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Mar 03, 2008, 02:19 AM
Crashing on a budget.
dusk's Avatar
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iMax B6 service menu

A while back, I bought the iMax B6 charger and like most people, found the voltages to be out. The I proceeded to change the resistors for ones of higher accuracy, using the kit from this thread:

However, even after the upgrade, the voltages were out. They were roughly balanced, but were still out by up to 0.05V each, finishing the charge in the vicinity of 12.48V for a 3 cell lipo.

That got me thinking about how resistor values would be compensated for in the factory and I came to the conclusion that the only option would be to adjust the voltages via some sort of service menu.

To cut things short, my search produced access to two "hidden" adjustments:

1. Press and hold "stop" and "inc" buttons during power on and you get a screen that looks as follows:

VOL(25.2) XXXX

The XXXX is a 4 digit number that constantly changes, until it settles on a value, and (at least for me) the following screen appears:


where the XXXX is the last number that was shown in the previous screen.

2. Press and hold "dec" and "enter" buttons while powering on and you get the following:

XX VOL: 12.52V

The XX value is adjustable from +20 to -20 and changes the value after VOL:
Then when you press "enter" the screen shows:

I had a play with these and the only thing that I could work out is that the second option adjusts the total voltage reading during charge.
I plugged in a battery and adjusted VOL: until the reading matched the actual voltage of the battery (I got -16).

After this, during charge, the battery voltage became accurate! However, the individual cell voltages were still out. What's worse, the charger appeared confused. It would alternate between the incorrect voltage from the individual cells and the correct total voltage. So only a partial success.

At a guess, the first service menu might be used to set the cell balancer voltages, but I have not figured out how to use it. Any ideas?


Menu 1 can be used to adjust the balancer calibration. However, it can only be used once. The menu disappears once the calibration is complete. If the values captured during calibration are incorrect, then you will be stuck with incorrect balancing.


Firmware compatible with this charger has now been posted within this thread (Post 167). If you feel capable of reprogramming the ATMega chip via an ISP programmer, then the calibration menu can be re-enabled.
Last edited by dusk; Jan 12, 2011 at 01:17 AM. Reason: Update
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Mar 03, 2008, 02:54 PM
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I checked my B6 charger: it has the same hidden menus indeed! I am not sure what they are meant for.
But still I think that changing the resistors to 0.1% types solves the unbalance. I have sold about 60 kits now, and the feedback I got was either they screwed up because the parts are quite small, or that the charger was accurate after the upgrade.

Is it possible that you overheated the parts while soldering?

Mar 03, 2008, 07:49 PM
Crashing on a budget.
dusk's Avatar
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Ben, your kit is great and works well. I would definitely recommend it.

However, even with very accurate components, there might still be a small error (and yes, I have checked my soldering and resistor values both in and out of circuit ). But more to the point, in my case it looks like the charger is applying an offset on some of the readings (ie. the more inaccurate components are giving a more accurate reading displayed by the charger).

Which is where we get to these service menus. My guess is that the manufacturer calibrates the charger at the factory, but because some of the components have very poor accuracy, even at maximum compensation the voltage readings are still out.

It would be nice if we could work out if these hidden menus are what the manufacturer uses for calibration. It is almost a dead certainty that menu 2 is used to calibrate the total voltage reading.

I'm wondering what would be the most efficient way for a manufacturer to calibrate the individual balancer voltage readings. My current guess is that they plug in a quality voltage source into the balancer plug and then let the software compare that against what it is measuring. That would correlate with the 25.2V displayed in the menu.
Mar 03, 2008, 10:12 PM
Registered User

Maybe they calibrate with a known 12.6 volts on the input.
Mar 03, 2008, 11:11 PM
I guess 4.2*6=25.2 it's a 6 cell charger.
Mar 04, 2008, 05:23 PM
-20 to +20 that's correction to value showing on display. If your battery is 12,42V (multimeter) but on B6 display is 12,51V you need correct it by -9.
That's simple and easy.
Mar 04, 2008, 08:50 PM
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Hazy's Avatar
Originally Posted by opit
-20 to +20 that's correction to value showing on display. If your battery is 12,42V (multimeter) but on B6 display is 12,51V you need correct it by -9.
That's simple and easy.

Does it affect the balancing of your lipo batt?
Mar 04, 2008, 09:16 PM
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dusk's Avatar
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No, menu 2 does not affect the balance tap voltages. Only the overall termination voltage.

The +/- 20 does not appear to directly correspond to 0.01V measured. I would say that it is more likely a propotional adjustment. So at 25.2V (6 cell), each step might be 0.01V, but at when using 12.6V battery (3 cell) it is only about 0.01V per two steps.

Either way, if you are more than about 0.2V out on a 6 cell, then you will not have enough adjustment available to correct for the error. In which case, the only option is to switch your resistors for more accurate ones, as per Ben's mod.
Mar 05, 2008, 06:25 AM
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dusk's Avatar
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Good news.

I used two 3S batteries in series to make a 6S pack. Plugged it into the charger and went into menu 1. The screen did it's usual scan through the numbers, then settled on a value and then proceeded to scan through each of the 6 cells, finishing with 6 OKs and a 4 digit value.

I had a quick go at "charging" the pack and the balance voltage readouts have different errors than before.

Next step, do the same thing but with properly charged and balanced batteries.
Mar 05, 2008, 08:39 AM
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dusk's Avatar
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Be warned. Since I have set the voltage readings using menu 1, I have not been able to access menu 1 again. The usual button combination no longer brings it up.

So if you are going to use it, make sure you do it with fully charged, accurate and well balanced batteries.
Mar 07, 2008, 02:19 AM
Registered User
Hi Dusk, I can only access the menu 2 on my B6.

It is interesting that how your menu 1 no longer appear after you did the calibration. Maybe there is a way to bring it back again. Just hope it can be found.
Mar 10, 2008, 12:27 AM
It is a good news!
Mar 16, 2008, 11:54 AM
Registered User
@Dusk: any news on the service menu?

Mar 16, 2008, 06:42 PM
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dusk's Avatar
Thread OP
No news. I'm no longer able to access the first menu. It looks like that menu is meant as a one-off run to be done at the factory.

As a guess, I would say that some chargers might be so far out of norm, that the factory calibration simply fails, leaving menu1 available to customers. Replacing components with more accurate ones enables the calibration to be completed.

The charger's micro-processor is locked, so there is no way to have a look at the code.

On a positive note, the ISP header is there and properly connected, so it would be easy enough to re-flash the processor with new firmware and start again. However, I cannot see the charger's firmware available anywhere. I have emailed SkyRC, but haven't had any response so far.
Mar 17, 2008, 01:40 PM
Registered User
I tried the same as you did Dusk, but with 2 quite good balanced 3S packs ( all cells about 4.17V).
The charger recognized the connected cells and set all measured cell voltages to 4.20V (so I should have used completely full packs of 4.20V per cell!!)
After that I couldn't reach service menu 1 either anymore.

So I think the best way to threat this charger is:

1. use my kit to exchange the bad resistors for good and stable ones
2. Connect 6 accurate 4.20V voltage sources to the balance connector and go to service menu 1 for calibrating (use fully charged perfectly balanced Lipo packs or powersupplies for this).
3. Connect an accurate 12.60V voltage source (Lipo or power supply) to the banana outputs, go to service menu 2 and adjust the voltage reading on the display to 12.60V

And like Dusk already said: be very careful when using service menu 1, because after calibrating it is not accessible anymore!!


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