How to measure Li-Po battery leakage?

Question

I'm working on a low-low power system, which is powered by a little solar panel and a Li-Po (Lithium-ion Polymer) battery, which can be \$20\$ or \$ 50\:\mathrm{ mAh}\$. The capacity by now is not important, since it's only a buffer to store the energy given by the panel, that should guarantee autonomy.

Today we were discussing the fact that Li-Po batteries have a leakage, that somewhere (I don't know the source) is indicated to be the 20% of the capacity in one month. So at first glance, seems that the smallest battery, as long as it can store the necessary energy to survive when there is no light.

But I'd like to measure how much is this leakage; the idea was to feed the battery with a small current (around \$ 1\:\mathrm{ \mu A}\$) and check if the battery voltage increases or decreases over a long time. But I'm not very aware of the chemical processes happening inside, and I think that this technique may involve other factors. So there is a better way than charging the battery and waiting to see it discharge?

Update

For reference, the battery we're currently using are Fullriver toy series 301213HS10C and 501417HS10C (same page). As can you seen, the manufacturer doesn't give these (and many other) specifications, so I want to measure it by myself.

The battery is charged by a LTC4071 shunt battery charger which has a really small (550 nA) operating current and which we have measured having less than 10 nA of leakage when the battery voltage is higher than the panel one.

And for me \$20 \:\mathrm{ \mu A} \$ are critical, since the panel in the worst case we consider (about \$50-100 \:\mathrm{Lx}\$, wich is in a room with no direct light on the panel) deliver \$10-20 \:\mathrm{ \mu A }\$, and the module consumes about \$10 \:\mathrm{\mu A} \$, taking the duty cycle into account.

Answer 1

The discharge rate dependes of several factors and will vary from manufacturer to manufacturer.

A good Li-Poly typically has 2-5% self-discharge rate per month. If it has a PCM (protection circuit module), that module will draw some current, expect less than 1uA for such a tiny cell. Parts of lesser quality may draw 30uA or more.

Self-discharge increases with temperature. I believe it also varies with cell construction (cylinder, prismatic etc.) Quality varies a lot from manufacturer to manufacturer so you have to ask for the exact specs.

UPDATE

I don't have much experience working with the current levels you mention. Wouldn't the ground current in the charger consume most of the 10-20uA? There is a lot of interest in energy harvesting and someone else can probably give a better answer. Slow-charging the battery is OK provided you cut off at 4.2V. My guess is that 10-20uA is not enough to offset self-discharge and other losses, but I'm frequently wrong.