Trim down charger voltage

[rg12]

I received this 88.2V (21S) charger and it puts out 88.6V
I want to trim it down to about 87.7V to not overcharge my 21S lipos.
I did it before, opened the charger and there was a trim pot and it worked.
This one has two 20 turn trim pots and none of them moves the voltage.
Once I trimmed one of them way up (or down, who knows) then I connected it to the socket and the fan was on like it was charging even though there was no load/battery connected to it, so that same pot down and the fan went back off.
Any ideas?

* Those two trim pots are on the output side of the circuit

 

[TommyCat]

The chargers I'm familiar with have three pots... One for the max voltage output for which you seek. One for the maximum charging amperage, and one for the shut-off current.
By turning down the current shut-off pot too low, say to shut off at a current level of almost zero. Perhaps that could keep the charger running with no battery.
If you can take a close look at the PCB and see what the pots are labeled. That might give you a clue as to what each one does.

 

[Chalo]

Put a power diode on the charger leads?

 

[rg12]

The chargers I'm familiar with have three pots... One for the max voltage output for which you seek. One for the maximum charging amperage, and one for the shut-off current.
By turning down the current shut-off pot too low, say to shut off at a current level of almost zero. Perhaps that could keep the charger running with no battery.
If you can take a close look at the PCB and see what the pots are labeled. That might give you a clue as to what each one does.

Just found the 3rd pot so will trim the voltage but now I messed with the other two, so I can measure amp output and trim it (since when can amps be trimmed with a pot?) but the third one for the shut off current, what is the use of it? and how can I put it back to where it was?

 

[TommyCat]

Give this thread a good read thu… Should answer all your questions. :thumb:

https://electricbike.com/forum/foru...-to-adjust-the-luna-charger-mini-and-advanced

 

[rg12]

Give this thread a good read thu… Should answer all your questions. :thumb:

https://electricbike.com/forum/foru...-to-adjust-the-luna-charger-mini-and-advanced

Got it thanks alot!
but one question though...What if it cuts current at the end of the charge but the pack is unbalanced and need a bit of charge to raise a few unbalanced cell groups through the bms?

 

[TommyCat]

but one question though...What if it cuts current at the end of the charge but the pack is unbalanced and need a bit of charge to raise a few unbalanced cell groups through the bms?

Now your getting into the question of how a BMS works. And an important one at that.

A standard BMS (exceptions for more expensive BMS's) will not begin to balance until at least one or more cell group reaches 4.2 volts or the maximum charge for that type of cell. That would require to have the output of the charger at least 88.2 volts. (21X4.2=88.2) At this point the BMS would allow the cell group with the HIGHEST voltage to discharge VERY slowly thru a bleed resistor, dissipating as heat. And allowing the lower voltage cell group(s) to catch up. With this slow discharging for balancing, and depending on the amount of imbalance, it would be unusual for the pack to be completely balanced when the charger first shuts off. Some chargers will automatically come back on when the the total charge is lowered enough thru the balancing process. But some you just let the battery sit for a couple hours, and then restart manually. The closer your current shut-off setting to optimal, the better.

If your charger is rated to charge LiPo batteries, this is no problem. And would allow it to charge to 100% and balance properly if adjusted to 88.2 volts output. Remember it is best to use it right after charging to 100% and NOT let it set with a full charge.

But with your original stated goal of only charging to 87.7 volts or under 4.18 volts per cell group, it may not balance at all.

This is why when one uses a charger that can also charge to 80, or 90 percent like my Luna Mini Charger. Which is used to extend battery service life. Needs to be charged at 100% occasionally depending on use, to balance the cells. :wink:

So if your goal is to occasionally charge to a lower voltage to extend battery life, as Chalo mentioned above. A Si diode or two can temporarily be put in series with the charger output that has been set to the proper 100% voltage charge to reduce the voltage output for that particular charge cycle. Reducing total charger voltage. ( I think about .7 volts per diode) Keeping in mind that this will allow the charger to keep charging, albeit very slowly. Giving you time to terminate the charge manually at the lower voltage.

