.

I took the top of the battery and got to work with the multimeter, testing voltages and connections. The flat battery was down to 34.4 volts, so the system had shut it down in plenty of time. Each cell was between 2.7v and 3.0v, most were 2.8v or 2.9v. Quite nicely balanced. I was getting 43.6V out of the charger, and all the connections between the charger and the battery were solid. A bit of a mystery. I had taken the XLR plug apart the day before so knew that the soldering was good and the wires were connected correctly, brown positive wire to pin 1, blue negative wire to pin 2. So what was wrong?

I put the multimeter back on the charger output, and noticed it was reading negative 43.6V, but I was convinced the plug was right, after all brown positive, blue negative is an international standard, right? I opened up the charger, and the blue wire came from the circuit board from a connection labelled U+, and went to the fuse before it went to the plug. That didn’t sound right to me. I tried reversing the charger connections in the battery temporarily, and low and behold, the battery was now charging perfectly, two red lights, fan on, all systems go. Within a few minutes, every cell was reading 3.34 or 3.33V exactly, so the cells are being beautifully balanced too.

When I took the RCA plug off the  Mebo battery charger, I should have paid more attention to the polarity of the wires, and not assume that a Chinese built charger will conform to what was maybe just a British wiring standard (power leads in the UK have a +ve brown, -ve blue, and earth yellow and green wire set). Is that some sort of bizarre cultural issue I have?

When the battery is charged I will need to reverse the polarity of both chargers, and put the battery back to its correct wiring. I am very impressed with the charging speed, and the fact that charger, cells and BMS are all running cool while charging, but after only an hour or so the battery is reading 40.5V (with the charger attached). I am very much more impressed with Chinese technology than I was a couple of hours ago!

Edit 23/06/2014 – the full charge took just under 4 hours. The total voltage at the terminals approached 43.6, and each cell was between 3.6 and 3.74 – that higher reading was a bit worrying. When I tested this morning before the ride to work, the voltage was down to 40.0v, so a bit more than the 39v below. The second battery was still reading 39.2V, but still took about 2 hours to charge.

Panic over, no need to buy new BMS or new battery chargers unless I want to upgrade. I have found some nice smart BMS units which may be more suitable, and they are only $20 and include the ability to switch the battery off with a low amperage switch, as well as being able to have the maximum cell voltage set to 3.6v. Watch this space!

The ride was good, but even on the lowest setting, the bikes want to do 20kph on the flat, which is a bit scary on a muddy track along a stream stop bank! I need to work on some better speed control. Back to the interweb to read some more technical stuff.

Posted in Electric Bikes

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