So I have THIS MAGICSHINE MJ808 LIGHT and ther battery has a display that shows the battery level and the voltage output. I’ve not really paid much attention to it but now I’ve noticed that as the battery level gets lower, so does the voltage. Is this normal? and if it is, why does this happen?

I’m not looking for a Magicshine/cheap Chinese light abuse about it, I am genuinely interested into why this happens.

as it power decreases it loses voltage – its normal but power curves vary between different batteries

Imagine you sprint flat out over time do you think your power reduces as you tire and you need a recharge- that is essentially what happens with the battery

YOU may tire abruptly and run out of power after say 30 minutes or it might trail of gently from the maximum over say an hour

Massively simplified analogy to explain the process not the physics/chemistry

does it tell you what the voltage is ?

I think around 3.7v (or 7.5 if 2 in series) is the typical stated “working” voltage when a Li-ion battery has a fair bit of charge left (full ones are a bit more, 4 or so I think). If it drops much below that – even to 3.5 or 3.6 I’d be looking to dim the beam and look for routes home

It starts at 8v and is empty at 6v or 6.1v.

right, so that’s dropping each cell (I assume 2 in series) from 4ish to 3ish

I’d call 3ish very empty and potentially damagingly so but I’m not any sort of expert on the subject. Course, your meter may not be accurate (though volts isn’t that hard to measure, I don’t think)

LiIon cells normally come off the charger around 4.2V and have a safe cutoff at 3.0V which is why the “nominal” voltage is normally quoted at 3.6V (sometimes 3.7V).

Obviously from fully charged, the voltage will drop over time when the light is in use. Usually, if there is control circuitry in the light, it will sense the voltage the battery pack is supplying and step down the light level when nearing some pre-set safety level. If you continued to use the light and run the cells right down then normally the control circuitry will cut the light at say 3.0V or if the cells have a protection circuit, that will cut-out (at the same 3.0V level) to prevent the cells being drained to the danger point which is usually around 2.7V.

When I say danger point I don’t mean flash-boom! but more likely internal damage to the cell shortening the life (assuming they could even be brought back from that point).

EDIT: Just to add, that’s why it’s not a good idea to use unprotected cells in a light which doesn’t have control circuitry. If your light comes with a sealed battery pack then there is a good chance that the manufacturer has ensured that there is protection in there somewhere to prevent over discharging (and over charging for that matter).

If you have a light which takes discrete individual cells then unless it’s specifically stated, it’s probably safer to use protected cells.

It’s completely normal, but the LEDs in bike lights are usually driven by a constant current driver so the brightness of the light doesn’t change as the battery voltage drops.

Does the reported voltage change at all with when the lights are turned on/off? I’ve come across some cells with significant internal resistance, such that their reported voltage drops quite a lot when loaded.

When I say danger point I don’t mean flash-boom!

Over discharging a Li Ion cell can be dangerous in the flash boom sense when you come to recharge them, as it’s possible for a short circuit to form inside the cell when it’s in this state which is why the protection circuit is so important.

So long as it’s normal that’s fine. I just wondered if I had a dodgy battery. Thanks all.