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I have a device that takes 2 disposable 3.6 V 1/2AA 1100 mAh lithium-thionyl batteries and I would like to replace them with rechargable ones. It is a device that measures angles in wireless mode, and I only get a life span of 10 hours for 2 disposable batteries, not nice for the environment at all.

I understand I need to make sure the voltage matches. But what about the mAh that it outputs to the device? Does it have to be the same? Can they be higher? I'm having problems finding an exact match...which is what brought me here.

And also does it have to be a lithium thionyl chloride rechargable or can it be a different type? I am clueless when it comes to electronics...

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And also does it have to be a lithium thionyl chloride rechargable or can it be a different type? I am clueless when it comes to electronics... –  Daniela Birkelbach Oct 29 '12 at 14:12
    
You should be editing the additional information you provided in the comments back into the original question. –  Dave Tweed Oct 29 '12 at 14:48

3 Answers 3

Few things to take into account when replacing the batteries, are the cells conneted in parallel (higher current) or in series (higher voltage)? Does your device have a regulator and reverse voltage protection? Shorting an LiSOCl will just make it warmer, shorting a LiPo cell might cause an explosion.

You said it is a wireless device, 2 1/2AA LiSOCl cells in parallel can provide a 200mA pulse, check if your intended replacement cell can provide it.

Self discharge: LiSOCl cells have low self discharge and very long shelf life. Rechargeable cells have much higher self discharge and if the cell drops below a certain threshold, it cannot be used anymore.

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maH Is just a way of the manufacturers trying to tell you how long the battery will run for. In general more maH = longer run time for you, in practice there's a lot of other factors that affect run time. Anyway in short if you find batteries that can do the same voltage you don't need the same maH rating to have it work, but you may get less battery life.

I don't know your application but lithium-thionyl batteries can supply a lot of current if they need to. So there may be a reason your manufacturer picked those, or they may just have liked the battery life.

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Great, thank you! So, if I can find 3.6V even with a much higher maH it should be okay? What I have is a device that measures angles in wireless mode, and I only have a life span of 10 hours for 2 disposable batteries, not nice for the environment at all. Thanks again. –  Daniela Birkelbach Oct 29 '12 at 14:29
    
Yeah I'd think you should be fine in that case. –  Some Hardware Guy Oct 29 '12 at 14:40

Lithium primary cells are intended for long life and the mAh capacity is rated for low drain. However efficiency losses are poor when draining much faster, so your capacity is much less than the rated 1100 mAh in 10 hours. Higher loading current also tends to reduce the cell voltage. The cut-off specs are often 2V but this might not be the case for your device. If it has a higher cut-off, then you get a lower capacity in mAh.

The open circuit cell voltage of Li-Th is around 3.67V but should not drop below 3V for a new battery under pulse load and also depends on size of cell.

IMHO I suspect you are getting less than 50% capacity in 10 hrs so I estimate your average current is 55mA for 550mAh and 1/2AA cell voltage may be as low as 3.2V under load.

If you can shut-off your device faster between usage or look for lithium metallic oxide cells in the same voltage range of 3.6 to 3.7V and higher capacity. Do not try to run cells in parallel without expert guidance and avoid water if they fail by leakage.

The cheaper "Li-FePO4" cells that operate at 3.2V might not be a good match for voltage, but it is hard to tell.

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