The DBMS worked fine for 50 or 60 km, but the behaviour of the various components – batteries, motor and controller and DBMS started to get erratic, and it started to get difficult to tell what was going wrong. I slowly removed items until I was back to a normal set up on both bikes, which seem to worlk normally with their own batteries and no DBMS. The DBMS seemed to be the main factor causing issues.
So having taken it off the bike I started bench testing with the lid off.
With just one battery connected, the Arduino fired up normally, and +5v was delivered to one of the relays, so the 50v-5v dc-dc converter appears to be working fine, and probably the diode too.
However, neither operating light was on on the relays. I tested the output of the relay which was receiving the switching signal, and it was giving 40V across it’s output terminals. However, when I tested the other relay, so was it!
When I disconnected the battery, I tested the resistance across the relay outputs, which should have been open connections, however I was getting zero ohms across both. So when the battery was connected, even without the computer connected, both relays were in the “On” status, so power was passing through one relay, to the common output, and back through the other relay.
I removed a relay from the DBMS and opened it up. This is where it gets disturbing. Firstly there was signs of overheating around the terminals of the MOS Field Effect Transistor (MOSFET) which provides the main switching. When I tested the MOSFET, it was a dead short across the output terminals.
I moved the MOSFET so that I could read the part number which was IRFP250N. This is an interesting device, but is only rated at 30amps, and that at only 10V for continuous use. Given that the relay is supposed to be 60V 40A, this single MOSFET was not going to manage that very well!
The rest of the circuitry is interesting. When I ordered the part from https://www.ebay.com/usr/fashionstyle6?ul_noapp=true I assumed I would get a Fotek device like in the photo. Now the Fotek device has a couple of good attibutes. Firstly, Fotek has a reasonably good reputation, they provide good equipment at good prices. Secondly, the SSR-40 DD is an optically isolated device. This means that the control circuit, which is 3v to 32v and which I drive at 5v from my arduino, is completely isolated from the swiched output side, which I put up to 40V 10A though. The isolation is done with an opto-coupler which is an LED and a photosensitive cell in a tiny device, the input circuit causes the LED to shine, and the output circuit detects the LED shining and switches the relay terminals on and off.
One of the big advantages of this is it removes the need for a common earth – the two circuits can be completely separate. The other advantage is that if anything goes wrong, like a huge current surge of the output side, it will never ever get back to the input side and my arduino microprocessor.
What arrived was a CNMF part. When I took the failed unit apart and saw the under specced MOSFET, I wondered what else was wrong. It all looked quite good, well soldered, lots of resin around the parts etc.
The first thing I looked for was the Opto-coupler. While I am not an expert, I am fairly certain that there isn’t one on the circuit board. In fact there is a small transformer like device which seems to take its place. I examined the circuit further and I realised that the two sides of the relay were not isolated at all.
Power from the 5V input passes through the LED power indicator, and connects directly to the signal terminal of the MOSFET, For this to work, both circuits must use a common ground, and when you trace the circuitry, this is in fact the case.
The result of this is that this is not a Relay at all, just a simple electronic switch. There is no isolation of the two circuits. A bit disappointing really, total rip off by these people https://www.ebay.com/usr/fashionstyle6?ul_noapp=true