You could also install an inline volt/amp meter like this...Post #47.

https://electricbike.com/forum/foru...t-52v-11-5ah-huffy-parkside?p=72784#post72784


and terminate manually exactly when you want to.

 

[rg12]

I have an issue with packs not balancing with 3 different BMS (different brands) and also different charger brands.
They are all trimmed to about 0.4v under max pack voltage but they don't charge all the way up nor balance because one of the cell groups reached 4.2v before the rest and a 20S pack sits at 82.5v and I left it in the charger for 24 hours and it didn't move even 0.01v with two different chargers.
What am I missing here?

 

[TommyCat]

I have an issue with packs not balancing with 3 different BMS (different brands) and also different charger brands.

So you must be checking the individual cell groups. How far off are they? Did you make the packs?

They are all trimmed to about 0.4v under max pack voltage

This sounds like your de-rating the chargers? If so,why are you doing this?

but they don't charge all the way up nor balance because one of the cell groups reached 4.2v before the rest and a 20S pack sits at 82.5v and I left it in the charger for 24 hours and it didn't move even 0.01v with two different chargers.

Did the fully charged cell group (4.2v) drop any after the 24 hour time period? Did the different chargers start up and run at all when first connected?

 

[rg12]

I have an issue with packs not balancing with 3 different BMS (different brands) and also different charger brands.

So you must be checking the individual cell groups. How far off are they? Did you make the packs?

They are all trimmed to about 0.4v under max pack voltage

This sounds like your de-rating the chargers? If so,why are you doing this?

but they don't charge all the way up nor balance because one of the cell groups reached 4.2v before the rest and a 20S pack sits at 82.5v and I left it in the charger for 24 hours and it didn't move even 0.01v with two different chargers.

Did the fully charged cell group (4.2v) drop any after the 24 hour time period? Did the different chargers start up and run at all when first connected?

They are pretty far off, I built 3 packs from used cells of same brand and model and matched the IR of all cells.
They range from 3.85v to 4.07v (I know a manual balance is necessary but the BMS doesn't even try to balance.

I didn't trim the chargers, they came like that, did it only once with my lipo pack which doesn't have a BMS.

The fully charger 4.20v didn't drop even 0.01v during the 24 hours, that's the problem, I know that BMS bleeds super slow but this is just standing still.

 

[rg12]

In the 16S they range from 3.95V to 4.10V and non of them reached 4.2V and the charge just stopped at 64.6V while the charger is putting out 66.8V
Don't know whats going on in here.

 

[TommyCat]

Well this has drifted a bit from the original thread title. Perhaps if you where to start a new thread. And try concentrating on one particular pack, charger and specs. You might get more helpful responses.

Battery building is something I have yet to attempt. Salute to you for doing so.

But something is truly amiss. Why is every charger .4 volts less than optimal? Why is ending charge stopping too low.
I would probably double check my BMS installation(and specs) with the fully charged cells not dropping their voltage to balance charge.
And a good look at how you weeded out your used cells for usage perhaps.

Glad you got the one charger sorted out. (I'd still recommend a BMS... )


Best regards,
T.C.

 

[TommyCat]

A typical resistive top balancer will only correct for minor imbalances - and only when held at 100% charge (4.2VPC) for a while. If you used old (disparate) laptop cells, for example, no amount of top balancing will give you good performance. Some cells will leak and drain on their own, some cells will have significantly lower capacity and some cells will have a higher ESR (thus triggering the protection function early during high rate discharge.) You will never see performance better than the weakest cell group, and sometimes even less than that (if you have a leaky cell.)

If you have a bunch of disparate cells you need something like a balancing charger and/or a full time balancer. Those are more expensive and complex.

Seems applicable...

 

[999zip999]

This can just be the behavior of your used cells. If you have a high cell you can bring that one down with a car tailight bulb .

 

[rg12]

This can just be the behavior of your used cells. If you have a high cell you can bring that one down with a car tailight bulb .

The BMS should lower the 4.20V group, even if super slow, it should still do it but it doesn't for some reason.

 

[TommyCat]

Do you have the specs on the BMS? Link, picture of it?

 

[rg12]

Do you have the specs on the BMS? Link, picture of it?