Simple Diy Lipo Balancer/charge Guard

[Dan Baldwin]

As always, the circuit depicted here is for experimental purposes only. Any person using this circuit, or any part of this circuit does so at their own risk.

It uses a latching relay, so basically no power is drawn (app 1 MA quiescent for LM431s) when a normal charge is in progress. The relay is rated at 5 amps max per contact, and 2 contacts are used in parallel, so it should be good for 10 amps max in theory, although I wouldn't try to push it that hard. Since it uses relay contacts instead of a FET, there is very little resistance in the charge path, so it should have almost no effect on charge duration. Once the relay trips out, it will disconnect the charger from the battery, light the 'tripped' LED, and it will stay in this condition until it is manually reset by pressing the reset button. The LED draws power from the battery (app 20 MA max), so don't leave it attached to the pack for long periods of time. The latching relay stays in the normal reset position, so the reset button should not have to be pressed when starting a charge unless there has been an overcurrent trip.

THEORY OF OPERATION

The 3 balancers operate exactly the same as the balancers I posted in a previous thread (linked below). They are shunt balancers that will begin conducting when the voltage on the cell being protected reaches 4.2 volts (or whatever voltage you set). Under normal circumstances, the individual cells within the pack should be very close to the same voltage, or in balance. Any currents needed to bring the pack back into balance should be very small, on the order of a few MA. If the balance current ever gets above 600 MA, the charge guard will trip out the latching relay, and disconnect the charger from the pack. Lower trip currents can be set by using a larger resistor for R10. 2 ohms would give approximately 300 MA trip current. The same circuit will work with 2 cell or 3 cell packs. Just leave the unused balancer disconnected if charging 2 cells. This circuit can be used for higher cell counts, but some values will have to be changed. Hookup and calibration of the balancers will be identical to the balancers shown here (http://www.rcgroups.com/forums/showthread.php?t=270580).

Caution
Some chargers may be damaged by being disconnected from the pack while charging. Use of this circuit could damage those chargers.

This circuit MAY work with an Astro Flight 109 charger, since it has a short trip delay, but I have not tested it. If I get a chance, I will test it later.


Mouser Parts List for 3 cell balancer/charge guard

1 655-V23079B1201B301 5 AMP DUAL COIL LATCHING 5 VOLT RELAY 2.60
3 512-1N4148 100 V 200 MA DIODE .04
3 652-MFR185 RESETABLE FUSE .58
3 511-TIP137 PNP POWER DARLINGTONS .68
3 531-PT10MH-1K 1K OHM POT .40
4 604-L934SGC T-1 GRN 300 MCD 50 DEG SUP BRITE LED .18
3 511-TL431ACZ ADJUSTABLE SHUNT REGULATOR .28
4 511-2N3906 PNP GENERAL PURPOSE SIGNAL XISTOR .49
4 511-2N3904 PNP GENERAL PURPOSE SIGNAL XISTOR .36
3 567-290-1AB T0-220 HEAT SINK .38
3 532-4880M TO-220 mounting kit for TIP137 .77
3 75-516D107M035 100 mfd cap .28
1 140-XRL50V1.0 1 UF 50 VOLT ELECTROLYTICS .06
6 581-BF014D0104K .1 mfd cap .11
3 72-RWM410-1-5 1 ohm 3 watt .32
3 271-10K 10 K 1% RESISTOR .09
3 271-15K 15 K 1% RESISTOR .09
6 660-CF1/4L104J 10OK 5% RESISTORS .05
6 660-CF1/4L333J 33K 5% RESISTORS .05
3 660-CF1/4L103J 10K RESISTORS .05
1 660-CF1/4L222J 2.2K RESISTORS .05
1 660-CF1/4L102J 1K 5% RESISTORS .05
1 660-CF1/4L621J 620 OHM RESISTOR .05
6 660-CF1/4L331J 330 OHM 5% RESISTORS .05
8 660-CF1/4L121J 120 OHM 5% RESISTOR .05
2 78-1N5231B 5.1 VOLT .5 W ZENER DIODES .04
1 N.O. PUSH BUTTON

THE FOLLOWING SIGNAL TRANSISTORS CAN BE SUBSTITUTED
FOR THE 2N3904 AND 2N3906.
4 512-KSA708YTA PNP 60 VOLT SIGNAL TRANSISTOR .06
4 512-KSC1008GTA NPN 400 HFE 60 V SIGNAL TRANSISTOR .06


Dan

[TMetalMan]

Thanks Dan! Looks like another project keeping me from doing my school work :p.

[Dan Baldwin]

Actually, this was supposed to go in DIY electronics, but I messed up. Just waiting for a moderator to move it.

Dan

[Acetronics]

HI, Dan

"Caution
Some chargers may be damaged by being disconnected from the pack while charging. Use of this circuit could damage those chargers."

Why not use , as it was done years ago for stereo amplifiers, a relay to connect a dummy load power resistor, instead of the battery ... ???
now mosfets could do it quicker ...
chargers would then only see a "batt failure" ...

Alain

[Dan Baldwin]

I will post a 'dummy load' circuit if anyone wants to see it.

Dan

[rick.benjamin]

I look forward to the "dummy load" circuit also.
I'll put your name on this project also.

[Dan Baldwin]

I'll post it later today

Dan

[Dan Baldwin]

A load resistor is not needed with any non-boosted charger, such as the DIY chargers that I have posted, or the chargers that use LM317s. It's my understanding that most boosted-computer controlled chargers are not damaged by disconnecting the battery while charging either, but I have read some accounts of people damaging their chargers in this way.

The schematic shown below was the simplest method I could come up with to put a dummy load on the charger when the charge guard trips out. I rearranged the contacts on the lower part of the latching relay so that both sets of contacts are still used in parallel for charging, but when it trips out, a resistor is put across the charger. The 15 ohm resistor is just a suggestion-a starting point. Hopefully the charger will see the voltage change when the charge guard trips out, and shut down with an error. Of course, I cannot guarantee that this circuit will save your charger.

I'm planning on disconnecting the battery while my Electrifly Polycharge 4 is charging later today to see if it dies. I'll post the results.


Dan

[Dan Baldwin]

I put a 3S 1800 MAHR pack on the electrifly polycharge 4, waited until it was in normal charge (flashing LED), and disconnected the battery. The voltage went up to abou 25 volts, and the charger stopped with an error. I tried the test twice, and there was no damage to the charger.

That's the only "factory" lipo charger I have, so it's the only one I can test. I don't think I want to borrow any charger to see if I can kill them.

Dan