Help please electronics gurus - Celestron LiFePO4 power tank circuitry repair

[Ships and Stars]

Hi all,

Anyone good at circuit board testing/repair or know what pops first on a Lithium power pack PCB? Batteries are fine and holding voltage.

Background: I received a new Celestron Lithium LiFePO4 7.2Ah powertank  with my scope when I bought it second hand, was really happy the seller chucked that in.

However, there was no charger with it (it was a 16V charger, and I couldn't find one online anywhere and the seller could not find his) so I tried a 17.5V charger under carefully controlled conditions in a safe area in case of explosion or fire. NO problems.

The voltage/amperage should have been within what was generally accepted range for this type of pack, albeit at the high end.

Anyway, I used this charger very carefully about five times, no overheating, etc but after charging it for about an hour one day I noticed the LEDs had stop emitting and I immediately disconnected it. Now the circuitry has a short somewhere and the tank won't power on, but the eight 26650 batteries - wired in series in two banks of four - are fine and holding voltage, no leaks etc.

 I've tested the coil and can see no damaged/melted solder, except at the base of the main copper wire coil, but this is probably from the manufacturing process as it's the largest area of solder requiring the most heat.

Given that these batteries can be very dangerous if they explode, I'll probably dispose of it safely and buy a new power tank, but hate to chuck it for a 10p resistor, etc. I'm ok with soldering etc but no electrical engineer.

Ideas?

Cheers all

 

 

[Ships and Stars]

This is what it look like assembled, by the way. Celestron 18771

[Dr_Ju_ju]

It looks like a switched mode regulator system and so will require further testing with a good multi-meter \ oscillator.... I doubt that the coil would be at fault as its usually rated for higher current....

If you can, check the diodes & capacitor voltages, but its more likely to be one of the transistors\IC's.

The IC adjacent to the red wire looks suspicious, with what looks like a burn mark, same with the 100 resistor by the red wire.... 

[Carbon Brush]

Re Dr Ju Ju comments.

R100 colouring could be flux residue.

The adjacent 8 pin soic chip may have a burn mark, or maybe just a flux splash from wire soldering.
 

What is not clear without further photos, or sight of the kit, is exactly what charge control there is.
But the amount of circuitry does suggest to me there is a proper charge circuit.

The following notes may help dispel some of the 'black magic' associated with charging.

In a ideal situation, the charger first measures the battery state. If well discharged, it allows a small current at first until things improve.

Then it puts in a high constant current. For this size pack, probably around 1 amp or 1.5 amps.

After some time, the battery pack voltage rises to a value (specified by the battery manufacturer) and the charger gradually reduces current to keep at constant voltage.

Eventually the charge current falls to a low value (specified by the battery manufacturer) and the charger switches off.

While all of the above is going on, the charge control measures the temperature. Ideally using a thermistor stuck into the battery pack, not on the charger board.
If the battery is outside safe charge limits, typically 0C to 45C, then charging is prevented.

If you used a plug top supply with too high voltage, then it may have prevented the voltage limit and switch off from working correctly.
Or it could have damaged the charger control circuit.
I'm only making an intelligent(?) guess here.
Don't forget some plug top supplies have unregulated or unsmoothed outputs. The peak voltage may be much higher than the label.

If you overcharge lithium cells, they don't recover.

I would do two things here.

First of all buy some similar cells.
Whether you go for a 2-parallel, 2-series pack as original, or individual cells depends on the available space.

Connect them on long wires (for easy access) to your charger.
If the cells are tagged then solder, avoiding too much cell heating.
If they are untagged, spend a couple of quid on holders.
Monitor charge voltage and current - hence the long wires.
If they don't limit and turn off as described above, then you have a flaky charger circuit.
By not abusing the new cells for too long, you will avoid serious harm.

If they behave as described, then you had flaky cells. You can then work out how to fit it all back together with the new cells.

During your tests, just keep a finger on temperature. If the cells feel too warm, then switch off.
By that I mean if you can't leave a fingertip permanently pressed on, it is too hot.
Explosions are usually a consequence of cells getting really hot. Hot enough to burn your fingers.

Hope this helps, David.

 

 

 

 

 

[Ships and Stars]

Hi Julian and David,

Many thanks, there's some good information in here. I have a bit of testing to do! The IC and R100 I believe both show simple resin discolouration, but I'll investigate around there again. 

I'll also check the transistors/ICs. 

Hoping it was something simple, but a close inspection of both sides of the circuit board (sorry, no reverse pics) didn't reveal any obvious signs of overheating or cracked solder. Oxidation or corrosion isn't a problem as it's practically a new unit.

I shall persevere a bit more on this, but may end up buying a simple 12v lead-acid battery or running a power lead from my van's 105Ah leisure battery, providing the voltage drop isn't too great (i.e. use the proper gauge wire and fuse it).

Fingers crossed and thanks for all the info!

 

[Dr_Ju_ju]

One other thought, if it shows no signs of life (lights etc.) there may be an in-line\PCB fuse somewhere which has done its job.....

[Ships and Stars]

A simple fuse would be lovely! I've included some slightly better photos, incl reverse side of circuit board. I know I'm probably better off time-wise just getting another battery, but I'd love to be able to fix something on this level so may put a little time into it. One other thing I forgot to add is that I have a Nitecore i4 Intellicharger for my 18650 LED torch batteries, and it says it also accepts up to the 26650s. I believe it has a built-in test circuit, but might try and charge them individually that way. It also has a safety cut-out for overheating/overcharging apparently. Will reread manual for the i4 charger. 

Thanks again! If I get lucky and fix this, I will let everyone know. 

[Ships and Stars]

And the reverse side for your viewing pleasure...

[Ships and Stars]

Finally, what I believe is possibly the charge controller? It's a separate unit off the main red and black wires. 

I uploaded these as 3 posts so the image file could be as large as possible. 3000px on the long edges. 

Cheers

[Ships and Stars]

PS: the L3 coil/inductor thingy looks burnt on the left side, but it's just the sun reflecting off the resin...

[Carbon Brush]

It looks like the small PCB is the charge balance circuit. The switcher supply/charge control being on the large PCB. Assuming the thick black & red wires go to the PCB, and the 6 way header goes to the battery assembly.

At first glance I don't see anything obviously wrong construction-wise or charred on any board. Build quality is standard chinese with someone applying some sort of QA.

Given a voltmeter and ammeter, you should be able to make some useful calls.

With a voltmeter only you could for example  comment on the individual cell voltages.

David.

Edited August 5, 2019 by Carbon Brush

[Ships and Stars]

Thanks David, shall give it a go. I have a digital multimeter that measures DC amperage but no oscilloscope. You are correct, the red & black wires run to the smaller PBC, didn't show a photo of them both together.

If I get lucky I'll let you know!  

[wcoastsands]

Did you ever figure this one out? On the larger board, it looks like the 4435 MOSFET near the red wire got cooked.

i'm in a similar situation, looking for insights in how to repair this.

[wcoastsands]

Actually, think I may have identified the issue with mine. The 4435 MOSFET labeled as U1 appears to be the point of failure.