| General > General Technical Chat |
| Lithium AA and AAA cells - interesting behaviour. |
| (1/6) > >> |
| paulca:
My mouse just told me that the battery in it is under 10% charge. All normal, except it is currently powered by a lithium replacement AA cell. My understanding is that these would produce close to bang on 1.5V until the lithium dropped under cut out and would then produce 0V. How does the mouse know it's 10%? The only thing I can think of is... if they had the brilliant idea to use a "Fuel gauge" type affair and then alter the buck converter output to, say, 1.2V when close to lithium cut out. Further they could just replicate the good part of the charge curve of a traditional AA cell. I wasn't expect it in a set of 'amazon' AA replacements though. |
| IanB:
I would say that when the lithium ion voltage drops below the minimum threshold of the step down converter, that it would not suddenly stop working (unless they designed an off switch into the system). Rather, what would happen is that the output voltage would gradually decrease as the input voltage decreases, especially when the mouse draws such a low current. This is the sort of behavior I think you would see if you connected a DC/DC buck converter to a variable voltage supply, and then gradually decreased the input voltage while watching the output voltage? The converter would gradually begin to lose regulation and its output voltage would start to sag without suddenly dropping off a cliff. |
| tom66:
It's possible it is clever enough to measure the lithium-ion battery voltage and adjust the 1.5V output as that gets low, I've not seen any that do that but it seems like an interesting idea to maintain battery SOC indication. I don't think the converter is dropping out as the cell voltage would have to fall below the safe limit for a lithium cell, around 2.5V. |
| IanB:
I think this could also be a deliberate feature of the cells, so that you have an early warning to change batteries, rather then experiencing a sudden failure at an inconvenient moment. |
| Monkeh:
Are you sure you're not using lithium primary cells? They have a discharge curve like any other cell, if not necessarily as steep as alkalines. They certainly don't just drop to 0. |
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