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Magellan not working with anythng but original battery


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07-05-16, 12:19 AM   #1 (permalink)  
Magellan not working with anythng but original battery

I am trying to wire a Magellan GPS to work with a set of 1.25 NIMH batteries, but the darned thing just won't work with them. I checked the polarities, measured the voltages. It seems the original battery turns it on when it has 3.5V or more. So I used 3 x 1.25 NIMH AA cells + 1 dummy placeholder to complete the circuit for about 3.8 volts (dropped to 3.4V after connection), but it still won't work. Connecting a charged 3.7 V cellphone battery had no effect. Even when I connected the original cell with the same alligator clips it did not work. But if I tape the harness back onto the original battery, for some reason it works. The only other pin on the battery harness is TH (temperature?), but where the harness attaches to the battery, there doesn't seem to be a metallic connection to the cell. So as long as the temperature sensor is present, it should be sufficient.

If I turn the unit on while it's connected via USB, it goes on, but turning off USB causes the unit to "hiss" and the screen to freeze and distort. Disconnecting the cells at that point results in shutdown.

Upping the voltage with another 1.25 cell had no effect.

I tested the harness pins and I know it's going through and that polarity is correct.

The only thing I can think of at this point is the alligator clips are not providing sufficient current due to low contact area.

Any ideas?

 
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07-05-16, 04:51 AM   #2 (permalink)  
What was the original battery pack? Is it NiMh or NiCd and how many cells?

 
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07-05-16, 11:53 AM   #4 (permalink)  
Your battery is lithium. That complicates matters a bit. Lithiums are very sensitive to being over discharged so most devices sense the battery voltage and turn off when the battery drops to a certain point. It's possible that the 3.4 and 3.5 volts you have tried are less than the GPS needs to sense a "good" battery.

Also, the third wire in your pack could be a temperature sensor or if the pack is a 1s2p (two cells in parallel) then the third wire is used for balance charging the two cells. But it being a single cell with a temperature sensor is most likely.

Now that we know your device originally had a lithium battery I would not recommend powering it off a different chemistry. You won't hurt the GPS but there are several safeguards built into devices that use lithiums that make them difficult to convert to other battery chemistries. The most notable are high and low voltage cutoffs during operation and cell balancing and temperature during charging.

 
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07-05-16, 12:14 PM   #5 (permalink)  
So should a 3.7 volt Lithium Ion cellphone battery work? Because that also didn't work, but I guess I could try again. It works in the phone.

 
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07-05-16, 12:44 PM   #6 (permalink)  
Are you certain your connections are good? Just holding a wire to a contact might not be good enough.

 
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07-05-16, 12:55 PM   #7 (permalink)  
I'll be sure to measure the voltage at the harness terminals near where they connect to the GPS unit.

 
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07-05-16, 04:18 PM   #8 (permalink)  
Just tried a 3.7 V Lion LGIP-580NV that had about 4V in it. Unit will not start. The only difference is that this cellphone cell has a little less capacity (9% less) but this wouldn't stop the GPS from starting. I took out the harness right after trying the gps and measured it at the harness pins where they connect to the GPS and got a 4V reading.


Last edited by kotterr; 07-05-16 at 04:39 PM.
 
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07-05-16, 05:20 PM   #9 (permalink)  
You mention "harness" several times but how are you making good electrical contact between a battery and device that have incompatible connectors?

 
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07-05-16, 06:52 PM   #10 (permalink)  
Alligator clips. I tested with a voltmeter and the voltage was there.

I realize this is not reliable for long-term use, but I was just trying to test it out. The voltmeter confirmed there was voltage there.


Last edited by kotterr; 07-05-16 at 08:27 PM.
 
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07-06-16, 08:47 PM   #11 (permalink)  
Finally got it to work! There were two issues:

First, 3 cells + a dummy was not enough voltage. It needs 4 AA batteries at 1.25 each for 5 volt total capacity Below that for some reason does not work despite the fact that the original cell indicates it should. Regardless, 4 AAs show as "fully charged" on the unit.

Second, there was a faulty connection at ground. No idea why. It must depend on the position of the wire which is why my voltmeter fooled me.

I must have been swinging back and forth from one problem to the next, not knowing that I had two at the same time.

4 cells should be a safe bet: The total voltage is 5V which equals USB voltage (the power source) meaning they will likely never be overcharged. The unit will just charge them until it reads sufficient power level and stop.

 
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07-06-16, 10:27 PM   #12 (permalink)  
The only thing I did not connect is the temp sensor. Will this cause any problems?

 
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07-07-16, 09:07 PM   #13 (permalink)  
So the batteries lasted a while, but then failed suddenly. I used a voltmeter and confirmed that the device was NOT charging the cells whether I had 3 cells or 4 cells. I put 3 cells in there + 1 dummy to complete the circuit and played around with the voltmeter. It appears that it will only charge the batteries (according to the voltmeter) when the white GPS harness wire is connected to TH on the battery harness and when P- is connected to ground. Connecting red (live) appears to have no effect so I soldered P- and TH as described based on my earlier findings. Hopefully it will now charge the cells. I figured I would use 3 x 1.25 for a nominal value of 3.75, which is very close to 3.7 (nominal value of original cell) so that the GPS can correctly estimate cell charge and charge accordingly.

 
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