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640 posts

Ultimate Geek
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Topic # 119676 10-Jun-2013 10:51 Send private message

Does anyone know what gives with these charging circuits? My Chinese GPS unit seemed as if it wouldn't hold a charge and kept wanting to shutdown, even when connected to a charging source. So, with some difficulty (much easier a second time!!), I got into the innards.

The bty has an integral charger circuit and has red, black and white wires going to the GPS mobo. Thinking that the bty was stuffed, I was surprised to find it was reading 3.9V. Not the bty then. The white wire comes from the charger cirduitry via an external small capacitor so it must carry an AC signal.

At this point I am stumped. Would very much like to know how these power systems work but it is not looking good at this stage for my GPS unit. Pity, as it otherwise works fine.

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2824 posts

Uber Geek
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  Reply # 833624 10-Jun-2013 16:08 Send private message

What is "bty"?

Most devices take 5V DC in, some 12V DC.

The battery has an internal circuit that protects it from over/under charge.

Also be very careful with cheap Li-ion batteries, after researching for a new product development I'll never again buy anything with an unbranded battery in it.




You can never have enough Volvos!




640 posts

Ultimate Geek
+1 received by user: 69


  Reply # 833643 10-Jun-2013 16:35 Send private message

I know about Li batteries (btys!!) and their charging needs and voltages but something is telling my unit that the battery is so low that the GPS needs to shutdown even when connected via a third party car USB charging socket.

I have fully charged the battery via a mains USB charger and it took the battery up to 4.15V. Close enough to fully charged for a Lipo cell.

The white wire seems to feed a voltage to the mobo. Fully charged the voltage on the white wire was 0.92V and it dropped as the battery charge went down. Maybe the component that looks like a capacitor isn't a cap after all.

The GPS has gone down to critical battery level after 20 min. Wow, a full 20 min! The battery voltage is 3.6V and the white wire is down to 0.02V.

Looks like the battery is a dog with very low capacity. Guess I will have to look around for a replacement. Could be tricky, especially as it will need to be hooked up to the original battery's charger circuitry.

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Uber Geek
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  Reply # 833697 10-Jun-2013 18:20 Send private message

Oh, so "bty" is an abbreviation for battery. Please don't use cryptic abbreviations or no one will try to help you. I think this is actually in the forum usage agreement.

All USB chargers are not the same. Do you still have the original?

All USB cables are also not the same, some have too much resistance and cannot handle the pulse currents from a (cheap) charger.

I have never seen a battery with an integral charger, you might be referring to the battery protection module (over voltage/current and under voltage/current disconnect). The charger will be on the main board and will have an inductor (for the switch mode charger) which could look to you like a capacitor.

So the white wire is a 3rd wire between the battery and circuit board? Most likely a temperature sensor, on brand name batteries it is often a digital interface to an authentication IC. On yours it can still be a digital 1-wire interface and the voltage you see is just the average of a very fast digital data stream, but unlikely.

4.15V is AVERAGE for a LiPoly battery, some are 4.1V and some are 4.2V termination voltage. It depends on what the manufacturer specifies for his specific mix of chemistry.

Have you left it discharged for long? This damages Li batteries. How old is it? Typical life if looked after is 300 cycles.

You can get batteries from eBay, Deal Extreme, etc., but I would not. The unbranded (and fake) batteries can be very dangerous as LiPoly cells release oxygen when they fail which "fuels the fire". Worst is a GPS etc. which is left in a car and thus higher at risk of catching fire.

If manufacturers would just wake up and switch to LiFePO4 which is totally safe (someone please tell Boeing!). At work we are short circuiting 12Ah cells and 20 AWG wire with no issues.




You can never have enough Volvos!




640 posts

Ultimate Geek
+1 received by user: 69


  Reply # 834532 11-Jun-2013 22:55 Send private message

This GPS is not that old and has not been through many charge cycles. It looks like it can't get to a low enough voltage to damage the battery (it squeals and shuts off at 3.6V).

I have tried several 12V to USB chargers and they all charge at 300mA with the gps turned off. With the charger and screen on, the current draw is 400mA. As I only do short car trips seems like the battery doesn't get charged fully so the GPS soon shuts off.

The only voltage in is 5V from the USB port so, yes, the circuitry on the battery will be for over/under control.

Yes, no-name batteries are a gamble and I don't want to gamble with my car and house (garage under house). Agree, Life technology is much safer but the development isn't there compared with Lipo.

The irony is that I don't want to be able to use this unit outside the car at all. (My phone has gps for portable use).

I really want it to be powered from the car battery, via the USB port, so that it will be instantly on for use as a rear camera monitor. And I want it to go into sleep mode when the car is not being used so it doesn't take several minutes to boot up and load the navigation software when the car is started. It does function like this for a short while till the battery runs low.

I am not at all sure this is possible now I have been thinking it through. The USB power would have to come from a direct lead from the car battery as the cigarette lighter power goes off when the car key is off. That's awkward but doable but if power was to stay on on the GPS, sleep mode would have to be manually set and manually overridden as sleep mode is normally triggered by the external power being removed. Not sure how pratical this would be and permanent power-on has safety issues.

To sum up, the GPS needs supplementary external charging which would be more practical with a higher capacity battery.

Back to the drawing board.


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