OpenEVSE Relay overheating?


I have an OpenEVSE unit, which I purchased here 1 year ago. I’m using it nearly every day and I really like it but when I opened the case recently, I discovered that the bottom left side of the relay is getting very hot (50+°C), up to the point that it is damaging the connected wires nearby (see photo: we changed wires and replaced ferrules 1 week ago but the plastic parts are now completely molten again).

The unit is operating at 32A but it seems to get similarly hot at 24A and even 16A or lower.

Also sometimes, when starting a charge, the relay “whistles”, something which can usually be avoided by interrupting the charge for a few seconds.

Have other people experienced the same ?
Shall I consider replacing the relay ? (also any idea how to get a replacement part ?)


Hello @mma68 thanks for posting, I’ve sent @glyn.hudson a message to take a look at this for you.

Hi Marcel,

Sorry for the delay, replying to this post. I received your support email today. A replacement relay has been shipped to you today.

I’ve not heard of any overheating issues like this providing ferrules are used. The only thing I can think of is possibly the screw clamp was slightly loose? Or overtightened crushing the conductors? It’s possible for terminals to loosen overtime. To negate this risk we have moved to using cage clamp terminals in our EmonEVSE. That said a correctly tightened screw clamp with ferrules should be totally fine.

I’ve been using the same relay daily for the past 3yrs in my own OpenEVSE charging at 32A with no issues. I’ve just checked and the wires only get gently warm during charging.

How is your unit wired in? What type of MCB/RCD are you using and what EV are you charging?

All versions of OpenEVSE has got integrated over temperature protection, charge rate will be throttled if temperature inside the enclosure reaches over 65 degC stopped altogether if temperature exceeds 71 degC. Firmware 4.12.3 (March 2018) onwards will display a warning message on the LCD if an over temperature event occurred.

Are you logging data to Emoncms? This makes it much easier to see the internal temperature during a charging session e.g here is a graph of normal internal temperature during a 8hr @ 30A charging session. Internal temperature increases 17 deg C from 30 deg C - 47 dec C.

It does sound like you’ve got a faulty contactor. Let me know how you get on with the new replacement.

Hello Glyn & Trystan,

Thanks very much for your replies and for the shipping of a replacement relay - that’s such a great customer service ! Of course I will keep you updated after the repair.

For information, my unit is wired directly from a new circuit in the house’s main wiring cabinet using 6mm² cables and protected by a dedicated 40A MCB and the general 300mA type-A RCD (a type-B RCD is not (yet) mandatory in Belgium AFAIK).

I knew about the charge rate throttling based on temperature but it never occurred here, to my knowledge. When charging at full 32A rate, the maximum I could observe, as reported to Emoncms, was 50.2°C (but the relay itself must have been significantly hotter than that, I believe).

That all sounds correct.

Yes, it sounds like the overall temperature inside the enclosure must not have reached the throttling temperature.

Thanks. And BTW my car is a Nissan Leaf 2.0 (40 kWh).

Hello @glyn.hudson,

I’ve repaired my OpenEVSE unit this Saturday with the replacement SSR you sent me (thanks again !) and I have good hope that the overheating issue is resolved.

Over the week-end, I used it to fully charge (at 32A) my 40-kWh Leaf and a 100-kWh Tesla. I checked inside connections twice and they were not even hot. Temperature inside enclosure stayed below 30°C all the time and it operated very silently.

I still don’t understand what the problem could have been with the other relay and I’m prepared to send it back to you if so desired for post-mortem analysis.

Also follows another photo of it. To my opinion and according to what I could feel when I swapped the relays, the big screws were correctly tightened, not loose at all and also not overtightened.

I will continue monitoring temperature and check terminal connections inside the enclosure over the next few weeks.


That’s great to hear. It must have been a faulty contactor, please send it back to use (we will refund shipping), I will then take it up with the manufacturer:

Intec F9
Ffordd y Parc, Parc Menai
LL57 4FG

For the record it’s not actually a SSR (solid state relay i.e power electronics). It’s a physical electromechanical contactor. SSRs can’t provide adequate isolation for use in an EVSE.

Looking carefully at the photo, I’d question whether the cable insulation was stripped back far enough, because it looks like there is some melted PVC on the terminal. The terminal (nearest the camera) itself has certainly got hot - I can see from the colour that the brass has started to oxidise.

@Robert.Wall Thanks for your comments. The melted PVC on the terminal must have come from the sleeve ferrule (see first photo in original post). Also I don’t think cable stripping was too short as wires were (still) visible at the end of the ferrule.

@glyn.hudson I’ll ship the contactor to you later this week. Given the contents, is it OK to use basic parcel service of the Belgian post which is only a few EUR for the UK but is not tracked and may take 1 or 2 extra days ? (also I don’t need a refund - whatever the chosen shipping service - but thanks for proposing it)

I just check again the interior of the enclosure, after 1 week of operation and all terminals are very clean, just as new. Also, the reported temperature always stays around 30C max and the contactor is now really silent.

@glyn.hudson The damaged contactor is on its way to you.

Thanks again to all of you for your help.