Mobility scooter power module won't charge

[Bill From Net]

My mother-in-law has a mobility scooter with 24VDC power modules that each contain 2 12V AGM lead acid batteries and a charger mounted internally that connects to utility power via a common power cord such as those used for personal computers with IEC 386 receptacles. This charging component is rated at 100-120VAC 50/60Hz 1.2A input, and 24VDC/1.5A output.

One of the units fails to charge now. Initially, the charge light showed green instead of red, and now it does not illuminate. I disassembled it and discovered that the scooter maker seems to have taken a charger that is commonly available in a similar form (it is identified as a High Power HP1201-2) and hacked it so that:

• The integral red/green charging/charged indicator LED is disconnected, and three additional wires were drawn out through the DC output end to drive a LED for the same function, but mounted through a hole in the larger case where it can be seen.
• The three prong external plug for DC output that the original unit was designed to use was replaced by a plastic connector for use inside the scooter power module. Only two conductors are used.

I initially suspected that this is the part that failed and prepared to replace it. That would be simple in concept, but I don't think it can be obtained conveniently due to the hacks. The alternative, if it is available, is to get it from the scooter maker that is semi-defunct and will likely gouge me for the parts. But maybe I don't need to, because there is another possibility for the cause of the failure, and I hope someone here can offer advice.

Please refer to the diagram in the inset of the photo attached to this post. The power supply DC output connects to the remainder of circuitry of the power module shown in the diagram via the connector depicted in the yellow shading. When the entire power module is mounted on the scooter, it connects via the four-prong receptacle depicted in the green shading. The final bit of shading, in pink, depicts an axial lead diode. That diode is shown in the photo. It was encased in shrink tubing that had fused to the surface so I had to scrape and peel to expose it with limited success. I would like to identify it and determine if it is faulty and replace it if appropriate.

I know very little about electronics, but I tried some tests with a multimeter. I hope this provides enough information for someone more experienced to render advice:

• I tested the DC output voltage of the power supply with no load. It indicates 65V ! Is this because there is no load, or is it malfunctioning?
• I also speculated that the diode might be failed in an open circuit state, So I tested it with the diode test function of my meter. It indicates open regardless of which lead I apply to which end.

So, I think I need to replace the diode. But do I need to replace the power supply too? Did it cause the diode to fail due to the high voltage, causing excessive current flow and heat? If this (voltage) isn't necessarily a cause for concern, is there some way to test the power supply under load, perhaps with an appropriate resistor? Should I do this or just look for a replacement, or not worry about it based on assurance that this is how they operate normally? If I must replace the power supply, I'll probably have to replace it with something external and less conveniently packaged for the user.

Finally, how do I identify the diode in its degraded condition? Failing that, what would be an appropriate diode for this circuit? And stretching your indulgence - where is a good place to get them?

Thanks for your attention.

 

[kellys_eye]

I tested the DC output voltage of the power supply with no load. It indicates 65V !

say what????

Can you double check that and/or show a pic of the charger itself?

The diode (which, from your information, does seem open) is a simple rectifier and anything rated at 100V (or upwards) and 3A (or upwards) will suffice - a typical diode would be 1N5402 [03, 04, 05, 06].

Clearly applying 60+ volts to the batteries is going to cause 'problems' to say the least - time to get the charger checked out and/or replaced!

 

[73's de Edd]

Sir Bill . . . . .

Back when you measured 65VDC . . . if you had the probes in the right place, along with that diode being open circuit, I could see 12VDC [PLUS] 12VDC [PLUS] 24VDC from the charger AND its additional surplus voltage put out since there was being NO LOADING placed upon it.
Thats getting on up to 48VDC voltage level plus that ? surplus voltage factor .

Check out your meter accuracy by placing in DC mode an probably in 50V range and measure each of the batteries to see if their measured voltage is realistically, reading in the 12VDC range.

Then check at the charger plug for ~ 24VDC or greater there, since it will be unloaded.

That bad diode being seen , with its case sizing and lead diameter is being at least a 6A or possibly more.
With them factoring in a safety factor.
If that scooter has had made a long shopping run on it, that 2.5 A coming into it just after being plugged in, will just barely be making a trickle charge level on the unit until hours later.
Since the batteries will be initially wanting all of the power that they can receive after they have been pulled down.
Also, a depleted battery may be pulling out all that is possible to come out from that charger.
A gripping of the diode and finger temp sensing should confirm that it is passing somewhat of a current.


73's de Edd

 

[Bill From Net]

Thanks for responding. Yes, it really seems to be over 60V from the power supply. I checked again this morning and also sanity checked against one of the old batteries that is depleted and a newer one that is only slight run down. 64V on the PS, 11-12V on the bad battery, 12-13V on the good one. So the meter seems to be functioning well enough. I've attached photos.

I appreciate the lead (no pun intended) on the diodes. It is easy to find such to order up to 3A, but beyond that seems more difficult. For those I did find, they seem to be skinnier in appearance than the one in question. But I did find something more similar in physical dimensional proportion at the link given below. It is described as a transient suppression diode with some very beefy specs. I wonder if something like this could be the ticket. Your thoughts?

• https://www.westfloridacomponents.c...000W+TVS+Diode+Axial+Lead+World+Products.html
  

 

[kellys_eye]

The diode is likely only required to stop the batteries back-feeding the charger and since the charger output is rated at 1.5A a 3A diode (100% over-rated) should suffice but there's nothing lost by fitting a higher rated one.

The charger is defo dud then!

Ideally you should use two matched batteries to get an equalized charge through them - this would usually mean buying two new batteries but that's down to you. If you have a 12V sealed lead-acid charger you could try charging them both separately to see how well they take a charge.

 

[Bill From Net]

I have four batteries. Two matched pairs. I assumed the diode was only protection in case somehow a charger was connected with reverse polarity, but seeing as how the maker integrated the charger, I suspect they might have had more scenarios in mind, and the phrase "transient suppression" seems to play into this.

I will have a charger with similar nominal electrical specs that is designed for similar application soon. I'll see how it behaves.

I wonder how a PS would fail in such away that its voltage spikes. Maybe a short in a transformer primary winding?

 

[Alec_t]

Maybe a short in a transformer primary winding?

Sounds likely, if the secondary voltage is now WAY too high.