Convert Alkaline Batteries-powered Multimeter to Li-Ion / Li-Pol

Tired of replacing batteries in your Multi-meter or other alkaline batteries-powered gadget? Here is a solution: convert it to Li-Ion/Li-Pol.

I’ve did it for my 3 multi-meters (9v and 4.5v) and a Transistor-Tester (9v), so, let me share my experience and help you to pick up the best option for your particular multi-meter.

I would consider several options:

1. Optocoupler as a power driver: recommended for modern multi-meters, 9v or 4.5v (AA/AAA batteries) powered with auto-switch off function;
2. MOSFET as a key with auto-switch off feature: recommended for old-style meters without auto-switch feature.

You would need:

• Li-Ion/Li-Po battery (with or without protection), salvaged from the old smart phone or a new one:
• 23x38mm 612338 3.7v 600mAh Li-Po Battery that fits 9v battery compartment:
• https://www.aliexpress.com/item/33059718615.html
• 20x50mm 102050 3.7V 1000mAh Li-Po Battery: https://www.aliexpress.com/item/33008307357.html
• Li-Ion/Li-Po TP4056 charging module with protection or without protection – depends on the battery you are going to use:https://www.aliexpress.com/item/1005001636470790.h...
• DC-DC step-up boost module, either one of:
• MT3608 DC-DC Boost modules: https://www.aliexpress.com/item/32862758581.html
• SX1308 DC-DC Boost module:https://www.aliexpress.com/item/1005001548151243.h...
• MT3608 & TP4056 Combined module Li-Io/Li-Po Battery Charger and DC-DC Step Up Boost: https://www.aliexpress.com/item/33014407148.html
• new MT3608-based miniature 22x11mm module (5pcs): https://www.aliexpress.com/item/4001054709058.htm...
• Opto-couplers:
• Regular PC817 (LED max current of 50mA): http://got.by/3acu6h
• PC817 in SMD case: https://www.aliexpress.com/item/32855579632.html
• NEC PS2501 (LED max current of 80mA):https://www.aliexpress.com/item/33005015558.html
• Connectors: XH2.54 connector set 230pcs: http://got.by/3dmbwb

The idea of using Li-Ion/Li-Po battery and a DC-DC step-up module to power a multi-meter is not new. However, the serious drawback of the simple solution is that the step-up boost module consumes power draining out a battery even if your devise is off. The current in idle mode is about 2mA (RE datasheet for MT3608) means that, for example, a 500mAh battery will be discharged in about 10 days whether you use your device or not… Good news is that almost all step-up chips like MT3608, or SX1308, are equipped with Enable pin (EN) and, if used, the consumption of the chip in shut down mode can be decreased to a marginal 1uA extending battery life to years. Unfortunately, EN pin is permanently enabled in Chinese modules.

Sure, you may simply add a power switch cutting the line from battery to the DC-DC boost module's input. Disadvantages of the simple solution:

• you may forget to switch off, so the DC-DC module will consume energy even though the meter may be auto switched off by timeout;
• need to cut/drill a gadget case adding a power switch (still, may be OK for old meters).

Let us discover more advance way of exploiting EN pin and make it work in background.

Optocoupler drives EN pin: How does it work

Please, refer to schematic above.

The idea is to use an optocoupler at the DC-DC module’s output. Optocoupler LED performs as regular diode then. When in a dormant state, positive potential goes through inductor and Schottky diode of the DC-DC boost module to the output OUT+. The output potential would be Vbat – Vschottky diode = 4.2 - 0.3 = ~3.9v for a fully charger Li-Ion/Li-Po battery. The meter is off, no current is consumed by the meter, optocoupler's LED is off, optocoupler's transistor is off/closed, EN pin is pulled down to GND by a resistor; overall consumption is minuscular, in uA range. (See picture above: 3.41v is a positive potential on at module's output with a gadget switched off and the DC-DC module in a dormaint state).

When you switch on the meter it starts consuming power, the current increases going through and lighting up opto-LED causing opto-transistor to open and pull EN pin to a positive potential. DC-DC starts working and increases output voltage to the required 9v or 4.5v, so multimeter performs as usual. (See picture above: 4.24v (a bit lower than 4.5v, still good enough for meter to operate) is a working voltage at module's output with a meter switched on and DC-DC module in a voltage boosting working mode).

When you switch off the meter, or it is switched off automatically by timeout, the power consumption drops, opto-LED goes off, opto-transistor closes and EN pin is pulled to GND by resistor switching off the DC-DC module. That’s it!

Limitation: gadget’s consumption should not exceed 50mA for a popular PC817 optocoupler (in most cases multi-meters consume 10-20mA).

