Hi everyone

I have a mystery I'd like your help with.

A little while ago I bought a cheap 1500W 12v-240v inverter from Amazon to use for occasional camping trips. I initially hooked it up to my car battery and everything appeared to work fine, powering everything I needed for several hours at a time. Maximum power consumption never exceeded 100w AC. So far, so good.

I then decided to invest in a 75Ah deep cycle battery to make the system a bit more portable and avoid dependence on the car battery. However, the first time I hooked it up to the inverter (without any load), there was a fizzing sound, a little cloud of blue smoke and the inverter closed down, never to work again. A quick peak inside showed that one of the voltage regulators was burnt out. The battery was freshly charged and producing a nice, steady 12.7v and the inverter was supposed to have over/under voltage/current/heat protection, so I put it down to a faulty component and sent it back for a refund.

Next, I decided to buy a replacement inverter from a different supplier (Ebay this time), choosing a 1000W unit with a different brand label on it. As soon as I connected it to the deep cycle battery, there was a hiss and a tiny cloud of smoke, but everything appeared to continue working so I put it down to some stray solder paste burning off and connected up an audio amplifier drawing about 11W AC. However, 20 mins later, the warning buzzer sounded and the unit closed down. On further tries I realised that the cooling fan did not appear to be operating so contacted the supplier to explain what had happened and he kindly sent a replacement. Unfortunately, when this arrived, exactly the same thing happened. No cooling fan and the unit operates for a short while until it shuts down.

I'm now really confused. Can I possibly be this unlucky? Are all of these units faulty? Have I done something wrong? Or is there something different about the deep cycle battery (still supplying a steady 12.7 v) that could cause this to happen when the original inverter worked fine with the car battery?

Thanks in advance for your help.

Matt

P.S. The Ebay seller had told me that he did not want the first unit back so I opened it up and investigated the problem myself. The pins connected to the fan appeared to be supplying 12V but as soon as a load was applied, this disappeared to nothing. Connecting the fan directly to the battery input made it work and the unit now works perfectly without overheating and closing down.

 

I can understand your confusion. It sounds like you're impossibly unlucky. Maybe if we saw your setup someone could see a problem that you have missed and therefore haven't told us about. Loose connections? Reverse polarity? Lack of cooling?

Know anyone with a good inverter you could borrow?

 

Hi wayneh and thanks for your quick response.

I'd be reluctant now to connect this battery to anyone else's inverter in case it went the same way.

Here's a picture grabbed from a movie the seller asked me to take to demonstrate the problem. You can see the DC voltage on the meter on the left and AC watts on the right. It's a pretty simple set up really, red to red across the board.

 

There certainly doesn't seem to be anything wrong, so everything I suggest is grasping at straws and challenging assumptions. Double check that red is positive for both the battery and the inverter. Verify whether the power strip and power meter are both working normally on other circuits. Use a 60W-100W lightbulb as the load to rule out anything odd happening on the load side.

Again, this is really grasping. The inverter probably protects itself against reversed polarity and should protect itself against an overload.

I would be stumped and confused just as you are.

 

What happens if you use a charger to bring the battery up to around 13.5v before turning on the inverter? Does the fan now work and the inverter continue to run?

Also what is the voltage at the inputs to the inverter? I see you're checking voltage at the battery posts, but what is the voltage at the input posts on the inverter?

 

My WAG.

The main difference between a car 'cranking' battery and a 'deep cycle' battery is the ability of the car battery to deliver a large peak current for a short period of time while still maintaining a usable voltage because of the internal construction. Crank battery, large surface area with thin plates for quick surface chemical reactions vs 'deep discharge', thicker plates with less exposed surface that forces the chemical reactions to move into the plates deeper during a discharge. I would suspect the deep cycle battery is not fully charged (static 12.7 is not a good indication of SOC) or is defective causing a initial voltage sag at power-up from the energy moving into the inverter capacitors/inductors fields from that battery that causes something tied to the the fan? controller circuit to over-current/overheat as it tries to stabilize output voltage or power due to bad design. The fan might have an RPM sensor that inhibits the inverter circuit from running if it stops or is slow.

 

That's a good WAG. I've had issues with using my (small) inverter off the car battery alone without the engine running. It didn't take long for an error light to come on as the battery voltage sagged. Maybe these big inverters are tweaked to assume a 13.5V alternator input at all times and are not really happy when it sags below 13V.

 

Thanks Mr Software and nsaspook.

That's something to go on at least. I charged the battery for several hours using my Halfords car battery charger and this seemed to think that it was fully charged, but I guess it might not be a reliable indication. I'm not sure if this is capable of taking it up to 13.5v but I'll give it a go.

I'll check the voltage at the inverter input and try monitoring the DC voltage while connecting up and switching on the inverter to see if I can detect a dip in voltage and report back. As you say, it would be very bad design if something as simple as this were to cause a component to burn out.

 

In general, an inverter should run fine from a deep cycle battery. I've got one of these:

https://www.amazon.com/gp/product/B000WGNNUQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

It's a 110v 1kw sine wave inverter. It will run a small window unit air conditioner for about 30 minutes from the deep cycle battery in my boat. I don't remember the exact capacity of the battery, but it's a little larger physically than what's in the typical car. The A/C unit takes about 700w once it's running. I use 4GA battery cables and the inverter turns off when the battery voltage sags to somewhere under 11.5v. So I guess my point is that expecting yours to run fine from a deep cycle battery is completely reasonable, especially with the small load that you have attached to it.

Does it run OK if you put it back on your car battery?

 

Thanks Mr Software and nsaspook.

That's something to go on at least. I charged the battery for several hours using my Halfords car battery charger and this seemed to think that it was fully charged, but I guess it might not be a reliable indication. I'm not sure if this is capable of taking it up to 13.5v but I'll give it a go.

I'll check the voltage at the inverter input and try monitoring the DC voltage while connecting up and switching on the inverter to see if I can detect a dip in voltage and report back. As you say, it would be very bad design if something as simple as this were to cause a component to burn out.

To properly charge a 'deep cycle' battery you need a charger with the correct multi-step profile but they seldom detect lost capacity in a sulfated battery that gives it high internal resistance. I would expect a nice sine wave inverter to handle low voltage conditions easily but the cheap MSW units are sometimes clones of cheap Chinese clones.

 

Yours is obviously rather higher quality than the ones I tried:

https://www.amazon.co.uk/gp/product/B00SZLPWRW/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

http://www.ebay.co.uk/itm/191851480661?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

I'll try it with the car battery as well.