Battery Capacity Tester

I found myself in need of a recharge­able bat­tery tester, so I went off to search the inter­net for a DIY tester.
I was pleas­ant­ly sur­prised to find that John Lowen, had built a Rechargeable Battery Capacity Tester (v2.0), and also devel­oped the PC inter­face soft­ware for desk­top use.
Even bet­ter I found that Paul V. had also built the same tester and had made sev­er­al improve­ment in the design and software.

I already had a spare INA219 high-side bi-direc­tion­al DC cur­rent and volt­age sen­sor break­out board made by Adafruit, a pro­to­type sin­gle L2 MOSFET board that I had built last year, and a Teensy 3.2 micro­proces­sor, so all I need­ed was a board to tie it all together.

The Teensy 3.2 has a DAC out­put so I decid­ed to uti­lize that instead of the orig­i­nal PWM scheme. I also added sev­er­al oth­er bells & whis­tles just for future upgrades if need­ed. I kept the orig­i­nal 10K cur­rent adjust poten­tiome­ter, along with sev­er­al switch inputs on head­er J5.
I added a encoder head­er J6, an alter­nate pow­er head­er J7, and an I2C dis­play head­er J1.
The IXTX200N10L2 MOSFET has a turn-on volt­age of around 3 volts, so I used an op-amp with an off­set volt­age of 2.5 V and a gain of 0.62 to max­i­mize the DAC’s out­put range of 0 to 3.3 volts. The off­set was pro­vid­ed by a MCP1525 2.5 volt ref­er­ence in a TO-92 pack­age.
The sys­tem is lim­it­ed by the INA219B board to 3.2 amps and 26 volts, but this could be increased to 6.4 amps by chang­ing the INA219 boards cur­rent sense resis­tor to 0.05 ohms with a degra­da­tion in sens­ing res­o­lu­tion. For my needs the 3.2 amp spec­i­fi­ca­tion works just fine.
The choice of com­po­nents was deter­mined main­ly by what parts I had in stock, and the only items pur­chased were the buzzer, the bias resis­tors for the op-amp, and the print­ing of the cir­cuit board by OSH Park.

I did have some issues with the ver­sion 2.2 board such as a bad U3 Eagle CAD foot­print, a miss­ing capac­i­tor, and a miss­ing con­nec­tion to the MOSFET ter­mi­nal block, which were all tak­en care of in the 2.4 board ver­sion. I also removed the C4 capac­i­tor as I had includ­ed it just in case the DAC did­n’t work out, and I need­ed to revert to PWM mode. 

I used a Bud Industries IP-6131 case for installing the boards and con­nec­tions. For mount­ing and sup­port­ing the boards I used arc-resis­tant GPO3 fiber­glass with a thick­ness of 1/8″ (3.2mm). A small piece of the GPO3 was also used to stiff­en the back of the banana jacks.

15 gauge strand­ed sil­i­cone test-lead wire was used from the pos­i­tive front pan­el jack to the fuse and then to the cur­rent sen­sor. Solid 16 gauge wire was used from the cur­rent sen­sor to the MOSFET drain con­nec­tion, and a dou­ble sol­id 16 gauge con­nec­tion from the MOSFET source to the front pan­el neg­a­tive jack.
The fuse hold­er is a Littlefuse Inc. 01550320ZXU which is capa­ble of 20 amps, but fused to 3 amps using an ATO 32 volt blade fuse.

For the L2 MOSFET assem­bly, the board was left unpop­u­lat­ed except for the MOSFET, a 14 gauge jumper from CS1 to CS4, a jumper across R1, and a 2 pin head­er at J1 con­nect­ed to CS1 (Gnd) and R1 (Gate). The sense ampli­fi­er and cur­rent sense resis­tor is not need­ed in this appli­ca­tion. I placed mul­ti­ple 1/4″ holes in the top sides of the case to allow air­flow for the heatsink.

A small 12 volt 60mm fan oper­at­ing at 5 volts was includ­ed for high­er pow­er test­ing, but is not need­ed for use below 2 watts. I may add a soft­ware con­trolled switch in the future that only turns the fan on when need­ed.
The dis­play is a RGB 4 line Negative LCD dis­play from Adafruit with one of my I2C to RGB LCD / EEPROM inter­face boards attached.
Everything is nor­mal­ly pow­ered thru the USB con­nec­tion to the host computer.

Both the LCD dis­play and the Windows PC soft­ware update the data in real-time. The LCD also dis­plays the DAC set­ting and the per­cent error from the tar­get mA setting.

The above graph was gen­er­at­ed by the Windows PC soft­ware after test­ing a 3600 mAh Lithium bat­tery with a 500 mA dis­charge cur­rent for 7 hours and 17 minutes. 

Link to John Lowen’s Version 2.0 Windows PC pro­gram
Eagle CAD V7.7 board and schemat­ic Ver. 2.4 Zip file
Eagle CAD board and schemat­ic files RGB I2C Display with EEPROM
Eagle CAD board and schemat­ic files Single L2 MOSFET and heatsink
Arduino sketch Battery Tester Rev. 3.3 for 4x20 dis­play Zip file