The limitation may be overcome by use of more "powerful" optocouplers like, for example, NEC PS2501 with LED max current of 80mA: https://www.aliexpress.com/item/33005015558.htm

1. MT3608’s EN pin is connected to IN+ by default. Cut the trace to the MT3608’s pin #4 to disconnect it from pin#5 (see pictures).
2. Clean the solder mask on the GND trace (OUT-) close to the pin#4
3. Solder 100k-560k resistor connecting pin#4 to GND (any resistor from that range would work)
4. Solder optocoupled to OUT+ pin as it is on the picture: pins 1 and 4 of the optocoupler to be connected to OUT+ of the module
5. Attach pin 3 of the optocoupler to EN pin#4 of the chip
6. Attach pin 2 of the optocoupler to IN+ of your gadget.
7. Attach OUT- of the module to IN- of your gadget.
8. Tune up/change the charging current to fit the Li-Ion/Li-Po battery you use (should be 0.5 of its value)
9. Done!

Same steps would be for SX1308-based modules since SX1308 chip is pin-2-pin compatible with MT3608.

MT3608’s EN pin is connected to IN+ by default. Cut the trace to the MT3608’s pin #4 to disconnect it from pin#5.

Clean/scrap the solder mask on the GND trace (OUT-) close to the pin#4, get ready to solder a pull-down resistor.

Any resistor in the range of 100k-560k would fit. I use 560k.

Pins 1 and 4 of the optocoupler to be connected to OUT+ of the module.

Also connect OUT- of the module to IN- of your gadget. IN- and OUT- and the same (common GND).

As you may see on pictures, I soldered a short pieces of a nikel strip to positive and negative contacts to connect them to batteries contacts in the battery compartment.

For reference: Li-Po battery size is 801730.

I also provide examples of a similar modules modifications by the same approach: classical "Blue pill" MT3608 module modification.

By default the charge current is 1A (a default resistor Rprog = 1.2k) which could be too high for a battery you use. It is recommended to set the charge current to or lower than 0.5 of the battery volume. For the popular TP4056 the formula for R=1200/Ibatt or according to the reference table above (refer to a charging chip's datasheet).

So, in my particular case for a small 100mA battery I set the charging current of 36mA by using 33k resistor.

Ready to go now!

Let's Explore Option 2!

The option is a bit more complex, requires more soldering and uses MOSFET as a switch between Li-Ion/Li-Po battery and DC-DC boost module as well as timeout switch off. The option is recommended for an old-style meter with a rotary multi-functional switch.

Two schematics above are with Common GND and Common Positive, they operate similarly but use different MOSFETs: P-channel and N-channel correspondingly.

How does it work

Let me explain based on a Common GND option: In a switch off mode MOSFET transistor is closed because its Gate is pulled up to positive potential by R3 relative to its Source. No power is at the input Vin+ of the DC-DC module, so no battery consumption at all even though the meter may be switched on in a working position.

There are two buttons: ON and OFF. So, to switch the meter you shortly press ON button. That pulls the Gate of the transistor to GND through a resistor R2, MOSFET opens delivering the power to input of the DC-DC module. It starts boosting and delivers a working voltage to the meter. The meter is ON and operates!

At the same time, pushing ON button causes an instant charging of capacitor C1 through a current limiting resistor R2 and ON button to a battery voltage potential. When you release the ON button, C1 holds the charge and keeps the Gate at a low potential keeping MOSFET open. Also, C1 starts discharging slowly through a large value resistor R3. With the values of C1 220uF and R3 of 1MegaOhm, C1 keeps MOSFET open for about 10 minutes. Eventually, C1 charge would vanish, transistor's Gate becomes positive to Source, and MOSFET goes off disconnecting DC-DC module from the battery. That's the automatic switch off function in action. If you'd like to extend the work time of the meter keeping it ON longer, just push ON button again any time refilling C1 capacitor's charge for another 10 mins.

I've also added the OFF button that essentially shorts the capacitor C1 trough a resistor R4 causing C1 instant discharge, pulling Gate to positive and closing the MOSFET. If you do not have space on your gadget for two buttons, you may still make it just with single ON button. Switching off will then happens automatically by time out only.

MOSFETS in SMD, P-channel:

• IRLML6402TRPBF - http://ali.pub/2dn0vg (20pcs), http://ali.pub/2dn0vg (50)

• AO3401A - http://ali.pub/2dn1l4 (100), http://ali.pub/2dn1l4 (100)

N-Channel:

• IRLML6344TRPBF - http://ali.pub/2dn2ev (10pcs)

Solder MOSFET and all other components to the DC-DC module on the example of classic "Blue pill" MT3608 module:

UNDER DEVELOPMENT. TO BE ADDED SOON!