Overcharging with power supply, any failsafe add-on option?

[blutow]

I've seen several reports of people overcharging cells using power supplies. I have some cells on order and recently bought the following power supply that I plan to use for top balancing and whatever other charging duties I might need it for:

Amazon.com

I've played with it a bit using a lead acid battery it seems pretty straight forward, but I'm concerned I'm missign something and will make a mistake and overcharge my cells when the arrive. It seems like you dial in max voltage and max current and one or the other will limit the output (but neither value can be exceeded). So, it might start out as constant current (current limited), but will eventually be constant voltage limited as the amps drop lower and lower at the max set voltage. That's how it seems to behave in my testing. Does the overcharging happen when the max voltage setting is somehow cleared? Is there something that can happen automatically that can change this max? I don't see any way to change the max voltage on my power supply without turning the knob. Even if I unplug it, it keeps the values.

Also, is there any device you can put inline that could serve as a failsafe to set a max (or min) voltage and disconnect if exceeded? I bought a victron smart battery disconnnect that disconnects when voltage drops too low, but doesn't work in the 3-4V range and you can't change the upper limit. It would be nice if there was a cheap device that could do this. I'd certainly spend $20-$30 as a safety net device to prevent charging over 3.65 or discharged under 2.5. Would there maybe be a way to use a BMS during parallel balancing to work as the safety device?

 

[snoobler]

Just follow the guide in Resources. Critical is setting VOLTAGE BEFORE connecting to battery and not touching it again.

People make the mistake of changing the voltage limit while connected. The display will only read what the PS is supplying at the given current.

 

[HRTKD]

Overcharging can happen because people get impatient, make mistakes or have subpar equipment.

My power supply was set just right and I happened to bump the voltage adjustment knob. The knobs are very sensitive. It threw off the charging. Fortunately, I caught it quickly. Otherwise, I had no problem getting my eight cells top balanced. That doesn't mean I wasn't worried and checking them frequently.

 

[Ampster]

My only suggestion is to independently check voltage at terminals. There will be some loss and that may cause you to become impatient and crank up voltage. One power supply I once had used separate leads for voltage measurement and that voltage corresponded to multimeter verification.

 

[Sillyputty]

My only suggestion is to independently check voltage at terminals. There will be some loss and that may cause you to become impatient and crank up voltage. One power supply I once had used separate leads for voltage measurement and that voltage corresponded to multimeter verification.

If the power supply has an adjustment mechanism, would it be a good idea to adjust the readout to show what the measured voltage is at the end of the charge leads resulting from voltage drop, rather than the voltage at the charger terminals? In other words, a charger may read X volts at the charger end, and Y volts at the ends of the leads that connect to the battery. My unskilled brain wants to think that the actual charging voltage is what is at the end of the charge leads, and not necessarily what the charger says it's supplying, and my skeptical brain is not wanting to see magic smoke appear.

 

[HRTKD]

It's been a few months since I top balanced my cells. My vague recollection is that the closer the charge came to being done, the less discrepancy there was between the reading at the charger and the reading at the terminals. Based on that, I would set the charger to what you want the final voltage to be and let it run, instead of trying to adjust the charger for the (initial) difference.

 

[Ampster]

If the power supply has an adjustment mechanism, would it be a good idea to adjust the readout to show what the measured voltage is at the end of the charge leads resulting from voltage drop, rather than the voltage at the charger terminals?

Yes, however I am not certain what the effect load would have. I think that is what @HRTKD is alluding to above.
A separate issue is where mistakes seem to be made is that the displayed voltage masks what the voltage setting is for the power supply. They turn the voltage knob and see the current increase but no increase in displayed voltage. Inadvertently they have increased the max voltage.

 

[Ampster]

Also, is there any device you can put inline that could serve as a failsafe to set a max (or min) voltage and disconnect if exceeded?

Yes, there are voltage adjustable relays. I bought one years ago and it was over $50. Usually their current capacity is small so an additional higher current relay would have to be used to disconnect the power supply source or the DC output. It has been used as a fail safe for repetitive charging situations but a pain to set up for a one time parallel top balance.

 

[Sillyputty]

where mistakes seem to be made is that the displayed voltage masks what the voltage setting is for the power supply.

Precisely my concern. It's hard enough keeping track of all the variables when building your own battery, and adding to those variables by changing what should be a constant you can rely on (the displayed voltage in CV mode) seems a recipe for magic smoke. I hope I listen to myself! ?

 

[Ampster]

I hope I listen to myself!

Hope springs eternal. Despite eight years messing around with Lithium batteries on ebikes, EVs and stationary packs I have to remind myself by posting these warnings to others. On more than one ocassion I have made the mistakes which I described above.

 

[Marinepower]

If the power supply has an adjustment mechanism, would it be a good idea to adjust the readout to show what the measured voltage is at the end of the charge leads resulting from voltage drop, rather than the voltage at the charger terminals? In other words, a charger may read X volts at the charger end, and Y volts at the ends of the leads that connect to the battery. My unskilled brain wants to think that the actual charging voltage is what is at the end of the charge leads, and not necessarily what the charger says it's supplying, and my skeptical brain is not wanting to see magic smoke appear.

What you are suggesting above is only a good idea if you measure the voltage at the battery terminal with the bench top power supply's current set zero or 100ma. The resistance in the wires will increase with current throughput, so you get a higher voltage drop at full current output giving you a false reading. When the charger switches to constant voltage and current limits, your voltage drop will reduce.

I assume you are worried about over charging during top balancing. I had the same concern when I got my cells.

IMHO - the safer way to do this is to charge the parallel pack in stages. This method is documented in many other parts of this forum.

First charge the cells in parallel to a high but safe voltage = 3.45v.

Then, find a time when you can spend a few hours with the pack without interruption. Disconnect the charger, reset the voltage to 3.5v and continue to charge while you are watching the charger and the batteries and check voltage with a volt meter at the cell terminals every so often. Depending on how big your bank is, this will only take about an hour. It's amazing how quickly the cells go up once you get into the knees. Then, disconnect the charger, reset the voltage to 3.6v and continue to charge until the current drops to about 0.5-0.3amps per cell.

Use short runs of large gauge wires (10-12 AWG) with crimped ring terminals btw the power supply and battery. Don't use the cheap leads that come with the power supply. Don't use small aligator clips.

Make sure your cell's busbars are tight and clean.

I recognize the above info is not what you originally asked in this thread. If you are looking for redundancy, maybe try a cheap HVC audible alarm?

 

[John Frum]

Current can only flow if there is a difference in potential so you can't really get a cell to 3.65 volts with a charge voltage of 3.65 volts.
You can get really close though.
Leaving a cell exposed to 3.65 volts beyond the its specified tail current is a bit stressful and less exposure is better.
IMO a few hours once in a cell's life is no big deal.