Any experts on Li-Ion Batteries? Need some help

[jonny round boy]

I've recently bought a small GPS tracker.

It comes with 2 identical batteries, both 3.7v 1000mAh Li-Ion. These are BL-5B cell phone batteries. With one of those installed, it will last a couple of days, but I want to increase that. The batteries would be charged via a permanently-attached USB cable, though power would only be available intermittently, hence the need to increase the battery life.

Here's how they fit into the tracker:

[attachimg=1]

As you can see there are three terminals; +ve, -ve, and one other. I don't know what this is for, possibly a temp sensor in the cell pack? Or some other charging circuitry within the battery?

Would it be feasible to connect both batteries in parallel by taping them together & soldering links across the terminals like this?

[attachimg=2]

I'd have to do it in a way that the terminals would still connect when installed, but that's not a problem. I'd also need to ensure (as accurately as I can) that both batteries were in the same charge state whilst making the connections, so there was no excessive current flow between the two.

Obviously the back cover would no longer fit on the tracker, but that is not a problem, it will be mounted inside an enclosure anyway.

Would this work? Specifically, would both batteries charge and discharge correctly (obviously it would take twice as long to charge)?

 

[Wuffles]

Wouldn't that double the voltage or the amps or both?

I know nothing by the way. Nothing.

 

[jonny round boy]

Wouldn't that double the voltage or the amps or both?

I know nothing by the way. Nothing.

If they were connected in series, that would double the voltage; you'd effectively end up with a 7.4v 1000mAh pack. Connecting them in parallel would result in a doubling of the capacity, i.e. a 3.7v 2000mAh pack (which is what I'm after).

 

[Wuffles]

Bought a book on electronics once, lost the plot at page 5, so I bought the Dummies Guide, managed about 20 pages.

Oddly I am fine with house electrics. Took me a while to get the water to come out of the 16mm2 cable I installed for the shower. Forgot to remove the earth strand didn't I, then there was enough room to squeeze the water through.

 

[Davej]

Bought a book on electronics once, lost the plot at page 5, so I bought the Dummies Guide, managed about 20 pages.

Oddly I am fine with house electrics. Took me a while to get the water to come out of the 16mm2 cable I installed for the shower. Forgot to remove the earth strand didn't I, then there was enough room to squeeze the water through.

You'd get a better flow rate if you ripped out the neutral. [smile]

 

[keithu]

I would be a bit nervous about connecting the batteries in parallel. There seem to be a few potential problems involving failure modes where one battery discharges through the other. In addition the third connection on each battery is for a temperature monitoring thermistor. If you connect them together the thermistors will be connected in parallel and the charging circuit will see a resistance of only half the correct value - I don't know what effect that would have.

Have you considered ditching the batteries and using a USB "power bank"? Something like one of these:
http://www.amazon.co.uk/Ultra-Compact-Lipstick-Sized-Flashlight-Lightning-Blackberry/dp/B001HG1ZFK
They have input and output USB ports so you can power the tracker through its USB port and charge the power bank at the same time. 

 

[GarryMartin]

Thinking a little differently, couldn't you use one of the high-power USB batteries to *charge* the GPS battery, with your external power source charging the high-power USB battery?

Power source >>>> USB Battery >>>> GPS Tracker

Then you wouldn't be messing with the GPS Tracker battery or circuitry at all. Clearly it's a Bluetooth Audio device and not a GPS Tracker in the photo - but you get the idea...

[attachimg=1]

 

[Wuffles]

Bought a book on electronics once, lost the plot at page 5, so I bought the Dummies Guide, managed about 20 pages.

Oddly I am fine with house electrics. Took me a while to get the water to come out of the 16mm2 cable I installed for the shower. Forgot to remove the earth strand didn't I, then there was enough room to squeeze the water through.

You'd get a better flow rate if you ripped out the neutral. [smile]

I was doing my bit for saving water. I'm not a monster!

 

[jonny round boy]

I would be a bit nervous about connecting the batteries in parallel. There seem to be a few potential problems involving failure modes where one battery discharges through the other. In addition the third connection on each battery is for a temperature monitoring thermistor. If you connect them together the thermistors will be connected in parallel and the charging circuit will see a resistance of only half the correct value - I don't know what effect that would have.

Have you considered ditching the batteries and using a USB "power bank"? Something like one of these:
http://www.amazon.co.uk/Ultra-Compact-Lipstick-Sized-Flashlight-Lightning-Blackberry/dp/B001HG1ZFK
They have input and output USB ports so you can power the tracker through its USB port and charge the power bank at the same time.

Keith,

Thanks for that, that is exactly the type of reply I was hoping for. Your point about the thermistor connection makes sense, and it's a good reason not to do what I was planning to do.

I hadn't even considered the power bank option - that makes much more sense!