One Step Closer: New DC/DC

In the seemingly never-ending parade of not-quite dialed in and failed components preventing the EV Miata from being able to be a car instead of a project the latest issue arose with the DC/DC converter. After spending a couple days working on the stupid battery balancing system, thinking it had a better chance of being finished than ever before, I went to go take the car for a drive. You can instantly tell if the DC/DC is online or not with the sound of the vacuum pump, if the DC/DC is running the pump runs much faster than if it’s not. It wasn’t. I scoped the connections out and realized that the DC SSR that was supposed to switch power to the DC/DC had failed closed and that the fuse was blown. I tried a spare fuse, which instantly blew. Then I tried a spare 25A fuse up from 12, which also blew instantly. Yuss- $20 of fuses blown in 5 minutes.

I was using a IOTA DLS55 as a DC/DC converter- so I tried plugging it into AC power- no circuit breaker popping, and 13.8v output. What the fuck. Not really wanting to try to troubleshoot the cause of the problem, and leery of the general suitability of the IOTA to vehicular DC/DC duty anyways I decided to replace it.

After shopping around I decided that the isolated 675W DC/DC from Belktronics was the best replacement, features and cost wise. I really wanted a 12v enable input so that I could skip worrying about switching the HV input, given the patterns of long periods of disuse this car gets I like to be able to disconnect all loads from the battery. Several chinese DC/DCs have come on the market lately with similar pricing to the Belktronics, some with enable inputs as well. The “product support” line at Cloud electric was unable to confirm if the enable input was on the 12v side, as a HV side enable input would be much less useful.

After confirming it would do what I want over Email, I ordered the Belktronics converter with remote enable and soft start options. I consider the fact that you can actually get in touch with the guy who designed and builds the Belktronics converter a selling point over the Chinese one, I have a feeling if it broke you would be pretty much without recourse, particularly considering the generally under-capitalized support-averse nature of EV part dealers.

A month later the DC/DC was built and shipped. I installed it and it appears to work exactly as I would expect and hope. It’s a bit smaller than the IOTA but similar weight. It’s in a sealed box, should fare better under the hood than the IOTA. I’m not a huge fan of the recessed faston terminals but I think they well do the job just fine.

I used an 80A MEGA fuse to protect the 12v side and an A60Q 10A 600VDC Ferraz-Shawmut fuse on the HV side.

I bypassed the old SSR and used the 12v signal used to switch it to feed the Enable input on the converter.

I had to grind down the plastic shroud on the 10GA crimp fastons to make them fit in the recessed connectors

I mounted the converter to a 1/4″ aluminum plate which I mounted to the bottom of my battery rack in a similar fashion to the IOTA mounting.

I took the car for a drive after this (first time in almost 6 months). I only got 22AH out of the batteries before the car was reduced to a crawl. I’ve never pulled the batteries down so low before. I expect the combination of the batteries being kind of tired after all the disuse and the pukert effect as well as one underperforming battery are mainly to blame. I think I made it about 10 miles. It was really nice and fast at first, but a 1/2 mile jaunt on the freeway at about the 15 AH mark (at 55mph) really killed the pack. I was getting about 4kw of power at the end. It was real slow.

I charged it back up and verified that indeed my battery balancing system is not ready for prime time, now it’s exhibiting the frustrating behavior of crashing out occasionally. I’m really tired of dealing with it and feeling like I can’t trust it and must babysit the batteries all the time. I’m seriously considering tossing it out and replacing it with something off the shelf, probably something I should have done a long time ago with the interests of actually getting the car running in mind. It’s a push-pull between finishing the project I’ve started, that I’ve invested a bunch of money and time in or accepting that its failure doesn’t define my abilities and tossing it out to enable me to move on to other better-conceived projects and actually have a car here instead of a never-ending project.

While I figure out that part at least it drives with its new DC/DC. A lot of things on the car work really well. It’s just the nagging last few issues that are causing all this grief.