How to Test LiPo Battery Packs

Lithium Polymer Batteries (LiPo’s) have now been with us for many years and have become an essential part of this hobby. Their low discharge rate is perfect for devices that don’t draw considerable current, and the cell’s compact footprint makes them easy to squeeze into tiny devices. The challenge with LiPo batteries lies in their charge and discharge profile because unlike nickel or lead-based batteries, LiPo cell voltage is not self-limiting. Without a specifically designed charger, the battery voltage would continually increase until it bursts into flames (a generally frowned upon outcome in electrical design). Discharging LiPo batteries without proper protection is only marginally better, and will result in cell damage without restricting the operating voltage to a very specific range. If you want to test the condition of your LiPo battery pack then use a multimeter and you can test the voltage and mAh.

Look on the label on the LiPo battery pack. You need to find out the output voltage and mAh rating. The voltage figure is always a multiple of 3.7 and the mAh always a multiple of 2,100. For example, a LiPo battery pack using three cells has a voltage of 11.1 and mAh of 6,300, it is 5000mah lipo 2s. The figures on the label are the figures you can expect to get when you test the battery using the multimeter.

Measuring Voltage

To start, let’s measure voltage on a AA battery: Plug the black probe into COM and the red probe into mAVΩ. Set the multimeter to “2V” in the DC (direct current) range. Almost all portable electronics use direct current), not alternating current. Connect the black probe to the battery’s ground or ‘-’ and the red probe to power or ‘+’. Squeeze the probes with a little pressure against the positive and negative terminals of the AA battery. If you’ve got a fresh battery, you should see around 1.5V on the display (this battery is brand new, so its voltage is slightly higher than 1.5V).

If you’re measuring DC voltage (such as a battery or a sensor hooked up to an Arduino) you want to set the knob where the V has a straight line. AC voltage (like what comes out of the wall) can be dangerous, so we rarely need to use the AC voltage setting (the V with a wavy line next to it). If you’re messing with AC, we recommend you get a non-contact tester rather than use a digital multimeter.

Set the multimeter to read voltage. Check the two terminals on the battery pack to determine which is positive and which is negative. They are labeled “+” and “-” respectively.

Read the voltage output on the multimeter display. It reads the same as on the battery pack label. If the voltage is more than five percent less then it’s likely the battery is losing charge quickly as you have only just charged it.

Set the multimeter to measure mAh. Place the two sensors on the battery terminals as before. Read the mAh on the multimeter display. It is the same as the label on the LiPo battery. If the mAh is more than five percent below the figure on the label it’s likely you have a problem with your 14.8 lipo battery pack as it should not lose energy after just getting charged.

Leave the LiPo battery overnight. Do not use it as you need to see if it loses charge while not in use. A good battery pack has the same readings the next day.

How to test for aging lipos

As a possible rule of thumb, you might find that if a 3s has an IR more than around 50-60 milliohms, it is probably past its best. Based on my experience with 6s lipos, they seem to show other signs of aging before getting up to around 20 milliohms per cell (like the 3s), so perhaps around the same value 50-60 milliohms for the 6s pack as well.

The overall % of capacity seemingly dropping (ie. the charger will tell you that you pack has, say 20% left when you put it on charge, and yet you only took out 62% based on mAh put back in compared with rated mAh, so it only adds up to 82%)

You can’t measure the battery voltage without taking the battery packs apart because the voltage converter in the battery pack will provide a constant voltage (5.0V for USB, etc.) regardless of the battery voltage. However, you can estimate the battery voltage by determining when the power packs shut off. Lipo and li-ion cells are considered depleted when then fall below 3.3-3.5V, so the circuitry inside the battery pack should shut off then.

If you have direct access to the cells, you can also rely on the voltage info given by your hobby charger. When discharging, the Accucel displays the voltage, and that’s the battery voltage under load.