EMC-ya-later

Out with the old – my EMC-900 charger finally gives up the ghost

In my last blog I neglected to mention an unfortunate incident that happened while I was getting everything fixed and ready for the bike’s MOT inspection. I was in my kitchen (which looks out onto where I’d left my bike charging), when there was an almighty bang – like a firecracker going off – that came from the general direction of the bike. I went out to investigate, and straight away could smell the telltale odour of fried electronic components.

At first I was worried that it might have been my BMS (battery management system) that had failed. I had built it from scratch from a circuit board and around 3000 components (see the ‘Zephyr’ BMS build), and so it was irreplaceable, and I didn’t really fancy the prospect of taking it apart and diagnosing it, especially since it would probably mean hassling Richard Fechter, one of the designers of the BMS, for help fixing it. Fortunately though, the source of the explosion turned out to be my trusty (until that moment) EMC-900 charger, which was easily replaceable, though not especially cheap, especially with the £50 international shipping, and obscene import charges, taxes etc.

The blown MOSFET

Opening the charger, I soon found the source of the problem: A FET had exploded. I briefly considered trying to repair it by just replacing the component, but quite often when a component fails, other things also fail that aren’t so easy to track down and diagnose. Checking my BMSbattery account while I browsed around for a replacement, I noticed that I’d bought my charger 2 years and 3 months previously, so I couldn’t really complain about the service life I’d gotten out of it, especially since I’d been using it almost on a daily basis for months at a time.

Looking through the chargers available, it looked as though the EMC-900 had now been retired, however the more powerful EMC-1200 was available for just a little bit more, so I ordered one of those instead, speccing 85.6V and 11A output, with an anderson connector for the power output cable. Since the outside of the unit was identical to the EMC-900, it would also fit fine in the topbox I made for it, so that I could have it fixed to the bike if I needed to go anywhere I’d need a recharge to get back from.

Unfortunately, I’d forgotten to specify “50A” anderson, as I did with the old charger, so the one I ordered ended up with the piddling little connector below left. Fortunately, I had some 50A connector crimps lying around, so just salvaged the plastic connector bits from the old connector and transplanted the connector over to the new charger.

D’Oh! – Okay, so I forgot to specify the connector size, but then why didn’t they ask? 

Once up and running though, it worked fine, even cutting off the charge cycle at the exact 1.4A threshold that I’d had the BMS working at with the old charger. The model number (1200) is the power rating, 1200W, so theoretically it should be able to charge a third faster than my old, 900W version.

I’ve since upped the current setting to 12A using the adjustor pots (variable resistors) inside the unit, and it’s cut my recharge time from 2 hours 50 minutes (with the old charger) to just over 2 hours (from ‘flat’), and it looks like I might be able to up the amperage a little more if I really want to push it. Lithium packs can charge at an enormous rate, and my BMS is rated for 20A, so there’s plenty of scope to improve the recharge time further if needs be.

And in with the new – visibly almost identical, my brand new EMC-1200

Since I also had some weak cells in my LiFePO4 battery pack that needed replacing, I also ordered 6 new Headway 38140S 12Ah LiFePO4 cells plus a mass of the orange, plastic holders used to secure them (the legs on the holders can break easily when removing them to replace cells) and a few connecting plates . I only just recently got the LVC (low voltage cutoff) throttle and alarm circuits on my Zephyr BMS working, and so had still been relying on manual checks with my pack monitor to warn me of any dangerously low cells (more about that later).

Headway 38140S 12Ah LiFePO4 cells

The plastic holders and connecting plates

Without these proper safeguards, running it too far without a recharger would damage the weakest cells and let others get lower than they ought to as well. Hopefully, with the new LVC and alarm system in place, I won’t have to service the pack for a good, long time.